Common problems and solutions in the tapping process

Friends who have done machine tool attachments are not unfamiliar with the word "tapping", do you really understand the methods and skills of tapping and the problems encountered? Let's take a look below.

 

 

Abstract: Thread is a kind of coordination commonly used in mechanical parts. There are many methods for processing threads. Among them, tapping threads with ordinary taps is the most widely used method for processing internal threads. Especially for small internal threads, tapping is the only processing method. Make an analysis on the skills and problems of ordinary tap tapping threads for peer reference.

 

 

Thread tapping with ordinary taps is a traditional method for processing threads, and it is also the most commonly used method for tapping. At the same time, it is a highly skilled processing method. A qualified fitter must be proficient in and apply it Processing skills, and can correctly deal with various problems encountered in processing.

1 tap

To master the tapping skills of ordinary taps, we should first have a clear understanding of taps.

The taps are generally made of alloy tool steel 9SiGr and are made by heat treatment. The taps for break-up and machine taps. Both have the same structure and are composed of a working part and a handle. The shank is the part that is clamped when tapping the thread, and its end usually has a square tenon, which is convenient for matching with the hinge bar.

 

When tapping the thread, in order to reduce the cutting force and extend the life of the tap, the entire cutting workload is generally distributed to several taps to act. Generally, there are two taps for each group of M6～M24, called head cone and two cones; there are three taps for each group of taps below M6 and above M24, namely head cone, two cone and triple cone; fine thread taps are two groups .

The tool used to hold the tap when tapping the thread manually is a hinge bar.

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2 Common tap tapping method

2.1 Determination of the diameter and depth of the threaded bottom hole

1) Determination of the diameter of the thread bottom hole. When tapping the thread, the diameter of the bottom hole of the thread should be slightly larger than the small diameter of the thread. Otherwise, due to the squeezing effect during thread tapping, there will not be enough chip space between the crest of the thread and the bottom of the tap, which will cause the tap to jam and even break the tap in severe cases . However, the diameter of the bottom hole should not be too large, otherwise the height of the thread profile will be insufficient and the strength will be reduced.

To determine the size of the bottom hole diameter, it is necessary to comprehensively consider the tool material plasticity, thread diameter and drilling expansion and calculate it according to the empirical formula.

 

2) Determining the depth of thread bottom hole. When threading through holes is not possible, the end part cannot usually cut a complete tooth shape, which is due to the tapped cutting part.

 

3 The method and skills of tapping threads with ordinary taps

3.1 Methods and skills for tapping threads manually with ordinary taps

In the current actual production, many threads can only be tapped manually due to their special shape and position. The small size thread has low strength and small aperture, and the machine tapping thread is easy to break the tap, so it is usually also manually tapped. However, manual tapping has relatively high technical requirements for workers, and the quality is not easy to guarantee, so it is necessary to master the correct operation method.

1) Before tapping the thread, use a scribing tool to scribble and make a bottom hole. The hole of the bottom hole must be chamfered. The diameter of the chamfer should be slightly larger than the diameter of the thread, so that the tap can be easily cut when starting cutting, and at the same time can prevent the hole Do not squeeze the raised edges.

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2) Workpiece clamping. Use a vice to clamp the workpiece and set it up. Generally, the side that needs to be tapped should be placed horizontally. If the shape of the workpiece is not allowed, you can consider placing the threaded surface in the vertical direction. The work clamp is conducive to balancing the force during tapping and controlling the tapping speed and ensuring the tapping quality.

3) Attack. When starting the attack, the tap should be straightened. The left and right hands should cooperate with each other, hold down the middle of the hinge bar with one hand and apply pressure along the axis of the tap, and the other hand should be screwed in the order of cooperation. When the tap taps into 1 to 2 turns, it is necessary to check with a square ruler from the front, back, left and right directions in time to ensure that the tap is perpendicular to the thread base. Generally, the direction of the tap can be determined after 3 to 4 turns. At this time, axial pressure is no longer applied, and only two hands are required to hold the two ends of the hinge to apply force evenly, and the tap is screwed forward. During the screwing process At the same time, we should try to make the center line of the tap coincide with the center line of the hole. When the cutting part of the tap completely enters the workpiece, it is often necessary to reverse 1/4 to 1/2 to remove the chips in time to avoid the jamming of the tap due to the blockage of the chips.

4) When tapping, it must be performed in the order of head attack, second attack, and third attack until the tapping reaches the standard size. For harder materials, the taps should be alternately tapped to reduce the chip load and prevent the taps from breaking.

 

3.2 Common problems in tapping threads

Problem 1: The thread surface quality is poor, and the roughness value is too large.

Reason analysis: There are many reasons for the low tapping quality, and the major impacts include low hardness of the workpiece material and difficulty in chip removal during tapping; unreasonable selection of the parameters of the taper sharpening, resulting in low tapping quality and tapping quality Poor; the cutting speed is too high when tapping, the choice of chip fluid is unreasonable; the tap is used for a long time, and the wear is serious.

 

Solution: In order to improve the thread surface quality, we should take certain precautions before tapping. For workpieces with lower material hardness, heat treatment should be performed before tapping, and the hardness should be appropriately increased through the heat treatment process; when sharpening the tap, the taper angle can be appropriately increased, the cutting taper angle can be reduced, and the quality of the taper grinding can be improved. Ensure that the taper rake face has a low surface roughness value; when tapping, the cutting speed is reasonably selected according to the material characteristics of the workpiece, and a chip fluid with good lubricity is used; after long-term use of the tap, the serious wear should be replaced and used in time The new taps are tapped.

 

Problem 2: The thread diameter is too large or too small

Reason analysis: The first reason that causes the thread diameter to be too large or too small is usually that the accuracy grade of the tap is not selected properly. Secondly, the inappropriate grinding parameter selection will also cause the thread diameter to be inconsistent. Finally, the cutting linear speed is too High or too low, the poor coaxiality of the tap and the workpiece are common factors that cause the thread diameter to be too large or too small.

 

Solution: Before tapping, the tapping process should be carefully analyzed according to the drawing requirements, the appropriate tap accuracy level should be selected according to the process, the taper grinding parameters and cutting speed should be selected reasonably, and the tap and the bottom of the thread should be corrected when tapping the thread Degree, the burrs generated when sharpening the tap should be removed in time.

 

Problem 3: The tap wears too fast, collapses or even breaks

Reason analysis: the cutting speed is too high during tapping, and the tap is used for too long to cause serious wear of the tap; the quenching hardness of the tap is too high, and the cutting thickness of each tooth is too large, it is easy to cause the tap to collapse; and the chip removal is not good, the tap and the thread The different axes of the bottom hole, the uneven quality of the processed material, and the cutting speed that is too high and too fast are the main reasons for the breakage of the tap.

 

Solution: When tapping taps, the cutting speed is reasonably selected, and the taps are heat-treated to reduce their hardness. The taps with severe wear should be replaced in time, and the coaxiality of the taps and the bottom hole of the thread should be ensured.

 

3.3 How to remove the broken tap from the screw hole

During tapping, the tap breaks frequently in the screw hole, especially when processing small internal threads, the tapping force is improper, the tap is used incorrectly, and the tap is easily broken and stuck in the threaded hole in. How to take out the broken tap in the screw hole smoothly? At present, the more commonly used methods are as follows:

 

1) When the tap is broken and the threaded hole is exposed, it can usually be screwed out with pliers or removed with a chisel. When the exposed part is short, a hexagon nut can be welded on the exposed broken tap and then unscrewed with a wrench.

2) When the broken part of the tap is in the hole, two nuts can be screwed on the broken tap with square tenon, and the steel wire (the number of taps is the same as the number of tap slots) is inserted into the empty slot of the broken tap and the nut, and then the hinge bar Pull the square tenon in the exit direction to take out the broken tap.

3) When the tap breaks and wedges tightly in the metal, it is generally difficult to separate the cutting edge of the tap from the metal. At this time, a sharp chisel can be used to touch the chip flute of the tap, and use a hammer to press the positive and negative threads Tap gently in the direction repeatedly until the tap becomes loose.

4) Annealing drilling method. First anneal the tap with an acetylene flame or a blowtorch, then drill a hole with a drill bit with a diameter smaller than the diameter of the bottom hole of the thread, and drill a flat or square punch after drilling the hole, then slowly unscrew it with a wrench Tap.

5) Corrosion method. The tap is usually made of alloy tool steel, and its resistance to nitric acid corrosion is weak. When stainless steel is tapped, because the stainless steel can resist nitric acid corrosion, the work piece with a broken tap can be placed in a nitric acid solution for corrosion. When corroded to a certain extent, the broken tap can be taken out smoothly.