CNC main processing objects

Milling is one of the most commonly used machining methods in mechanical processing. It is mainly used for face milling and contour milling, as well as drilling, extending, reaming, boring and tapping operations on parts. Parts suitable for use with CNC include:

(1) Flat parts

The characteristic of plane parts is that each machining surface can be flat or flat. At present, most parts machined on CNC milling machines are flat parts. Flat parts are the simplest category of CNC machining china objects, and can usually be processed by 2-axis simultaneous machining (ie 2-axis half-coordinate machining) of a 3-axis CNC milling machine.

Plane parts with plane contoursPlane parts with inclined planesPositive and rib plane parts

(2) Variable tilt parts

The part where the angle between the processing surface and the horizontal plane changes continuously is called a variable-angle part. To process variable tilt parts, it is best to use a 4-axis or 5-axis CNC milling machine for angle processing. If there is no such machine, you can use a 2-axis half-control line machining on a 3-axis CNC milling machine to generate approximate values, but the accuracy is slightly lower.

(3) Surface type (three-dimensional type) parts

Parts whose processing surfaces are spatial curved surfaces are called curved surface parts. The machining surface of the curved part and the milling tool are always in point contact. Usually use 3-axis CNC milling machine, there are two commonly used processing methods:

A. The processing adopts 2-axis half linkage wire cutting method. The line tangential method is to connect only two coordinates during processing, and the other coordinates are carried out with a certain line spacing period. This method is often used to process less complex spatial surfaces.

B, processing with three-axis linkage method. The milling machine used must have x, y, z 3-axis simultaneous machining function in order to perform spatial linear interpolation. This method is often used to process more complex spatial surfaces, such as engines or molds.

Second section cnc machining workpiece installation

1. Principles for cnc processing selection and positioning standards

(1) In the parts, select the design criterion as the position criterion as much as possible

Choosing the design datum as the positioning datum position can prevent positioning errors caused by mismatching datums, ensure machining accuracy, and simplify programming. When developing a machining plan for a part, first select the best finishing conditions according to the principle of meeting the conditions to specify the machining path of the part. For this reason, in the initial processing, the surface processed according to the roughness standard must be considered. (2) When the positioning datum of the part does not match the design datum, when the processing surface and the design datum are different, when processing in one installation, the part graphics must be carefully analyzed to determine the design function of the part design datum. Through the calculation of the dimension chain, strict regulations The tolerance range between the positioning datum and the design datum guarantees the machining accuracy.

(3) If the CNC milling machine cannot complete the entire surface processing including the design basis at the same time, it should be considered that the selected reference can be used for positioning, and then all the main precision parts are processed at one time.

(4) The selection of positioning standards should ensure the completion of as much processing content as possible. For this reason, the positioning method that a single curved surface can be processed must be considered. For non-rotating parts, it is best to use one and two hole positioning schemes so that the tool can machine another surface. If the workpiece does not have suitable holes, process holes can be added and placed.

(5) During the batch process, the part position reference should match the tool reference (the dimension value between the origin of the work coordinate system and the position reference after tool processing) as much as possible.

In the batch process, the fixture is used to position and install the workpiece. The tool sets one workpiece coordinate system at a time, and then processes a series of workpieces. If the tool datum set in the workpiece coordinate system matches the part positioning datum, the positioning datum is passed directly to reduce positioning errors. .

(6) If multiple installations are required, the principles of uniform standards must be followed.

Section 3 cnc machining tool exchange

Decision about knife point and knife point

For Cnc machine tools, it is important to determine the relative position of the tool and the workpiece at the beginning of the processing, and execute the tool point. “Tool setting point” means the reference point for determining the position of the tool relative to the workpiece by the tool setting. When programming, regardless of whether the tool actually moves relative to the workpiece or the workpiece moves relative to the tool, the workpiece is considered stationary and the tool is moving. The tool point is also the origin of the part.

The selection principle of knife point is as follows:

(1) Facilitates mathematical processing and simplifies programming.

(2) It is easy to find on the machine tool, and it is convenient to determine the position of the origin of parts processing;

(3) It is convenient to check during the process.

The processing error caused by (4) is small.

You can set an example tool point on a part, fixture, or machine tool, but it must have a known and precise relationship with the position reference of the part. If the tool precision is required to be high, the tool point should be selected as much as possible in the design or process benchmark of the part. For parts placed as holes, you can use the center of the hole as the tool setting point.

If facing the tool, the tool point must match the tool position. The tool position is the reference point for determining the position of the tool. For example, the processing position of the flat end mill is usually the center of the plane. The position of the ball-end mill is the center of the ball. The drill bit is the drill tip.

“Changing tool point” must be placed according to the technical content, and the principle of positioning that the workpiece, fixture and machine tool will not be encountered when the tool is exchanged. The tool point is often a fixed point, located away from the workpiece.

2. Tool setting method

The tool accuracy directly affects the machining accuracy, so the tool movement must be careful, and the tool method must meet the machining accuracy requirements of the parts.

If the part machining accuracy is high, you can use the dial indicator to find the correct tool path. The tool position is consistent with the tool point. But this method is inefficient.

At present, some factories adopt new methods such as optical or electronic equipment in order to reduce working hours and improve accuracy.

Some commonly used tool setting methods are as follows:

(1) The origin of the workpiece coordinate system (in the tool point) is the center line of the cylindrical hole (or cylindrical surface)

A, lever dial indicator (or dial indicator) tool

This working method is relatively cumbersome and inefficient, but the tool accuracy is high, and the accuracy of the tested hole is also high. Try not to use only holes that have been hinged or boring or rough holes.

B, use the edge finding knife

This method is easy to operate, intuitive, and high-precision tool, but the measured hole must be high-precision.

(2) The origin of the workpiece coordinate system (in the tool point) is the intersection of two perpendicular lines

A、Using touch knife (or test cutting) method

This working method is relatively simple, but it leaves marks on the surface of the workpiece, and the precision of the big sword is not high. To avoid damage to the workpiece surface, it is necessary to add a scale between the tool and the workpiece to subtract the thickness of the tool. In this way, standard mandrels and block gauges can also be used with knives.

B, use the edge finding knife

This operation step is similar to the tool matching tool, except that the tool moves to the radius of the contact point of the finder. The method is simple and the knife precision is high.

(3) Tool z direction tool

Tool data in the z direction of the tool is determined by the trimming length of the tool on the tool holder and the zero position of the workpiece coordinate system in the z direction. It is the zero position of the workpiece coordinate system in the machine tool coordinate system.

You can use the tool to directly touch the tool, or use the z-direction setting manager to create an accurate tool. It works in the same way as “find edges”. The tool is also used to determine the tool value using the coordinate display of the machine tool by touching the end point of the tool with the surface of the workpiece or the side head of the setter in the z direction. When fitting the tool with the “z-direction setting manager”, please consider the height of the z-direction setter.

In addition, if different tools are used as tools in machining the workpiece, the distance from each tool to the zero point of the z coordinate is different. The difference in these distances is the length compensation value of the tool, so the length of each tool (such as tool pre-adjustment) must be measured in the machine tool or special tool, and recorded in the tool list for use by machine tool workers.

Section 4 Development of cnc processing technology

Cnc processing content selection and determination

Because CNC machining has unique characteristics and application objects, in order to make full use of the advantages and key functions of CNC milling machines, the type of CNC milling machine, CNC machining objects and process content must be selected correctly. Generally, the following blanks are used as the main choice for CNC machining:

(1) The curve contour in the workpiece, especially the non-circular curve and the list curve specified by the mathematical expression;

(2) gives the space surface of the mathematical model.

(3) Complex shapes, various sizes, marking and difficult part testing;

(4) When machining with a universal milling machine, it is difficult to observe, measure, and control the inner and outer feed slots;

(5) High-precision hole or surface adjusted to size;

(6) A simple surface or shape that can be milled in a single installation;

(7) The use of CNC can increase productivity and greatly reduce the general processing content of manual labor intensity.

Vertical CNC milling machines and vertical machining centers are also suitable for processing boxes, covers, flat cams, templates, flat or three-dimensional parts with complex shapes, and the inside and outside of molds. Horizontal CNC milling machine and horizontal machining center are suitable for processing complex box parts, pump body, car body, shell, etc. Multi-coordinate linkage horizontal machining center can also be used to process various complex curves, curved surfaces, impellers, molds, etc.

Cnc processing technology analysis

(a) Graphic analysis of parts

1. Verify the completeness and accuracy of the parts drawing

The machining program is written with precise coordinate points

(1) The relationship between each geometric figure element (such as tangent, intersection, vertical, parallel and concentric) must be clear.

(2) The conditions of various geometric primitives must be sufficient, and there are no additional dimensions that cause conflicts or closed dimensions that affect the process layout.

2. Mathematical model confirmation of automatic programming parts

After establishing a complex mathematical model of curved surfaces, we must carefully study the logic of the integrity, rationality and geometric topology of the mathematical model.

Integrity——Indicate whether to express the designer’s overall intention.

Rationality—Indicate whether the surface of the created mathematical model meets the requirements of surface modeling.

The logic of geometric topological relationship-indicates whether the mutual relationship between surfaces and surfaces (such as positional continuity, tangent continuity, curvature continuity, etc.) meets the specified requirements, whether the surface is trimmed clean, whether it is thorough, etc. The first teacher of the tool motion path must be the correct mathematical model. Therefore, the mathematical model required for NC programming must meet the following requirements:

(1) The mathematical model is a complete geometric model, and the surface cannot be repeated or missing.

(2) Mathematical models cannot have multiplicity or surface overlap;

(3) The mathematical model should be a smooth geometric model.

(4) The mathematical model of the outer surface must be smooth to eliminate the subtle defects inside the curved surface;

(5) The distribution of the surface parameter curve in the mathematical model is reasonable, and the surface must have no abnormal protrusions or depressions.

(B) Process analysis and treatment of component structure

1. The size of the part drawing should be easy to program.

In actual production, the size of the part drawing greatly affects the process. Therefore, different requirements should be put forward for the part design drawing.

2. Analyze the deformation of the parts to ensure the required machining accuracy

The cutting force of the thin substrate or the ribs during the processing and the elastic retreat of the thin plate will cause the vibration of the processed surface to be very large, so the thickness tolerance of the thin plate is difficult to guarantee, and the surface roughness increases. In CNC machining, the deformation of the parts not only affects the quality of the machining, but if the deformation is large, the machining cannot continue.

Precaution:

(1) For wide and thin plate parts, improve the clamping method and use proper machining sequence and tools.

(2) Use appropriate heat treatment methods: tempering of steel parts, annealing of cast aluminum parts;

(3) Separation of rough finishing and symmetry removal to reduce or eliminate the deformation effect.

3. Unify the relevant dimensions of the arc in the part contour as much as possible

(1) Within the contour, the arc radius R often limits the tool diameter.

In a part, the numerical consistency of the concave arc radius is very important to the process performance of CNC. It is best to use uniform geometric types or dimensions for the shape and cavity of the parts, so as to reduce the number of tool changes.

In general, even if you cannot seek complete unification, you must group arc radii with similar values together to achieve local unification, minimize the cutter size and number of tool exchanges, and prevent frequent tool exchanges from The number of pickups increased and the surface quality decreased.

(2) The effect of converting the radius value of arc

The radius of the conversion arc is large, and the use of the big finger to finish the milling tool can improve the efficiency and the surface quality is better, so the process is better.

The larger the radius r of the groove bottom of the milling surface or the radius of the corner where the bottom plate and the rib intersect, the worse the function of the milling tool end milling plane and the lower the efficiency. When r reaches a certain level, it must also be processed with a ball end mill.

If the area of the milled bottom surface is large and the bottom arc r is also large, only two milling cutter points with different r can be cut.

4. Ensure the principle of uniformity of benchmarks

Some parts need to be reinstalled during processing, but the CNC cannot use “trial cutting” to pick up the tool, so reinstalling the parts usually results in the knife not being accessible. In this case, it is better to use a uniform reference position, so the part must contain an appropriate hole as a reference hole. If the part does not have a reference hole, you can also set the process hole as the reference.

(C) Process analysis of part blank

1. The blank should have a sufficiently stable machining allowance.

Rough mainly refers to forging and casting. Forging During the forging process, the residual amount may be uneven due to uncompressed pressure and tolerance coefficient. In the casting, the sand material error, shrinkage and fluidity difference of the metal liquid are not filled into the hollow, and the remaining amount is uneven. In addition, the difference between the deformation of the blank and the deformation distortion may lead to an inappropriate and unstable processing residue.

For this reason, when designing to add a suitable amount of blank to the unprocessed surface represented by the part array, it must be fully considered.

2. Analysis of the adaptability of the blank folder

Mainly consider the position of the blank on the processing surface. For blanks without editing, it is recommended to add additional editing margin or auxiliary standards such as process studs and process lugs in the blank.

3. Analysis of blank deformation, margin size and uniformity

Analyze the degree of deformation during and after rough machining, and consider whether preventive measures and improvement measures are needed. In the hot rolling process, the thick plate is easily processed and deformed after quenching and aging, preferably the quenched plate after stretching treatment.

For the size and uniformity of the blank, the main consideration is whether to perform layered milling during processing and multi-layer milling. This problem is particularly important in automatic programming.

Processing process segmentation

In Cnc machine tools, especially in the machining center, the process of processing parts is very concentrated, and many parts only need to install a card to complete all processes. However, the rough machining of the parts, especially the reference plane and positioning surface of the blank parts, must be completed in the general workpiece machine tool, and then installed in the CNC workpiece machine tool for processing. In this way, the characteristics of the CNC machine tool can be exerted, the accuracy of the CNC machine tool can be maintained, the service life of the CNC machine tool can be extended, and the use cost of the CNC machine tool can be reduced. The method of machining parts in Cnc machine tools is as follows:

1. Method for centralized sorting of tools

is to use the same knife to process all possible parts of the part, and use the second knife and the third knife to complete the other parts of the tool to divide the process. This split sequence method can reduce the number of tool changes and compress the empty path time, thereby reducing unnecessary positioning errors. 2. Roughing and finishing methods

This sequence method is based on the principle of distinguishing between roughing and finishing of the shape and dimensional accuracy of the parts. Rough machining, semi-finishing and finishing machining individual parts or parts placement. It is best to distinguish between the arrangement and the reliability and convenience of the fixture at any time between roughing and finishing, so that more surfaces can be processed by one installation. For blanks without editing, it is recommended to add additional editing margin or auxiliary standards such as process studs and process lugs in the blank.

3. Analysis of blank deformation, margin size and uniformity

Analyze the degree of deformation during and after rough machining, and consider whether preventive measures and improvement measures are needed. In the hot rolling process, the thick plate is easily processed and deformed after quenching and aging, preferably the quenched plate after stretching treatment.

For the size and uniformity of the blank residual amount, the main consideration is whether to perform layered milling in processing and multi-layer milling. This problem is particularly important in automatic programming.

Select path path

The tool path is the movement path and direction of the tool during NC machining. The path is very important, because it is closely related to the machining accuracy and surface quality of the part. The general principles for determining the path are as follows:

(1) Ensure the machining accuracy and surface roughness of parts.

(2) Facilitates numerical calculation and reduces programming workload.

(3) Reduce the access path, reduce the lead time and other auxiliary time.

(4) Try to reduce the number of blocks.

In addition, when choosing a route, please pay attention to the following aspects.

Cnc processing technology parameter determination

Determining process parameters is important in process development, and the use of automatic programming is more important for the success of the program.

(a) When using a ball milling tool to process a curved surface, determine the process parameters related to the cutting accuracy

1, the step size is determined l (step distance)

Step length l (step distance)——The length of the distance between each two tool addresses determines the amount of machining address data.

The method of determining the curve track step l:

Directly define the step size method: by directly providing the step value during programming, it is determined by the machining accuracy of the part

Indirectly define the step size method: define approximate error and indirectly define the step size

2. Determination of approximate error er

Approximate error er——the maximum tolerance of the actual cutting trajectory from the theoretical trajectory

Three methods for defining approximate errors (see Figure 16-4):

Specify the external approximate error value: Use the remaining material left on the surface of the part as the error value

(If accuracy is high, usually select 0.0015 to 0.03mm) Specify the internal approximate error value: indicate the allowable amount of surface overcut

also specifies internal and external approximation errors

3, determine the line spacing s (cutting distance)

Line distance s (cutting distance)——the distance between the machining path and two adjacent tool paths.

Influence: small line spacing: high processing accuracy, but long processing time and high cost

Large line spacing: low machining accuracy, distorted part type surface, but short processing time.

There are two ways to define line spacing:

(1) Directly define the line spacing

The algorithm is simple and the calculation speed is fast. It is suitable for creating the tool trajectory of roughing, semi-finishing and finishing of relatively flat parts.

(2) Use the remaining height h to define the line spacing

Remaining height h——the height of the remaining groove between two adjacent cut lines in the normal vector direction of the processing surface.

H is greater than: large surface processing value

H is smaller: processing accuracy can be improved, but the program is longer, doubling the machine time will reduce efficiency

Select precautions.

When roughing, the line spacing becomes larger, and when finishing, the line spacing becomes smaller. Sometimes, to reduce the minimum height, you can cut the encrypted line once between the original two lines. That is to say, by performing curve peak processing, s is reduced by half, and the actual effect is better.

(B) Determine the process parameters related to the cutting amount

1, back intake AP and side intake AE

Back tool amount ap——It is the dimension of the processing layer measured parallel to the milling tool axis.

Side processing amount ae——the size of the processing layer measured perpendicular to the milling cutter axis.

Choose the cutting amount from the perspective of tool durability as follows:

First select the back tool volume AP or side tool volume AE, then determine the feed rate, and finally determine the cutting speed.

In the case where the accuracy of the parts is not high, if the rigidity of the process system allows, it is recommended to cut the machining allowance at one time to improve the machining efficiency. If the accuracy of the part is high, multiple passages must be used to ensure accuracy and surface roughness.

2. Determine the parameters related to the feed

Automatic programming of complex curved surfaces requires the following five feed rates:

(1) Rapid feed speed (empty feed rate)

In order to save non-cutting time, usually choose G00 speed, which is the maximum feed rate allowed by the machine tool.

(2) Bottom tool speed (close to the workpiece surface feed speed)

In order to approach the workpiece safely without damaging the machine tool, the tool and the workpiece, the processing speed must not be too high, and must be less than or equal to the cutting feed speed. For soft materials, usually 200mm/min; yes. For steel or cast iron, it is usually 50mm/min.

(3) Cutting feed rate f

The cutting feed rate should be comprehensively determined according to the performance of the machine tool used, the tool material and size, the cutting performance of the processing material and the size of the machining allowance.

The general principle is that the machining margin of the workpiece surface is large and the cutting feed rate is low. in contrast. The processing feed rate can be manually adjusted by the processing worker (depending on the workpiece surface being processed) to obtain the best cutting state. The cutting feed rate cannot exceed the allowable feed rate calculated by the approximate error and interpolation cycle.

suggested value:

Processed plastic parts: 1500 mm/min

Machining a large number of steel parts: 250 mm/min

Finishing of small clearance steel parts: 500 mm/min

Finishing of castings: 600 mm/min

(4) Inter-line connection speed (feedrate span)

Line-to-line connection speed-the speed of movement of the tool from one cutting line to the next.

This speed is usually less than or equal to the cutting feed speed.

(5) Retraction feed rate (retraction speed)

In order to save non-cutting time, usually choose G00 speed, which is the maximum feed rate allowed by the machine tool.

3. Determine the parameters related to the cutting speed

(1) Cutting speed c

Cutting speed c is mainly determined by the precision and material of the processed parts, the material and durability of the tool, etc.

(2) Spindle speed n

The spindle speed n is determined according to the allowable cutting speed c. n=1000c/d

Theoretically, the larger the c, the higher the productivity, avoiding the critical speed of generating chip tumors, and obtaining low surface roughness values. However, due to the limitations of actual machine tools and tools, the cutting speeds allowed when using domestic machine tools and tools can usually only be selected within the range of 100 to 200 m/min.

Tool exchange

Section IV Development of CNC Machining Technology

Cnc processing content selection and determination

Cnc processing technology analysis

CNC processing process segmentation

Cnc processing selection path

Cnc processing technology parameter determination

CNC main processing objects

Milling is one of the most commonly used machining methods in mechanical processing. It is mainly used for face milling and contour milling, as well as drilling, extending, reaming, boring and tapping operations on parts. Parts suitable for use with CNC include:

(1) Flat parts

The characteristic of plane parts is that each machining surface can be flat or flat. At present, most parts machined on CNC milling machines are flat parts. Flat parts are the simplest category of CNC machining objects, and can usually be processed by 2-axis simultaneous machining (ie 2-axis half-coordinate machining) of a 3-axis CNC milling machine.

Plane parts with plane contoursPlane parts with inclined planesPositive and rib plane parts

(2) Variable tilt parts

The part where the angle between the processing surface and the horizontal plane changes continuously is called a variable-angle part. To process variable tilt parts, it is best to use a 4-axis or 5-axis CNC milling machine for angle processing. If there is no such machine, you can use a 2-axis half-control line machining on a 3-axis CNC milling machine to generate approximate values, but the accuracy is slightly lower.

(3) Surface type (three-dimensional type) parts

Parts whose processing surfaces are spatial curved surfaces are called curved surface parts. The machining surface of the curved part and the milling tool are always in point contact. Usually use 3-axis CNC milling machine, there are two commonly used processing methods:

A. The processing adopts 2-axis half linkage wire cutting method. The line tangential method is to connect only two coordinates during processing, and the other coordinates are carried out with a certain line spacing period. This method is often used to process less complex spatial surfaces.

B, processing with three-axis linkage method. The milling machine used must have x, y, z 3-axis simultaneous machining function in order to perform spatial linear interpolation. This method is often used to process more complex spatial surfaces, such as engines or molds.

Second section cnc machining workpiece installation

1. Principles for cnc processing selection and positioning standards

(1) In the parts, select the design criterion as the position criterion as much as possible

Choosing the design datum as the positioning datum position can prevent positioning errors caused by mismatching datums, ensure machining accuracy, and simplify programming. When developing a machining plan for a part, first select the best finishing conditions according to the principle of meeting the conditions to specify the machining path of the part. For this reason, in the initial processing, the surface processed according to the roughness standard must be considered. (2) When the positioning datum of the part does not match the design datum, when the processing surface and the design datum are different, when processing in one installation, the part graphics must be carefully analyzed to determine the design function of the part design datum. Through the calculation of the dimension chain, strict regulations The tolerance range between the positioning datum and the design datum guarantees the machining accuracy.

(3) If the CNC milling machine cannot complete the entire surface processing including the design basis at the same time, it should be considered that the selected reference can be used for positioning, and then all the main precision parts are processed at one time.

(4) The selection of positioning standards should ensure the completion of as much processing content as possible. For this reason, the positioning method that a single curved surface can be processed must be considered. For non-rotating parts, it is best to use one and two hole positioning schemes so that the tool can machine another surface. If the workpiece does not have suitable holes, process holes can be added and placed.

(5) During the batch process, the part position reference should match the tool reference (the dimension value between the origin of the work coordinate system and the position reference after tool processing) as much as possible.

In the batch process, the fixture is used to position and install the workpiece. The tool sets one workpiece coordinate system at a time, and then processes a series of workpieces. If the tool datum set in the workpiece coordinate system matches the part positioning datum, the positioning datum is passed directly to reduce positioning errors. .

(6) If multiple installations are required, the principles of uniform standards must be followed.

Section 3 cnc machining tool exchange

Decision about knife point and knife point

For Cnc machine tools, it is important to determine the relative position of the tool and the workpiece at the beginning of the processing, and execute the tool point. “Tool setting point” means the reference point for determining the position of the tool relative to the workpiece by the tool setting. When programming, regardless of whether the tool actually moves relative to the workpiece or the workpiece moves relative to the tool, the workpiece is considered stationary and the tool is moving. The tool point is also the origin of the part.

The selection principle of knife point is as follows:

(1) Facilitates mathematical processing and simplifies programming.

(2) It is easy to find on the machine tool, and it is convenient to determine the position of the origin of parts processing;

(3) It is convenient to check during the process.

The processing error caused by (4) is small.

You can set an example tool point on a part, fixture, or machine tool, but it must have a known and precise relationship with the position reference of the part. If the tool precision is required to be high, the tool point should be selected as much as possible in the design or process benchmark of the part. For parts placed as holes, you can use the center of the hole as the tool setting point.

If facing the tool, the tool point must match the tool position. The tool position is the reference point for determining the position of the tool. For example, the processing position of the flat end mill is usually the center of the plane. The position of the ball-end mill is the center of the ball. The drill bit is the drill tip.

“Changing tool point” must be placed according to the technical content, and the principle of positioning that the workpiece, fixture and machine tool will not be encountered when the tool is exchanged. The tool point is often a fixed point, located away from the workpiece.

2. Tool setting method

The tool accuracy directly affects the machining accuracy, so the tool movement must be careful, and the tool method must meet the machining accuracy requirements of the parts.

If the part machining accuracy is high, you can use the dial indicator to find the correct tool path. The tool position is consistent with the tool point. But this method is inefficient.

At present, some factories adopt new methods such as optical or electronic equipment in order to reduce working hours and improve accuracy.

Some commonly used tool setting methods are as follows:

(1) The origin of the workpiece coordinate system (in the tool point) is the center line of the cylindrical hole (or cylindrical surface)

A, lever dial indicator (or dial indicator) tool

This working method is relatively cumbersome and inefficient, but the tool accuracy is high, and the accuracy of the tested hole is also high. Try not to use only holes that have been hinged or boring or rough holes.

B, use the edge finding knife

This method is easy to operate, intuitive, and high-precision tool, but the measured hole must be high-precision.

(2) The origin of the workpiece coordinate system (in the tool point) is the intersection of two perpendicular lines

A、Using touch knife (or test cutting) method

This working method is relatively simple, but it leaves marks on the surface of the workpiece, and the precision of the big sword is not high. To avoid damage to the workpiece surface, it is necessary to add a scale between the tool and the workpiece to subtract the thickness of the tool. In this way, standard mandrels and block gauges can also be used with knives.

B, use the edge finding knife

This operation step is similar to the tool matching tool, except that the tool moves to the radius of the contact point of the finder. The method is simple and the knife precision is high.

(3) Tool z direction tool

Tool data in the z direction of the tool is determined by the trimming length of the tool on the tool holder and the zero position of the workpiece coordinate system in the z direction. It is the zero position of the workpiece coordinate system in the machine tool coordinate system.

You can use the tool to directly touch the tool, or use the z-direction setting manager to create an accurate tool. It works in the same way as “find edges”. The tool is also used to determine the tool value using the coordinate display of the machine tool by touching the end point of the tool with the surface of the workpiece or the side head of the setter in the z direction. When fitting the tool with the “z-direction setting manager”, please consider the height of the z-direction setter.

In addition, if different tools are used as tools in machining the workpiece, the distance from each tool to the zero point of the z coordinate is different. The difference in these distances is the length compensation value of the tool, so the length of each tool (such as tool pre-adjustment) must be measured in the machine tool or special tool, and recorded in the tool list for use by machine tool workers.

Section 4 Development of cnc processing technology

Cnc processing content selection and determination

Because CNC machining has unique characteristics and application objects, in order to make full use of the advantages and key functions of CNC milling machines, the type of CNC milling machine, CNC machining objects and process content must be selected correctly. Generally, the following blanks are used as the main choice for CNC machining:

(1) The curve contour in the workpiece, especially the non-circular curve and the list curve specified by the mathematical expression;

(2) gives the space surface of the mathematical model.

(3) Complex shapes, various sizes, marking and difficult part testing;

(4) When machining with a universal milling machine, it is difficult to observe, measure, and control the inner and outer feed slots;

(5) High-precision hole or surface adjusted to size;

(6) A simple surface or shape that can be milled in a single installation;

(7) The use of CNC can increase productivity and greatly reduce the general processing content of manual labor intensity.

Vertical CNC milling machines and vertical machining centers are also suitable for processing boxes, covers, flat cams, templates, flat or three-dimensional parts with complex shapes, and the inside and outside of molds. Horizontal CNC milling machine and horizontal machining center are suitable for processing complex box parts, pump body, car body, shell, etc. Multi-coordinate linkage horizontal machining center can also be used to process various complex curves, curved surfaces, impellers, molds, etc.

Cnc processing technology analysis

(a) Graphic analysis of parts

1. Verify the completeness and accuracy of the parts drawing

The machining program is written with precise coordinate points

(1) The relationship between each geometric figure element (such as tangent, intersection, vertical, parallel and concentric) must be clear.

(2) The conditions of various geometric primitives must be sufficient, and there are no additional dimensions that cause conflicts or closed dimensions that affect the process layout.

2. Mathematical model confirmation of automatic programming parts

After establishing a complex mathematical model of curved surfaces, we must carefully study the logic of the integrity, rationality and geometric topology of the mathematical model.

Integrity——Indicate whether to express the designer’s overall intention.

Rationality—Indicate whether the surface of the created mathematical model meets the requirements of surface modeling.

The logic of geometric topological relationship-indicates whether the mutual relationship between surfaces and surfaces (such as positional continuity, tangent continuity, curvature continuity, etc.) meets the specified requirements, whether the surface is trimmed clean, whether it is thorough, etc. The first teacher of the tool motion path must be the correct mathematical model. Therefore, the mathematical model required for NC programming must meet the following requirements:

(1) The mathematical model is a complete geometric model, and the surface cannot be repeated or missing.

(2) Mathematical models cannot have multiplicity or surface overlap;

(3) The mathematical model should be a smooth geometric model.

(4) The mathematical model of the outer surface must be smooth to eliminate the subtle defects inside the curved surface;

(5) The distribution of the surface parameter curve in the mathematical model is reasonable, and the surface must have no abnormal protrusions or depressions.

(B) Process analysis and treatment of component structure

1. The size of the part drawing should be easy to program.

In actual production, the size of the part drawing greatly affects the process. Therefore, different requirements should be put forward for the part design drawing.

2. Analyze the deformation of the parts to ensure the required machining accuracy

The cutting force of the thin substrate or the ribs during the processing and the elastic retreat of the thin plate will cause the vibration of the processed surface to be very large, so the thickness tolerance of the thin plate is difficult to guarantee, and the surface roughness increases. In CNC machining, the deformation of the parts not only affects the quality of the machining, but if the deformation is large, the machining cannot continue.

Precaution:

(1) For wide and thin plate parts, improve the clamping method and use proper machining sequence and tools.

(2) Use appropriate heat treatment methods: tempering of steel parts, annealing of cast aluminum parts;

(3) Separation of rough finishing and symmetry removal to reduce or eliminate the deformation effect.

3. Unify the relevant dimensions of the arc in the part contour as much as possible

(1) Within the contour, the arc radius R often limits the tool diameter.

In a part, the numerical consistency of the concave arc radius is very important to the process performance of CNC. It is best to use uniform geometric types or dimensions for the shape and cavity of the parts, so as to reduce the number of tool changes.

In general, even if you cannot seek complete unification, you must group arc radii with similar values together to achieve local unification, minimize the cutter size and number of tool exchanges, and prevent frequent tool exchanges from The number of pickups increased and the surface quality decreased.

(2) The effect of converting the radius value of arc

The radius of the conversion arc is large, and the use of the big finger to finish the milling tool can improve the efficiency and the surface quality is better, so the process is better.

The larger the radius r of the groove bottom of the milling surface or the radius of the corner where the bottom plate and the rib intersect, the worse the function of the milling tool end milling plane and the lower the efficiency. When r reaches a certain level, it must also be processed with a ball end mill.

If the area of the milled bottom surface is large and the bottom arc r is also large, only two milling cutter points with different r can be cut.

4. Ensure the principle of uniformity of benchmarks

Some parts need to be reinstalled during processing, but the CNC cannot use “trial cutting” to pick up the tool, so reinstalling the parts usually results in the knife not being accessible. In this case, it is better to use a uniform reference position, so the part must contain an appropriate hole as a reference hole. If the part does not have a reference hole, you can also set the process hole as the reference.

(C) Process analysis of part blank

1. The blank should have a sufficiently stable machining allowance.

Rough mainly refers to forging and casting. Forging During the forging process, the residual amount may be uneven due to uncompressed pressure and tolerance coefficient. In the casting, the sand material error, shrinkage and fluidity difference of the metal liquid are not filled into the hollow, and the remaining amount is uneven. In addition, the difference between the deformation of the blank and the deformation distortion may lead to an inappropriate and unstable processing residue.

For this reason, when designing to add a suitable amount of blank to the unprocessed surface represented by the part array, it must be fully considered.

2. Analysis of the adaptability of the blank folder

Mainly consider the position of the blank on the processing surface. For blanks without editing, it is recommended to add additional editing margin or auxiliary standards such as process studs and process lugs in the blank.

3. Analysis of blank deformation, margin size and uniformity

Analyze the degree of deformation during and after rough machining, and consider whether preventive measures and improvement measures are needed. In the hot rolling process, the thick plate is easily processed and deformed after quenching and aging, preferably the quenched plate after stretching treatment.

For the size and uniformity of the blank, the main consideration is whether to perform layered milling during processing and multi-layer milling. This problem is particularly important in automatic programming.

Segmentation of processing

In Cnc machine tools, especially in the machining center, the process of processing parts is very concentrated, and many parts only need to install a card to complete all processes. However, the rough machining of the parts, especially the reference plane and positioning surface of the blank parts, must be completed in the general workpiece machine tool, and then installed in the CNC workpiece machine tool for processing. In this way, the characteristics of the CNC machine tool can be exerted, the accuracy of the CNC machine tool can be maintained, the service life of the CNC machine tool can be extended, and the use cost of the CNC machine tool can be reduced. The method of machining parts in Cnc machine tools is as follows:

1. Method for centralized sorting of tools

is to use the same knife to process all possible parts of the part, and use the second knife and the third knife to complete the other parts of the tool to divide the process. This split sequence method can reduce the number of tool changes and compress the empty path time, thereby reducing unnecessary positioning errors. 2. Roughing and finishing methods

This sequence method is based on the principle of distinguishing between roughing and finishing of the shape and dimensional accuracy of the parts. Rough machining, semi-finishing and finishing machining individual parts or parts placement. It is best to distinguish between the arrangement and the reliability and convenience of the fixture at any time between roughing and finishing, so that more surfaces can be processed by one installation. For blanks without editing, it is recommended to add additional editing margin or auxiliary standards such as process studs and process lugs in the blank.

3. Analysis of blank deformation, margin size and uniformity

For the size and uniformity of the blank residual amount, the main consideration is whether to perform layered milling in processing and multi-layer milling. This problem is particularly important in automatic programming.

Select path path

The tool path is the movement path and direction of the tool during NC machining. The path is very important, because it is closely related to the machining accuracy and surface quality of the part. The general principles for determining the path are as follows:

(1) Ensure the machining accuracy and surface roughness of parts.

(2) Facilitates numerical calculation and reduces programming workload.

(3) Reduce the access path, reduce the lead time and other auxiliary time.

(4) Try to reduce the number of blocks.

In addition, when choosing a route, please pay attention to the following aspects.

Cnc processing technology parameter determination

Determining process parameters is important in process development, and the use of automatic programming is more important for the success of the program.

(a) When using a ball milling tool to process a curved surface, determine the process parameters related to the cutting accuracy

1, the step size is determined l (step distance)

Step length l (step distance)——The length of the distance between each two tool addresses determines the amount of machining address data.

The method of determining the curve track step l:

Directly define the step size method: by directly providing the step value during programming, it is determined by the machining accuracy of the part

Indirectly define the step size method: define approximate error and indirectly define the step size

2. Determination of approximate error er

Approximate error er——the maximum tolerance of the actual cutting trajectory from the theoretical trajectory

Three methods for defining approximate errors (see Figure 16-4):

Specify the external approximate error value: Use the remaining material left on the surface of the part as the error value

(If accuracy is high, usually select 0.0015 to 0.03mm) Specify the internal approximate error value: indicate the allowable amount of surface overcut

also specifies internal and external approximation errors

3, determine the line spacing s (cutting distance)

Line distance s (cutting distance)——the distance between the machining path and two adjacent tool paths.

Influence: small line spacing: high processing accuracy, but long processing time and high cost

Large line spacing: low machining accuracy, distorted part type surface, but short processing time.

There are two ways to define line spacing:

(1) Directly define the line spacing

The algorithm is simple and the calculation speed is fast. It is suitable for creating the tool trajectory of roughing, semi-finishing and finishing of relatively flat parts.

(2) Use the remaining height h to define the line spacing

Remaining height h——the height of the remaining groove between two adjacent cut lines in the normal vector direction of the processing surface.

H is greater than: large surface processing value

H is smaller: processing accuracy can be improved, but the program is longer, doubling the machine time will reduce efficiency

Select precautions.

When roughing, the line spacing becomes larger, and when finishing, the line spacing becomes smaller. Sometimes, to reduce the minimum height, you can cut the encrypted line once between the original two lines. That is to say, by performing curve peak processing, s is reduced by half, and the actual effect is better.

(B) Determine the process parameters related to the cutting amount

1, back intake AP and side intake AE

Back tool amount ap——It is the dimension of the processing layer measured parallel to the milling tool axis.

Side processing amount ae——the size of the processing layer measured perpendicular to the milling cutter axis.

Choose the cutting amount from the perspective of tool durability as follows:

First select the back tool volume AP or side tool volume AE, then determine the feed rate, and finally determine the cutting speed.

In the case where the accuracy of the parts is not high, if the rigidity of the process system allows, it is recommended to cut the machining allowance at one time to improve the machining efficiency. If the accuracy of the part is high, multiple passages must be used to ensure accuracy and surface roughness.

2. Determine the parameters related to the feed

Automatic programming of complex curved surfaces requires the following five feed rates:

(1) Rapid feed speed (empty feed rate)

In order to save non-cutting time, usually choose G00 speed, which is the maximum feed rate allowed by the machine tool.

(2) Bottom tool speed (close to the workpiece surface feed speed)

In order to approach the workpiece safely without damaging the machine tool, the tool and the workpiece, the processing speed must not be too high, and must be less than or equal to the cutting feed speed. For soft materials, usually 200mm/min; yes. For steel or cast iron, it is usually 50mm/min.

(3) Cutting feed rate f

The cutting feed rate should be comprehensively determined according to the performance of the machine tool used, the tool material and size, the cutting performance of the processing material and the size of the machining allowance.

The general principle is that the machining margin of the workpiece surface is large and the cutting feed rate is low. in contrast. The processing feed rate can be manually adjusted by the processing worker (depending on the workpiece surface being processed) to obtain the best cutting state. The cutting feed rate cannot exceed the allowable feed rate calculated by the approximate error and interpolation cycle.

suggested value:

Processed plastic parts: 1500 mm/min

Machining a large number of steel parts: 250 mm/min

Finishing of small clearance steel parts: 500 mm/min

Finishing of castings: 600 mm/min

(4) Inter-line connection speed (feedrate span)

Line-to-line connection speed-the speed of movement of the tool from one cutting line to the next.

This speed is usually less than or equal to the cutting feed speed.

(5) Retraction feed rate (retraction speed)

3. Determine the parameters related to the cutting speed

(1) Cutting speed c

Cutting speed c is mainly determined by the precision and material of the processed parts, the material and durability of the tool, etc.

(2) Spindle speed n

The spindle speed n is determined according to the allowable cutting speed c. n=1000c/d

Theoretically, the larger the c, the higher the productivity, avoiding the critical speed of generating chip tumors, and obtaining low surface roughness values. However, due to the limitations of actual machine tools and tools, the cutting speeds allowed when using domestic machine tools and tools can usually only be selected within the range of 100 to 200 m/min.

Tool exchange

Section IV Development of CNC Machining Technology

Cnc processing content selection and determination

Cnc processing technology analysis

CNC processing process segmentation

Cnc processing selection path

Cnc processing technology parameter determination

Section 1 cnc main processing objects

CNC main processing objects

Milling is one of the most commonly used machining methods in mechanical processing. It is mainly used for face milling and contour milling, as well as drilling, extending, reaming, boring and tapping operations on parts. Parts suitable for use with CNC include:

(1) Flat parts

The characteristic of plane parts is that each machining surface can be flat or flat. At present, most parts machined on CNC milling machines are flat parts. Flat parts are the simplest category of CNC machining objects, and can usually be processed by 2-axis simultaneous machining (ie 2-axis half-coordinate machining) of a 3-axis CNC milling machine.

Plane parts with plane contoursPlane parts with inclined planesPositive and rib plane parts

(2) Variable tilt parts

The part where the angle between the processing surface and the horizontal plane changes continuously is called a variable-angle part. To process variable tilt parts, it is best to use a 4-axis or 5-axis CNC milling machine for angle processing. If there is no such machine, you can use a 2-axis half-control line machining on a 3-axis CNC milling machine to generate approximate values, but the accuracy is slightly lower.

(3) Surface type (three-dimensional type) parts

Parts whose processing surfaces are spatial curved surfaces are called curved surface parts. The machining surface of the curved part and the milling tool are always in point contact. Usually use 3-axis CNC milling machine, there are two commonly used processing methods:

A. The processing adopts 2-axis half linkage wire cutting method. The line tangential method is to connect only two coordinates during processing, and the other coordinates are carried out with a certain line spacing period. This method is often used to process less complex spatial surfaces.

B, processing with three-axis linkage method. The milling machine used must have x, y, z 3-axis simultaneous machining function in order to perform spatial linear interpolation. This method is often used to process more complex spatial surfaces, such as engines or molds.

Second section cnc machining workpiece installation

1. Principles for cnc processing selection and positioning standards

(1) In the parts, select the design criterion as the position criterion as much as possible

Choosing the design datum as the positioning datum position can prevent positioning errors caused by mismatching datums, ensure machining accuracy, and simplify programming. When developing a machining plan for a part, first select the best finishing conditions according to the principle of meeting the conditions to specify the machining path of the part. For this reason, in the initial processing, the surface processed according to the roughness standard must be considered. (2) When the positioning datum of the part does not match the design datum, when the processing surface and the design datum are different, when processing in one installation, the part graphics must be carefully analyzed to determine the design function of the part design datum. Through the calculation of the dimension chain, strict regulations The tolerance range between the positioning datum and the design datum guarantees the machining accuracy.

(3) If the CNC milling machine cannot complete the entire surface processing including the design basis at the same time, it should be considered that the selected reference can be used for positioning, and then all the main precision parts are processed at one time.

(4) The selection of positioning standards should ensure the completion of as much processing content as possible. For this reason, the positioning method that a single curved surface can be processed must be considered. For non-rotating parts, it is best to use one and two hole positioning schemes so that the tool can machine another surface. If the workpiece does not have suitable holes, process holes can be added and placed.

(5) During the batch process, the part position reference should match the tool reference (the dimension value between the origin of the work coordinate system and the position reference after tool processing) as much as possible.

In the batch process, the fixture is used to position and install the workpiece. The tool sets one workpiece coordinate system at a time, and then processes a series of workpieces. If the tool datum set in the workpiece coordinate system matches the part positioning datum, the positioning datum is passed directly to reduce positioning errors. .

(6) If multiple installations are required, the principles of uniform standards must be followed.

Section 3 cnc machining tool exchange

Decision about knife point and knife point

For Cnc machine tools, it is important to determine the relative position of the tool and the workpiece at the beginning of the processing, and execute the tool point. “Tool setting point” means the reference point for determining the position of the tool relative to the workpiece by the tool setting. When programming, regardless of whether the tool actually moves relative to the workpiece or the workpiece moves relative to the tool, the workpiece is considered stationary and the tool is moving. The tool point is also the origin of the part.

The selection principle of knife point is as follows:

(1) Facilitates mathematical processing and simplifies programming.

(2) It is easy to find on the machine tool, and it is convenient to determine the position of the origin of parts processing;

(3) It is convenient to check during the process.

The processing error caused by (4) is small.

You can set an example tool point on a part, fixture, or machine tool, but it must have a known and precise relationship with the position reference of the part. If the tool precision is required to be high, the tool point should be selected as much as possible in the design or process benchmark of the part. For parts placed as holes, you can use the center of the hole as the tool setting point.

If facing the tool, the tool point must match the tool position. The tool position is the reference point for determining the position of the tool. For example, the processing position of the flat end mill is usually the center of the plane. The position of the ball-end mill is the center of the ball. The drill bit is the drill tip.

“Changing tool point” must be placed according to the technical content, and the principle of positioning that the workpiece, fixture and machine tool will not be encountered when the tool is exchanged. The tool point is often a fixed point, located away from the workpiece.

2. Tool setting method

The tool accuracy directly affects the machining accuracy, so the tool movement must be careful, and the tool method must meet the machining accuracy requirements of the parts.

If the part machining accuracy is high, you can use the dial indicator to find the correct tool path. The tool position is consistent with the tool point. But this method is inefficient.

At present, some factories adopt new methods such as optical or electronic equipment in order to reduce working hours and improve accuracy.

Some commonly used tool setting methods are as follows:

(1) The origin of the workpiece coordinate system (in the tool point) is the center line of the cylindrical hole (or cylindrical surface)

A, lever dial indicator (or dial indicator) tool

This working method is relatively cumbersome and inefficient, but the tool accuracy is high, and the accuracy of the tested hole is also high. Try not to use only holes that have been hinged or boring or rough holes.

B, use the edge finding knife

This method is easy to operate, intuitive, and high-precision tool, but the measured hole must be high-precision.

(2) The origin of the workpiece coordinate system (in the tool point) is the intersection of two perpendicular lines

a. Use touch knife (or test cutting) method

This working method is relatively simple, but it leaves marks on the surface of the workpiece, and the precision of the big sword is not high. To avoid damage to the workpiece surface, it is necessary to add a scale between the tool and the workpiece to subtract the thickness of the tool. In this way, standard mandrels and block gauges can also be used with knives.

B, use the edge finding knife

This operation step is similar to the tool matching tool, except that the tool moves to the radius of the contact point of the finder. The method is simple and the knife precision is high.

(3) Tool z direction tool

Tool data in the z direction of the tool is determined by the trimming length of the tool on the tool holder and the zero position of the workpiece coordinate system in the z direction. It is the zero position of the workpiece coordinate system in the machine tool coordinate system.

You can use the tool to directly touch the tool, or use the z-direction setting manager to create an accurate tool. It works in the same way as “find edges”. The tool is also used to determine the tool value using the coordinate display of the machine tool by touching the end point of the tool with the surface of the workpiece or the side head of the setter in the z direction. When fitting the tool with the “z-direction setting manager”, please consider the height of the z-direction setter.

In addition, if different tools are used as tools in machining the workpiece, the distance from each tool to the zero point of the z coordinate is different. The difference in these distances is the length compensation value of the tool, so the length of each tool (such as tool pre-adjustment) must be measured in the machine tool or special tool, and recorded in the tool list for use by machine tool workers.

Section 4 Development of cnc processing technology

Cnc processing content selection and determination

Because CNC machining has unique characteristics and application objects, in order to make full use of the advantages and key functions of CNC milling machines, the type of CNC milling machine, CNC machining objects and process content must be selected correctly. Generally, the following blanks are used as the main choice for CNC machining:

(1) The curve contour in the workpiece, especially the non-circular curve and the list curve specified by the mathematical expression;

(2) gives the space surface of the mathematical model.

(3) Complex shapes, various sizes, marking and difficult part testing;

(4) When machining with a universal milling machine, it is difficult to observe, measure, and control the inner and outer feed slots;

(5) High-precision hole or surface adjusted to size;

(6) A simple surface or shape that can be milled in a single installation;

(7) The use of CNC can increase productivity and greatly reduce the general processing content of manual labor intensity.

Vertical CNC milling machines and vertical machining centers are also suitable for processing boxes, covers, flat cams, templates, flat or three-dimensional parts with complex shapes, and the inside and outside of molds. Horizontal CNC milling machine and horizontal machining center are suitable for processing complex box parts, pump body, car body, shell, etc. Multi-coordinate linkage horizontal machining center can also be used to process various complex curves, curved surfaces, impellers, molds, etc.

Cnc processing technology analysis

(a) Graphic analysis of parts

1. Verify the completeness and accuracy of the parts drawing

The machining program is written with precise coordinate points

(1) The relationship between each geometric figure element (such as tangent, intersection, vertical, parallel and concentric) must be clear.

(2) The conditions of various geometric primitives must be sufficient, and there are no additional dimensions that cause conflicts or closed dimensions that affect the process layout.

2. Mathematical model confirmation of automatic programming parts

After establishing a complex mathematical model of curved surfaces, we must carefully study the logic of the integrity, rationality and geometric topology of the mathematical model.

Integrity——Indicate whether to express the designer’s overall intention.

Rationality—Indicate whether the surface of the created mathematical model meets the requirements of surface modeling.

The logic of geometric topological relationship-indicates whether the mutual relationship between surfaces and surfaces (such as positional continuity, tangent continuity, curvature continuity, etc.) meets the specified requirements, whether the surface is trimmed clean, whether it is thorough, etc. The first teacher of the tool motion path must be the correct mathematical model. Therefore, the mathematical model required for NC programming must meet the following requirements:

(1) The mathematical model is a complete geometric model, and the surface cannot be repeated or missing.

(2) Mathematical models cannot have multiplicity or surface overlap;

(3) The mathematical model should be a smooth geometric model.

(4) The mathematical model of the outer surface must be smooth to eliminate the subtle defects inside the curved surface;

(5) The distribution of the surface parameter curve in the mathematical model is reasonable, and the surface must have no abnormal protrusions or depressions.

(B) Process analysis and treatment of component structure

1. The size of the part drawing should be easy to program.

In actual production, the size of the part drawing greatly affects the process. Therefore, different requirements should be put forward for the part design drawing.

2. Analyze the deformation of the parts to ensure the required machining accuracy

The cutting force of the thin substrate or the ribs during the processing and the elastic retreat of the thin plate will cause the vibration of the processed surface to be very large, so the thickness tolerance of the thin plate is difficult to guarantee, and the surface roughness increases. In CNC machining, the deformation of the parts not only affects the quality of the machining, but if the deformation is large, the machining cannot continue.

Precaution:

(1) For wide and thin plate parts, improve the clamping method and use proper machining sequence and tools.

(2) Use appropriate heat treatment methods: tempering of steel parts, annealing of cast aluminum parts;

(3) Separation of rough finishing and symmetry removal to reduce or eliminate the deformation effect.

3. Unify the relevant dimensions of the arc in the part contour as much as possible

(1) Within the contour, the arc radius R often limits the tool diameter.

In a part, the numerical consistency of the concave arc radius is very important to the process performance of CNC. It is best to use uniform geometric types or dimensions for the shape and cavity of the parts, so as to reduce the number of tool changes.

In general, even if you cannot seek complete unification, you must group arc radii with similar values together to achieve local unification, minimize the cutter size and number of tool exchanges, and prevent frequent tool exchanges from The number of pickups increased and the surface quality decreased.

(2) The effect of converting the radius value of arc

The radius of the conversion arc is large, and the use of the big finger to finish the milling tool can improve the efficiency and the surface quality is better, so the process is better.

The larger the radius r of the groove bottom of the milling surface or the radius of the corner where the bottom plate and the rib intersect, the worse the function of the milling tool end milling plane and the lower the efficiency. When r reaches a certain level, it must also be processed with a ball end mill.

If the area of the milled bottom surface is large and the bottom arc r is also large, only two milling cutter points with different r can be cut.

4. Ensure the principle of uniformity of benchmarks

Some parts need to be reinstalled during processing, but the CNC cannot use “trial cutting” to pick up the tool, so reinstalling the parts usually results in the knife not being accessible. In this case, it is better to use a uniform reference position, so the part must contain an appropriate hole as a reference hole. If the part does not have a reference hole, you can also set the process hole as the reference.

(C) Process analysis of part blank

1. The blank should have a sufficiently stable machining allowance.

Rough mainly refers to forging and casting. Forging During the forging process, the residual amount may be uneven due to uncompressed pressure and tolerance coefficient. In the casting, the sand material error, shrinkage and fluidity difference of the metal liquid are not filled into the hollow, and the remaining amount is uneven. In addition, the difference between the deformation of the blank and the deformation distortion may lead to an inappropriate and unstable processing residue.

For this reason, when designing to add a suitable amount of blank to the unprocessed surface represented by the part array, it must be fully considered.

2. Analysis of the adaptability of the blank folder

Mainly consider the position of the blank on the processing surface. For blanks without editing, it is recommended to add additional editing margin or auxiliary standards such as process studs and process lugs in the blank.

3. Analysis of blank deformation, margin size and uniformity

For the size and uniformity of the blank, the main consideration is whether to perform layered milling during processing and multi-layer milling. This problem is particularly important in automatic programming.

Processing process segmentation

In Cnc machine tools, especially in the machining center, the process of processing parts is very concentrated, and many parts only need to install a card to complete all processes. However, the rough machining of the parts, especially the reference plane and positioning surface of the blank parts, must be completed in the general workpiece machine tool, and then installed in the CNC workpiece machine tool for processing. In this way, the characteristics of the CNC machine tool can be exerted, the accuracy of the CNC machine tool can be maintained, the service life of the CNC machine tool can be extended, and the use cost of the CNC machine tool can be reduced. The method of machining parts in Cnc machine tools is as follows:

1. Method for centralized sorting of tools

is to use the same knife to process all possible parts of the part, and use the second knife and the third knife to complete the other parts of the tool to divide the process. This split sequence method can reduce the number of tool changes and compress the empty path time, thereby reducing unnecessary positioning errors. 2. Roughing and finishing methods

This sequence method is based on the principle of distinguishing between roughing and finishing of the shape and dimensional accuracy of the parts. Rough machining, semi-finishing and finishing machining individual parts or parts placement. It is best to distinguish between the arrangement and the reliability and convenience of the fixture at any time between roughing and finishing, so that more surfaces can be processed by one installation. For blanks without editing, it is recommended to add additional editing margin or auxiliary standards such as process studs and process lugs in the blank.

3. Analysis of blank deformation, margin size and uniformity

For the size and uniformity of the blank residual amount, the main consideration is whether to perform layered milling in processing and multi-layer milling. This problem is particularly important in automatic programming.

Select path path

The tool path is the movement path and direction of the tool during NC machining. The path is very important, because it is closely related to the machining accuracy and surface quality of the part. The general principles for determining the path are as follows:

(1) Ensure the machining accuracy and surface roughness of parts.

(2) Facilitates numerical calculation and reduces programming workload.

(3) Reduce the access path, reduce the lead time and other auxiliary time.

(4) Try to reduce the number of blocks.

In addition, when choosing a route, please pay attention to the following aspects.

Cnc processing technology parameter determination

Determining process parameters is important in process development, and the use of automatic programming is more important for the success of the program.

(a) When using a ball milling tool to process a curved surface, determine the process parameters related to the cutting accuracy

1, the step size is determined l (step distance)

Step length l (step distance)——The length of the distance between each two tool addresses determines the amount of machining address data.

The method of determining the curve track step l:

Directly define the step size method: by directly providing the step value during programming, it is determined by the machining accuracy of the part

Indirectly define the step size method: define approximate error and indirectly define the step size

2. Determination of approximate error er

Approximate error er——the maximum tolerance of the actual cutting trajectory from the theoretical trajectory

Three methods for defining approximate errors (see Figure 16-4):

Specify the external approximate error value: Use the remaining material left on the surface of the part as the error value

(If accuracy is high, usually select 0.0015 to 0.03mm) Specify the internal approximate error value: indicate the allowable amount of surface overcut

also specifies internal and external approximation errors

3, determine the line spacing s (cutting distance)

Line distance s (cutting distance)——the distance between the machining path and two adjacent tool paths.

Influence: small line spacing: high processing accuracy, but long processing time and high cost

Large line spacing: low machining accuracy, distorted part type surface, but short processing time.

There are two ways to define line spacing:

(1) Directly define the line spacing

The algorithm is simple and the calculation speed is fast. It is suitable for creating the tool trajectory of roughing, semi-finishing and finishing of relatively flat parts.

(2) Use the remaining height h to define the line spacing

Remaining height h——the height of the remaining groove between two adjacent cut lines in the normal vector direction of the processing surface.

H is greater than: large surface processing value

H is smaller: processing accuracy can be improved, but the program is longer, doubling the machine time will reduce efficiency

Select precautions.

When roughing, the line spacing becomes larger, and when finishing, the line spacing becomes smaller. Sometimes, to reduce the minimum height, you can cut the encrypted line once between the original two lines. That is to say, by performing curve peak processing, s is reduced by half, and the actual effect is better.

(B) Determine the process parameters related to the cutting amount

1, back intake AP and side intake AE

Back tool amount ap——It is the dimension of the processing layer measured parallel to the milling tool axis.

Side processing amount ae——the size of the processing layer measured perpendicular to the milling cutter axis.

Choose the cutting amount from the perspective of tool durability as follows:

First select the back tool volume AP or side tool volume AE, then determine the feed rate, and finally determine the cutting speed.

In the case where the accuracy of the parts is not high, if the rigidity of the process system allows, it is recommended to cut the machining allowance at one time to improve the machining efficiency. If the accuracy of the part is high, multiple passages must be used to ensure accuracy and surface roughness.

2. Determine the parameters related to the feed

Automatic programming of complex curved surfaces requires the following five feed rates:

(1) Rapid feed speed (empty feed rate)

(2) Bottom tool speed (close to the workpiece surface feed speed)

In order to approach the workpiece safely without damaging the machine tool, the tool and the workpiece, the processing speed must not be too high, and must be less than or equal to the cutting feed speed. For soft materials, usually 200mm/min; yes. For steel or cast iron, it is usually 50mm/min.

(3) Cutting feed rate f

The cutting feed rate should be comprehensively determined according to the performance of the machine tool used, the tool material and size, the cutting performance of the processing material and the size of the machining allowance.

The general principle is that the machining margin of the workpiece surface is large and the cutting feed rate is low. in contrast. The processing feed rate can be manually adjusted by the processing worker (depending on the workpiece surface being processed) to obtain the best cutting state. The cutting feed rate cannot exceed the allowable feed rate calculated by the approximate error and interpolation cycle.

suggested value:

Processed plastic parts: 150

Processing a large number of steel parts: 250 mm/min

Finishing of small clearance steel parts: 500 mm/min

Finishing of castings: 600 mm/min

(4) Inter-line connection speed (feedrate span)

Line-to-line connection speed-the speed of movement of the tool from one cutting line to the next.

This speed is usually less than or equal to the cutting feed speed.

(5) Retraction feed rate (retraction speed)

3. Determine the parameters related to the cutting speed

(1) Cutting speed c

Cutting speed c is mainly determined by the precision and material of the processed parts, the material and durability of the tool, etc.

(2) Spindle speed n

The spindle speed n is determined according to the allowable cutting speed c. n=1000c/d

Theoretically, the larger the c, the higher the productivity, avoiding the critical speed of generating chip tumors, and obtaining low surface roughness values. However, due to the limitations of actual machine tools and tools, the cutting speeds allowed when using domestic machine tools and tools can usually only be selected within the range of 100 to 200 m/min.

Tool exchange

Section IV Development of CNC Machining Technology

Cnc processing content selection and determination

Cnc processing technology analysis

CNC processing process segmentation

Cnc processing selection path

Cnc processing technology parameter determination

Section 1 cnc main processing objects

CNC main processing objects

(1) Flat parts

The characteristic of plane parts is that each machining surface can be flat or flat. At present, most parts machined on CNC milling machines are flat parts. Flat parts are the simplest category of CNC machining objects, and can usually be processed by 2-axis simultaneous machining (ie 2-axis half-coordinate machining) of a 3-axis CNC milling machine.

Plane parts with plane contoursPlane parts with inclined planesPositive and rib plane parts

(2) Variable tilt parts

(3) Surface type (three-dimensional type) parts

Second section cnc machining workpiece installation

1. Principles for cnc processing selection and positioning standards

(1) In the parts, select the design criterion as the position criterion as much as possible

(6) If multiple installations are required, the principles of uniform standards must be followed.

Section 3 cnc machining tool exchange

Decision about knife point and knife point

The selection principle of knife point is as follows:

(1) Facilitates mathematical processing and simplifies programming.

(3) It is convenient to check during the process.

The processing error caused by (4) is small.

If facing the tool, the tool point must match the tool position. The tool position is the reference point for determining the position of the tool. For example, the processing position of the flat end mill is usually the center of the plane. The position of the ball-end mill is the center of the ball. The drill bit is the drill tip.

2. Tool setting method

Some commonly used tool setting methods are as follows:

A, lever dial indicator (or dial indicator) tool

B, use the edge finding knife

A、Using touch knife (or test cutting) method

B, use the edge finding knife

(3) Tool z direction tool

Section 4 Development of cnc processing technology

Cnc processing content selection and determination

Because CNC machining has unique characteristics and application objects, in order to make full use of the advantages and key functions of CNC milling machines, the type of CNC milling machine, CNC machining objects and process content must be selected correctly. Generally, the following blanks are used as the main choice for CNC machining:

(2) gives the space surface of the mathematical model.

(3) Complex shapes, various sizes, marking and difficult part testing;

(5) High-precision hole or surface adjusted to size;

(6) A simple surface or shape that can be milled in a single installation;

Cnc processing technology analysis

(a) Graphic analysis of parts

1. Verify the completeness and accuracy of the parts drawing

The machining program is written with precise coordinate points

2. Mathematical model confirmation of automatic programming parts

Integrity——Indicate whether to express the designer’s overall intention.

(2) Mathematical models cannot have multiplicity or surface overlap;

(3) The mathematical model should be a smooth geometric model.

(B) Process analysis and treatment of component structure

1. The size of the part drawing should be easy to program.

2. Analyze the deformation of the parts to ensure the required machining accuracy

Precaution:

3. Unify the relevant dimensions of the arc in the part contour as much as possible

(1) Within the contour, the arc radius R often limits the tool diameter.

(2) The effect of converting the radius value of arc

The larger the radius r of the groove bottom of the milling surface or the radius of the corner where the bottom plate and the rib intersect, the worse the function of the milling tool end milling plane and the lower the efficiency. When r reaches a certain level, it must also be processed with a ball end mill.

If the area of the milled bottom surface is large and the bottom arc r is also large, only two milling cutter points with different r can be cut.

(c) Process analysis of parts blank

1. The blank should have a sufficiently stable machining allowance.

2. Analysis of the adaptability of the blank folder

3. Analysis of blank deformation, margin size and uniformity

Processing process segmentation

1. Method for centralized sorting of tools

3. Analysis of blank deformation, margin size and uniformity

Select path path

(1) Ensure the machining accuracy and surface roughness of parts.

(2) Facilitates numerical calculation and reduces programming workload.

(3) Reduce the access path, reduce the lead time and other auxiliary time.

(4) Try to reduce the number of blocks.

In addition, when choosing a route, please pay attention to the following aspects.

Cnc processing technology parameter determination

1, the step size is determined l (step distance)

The method of determining the curve track step l:

Indirectly define the step size method: define approximate error and indirectly define the step size

2. Determination of approximate error er

Three methods for defining approximate errors:

also specifies internal and external approximation errors

3, determine the line spacing s (cutting distance)

Influence: small line spacing: high processing accuracy, but long processing time and high cost

Large line spacing: low machining accuracy, distorted part type surface, but short processing time.

There are two ways to define line spacing:

(1) Directly define the line spacing

(2) Use the remaining height h to define the line spacing

H is greater than: large surface processing value

Select precautions.

(B) Determine the process parameters related to the cutting amount

1, back intake AP and side intake AE

Choose the cutting amount from the perspective of tool durability as follows:

2. Determine the parameters related to the feed

Automatic programming of complex curved surfaces requires the following five feed rates:

(1) Rapid feed speed (empty feed rate)

(2) Bottom tool speed (close to the workpiece surface feed speed)

(3) Cutting feed rate f

suggested value:

Processed plastic parts: 1500 mm/min

Machining a large number of steel parts: 250 mm/min

Finishing of small clearance steel parts: 500 mm/min

Finishing of castings: 600 mm/min

(4) Inter-line connection speed (feedrate span)

Line-to-line connection speed-the speed of movement of the tool from one cutting line to the next.

This speed is usually less than or equal to the cutting feed speed.

(5) Retraction feed rate (retraction speed)

3. Determine the parameters related to the cutting speed

(1) Cutting speed c

(2) Spindle speed n

The spindle speed n is determined according to the allowable cutting speed c. n=1000c/d