Common problems and solutions in tapping

Thread is a commonly used fit in mechanical parts, and there are many ways to process threads. Among them, tapping with ordinary taps is the most widely used internal thread processing method. Especially for small-sized internal threads, tap tapping is the only machining method. Make a brief analysis on the skills and problems of ordinary tap tapping for reference by peers.

Using ordinary taps to tap threads is a traditional method of processing threads, and it is also the most commonly used processing method for tapping. At the same time, it is a processing method with strong skills. A qualified fitter must master and apply it Processing skills, and can correctly deal with various problems encountered in processing.

1 Tap
To master the skills of ordinary tap tapping, we should first have a clear understanding of the tap.
Taps are generally made of alloy tool steel 9SiGr, and are made by heat treatment. They are divided into taps 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, and its end usually has a square tenon, which is convenient for matching with the hinge rod.

When tapping, in order to reduce the cutting force and prolong the life of the tap, the entire cutting workload is generally allocated to several taps. Usually, there are two taps in each group of M6~M24 taps, called head taps and two taps; taps below M6 and above M24 have three taps in each group, namely head taps, two taps and three taps; fine thread taps are two in a group .
The tool used to hold the tap when hand tapping is the threaded rod.
1/5
2 The method of ordinary tap tapping
2.1 Determination of threaded bottom hole diameter and depth
1) Determination of the diameter of the threaded bottom hole. For tapping, the diameter of the bottom hole of the thread should be slightly larger than the small diameter of the thread. Otherwise, due to the extrusion effect during tapping, there will not be enough chip space between the thread crest and the tap bottom, which will cause the tap to be stuck, 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 diameter of the bottom hole, it is necessary to comprehensively consider the plasticity of the tool material, the diameter of the thread and the expansion of the hole, and calculate it according to the empirical formula.

2) Determination of the depth of the threaded bottom hole. When a through-hole thread cannot be tapped, the end usually cannot be cut with a complete profile, which is due to the cutting part of the tap.

3 Methods and skills of ordinary tap tapping
3.1 Methods and skills for manual tapping with ordinary taps
In the current actual production, many threads can only be tapped manually due to their special shapes and positions. Small-sized threads have low strength and small hole diameters. Machine-tapping threads are easy to break taps, so they are usually manually tapped. However, manual thread 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 scribe and make a bottom hole. The bottom hole must be chamfered, and the diameter of the chamfer should be slightly larger than the diameter of the thread, so that the tap can be easily cut when it starts cutting, and it can prevent holes Do not extrude the lip.
2/5

2) Workpiece clamping. Use a bench vise to clamp and align the workpiece. In general, the side that needs to be tapped should be placed horizontally. If the shape of the workpiece does not allow, consider placing the thread surface in the vertical direction. The workpiece clamp is beneficial to balance the force and control the tapping speed and ensure the tapping quality during tapping.
3) Attack. When attacking, the tap should be placed in the right direction. The left and right hands should cooperate with each other, press the middle of the hinge bar with one hand and apply pressure along the axis of the tap, and the other hand cooperate to advance in sequence. After the tap has been tapped for 1 to 2 turns, it should be checked with a square ruler from the front, rear, left and right directions in time to ensure that the tap and the base surface of the thread are perpendicular. Usually, the direction of the tap can be determined after it has been tapped for 3 to 4 turns. At this time, no axial pressure is applied. It is only necessary to hold the two ends of the hinge rod with both hands and apply force evenly to screw the tap forward. During the screwing process , try to make the center line of the tap coincide with the center line of the hole. When all the cutting part of the tap enters the workpiece, it is necessary to reverse 1/4 to 1/2 frequently to remove the chips in time to avoid the tap being stuck due to the blockage of the chips.
4) When tapping, it must be carried out in the order of first tapping, second tapping, and third tapping, until the tapping reaches the standard size. For harder materials, the taps should be alternately tapped to reduce the partial load of the chips and prevent the taps from breaking.

3.2 Common problems in ordinary tap tapping
Problem 1: The surface quality of the thread is poor, and the roughness value is too large.
Reason analysis: There are many reasons for the low tapping quality. The major influences include low hardness of the workpiece material and difficulty in chip removal during tapping; unreasonable selection of tap sharpening parameters, resulting in low tap sharpening quality and tapping quality. Poor; the cutting speed is too high when tapping, and the selection of chip fluid is unreasonable; the tap is used for a long time, and the wear is serious.

Solution: In order to improve the surface quality of the thread, we should take certain preventive measures before tapping. For workpieces with low material hardness, they should be heat treated before tapping, and the hardness should be appropriately increased through the heat treatment process; when sharpening taps, the rake angle of the tap can be appropriately increased, the cutting taper angle can be reduced, and the sharpening quality of the tap can be improved. Ensure that the rake face of the tap has a low surface roughness value; when tapping, select the cutting speed reasonably according to the material properties of the workpiece, and use the chip fluid with good lubricity; Tapping with a new tap.

Problem 2: The pitch diameter of the thread is too large or too small
Reason analysis: The first reason that causes the thread pitch diameter to be too large or too small is usually the inappropriate selection of the accuracy grade of the tap. Secondly, the improper selection of the sharpening parameters will also cause the thread pitch diameter to be inconsistent. Finally, the cutting line speed is too high. Too high or too low, and poor coaxiality between the tap and the workpiece are common factors that cause the pitch diameter of the thread to be too large or too small.

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

Problem 3: Taps are worn out too fast, chipped or even broken
Reason analysis: The cutting speed is too high when tapping, and the tap is used for too long, which will easily cause the tap to wear seriously; the tap quenching hardness is too high, and the cutting thickness of each tooth is too large, which will easily cause the tap to collapse; The main reasons for tap breakage are that the bottom hole is not on the same axis, the quality of the processed material is uneven, and the cutting speed is too high and too fast.

Solution: When using tap tapping, due to the reasonable selection of cutting speed and heat treatment of the tap to reduce its hardness, the tap with serious wear should be replaced in time, and the coaxiality of the tap and the bottom hole of the thread should be ensured.

3.3 How to remove the broken tap from the screw hole
During the tapping process, it often happens that the tap breaks 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. middle. How to remove the broken tap in the screw hole smoothly? The most commonly used methods are as follows:

1) When the tap is broken to expose the threaded hole, it can usually be unscrewed with pliers or removed with a chisel. When the exposed part is short, a hexagonal nut can be welded on the exposed tap and then unscrewed with a wrench.
2) When the broken part of the tap is in the hole, screw two nuts on the broken tap with a square tenon, insert the steel wire (the number of which is the same as the number of the tap grooves) into the empty groove of the broken tap and the nut, and then use a hinge rod. Pull the square tenon in the exit direction and take out the broken tap.
3) When the tap is broken and tightly wedged in the metal, it is generally difficult to disengage the cutting edge of the tap from the metal. At this time, a sharp chisel can be used to place it in the chip groove of the tap, and press the front and back of the thread with a hammer. The direction is repeatedly tapped lightly until the tap is loose.
4) Annealing drilling method. First anneal the tap with an acetylene flame or a blowtorch, then use a drill bit smaller than the diameter of the threaded bottom hole to drill a hole in the center of the threaded hole. After drilling the hole, insert a flat or square punch, and then slowly unscrew it with a wrench tap.
5) Corrosion method. Taps are usually made of alloy tool steel, which has weak resistance to nitric acid corrosion. When tapping stainless steel materials, because stainless steel is resistant to nitric acid corrosion, the workpiece with broken taps can be put into nitric acid solution for corrosion. When the corrosion reaches a certain level, the broken tap can be removed smoothly.