Common Problems and Solutions in Machining Small Size Internal Threads

The manufacturing cell of a machine is a part, which is connected to form a machine through a certain form. And one of the connection forms between parts is screw connection, therefore, screw is often one of the most common structures on parts. The common methods for machining thread include turning, tapping and rolling, and tapping is the most widely used internal thread processing method. Especially for small size internal threads, tapping is almost the only processing method. There are generally two methods of tapping threads, namely, hand tapping and machine tapping. In order to facilitate thread processing, the following will discuss a series of problems in thread processing, such as precautions, common problems, causes, solutions, and common methods of taking out broken taps.

1. Methods and precautions for tapping threads with common taps

1) Manual tapping method and precautions

At present, in mechanical processing, hand tapping thread still occupies a certain position. Because in actual production, some threaded holes are not applicable to machine tapping threads due to their location or part shape. For small hole threads, because the diameter of the threaded hole is small and the tap strength is low, it is easy to break the tap when tapping the thread with a machine. Generally, manual tapping is also used. However, the quality of hand tapping thread is greatly affected by human factors, so we can only ensure the processing quality of hand tapping thread by adopting correct tapping methods.

a. The clamping of the workpiece shall be aligned with the clamping of the processed workpiece. In general, the side of the workpiece to be threaded shall be placed in a horizontal or vertical position. It is convenient to judge and keep the tap perpendicular to the workpiece base plane.

b. The initial position of the screw tap shall be at the beginning of thread tapping. The screw tap shall be placed correctly, and then the screw tap shall be upright with one hand, and the hinge rod shall be rotated gently with the other hand. When the tap is rotated for 1~2 turns, observe whether the tap is perpendicular to the workpiece base plane from the front or side. If necessary, use a square to correct. Generally, after tapping 3~4 turns of thread, the direction of the tap is basically determined.

If the thread tapping is not correct at the beginning, screw out the tap, correct it with a biconical tap, and then tap the thread with a head tap. When all the cutting parts of the tap enter the workpiece, it is no longer necessary to apply axial force, and the thread can be naturally screwed in.

When tapping, it is generally appropriate to screw in 1/2~1 turn each time. However, under special circumstances, specific analysis should be made for specific problems, for example: the screw tap with the size below M5 should not be rotated more than 1/2 turn at a time; When manually tapping fine thread or thread with high precision requirements, the feed rate shall be appropriately reduced each time; The tapping speed of cast iron can be appropriately faster than that of steel. After each precession, the reverse rotation is about 1/2 stroke of precession; When tapping deeper threads, in order to facilitate chip breaking and chip removal, reduce chip sticking on the cutting edge, ensure a sharp cutting edge, and make the cutting fluid smoothly enter the cutting part for cooling and lubrication. The rotary stroke is larger and needs to be rotated repeatedly. In addition, when tapping blind hole threads, the tap should always be withdrawn to remove chips to ensure the effective length of the threaded hole.

c. When the hinge rod is rotated evenly with force, the operator’s two hands should be balanced with force, and excessive force and shaking from left to right should be avoided, otherwise the thread profile may be torn, the threaded hole may be enlarged and the taper may occur. If it is hard to tap the thread forcibly, it is not allowed to do so. The tap should be turned upside down to remove the chip, or the two taper should be used to tap for several cycles to reduce the load on the cutting part of the tap, and then the tap should be used to continue tapping. If it is still hard to tap the thread or the tap sounds “cluck” intermittently, the cutting is abnormal or the tap is worn, and the tapping should be stopped immediately to find out the cause, otherwise the tap may break.

d. When tapping blind hole threads by withdrawing the tap operation mode, when the end tap tapping is completed and the reaming screw reverses the tap loosening, it is convenient to screw the tap out. Because the tapped threaded hole and the tap are loosely matched, and the reaming screw is heavy, if the reaming screw is used to screw out the tap, it is easy to produce swing and vibration, thus damaging the surface roughness of the thread. When tapping through hole threads, the calibration part of the tap should not be fully out of the hole to avoid expanding or damaging the last threads.

e. The application of group taps When tapping threads with group taps, after the head tap tapping is completed, the two or three taper taps shall be screwed into the threaded hole by hand first, and the reaming screw can be used only when the screw cannot be rotated, to prevent the thread tapped by the previous tap from being misaligned.

2) Thread tapping method and precautions of machine tap

Due to the problems of low efficiency and unstable quality of hand tapping thread, machine tapping thread with good quality, high efficiency and low production cost is mainly used in actual mass production. But in the process of machine tapping, we must also correctly use machines and tools, otherwise, the processing quality of threaded holes will also be affected.

a. The self accuracy of the machine tool The radial runout of the drilling machine spindle should generally be adjusted within 0.05mm. If the accuracy of tapping threaded holes is high, the radial runout of the spindle should not be greater than 0.02mm. The perpendicularity error between the positioning bearing surface of the fixture that clamps the workpiece and the center of the drilling machine spindle or the center of the tap should not be greater than 0.05mm/100. The coaxiality between the threaded bottom hole of the workpiece and the tap should generally not be greater than 0.05mm.

b. Operation mode of tapping When the tap is about to finish tapping, the feed should be light and slow to prevent interference between the front end of the tap and the depth of the thread bottom hole of the workpiece and damage the tap; When tapping blind hole threads or deep screw holes, thread tapping safety clamps shall be used to bear the cutting force. The cutting force borne by the safety chuck must be adjusted according to the size of the tap to avoid breaking the tap or failing to penetrate; Within the tapping stroke of the length of the cutting part of the tap, evenly apply appropriate pressure on the feed handle of the drilling machine to help the tap enter the bottom hole, so as to avoid scraping the thread when pulling down the spindle with the first few incomplete threads. When the calibration part enters the workpiece, reliably screw the thread in naturally for tapping, so as to avoid thinning the tooth profile. When tapping through hole threads, it should be noted that the calibration part of the tap should not be exposed completely, otherwise, when the tap is withdrawn in reverse, the phenomenon of disordered threading will occur.

c. The selection of cutting speed The cutting speed of tapping thread is mainly determined according to the cutting material, tap pitch, pitch, depth of threaded hole, etc. Generally, when the depth of the threaded hole is within 10~30mm and the workpiece is made of the following materials, the cutting speed is roughly as follows: steel v=6~15m/min, quenched and 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 relatively high speed is used for smaller tap diameter, the relatively low speed is used for larger tap diameter, and the low speed is used for larger pitch.

D. Selection of cutting fluid When tapping threads, the cutting fluid is mainly selected according to the material to be machined, and sufficient cutting fluid shall be maintained. For metal materials, emulsion is generally used; For plastic materials, emulsified oil or vulcanized cutting oil can be generally used. If the surface roughness of the threaded hole on the workpiece is required to be low, rapeseed oil and molybdenum disulfide can be used, and soybean oil is also effective.

2. Common Problems, Causes and Solutions of Tapping Thread with Common Tap

According to the actual production, it can be found through induction and summary that tap breaking and thread pitch diameter out of tolerance often occur when tapping threads with ordinary taps. For these problems, we have summarized and analyzed them as follows and provided them in the form of tables to guide the actual production, reduce production accidents and improve processing efficiency.

Solution to the problem: When the tap breaks the thread and the bottom hole is processed, the diameter of the bottom hole is too small, and the chip is blocked due to poor chip removal; When tapping blind hole threads, the drilling depth is not enough; Tapping speed is too high and too fast; The diameter of the tap for tapping is different from that of the thread bottom hole; Improper selection of tap grinding parameters; The hardness of the machined part is unstable; The tap has been used for a long time and is excessively worn. Correctly select the diameter of the threaded bottom hole; Inclination of grinding edge or spiral groove tap; The depth of bottom hole shall meet the specified standard; Reduce the cutting speed appropriately and select according to the standard; When tapping, correct the tap and bottom hole to ensure that their coaxiality meets the requirements, and select a floating tapping chuck; Increase the front angle of tap and shorten the length of cutting tap; Ensure that the hardness of the workpiece meets the requirements, and select the safety chuck; If the tap is worn, it shall be replaced in time. The front angle of the tap with broken teeth is too large; The cutting thickness of each tooth of the tap is too large; The quenching hardness of the tap is too high; The tap is seriously worn. Properly reduce the tap front angle; Increase the length of the cutting cone appropriately; Reduce the hardness and replace the tap in time. The speed of tapping thread is too high when the tap is worn too fast; Improper selection of tap grinding parameters; Improper selection of cutting fluid and insufficient use; The material hardness of the workpiece is too high; Burn occurs when the tap is sharpened.

Reduce the cutting speed appropriately; Reduce the front angle of tap and lengthen the length of cutting cone; Select cutting fluid with good lubricity; Carry out appropriate heat treatment for the workpiece to be processed; Correctly sharpen the tap. The pitch diameter accuracy grade of taps with excessive pitch diameter of threads is improperly selected; The selection of cutting fluid is unreasonable; The tapping speed is too high; Coaxiality difference between tap and workpiece thread bottom hole; Improper selection of tap grinding parameters; Burr occurs when grinding the tap; The length of tap cutting tap is too short. Select the pitch diameter of tap with appropriate accuracy grade; Select appropriate cutting fluid and reduce cutting speed appropriately; Correct the coaxiality of the tap and the thread bottom hole when tapping; Use floating chuck; Properly reduce the front angle and the back angle of the cutting cone; Eliminate burrs generated by grinding tap, and appropriately increase the length of cutting tap. The pitch diameter accuracy grade of the tap with too small pitch diameter of thread is improperly selected; Improper tap grinding parameters; Worn tap; The cutting fluid is not suitable. Select the pitch diameter of tap with appropriate precision grade; Appropriately increase the tap rake angle and cutting taper; Replace the excessively worn tap; Select cutting fluid with good lubrication. The grinding parameters of large thread taper with thread surface roughness are inappropriate; The material hardness of the workpiece is too low; Poor tap grinding quality and improper cutting fluid selection; The cutting speed of tapping thread is too high; The tap is severely worn. Appropriately increase the front angle of tap and reduce the cutting taper; Heat treatment shall be carried out to properly improve the hardness of the workpiece; Ensure that the tap rake face has a lower surface roughness value; Select cutting fluid with good lubricity; Reduce the cutting speed appropriately; Replace the worn tap.

3. Method of taking out the broken tap from the screw hole

When machining small size internal threads, tap breaking often occurs due to small tap diameter and low strength. Therefore, how to correctly take out the broken tap is also an important problem that needs to be studied. First of all, before taking out the broken tap, the chips in the screw hole and the tap chips should be cleaned up to prevent the tap from withdrawing from being clamped between the thread and the tap. Specific methods are as follows:

When the broken part of the broken tap is exposed outside the hole, it can be screwed out with pliers; Or use a sharp chisel to gently pick out; You can also weld a hexagon nut on the broken cone, and then gently pull the hexagon nut with a wrench to pull the broken wire out.

When the broken part of the tap is in the hole, two nuts can be screwed on the broken tap with square tenon, and steel wires (the number of wires is the same as the number of tap slots) can be inserted into the empty slots of the broken tap and nut, and then the square tenon can be pulled by the reamer in the exit direction to take out the broken tap.

The breaking of the tap often happens suddenly under great stress, so that the cutting edge of the half of the tap broken in the screw hole is tightly wedged in the metal, and it is generally difficult to make the cutting edge of the tap come out of the metal. In order to make the tap loose in the screw hole, a sharp chisel can be used to reach the chip holding slot of the tap, and a hammer can be used to repeatedly tap gently in the positive and negative directions of the thread until the tap is loose.

Anneal the tap with acetylene flame or blowtorch, and then drill with a drill bit. At this time, the diameter of the drill bit should be smaller than the diameter of the bottom hole. The drill hole should also be aligned with the center to prevent the thread from being damaged. After drilling the hole, drive a flat or square punch and then use a wrench to screw out the tap. Use EDM equipment to corrode the broken tap.

When tapping stainless steel materials, because stainless steel can resist nitric acid corrosion, while the tap made of high-speed steel can be quickly corroded in nitric acid solution, the broken tap can be taken out by putting the workpiece into nitric acid solution for corrosion.