Fig.1 Square thread thread parameters
Our factory externally processes the bed nuts of the J53-300 friction press. The nut is assembled from a jacket (material ZG45) and a copper nut. The nut cover is connected to the brass nut by the thread of M265×4 and the pin is driven. The copper nut bore has a 630mm four-thread square thread with thread parameters (see Figure 1): pitch 44mm, lead 176±0.07mm, thread outer diameter Ø227D7+0.6mm, thread diameter Ø191D4+0.09mm, helix 14°25'. Four-line square thread is a kind of non-standard thread and it is difficult to process. To this end, we designed and manufactured a turning tool that uses a cutting and extruding process to form threads (see Figure 2). The knife has the following characteristics: In order to meet the processing needs of large-aperture thread, the head of the tool holder is equipped with a sliding bearing. In radial infeed, the arbor moves in the flat bore of the sliding bearing. Because the sliding bearing keeps contact with the inner hole during the cutting process, the rigidity of the cutter bar can be enhanced and the vibration of the cutter bar can be eliminated; 
1. Arbor 2. Rear pressure plate 3. Sliding bearing 4. Tool 5. Front pressure plate
6. Press the screw (M16) 7. Countersunk screw (M10)
Figure 2 four-wire thread forming tool structure
The tool is the final finishing tool. The rake part (about 3mm) of the two edges of the tool head acts as a cutting. The rear part of the tool finishes the extruding light. The two side blades form a negative of about 1° during processing. The back corner (about 0.5mm in width) makes the flank face of the tool fit tightly with the thread surface of the workpiece, which reduces vibration and increases the smoothness of cutting. At the same time, it also has a certain calendering effect, which can improve the surface quality of the workpiece; The top edge of the tool has a negative rake angle of 3°, so both side edges have a negative edge angle of 3°, which can force the chip to the top edge during cutting. The direction of flow, so that the surface has not been scratched; Behind the side edge (right edge of the top surface) grinding crescent slot, so that the rake angle of the blade increases, the cutting edge can make the work of the two front edge angle Equal to, so as to maintain stable cutting and avoid the sickle phenomenon. When processing copper nuts, the workpiece speed is 50r/min, and three cutters are used for cutting: the first rough cutter width is 8mm, the front rake is larger, the feed amount is 1.5mm, and the second rough cutter width is 15mm, each feed 1.5mm; the third is a fine knife (see Figure 3), the width is 16 + 0.2mm, 3mm per feed. 
Figure 3 fine knife
Prior to cutting, apply a knife to draw a groove on the workpiece. Place the first knife in the middle of the groove to ensure that the balance on both sides is equal; when starting cutting and when it will be cut, force is applied on one side, resulting in the first buckle and the last An oversized thread is used. To solve this problem, the blank can be made to have a slightly larger size than the workpiece and then cut at both ends.
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