Today, the popularity of stainless steel pipe fittings is mainly due to its environmental protection. The internal thread on stainless steel pipe fittings is usually tapped to stop tapping. Due to the high viscosity of stainless steel and poor chip breaking performance, it is easy for chips to scratch the workpiece thread or tap chipping during tapping, which affects processing efficiency and thread quality. In order to prolong the service life of taps and improve the quality of thread processing, the following points should be paid attention to.
1.Design and manufacture of taps
(1) Select a better tap material. Participating in special alloy elements in ordinary high-speed tool steel can significantly improve the wear resistance and toughness of taps.
(2) The titanium nitride coating on the surface of the tap thread can significantly improve the wear resistance, heat resistance and smoothness of the tap.
(3) Properly increase the rake angle of the tap. However, it should be noted that if the rake angle of the tap is too large, it is easy to form the tap collapse edge and the taper of the thread when the tool is retracted.
(4) Properly increase the amount of tap back. However, it should be noted that if the back of the shovel is too large, it is easy to cause the phenomenon that chips are inserted into the back angle of the tap when retracting the tool, and the brightness of the tapped thread is not good.
(5) Reasonably select the thermal treatment method of the cutter to coordinate the hardness and toughness of the tap.
2.Pre-JJN-R of the bottom hole of the workpiece
(1) According to the rule that the tapping processing amount is as small as possible, the bottom dimension of the pre-processed workpiece, especially the American standard NPT thread and British standard F, R thread, has a taper of 1:16. According to the small end diameter of the thread, a straight hole is cast as the tapping bottom hole, which causes a large amount of local processing in the second half of the tapping, which easily causes the following undesirable results:
① The tapping torque is large, and the tap is easy to break or chip;
② The brightness of the tapped thread is poor;
③ The diameter of the tapped cylindrical thread is too small when tested with a plug gauge. The reason is not that the middle diameter of the thread does not diverge, but that the thread is scratched by chips and there are burrs.
(2) The size of the bottom hole is preferably the small diameter size of the thread to be processed plus a margin of 0.10.2MM.
(3) Custom reamers are best used for reaming before tapping. For American standard NF, R thread and British standard F, R thread, should be machined into a tapered bottom hole. At present, many companies use waste taps to manually grind off the threads to make reamer, so that the waste taps can be used twice. However, there are two issues to be aware of when restructuring the reamer:
① Be sure to grind the back angle when grinding the thread;
② The front corner of the tap edge should be straight.
3. Reasonable use of taps
(1) Consider different chip directions when processing different types of workpieces. New taps also need to be ground when necessary. The grinding guideline is to make the chips sink as far as possible when tapping to avoid scratching the processed threads. An example is explained as follows: Processing coupling (SPU): The vertical tapping machine (or three-head tapping machine) is best used for the tapping processing of the coupling. The tap grinding method is:
① Lengthen the front guide of the tap to make the tap gradually enter the cutting to improve the service life of the tap;
② Grind the blade inclination on each blade of the tap, but the initial sharpening should not be too long (about 34MM), and it can be gradually increased in the future. The criterion for judging the quality of the inclination of the blade is that the chips are discharged from the end face of the coupling in a trumpet shape during tapping, which will not damage the tapped thread. When the tapping stops for a period of time, if you find that the quality of the tapped thread is not good, you can increase the blade inclination.
Automatic tube processing