threading

industrial processes for producing threads through removal of a chip from the surface of various articles. Threading is performed using special thread-cutting, nut-tapping, bolt-screwing, and thread-grinding machines, as well as automatic, semiautomatic, and turret lathes.

Figure 1. Threading on a lathe: (a) by the copying method; (b) by the generating method; (1) thread profile; (2) cutting tool; (S) cutting-tool feed, equal to the thread pitch; (S_rad) radial feed; (S_tat) lateral feed

The simplest threading method uses taps and dies and is done manually, using nut-tapping and bolt-screwing tools. Threading on lathes is performed with cutting tools in several passes (Figure 1). Thread with a pitch S ≦ mm is cut by the copying method with radial feed, whereas thread with S ≧ 2.5 mm is cut first by the generating method with lateral feed (rough passes) and then by the copying method (finishing passes). Threading with cutting tools is a process of low productivity, but it is capable of producing thread with low surface roughness (grade ▽8-▽9), which corresponds to precision class 4. Threading by means of thread chasers and cutters increases productivity, since such methods make possible production of the thread in a single pass.

The most advanced method of threading, called the highspeed or vortex method, makes possible an increase in productivity’by a factor of 2.0–2.5 as compared to the production rate with thread cutters and by a factor of more than 10 as compared to methods using cutting tools. Threading by the high-speed method is performed using methods of generating and envelopment (Figure 2). The cutting head, with one or more cutters equipped with hard-alloy cutting bars, is mounted eccentrically with respect to the blank being machined. The blank rotates at 30–300 rpm in one direction, and the cutting head rotates at 1,000–3,000 rpm in the opposite direction. The blank is advanced axially by one step, corresponding to the pitch of the thread, during each revolution. Each cutter touches the blank once each revolution. The cutting speed may be as high as 400 m/min. The method is applicable to threading on both exterior and interior surfaces with diameters of more than 50 mm and provides surface roughness of class 6 (▽6) and precision of class 5–6. Threading may also be performed by plastic deformation (thread knurling).

Figure 2. High-speed threading: (a) envelopment method, (b) generating method, (1) blank, (2) cutting head, (n_b) direction of rotation of blank, (n_c) direction of rotation of cutting head