RATIONAL GEOMETRY OF HIGH-SPEED CUTTERS IN MACHINING STAINLESS STEEL

K. A. NASSONOV

(Presented by Professor, Doctor A. M. Rozenberg)

A distinctive feature of machining stainless, acid-resistant, and heat-resistant steels and alloys by cutting is their poor machinability.

The latter is associated with the physical properties of this class of steels, namely: high toughness, a high work-hardening index, and low thermal conductivity. As a result, the use of carbide tools in cutting stainless steels is limited, since, owing to the brittleness of hard alloys, the cutting edge of the tool chips before normal wear develops, which leads to rapid destruction of the cutting tool, an increase in cutting force, and deterioration in the finish of the machined surface. This phenomenon is observed especially often in interrupted machining processes, which include, in particular, milling.

The chief advantage of high-speed steels in comparison with hard alloys is their high strength. Therefore we attempted to use high-speed steels in milling stainless steel IX18H9T. The results of the investigation are presented in this article.

Endurance tests of high-speed cutters were carried out on a 6H12 vertical milling machine with cooling by a five-percent solution of emulsol in water. The tests were carried out with a single-tooth cutter 200 mm in diameter; the width of the specimens being milled was taken as 160 mm.

As the cutting material, forged high-speed steel P-18 and cast high-speed steels with additions of cobalt, titanium, boron, and sulfur were used.

The best endurance characteristics were obtained for forged high-speed steel P-18 and for a cast high-speed steel containing 12% cobalt, 20% tungsten, 4% chromium, 2% vanadium, and 0.8% carbon, with hardness \(R_c = 65\).

For example, when using cast high-speed steel modified with cobalt, at a cutting speed of 18 m/min, a depth of cut of 2 mm, and a feed of 0.63 mm/tooth, an endurance of 60 min of machine time was achieved (in all cases the wear criterion was ...

the width of the wear land on the flank face, equal to 1 mm, was taken. It should be noted here that, for such large feeds in milling, the life of a carbide cutting tool (VK8, T5K10, T15K6) usually does not exceed 5–15 minutes over a wide range of cutting speeds.

Laboratory tests of forged high-speed steel R-18 ($R_c = 64—65$) showed that milling steel 1Kh18N9T is expediently carried out with large positive rake angles and angles of inclination of the main cutting edge.

Fig. 1.

Figure 1 presents the dependence of the life of a milling-cutter tooth on the magnitude of the rake angle. From consideration of this figure it follows that the optimum value of the rake angle is $25^\circ \pm 2^\circ$.

Figure 2 gives the wear curves of a milling-cutter tooth obtained at various angles of inclination of the main cutting edge. Analysis of these curves shows that the milling cutter had the greatest life at an inclination angle of the cutting edge equal to $30^\circ$. In this case, the life of the cutter, at a wear-land width of 1 mm, was 85 min.

It should be emphasized that deviation from the optimum value of the angle of inclination of the cutting edge caused a significant reduction in tool life. Thus, at an inclination angle of the cutting edge equal to $39^\circ$, the life of the cutter was 50–60 min, and at $21^\circ$—only 17 min.

Fig. 2.

Experiments established that on the rake face it is expedient, along the main cutting edge, to grind a chamfer 0.2 mm wide with a rake angle on the chamfer of $5—7^\circ$, and at the tip of the tooth to create a transitional cutting edge up to one millimeter wide with an angle in plan of $25—30^\circ$.

The indicated elements make it possible to reduce significantly the magnitude of the initial wear of the cutting tool.

Fig. 3.

Figure 3 shows the dependence of the tool life of a high-speed milling cutter on cutting speed. The graph presented makes it possible to judge the permissible level of cutting speeds when milling stainless steel. In particular, the arrangement of the experimental points on the graph shows that the use of cutting speeds exceeding 18 m/min is undesirable, since this leads to a very substantial reduction in the life of the cutting tool.