The method of prolonging the tool life of MJ thread hole machining

MJ thread is a metric Aerospace screw thread. Its basic profile is different from that of metric common thread. The tooth base of external thread adopts a large radius of tooth base arc, and its tooth base is a continuous and smooth curve. The radius of arc at any place on the curve shall not be less than 0.15011 P. its precision is higher than that of ordinary thread. Because of the high precision requirement, it is difficult to process. This kind of threaded hole is used in Cummins natural gas engine block and main bearing block of EFI engine block.
This paper introduces how to improve the service life of 14 MJ12 bolt hole tapping taps on the main bearing block of Cummins EFI engine.

Since January 2008, there have been slight tooth drop and high surface roughness of MJ thread hole, and the thread after processing can not pass through the go gauge. The tool wear is accelerated, and the average machining of 15 ~ 30 pieces requires tool change. According to the field situation, the following two improvement schemes are put forward from the design of taps.

(1) In scheme 1, the worn taps are treated with surface coating after regrinding.
At present, it is considered to control the bottom hole diameter at the upper limit, so as to reduce the wear of the tap and improve the tap life. Under the condition that tap stock is allowed, regrinding taps should be used as little as possible at present, because the service life of taps after regrinding is inconsistent, it is easy to cause unqualified thread of individual thread holes in batch production, and at the same time, the workload of inspection and repair will be increased. If it is necessary to use regrinding taps, the surface coating treatment shall be carried out after regrinding, and the tap pitch diameter shall be increased to ensure that the tap pitch diameter is qualified. At present, the taps with small pitch diameter, or those with unqualified first thread processed by OSG in Japan, are entrusted to Dongfeng measuring instrument factory to conduct surface coating treatment on the used taps after regrinding. The surface coating can be carried out directly on the taps with a pitch diameter of 6 μ m, and the service life of the taps can be collected at the same time.


(2) In scheme 2, the pitch diameter of the new tap is increased by 0.01mm, and the diamond carbide taps of Dongfeng company, OSG of Japan, Wuxi New Era and Zhuzhou Diamond carbide taps are tested respectively.
① The material of tap and cutting tool of Dongfeng measuring instrument factory belongs to ordinary high speed steel. Before improvement, the new tool can process 30 pieces at a time. The surface roughness of the processed thread hole can meet the requirements and ensure the pass no go gauge inspection. The service life of the worn tap is about 30 pieces, and the surface roughness value is relatively high. The main reason may be that the cutting edge will become blunt after coating on the tap surface, resulting in the cutting edge not very sharp, resulting in high surface roughness.


The pitch diameter of taps was increased by 0.01mm and 0.02mm respectively on the original basis, and 10 taps were made for test. Firstly, the taps (k23d2-128c) with pitch diameter increased by 0.01mm (test hole numbers: 6003, 6005, 6007, 6009, 6011, 6013 and 6015) were tested, In the first piece inspection, except for 2 threads of 6007 hole no-stop gauge, the rest are qualified. Up to now, more than 220 pieces have been processed (about 25 pieces can be processed before the pitch diameter of tap is not increased). Judging from the wear condition of taps, the processing can be continued. Later, the tap with 0.02mm pitch diameter increased was tested (k23d2-128d) (test hole numbers are 6003, 6005, 6007, 6009, 6011, 6013 and 6015). In the first inspection, except for about 4 threads of no go gauge of 6007 hole, the rest are qualified, and the final service life is more than 300 pieces.