Comparative Tool-Life Tests of High-Speed Steel Drills Made from Sector-Rolled Stock and Milled Drills

A. M. Rosenberg, D. V. Kozhevnikov

In recent years, the tool industry has been widely introducing the production of high-speed steel drills by the method of sector rolling. Compared with milling, this method provides higher productivity and saves high-speed steel. According to data from the Frezer plant [1], the cost of sector-rolled and milled drills is still approximately the same; this is explained by the low level of mechanization and automation of the rolling process, as well as by the need to introduce a number of new operations (straightening, heating, special centering, etc.). Nevertheless, there is reason to believe that, as rolling technology is improved, the cost of rolled drills will be substantially reduced.

From the standpoint of assessing the economic effectiveness of the new technology, the question of drill life is also of considerable importance.

At the request of the Tomsk Tool Plant, comparative tool-life tests were carried out in the metal-cutting laboratory of TPI on 17 mm drills milled from high-speed steel R18 and manufactured by the sector-rolling method from steels R18 and R9. The milled drills and the drills rolled from steel R9 were taken from the plant’s commercial production; the drills rolled from R18 were from an experimental batch.

All drills were checked for geometric parameters and selected with identical austenitometer readings.

The tests were conducted on a 2A135 vertical drilling machine; in order to obtain a large number of spindle-speed steps, the pulleys on the electric-motor shaft were changed. Drilling was carried out to a depth equal to three drill diameters, with cooling by a 5% emulsol solution. Steel 45 rolled stock with hardness \(H_3 = 210—217\) was used as the work material.

A few words should be said about the blunting criterion.

In conducting tool-life tests of drills, the criterion adopted is either a definite amount of wear on individual elements of the drill or a characteristic sharp squeal, which usually indicates a loss of the drill’s cutting ability. In the latter case, an additional criterion is also the appearance of a crushed bottom in the drilled hole.

As numerous studies show, including our own experiments, drill blunting is, as a rule, nonuniform in character, especially under different operating conditions. For this reason, the second criterion

is more convenient. In addition, it is widely used under production conditions.

The application of this criterion in our experiments showed that the dependence of tool life on cutting speed is obtained with sufficient clarity for the different types of dulling, with the exception of one, which will be discussed below.

Therefore, in carrying out the tests we adopted precisely this criterion. The results of the comparative tests are presented in Figs. 1, 2 and in Table 1. A study of the nature of drill dulling showed that it occurred either as a result of wear and destruction of the margin under noticeable wear on the flank and rake surfaces, or as a result of dulling of the main cutting edge without noticeable traces of wear on these surfaces. Several drills lost their cutting ability as a result of burning at the corners.

Fig. 1.

Fig. 2.

In the course of testing the drills, especially those milled from R18, a distinctive type of wear was also observed, previously described in the article by A. M. Rozenberg, A. N. Eremin, and A. N. Dobrovidov [2]. It consists of the following. After the drill has operated for some time, an annular groove (mark) appears on both lands, located almost at the very junction of the land and the main cutting edge. As wear progresses, this groove increases in depth and spreads over the entire width of the land (Fig. 3).

Drills with this type of land wear, despite considerable wear on the front and rear surfaces and almost complete wear of the web, continued to operate normally. Their life before complete dulling proved to be four times higher than that of drills with the usual type of wear (Fig. 2).

This specific wear of the land was also observed in rolled drills made of R18 and R9, but there it was expressed to a lesser degree. Nevertheless, all drills with such wear had a somewhat higher life in comparison with the others.

In rolled drills made of R18 the life proved to be lower by approximately a factor of 8 in comparison with milled drills with normal wear, and by a factor of 30 in comparison with drills having specific land wear.

Fig. 3.

Drills rolled from R9 showed high life in comparison with those rolled from R18, but lower life than milled drills made of R18.

profile of a rolled drill

profile of a milled drill

Fig. 4.

The reduced life of rolled drills made of R18 in comparison with milled drills and those rolled from R9 is, in our opinion, explained by two circumstances:

1. The worsened geometry of the rolled drills. Because of the reduced width of the land, the auxiliary part of the flute profile, whose purpose is to promote better chip removal, is greatly opened up. As a result, the chip, coming off approximately perpendicular to the cutting edge, strikes the wall of the hole being drilled, which leads to poorer conditions for its removal, chip packing, increased chip shrinkage, and poorer heat removal from the main cutting edge. Figure 4 gives a comparison of the flute profiles of rolled and milled drills. In addition, rolled drills often show asymmetry of the helical flutes and considerable runout of the main cutting edges and of the lands, which is explained by the low accuracy of the hot mechanical-working operation.

2. The poorer machinability in the hot state (during rolling). Evidence of this is provided by tests of rolled drills made of R9 steel, which has better deformability: despite the same shortcomings in geometry, their tool life was 2.5 times higher than that of rolled drills made of R18 steel.

Moreover, experiments conducted earlier by Yu. A. Rozenberg with rolled drills of R9 steel, diameter 19 mm, having a flute shape more favorable for chip removal, showed that twisted drills made of R9 were not inferior in tool life to milled drills [3]. At present it is not possible to explain the considerable increase in the tool life of milled drills made of R18 with the specific wear of the lands. It can only be said that the matter here is apparently the tool material, since attempts to artificially create an annular groove on the lands produced no effect on tool life.

Conclusions

1. Comparative tool-life tests of milled and rolled drills made of R18 showed that the latter have a tool life 8 times lower than the milled drills.

2. Drills rolled from R9 showed higher tool life in comparison with the same drills made of R18.

3. Although the manufacture of drills by the rolling method is more productive and, as the production technology is improved, can ensure a considerable reduction in their cost, it must not lead to a reduction in tool life. Otherwise, in mass production of drills, the loss in their tool life may nullify the economic effect of introducing this method. The tests carried out make it possible to state that much more extensive and painstaking work is required to improve the technology of drill rolling in order to increase their tool life. The main directions of this work are: increasing the accuracy of drill manufacture and finding optimal temperature conditions for rolling and heat treatment. This applies especially to drills made of R18 steel.

Literature

1. VNII. Collection of Works of the Institute, issues I–II, Moscow, 1958.

2. A. M. Rozenberg, A. N. Eremin, A. N. Dobrovidov. Experimental investigation of methods for increasing the tool life of cutting tools, Izvestiya TPI, vol. 61, no. 3, 1948.

3. Report on cost-accounting contract No. 27/56 concluded with the Tomsk Tool Plant.

Table 1