Problems and solutions that often occur when machining small internal threads

To facilitate thread processing, this article discusses and summarizes the precautions to be taken in small-size thread processing, the problems that often occur during tapping, the causes and solutions, and the common methods for removing broken taps.

The manufacturing unit of a machine is the part, and the parts are connected to each other in a certain way to form the machine. One of the forms of connection between the parts is the threaded connection, so the thread is often one of the most common structures on the parts. The most commonly used methods for machining threads are turning, tapping and lapping, but tapping is the most widely used method for machining internal threads. Especially for small internal threads, tapping is almost the only method. There are two general methods of tapping, namely, hand tapping and machine tapping. For the purpose of threading, the following is a list of considerations, common problems, causes, solutions, and common methods for removing broken taps.

1 Method and precautions for tapping threads with a common tap

Hand tapping method and precautions

At present, hand tapping still occupies a certain position in machining. In practice, some threaded holes are not suitable for machine tapping due to their location or the shape of the part. For small hole threads, due to the small diameter of the threaded hole and the low strength of the tap, it is easy to break the tap with machine tapping, so hand tapping is often used. However, the quality of hand tapped threads is affected by human factors, so we can only ensure the quality of hand tapped threads if we adopt the correct tapping method.
The workpiece should be clamped correctly. In general, the side of the workpiece to be tapped should be placed in a horizontal or vertical position. This makes it easier to judge and keep the tap perpendicular to the workpiece base.

The initial position of the tap is to be set correctly at the beginning of the tapping process, and then the tap is held in place with one hand while the other hand gently turns the reamer. When the tap is rotated 1 or 2 times, observe from the front or side whether the tap is perpendicular to the base surface of the workpiece, and correct it with a right-angle ruler if necessary.
If you start tapping incorrectly, turn the tap out and correct it with a second tap, then use the head tap to tap the thread. When the cutting part of the tap enters the workpiece, it is no longer necessary to apply axial force, and the thread can be naturally screwed in.

When tapping the thread, it is generally appropriate to turn the thread in 1/2 to 1 turn at a time. However, in special cases, specific analysis should be made, for example: for taps below m5, the amount of feed should not be greater than 1/2 turn at a time; when tapping fine threads or threads with high accuracy requirements, the amount of feed should be reduced at a time; tapping cast iron can be faster than tapping steel, and after each turn, reversing about 1/2 stroke of the turn; when tapping deeper threads, in order to facilitate chip breaking and chip evacuation, it is necessary to reduce the phenomenon of chip sticking to the cutting edge to ensure that the threads can be tapped. When tapping deeper threads, in order to facilitate chip breaking and chip removal, reduce the phenomenon of chip sticking on the cutting edge, ensure a sharp edge, and make the cutting fluid enter the cutting part smoothly and play a cooling and lubricating role. In addition, when tapping blind hole threads, the tap should be withdrawn frequently to remove the chips to ensure the effective length of the threaded hole.

When turning the reamer, the operator should use a balanced force with both hands, avoiding excessive force and swaying from side to side, otherwise the threads will be torn and the threaded hole will be enlarged and tapered. If you feel that you are struggling, do not force the thread to be tapped. Turn the tap upside down to remove the chips, or use the second cone to tap a few turns to reduce the load on the cutting part of the head cone, and then continue tapping with the head cone. Stop tapping immediately and find the cause, otherwise the tap may break.

When tapping blind hole threads, use the reamer to spin the taps after the taps are tapped, because the tapped hole and taps are loose, and the reamer is heavy. When tapping through-hole threads, try not to head out all of the calibrated part of the tap to avoid expanding or damaging the last few buckles of threads.

When tapping with a tap in a group, after tapping the first tap, the second or third tap should be screwed into the threaded hole by hand until it does not move, then the reamer can be used to prevent misalignment of the thread tapped by the previous tap.
Method and precautions for tapping threads with a machine tapping machine

Because of the low efficiency and unstable quality of hand-tapped threads, machine tapping is mainly used in mass production for good quality, high efficiency and low production cost. However, during the process of machine tapping, we must also use the machine and tools correctly, otherwise, the quality of the threaded hole processing will also be affected.

The radial runout of the drill press spindle should be adjusted to within 0.05mm. If the accuracy of the tapped hole is high, the radial runout of the spindle should not be greater than 0.02mm. The verticality error between the positioning support surface of the clamping fixture and the drill press spindle center or tap center should not be greater than 0.05mm/100. 0.05mm.

When the tapping tap is about to finish tapping, the tool should be fed gently and slowly to prevent the front end of the tap from interfering with the depth of the bottom hole of the workpiece and damaging the tap; when tapping a blind thread or a deep screw hole, a tapping safety collet should be used to withstand the cutting force. The cutting force of the safety chuck must be adjusted according to the size of the tap to avoid breaking the tap or not tapping in; during the tapping stroke of the cutting part of the tap, an even and suitable pressure should be added to the drill press feed handle to assist the tap to enter the bottom hole, so as to avoid scraping the threads when pulling the spindle downward by the first few incomplete threads. Tapping is performed with natural rotation so as not to cut the threads thin. When tapping through-hole threads, be careful not to expose all of the tap’s calibration part, otherwise, when reversing the tap, a messy buckle will occur.

The cutting speed of tapping is mainly based on the cutting material, tap diameter, pitch and depth of the threaded hole. Generally, when the depth of the threaded hole is within 10-30mm and the workpiece is the following materials, the cutting speed is as follows: steel v=6-15m/min, tempered steel or harder steel v=5-10m/min, stainless steel v=2-7m/min, cast iron v=8-10m/min, under the same conditions, the tap diameter is small to take the relative high speed, the tap diameter is large to take the relative Under the same conditions, a small tap diameter takes a relatively low speed, a large tap diameter takes a relatively low speed, and a large pitch takes a low speed.

When selecting the cutting fluid, the cutting fluid is mainly selected according to the material to be machined, and sufficient cutting fluid needs to be maintained. For metal materials, emulsified fluid is generally used; for plastic materials, emulsified oil or vulcanized cutting oil can be used. If the surface roughness value of the threaded hole on the workpiece is required to be low, vegetable oil and molybdenum disulfide can be used, and the effect of soybean oil is also better.

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