Machining Methods

Causes And Elimination Of Common Injuries In Metal Cutting Machine Tools

Causes And Elimination Of Common Injuries In Metal Cutting Machine ToolsCauses And Elimination Of Common Injuries In Metal Cutting Machine Tools

Some parts and components of the machine tool that are relatively sliding, such as skateboards and guide rails, shafts and sliding bearings, worms and worm gears, etc., often have scratches or grooves on their surface after a period of operation. We call this phenomenon a research injury . Grinding damages the precision of the machine tool and affects the service life of the machine tool. If the repair is not timely, the particles generated by the grinding will also increase the grinding damage. When it is serious, it can make the sliding parts of each other stop sliding, resulting in the phenomenon of seizure. Flaws are essentially wear under abnormal conditions. The common flaws on machine tools can be divided into two types according to the causes: one is adhesive wear type grinding, and the other is abrasive wear type grinding. . Here is a brief introduction to the causes, preventive measures and repair methods of these two types of injuries.

Adhesive Wear Type Research

This type of grinding refers to the friction of the friction pair during the relative movement, due to the friction between each other, the contact surface material is transferred from one surface to another surface, resulting in scratches and grooves on the friction surface.

1. The mechanism of adhesive wear type research

Studies have shown that: the solid surface condition, from the microscopic point of view, has uneven defects, even if it is polished, it cannot completely eliminate the peaks and valleys. When the two friction surfaces are in contact, the peaks of the two friction surfaces are actually in contact with each other. The contact stress is so great that elastic and plastic deformations occur, increasing the contact area until it can withstand all loads. In this case, a firm adhesion point will appear on the metal contact surface, a phenomenon known as cold welding adhesion. These adhesion points are generated without a surface film. When there is a surface film on the surface of the friction pair, only the normal force is applied, and the cold welding adhesion will not occur. If there is a tangential force at the same time, and the normal force and the tangential force are both very large, and when doing relative sliding, the temperature of the friction surface will rise, under high temperature and high pressure, it will cause the oil film to break and contact the metal surface It will soften or melt, and the contact point will produce a stick-tear-stick-tear cycle process, so that the material on the contact surface is transferred from one surface to the other surface, so that one of the surfaces (or both surfaces) The formation of scratches and grooves, that is, the formation of adhesive wear-type scratches.

2. Classification of adhesive wear type

According to the degree of damage of the surface damage of the friction pair, we can classify the common adhesive wear-type flaws on the machine tool into four categories:

(1) The smearing injury only occurs on the shallow surface of the soft metal, and the thin layer of the scratched soft metal is transferred to the hard metal surface by smearing. On the surface of the worm.

(2) Scratches and scratches occur in the shallow part below the surface layer of the soft metal. The damage method is to produce small scratches along the direction of movement. Sometimes the hard metal surface may also be scratched. The most common is at the beginning of the operation of the machine tool, the shaft and the sliding bearing are in the wear-in period of wear.

(3) The gluing (or tearing) flaw occurs on one of the two parts rubbing each other or the base of the two sides is deep. Due to the high local temperature and high pressure on the surface, the bonding strength of any base metal is sheared. Strength, when the friction pair is relatively sliding, the surface is made for one day and the monk hits the clock for one day, causing tearing damage and gluing damage. Such as cam pair, worm pair, gear pair, are more common; when the machine tool slide and guide rail lack of lubricating oil and lead to dry friction, this kind of injury often also occurs.

(4) The temperature of the instant flash on the surface of the friction pair is quite high, the adhesion area is large, and the strength of the adhesion point is also very high. The bond cannot be cut off from the substrate, which causes the phenomenon of relative motion being stopped. Bite is the most serious type of injury. For example, the “holding shaft” that appears when the lubrication of the shaft and the sliding bearing is poor, this kind of injury will occur; the large area of the injury caused by the lack of oil on the guide rails of large machine tools will also cause bite.

3. Influencing factors and preventive measures of adhesive and wear-type research injuries

There are many factors that affect such injuries, and we can take corresponding measures against these factors to prevent their occurrence and development.

(1) The factors of lubricating grease have a great influence on the adhesion and wear-type injuries, as long as the friction surface always maintains a sufficient strength lubricating film to avoid the direct contact between the friction surface and the surface of the metal to form dry friction or semi-dry friction. Effectively prevent and control the occurrence and development of adhesive wear-type grinding injury. Among the various factors that affect this type of injury, ensuring good lubrication of the friction surface is the most important factor. Adding oily or viscosity additives to the lubricating oil can improve the absorption capacity of the lubricating oil film and the strength of the oil film, which can multiply improve the ability to resist research injury.

(2) The pressure factor adhesion and wear-type flaws generally increase with the increase of pressure. When the pressure load exceeds a certain value of the hardness of the friction pair material, the oxide film on the surface of the friction pair is crushed, and the protruding points of the new surface between the two surfaces are embedded with each other. When they move relatively, this type of injury will increase sharply. Cause the surface of the friction pair to be glued or even bite. Therefore, when using the machine tool, the worktable and sliding guide rails should avoid overloading. For example, the gantry planer, milling machine, boring machine, etc., clamp the workpiece on the worktable, which must be reasonably distributed and not overweight. When the machine tool is overhauled, its guide rails are quenched, which also has a great effect on preventing such injuries.

(3) Factors of temperature Temperature also has a great influence on the occurrence of adhesive wear-type flaws. The heat generated during the friction process raises the temperature of the friction surface to a certain level. If it is light, it will damage the lubricating film and cause the metal surface to directly contact to form dry friction or semi-dry friction. Fire state reduces the hardness of the material; heavier, the local area can make the material on the friction surface in a molten state. These will promote the generation of such injuries. Selecting materials with high thermal stability or strengthening cooling is an effective method to prevent adhesive wear-type grinding damage caused by temperature.

(4) Factors of sliding speed When the pressure is constant, the sliding speed is small, the role of forming a lubricating oil film is reduced, the thickness of the oil film is small, and the oil film is often partially damaged due to the pressure of the moving parts, causing the two metals to directly contact , Easy to cause injury. However, when the sliding speed is high to a certain degree, the temperature of the friction pair is increased, which is easy to damage the lubricating oil film and also easy to cause flaws. Therefore, choosing an appropriate sliding speed can reduce the tendency of adhesive wear-type flaws.

(5) Factors of surface roughness Generally speaking, the smaller the surface roughness of the friction pair, the greater the ability to resist adhesive wear-type flaws. Properly reducing the surface roughness can prevent such flaws. For new machine tools, it is often used The purpose of gradually loading and running is to reduce the surface roughness, so as to reduce the occurrence of adhesive wear-type flaws in the early stage. However, the surface roughness of the friction pair is too low, the lubricant cannot be stored between the friction surfaces, and it is easy to cause scratches. Therefore, when the pattern on the machine tool guide rail is worn, when the machine tool is maintained, the pattern is often scraped on the top. , The purpose is to facilitate oil storage and prevent injury.

(6) Factors of materials Brittle materials are more resistant to adhesion and wear than plastic materials; friction pairs composed of materials with large mutual solubility are more likely to produce wear-type flaws than friction pairs composed of materials with low mutual solubility; metals and Non-metallic friction pairs have less tendency to produce such flaws than friction pairs composed of two metals. When overhauling the machine tool, non-metallic coating can be used to repair the slide rail, shaft and sleeve using different materials, or the surface treatment process of the shaft, etc., to reduce the occurrence of adhesive wear-type injuries.

Abrasive Abrasive Grinding

This kind of grinding refers to the hard particles polluted between the two friction surfaces, which play a significant cutting role when the surface moves, causing scratches and grooves on the friction surface.

1. The mechanism of abrasive wear-type grinding

After the hard particles enter between the two friction surfaces, they are subjected to two forces, perpendicular to the surface and parallel to the surface. Hard particles penetrate into the surface under the action of the former, but produce tangential movement under the action of the latter, which causes the surface to be plowed or cut, leaving scratches and grooves, that is, injuries.

2. Classification of abrasive abrasive wear

(1) After the bumps caused by bumps, such as the guide rails of the machine tool, are bumped by high stress of workpieces or tools, pits will appear on the surface of the rails, and bumps or burrs will appear around the pits. If it is not repaired in time, when the skateboard slides on the guide rail, the protrusions or burrs on the guide rail will inevitably grind the skateboard.

(2) Chips or molding sand between the friction surfaces of hard particles, such as cutting chips or molding sand, enters between the guide rail and the sliding plate of the machine tool, which will form a typical abrasive wear-type grinding injury: the cutting chips or molding sand crush the guide rail or the sliding plate under vertical pressure Surface, the guide rail or skateboard will be injured when moving.

(3) If dirt enters between friction surfaces, for example, dirt enters between friction surfaces through lubricating oil, although it does not press the friction surface, it can change the contact condition of the friction surface. Due to the support of the dirt, the contact position with the dirt The compressive stress increases, and after a long time of operation, it will inevitably cause scratches or tiny scratches on the friction surface.

3. Prevention of abrasive wear-type grinding injury

(1) First of all, it is necessary to prevent knocking and rubbing on the surface of the friction pair, such as not depositing tools, gauges, workpieces and other debris on the machine tool guide rails, and secondly, repairing in time after knocking and rubbing on the surface of the friction pair, such as the protrusions and burrs generated It should be smoothed in time with a whetstone or a spatula to prevent rubbing the surface of the scratch; the pits that appear should be repaired as much as possible to prevent the dirt and dirt contained in the pits from leaving hidden dangers to the scratches.

(2) Fortify and configure guards to prevent hard particles such as chips and molding sand from entering the friction surface. Once hard particles or debris are found to enter the friction surface, they should be eliminated immediately to prevent the occurrence or expansion of the flaw.

(3) Maintain the machine tool on time, remove the dirt, grease and dirt on the guide rails, slide plates, oil holes and dead corners in time to prevent the dirt from entering between the friction surfaces and forming abrasive wear-type injuries.

Repair Of Injury

Once the friction pair of the machine tool has been damaged, it must immediately stop running, find out the reason, the degree of injury, and take different measures. In general, the scratches of the smear type can be repaired, but the lubrication should be strengthened to prevent expansion; for the scratches (including the scratches caused by dirt), the dirt should be removed, and the oilstone or scraper will be used to scratch After the part is leveled, it can continue to run: while the injuries caused by gluing, biting and hard particles must be repaired. Now there are some brief introductions to the repair of several friction pairs:

1. Repair of sliding bearings and shafts

(1) After the surface of the sliding bearing is ground, the preferred repair plan should be repaired by the stonecutter method. Generally speaking, the sliding bearing has a margin for adjustment and repair, which can meet a repair cycle of the machine tool. If the research injury is very serious and the repair margin cannot be met, you need to consider replacing it. After the two-half tile sliding bearing is ground and injured, the gasket can be thinned and reassembled, and the inner hole of the ground bush can be scraped to require accuracy, and it can be used after repair.

(2) When the journal is damaged, the method of grinding the journal and replacing the sliding bearing can be used. However, the grinding of the journal should not be too large, such as the spindle of the machine tool, the amount of grinding should not exceed the quenching of the surface of the journal, or the thickness of carburizing, nitriding, cyanide, and the hardness of the surface of the journal after grinding should not be lower than the lower limit of the original design hardness ; For the drive shaft, the journal should not be smaller than the size of the front journal or boss in the assembly direction during assembly.

If the repair method is used to repair the journal, the influence of the welding repair process on the geometric accuracy, mechanical strength and surface hardness of the shaft must be considered. After the journal is repaired, it is generally necessary to reconfigure the journal as a sliding bearing (or sleeve).

2. Repair of machine tool guide rails

For minor grinding and abrasions of the machine tool guide rail, as long as it is repaired with a Qiao knife or whetstone, it can continue to run. For gluing flaws, it is necessary to repair them by welding or filling. Even for larger flaws, as long as a large part of the rail surface is kept intact, this method can also be used to repair. After the machine tool has made three orders and five claims, it will be repaired by means of fine planing, fine grinding, or glued boards and paneling.

The welding repair and stick repair of the machine tool guide rails are relatively more complicated, especially for the repair of large-area flaws. Before construction, more careful consideration should be considered; first of all, the accuracy of the repair process on the guide rail, Whether the mechanical properties have an influence, such as welding and sticking repairs mostly need to be heated, then whether the temperature during the process will cause the rail to deform, will it change the mechanical strength and surface hardness of the rail, etc. Second, it is necessary to consider the adaptability of the repair process used to the guide rail materials. For example, the guide rails made of cast iron generally do not use cast iron surfacing welding. Due to the poor weldability of castings, defects such as cracks and porosity are easy to produce, and the machining is also difficult. The process temperature is high (referring to gas welding), which is easy to cause deformation of the guide rail. The third is to consider the mechanical strength of the repair layer itself and the actual working environment of the guide rail. For example, we also used tin-bismuth alloy overlay welding to repair 4m gantry planer guide rails and T68 boring guide rails. Because the former guide rail has good lubrication conditions, it has been used for 20 years and the situation is still good; while the latter is due to poor lubrication conditions and poor working environment. Three years after shedding off. In short, it is necessary to make the repair layer meet a certain life cycle as much as possible.

In the overhaul of the machine tool, the guide rails are generally repaired by the following two methods: first, the bed guide rails are planed, milled or polished, and the injury is flattened, according to the size of the bed guide rails repaired and the wear of the skateboard , Or replace the skateboard, or glue other compensation materials on the slide rail, such as Teflon soft tape, epoxy wear-resistant coating, nylon plate or metal plate, etc., to compensate the dimensional chain. Another method is to fine-tune The machined bed rails are pre-heat-treated steel rails or hardened steel strips.

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