Tool Life of Corn Cob Cutters

A. M. Rozenberg
Professor, Doctor of Technical Sciences

A corn cob roughing cutter operates on the principle of non-free cutting and, as we established in a special study [1], removes, by part of its cutting edge, a substantially thicker chip than an ordinary cylindrical cutter under the same cutting conditions. This circumstance leads to a reduction in the specific cutting work and in the power consumed. When working with small feeds, when the share of work performed by the auxiliary cutting edges is insignificant, the corn cob cutter reduces the power consumed, in comparison with a cylindrical cutter, by 30–40%, which is substantial in cases where milling productivity is limited by the power of the machine tool. In this case, the corn cob cutter makes it possible to increase the productivity of the milling machine. When shifting to larger feeds (thicker chips), in the operation of the corn cob cutter the influence of the non-free cutting process begins to be felt; the share of participation of the auxiliary cutting edges in the cutting process increases, and this raises the specific work, so that the corn cob cutter is then characterized by a less significant saving in consumed power. Nevertheless, throughout the range of feeds practically used in milling, the corn cob cutter is a more economical tool from the standpoint of power consumption than the cylindrical cutter. In addition, the corn cob cutter, removing a chip divided across its width into separate narrow pieces, in contrast to the cylindrical cutter, which when machining ductile metals removes one continuous chip over the entire milling width, operates more smoothly and is less prone to vibrations and chattering, especially at large milling depths. All this together creates in production the impression that the corn cob cutter works more easily, smoothly, and productively than the cylindrical cutter. This impression corresponds to reality when productivity is limited by the power of the machine tool or by its stability.

In the case where work is carried out on a stable machine tool of high power, productivity will be determined by the life of the tool. In this case the corn cob cutter will give lower productivity than the cylindrical cutter, since its life will be lower. The lower life of the corn cob cutter is due to the fact that part of its cutting edges removes a thicker chip and is therefore subject to more intensive wear than the cutting edges of a cylindrical cutter. In addition, the presence of the non-free cutting process is inevitably associated with impeded chip formation and chip removal at the points of junction of the main and auxiliary cutting edges, where increased wear also occurs.

The present investigation was undertaken for the purpose of experimentally verifying the stated assumption concerning the lower life of corn cob cutters in comparison with cylindrical cutters.

For carrying out experiments to compare the life of corn-cob and cylindrical cutters, the Tomsk Tool Plant made, from one bar of EI-262 steel, three each of both types of cutters, with the same heat treatment and identical dimensions. The cutter dimensions are given in Table 1.

Table 1

The profile of the cutting edge of a tooth of the corn-cob cutter, with the corresponding designations, is shown in Fig. 1.

Fig. 1

The chemical composition of the cutter material and its structure are not given here, since both the cylindrical and the corn-cob cutters were made from one bar, underwent identical heat treatment, and, as Table 2 shows, had sufficiently close hardness values. Hardness was determined on the end face of the cutter teeth by 8–10 measurements on each cutter, and Table 2 gives the range of hardness variation for each cutter.

Table 2

The cylindrical cutters had a pointed tooth and were sharpened on the back face. The corn-cob cutters had a relieved tooth and were sharpened on the front face. The back angles of both types of cutters, as Table 1 shows, were the same; this is important in view of the known influence of the magnitude of the back angle on wear and tool life. The corn-cob relieved cutters had an unground relief surface on the teeth. It might have been feared that during heat treatment some decarburization of the surface could occur there, which would be associated with a reduction in surface hardness and, as a result, in cutter life. As a check, the hardness of the corn-cob cutters was measured not only on the end face of the teeth, but also on the relief surfaces. The measurement showed that the hardness on the relief surfaces was 1–2 units lower than the hardness on the end faces. We considered this permissible, since Table 2 shows that the hardness on the end faces of the corn-cob cutters is somewhat higher than that of the cylindrical cutters.

The endurance testing of the cutters was carried out in the metal-cutting laboratory of the Tomsk Polytechnic Institute on a universal milling machine from the Tula plant. The machine was equipped with a direct-current motor, which made it possible to obtain stepless speed regulation by introducing a rheostat into the circuit of the motor excitation winding.

Before testing, the cutters were carefully sharpened so that the eccentricity of the individual teeth did not exceed 0.02 mm. Eccentricity was checked on the machine after mounting the sharpened cutter on the spindle mandrel.

The endurance tests were carried out on blanks of 3KhV-8 steel, which had previously been annealed and had a hardness of \(H_b = 240\). In order to eliminate the influence of any possible, even slight, change in the hardness of the blanks (which was not found when the hardness of the blanks was measured), each blank was cut lengthwise into two halves; on one of the halves an experiment was carried out testing the cylindrical cutter, and on the other, the corn-cob cutter. Hardness was measured both on the surface of the blanks and inside them, after a number of chips had been removed. Everywhere the hardness proved to be the same.

The cutting conditions during the test were as follows:

depth of cut \(t = 2\) mm;
feed per tooth \(S_z = 0.19\) mm;
feed per one revolution of the cutter \(S_n = 1.52\) mm/rev;
milling width \(b = 30\) mm.

The work was carried out without cooling.

The comparatively small depth of milling was chosen because of the insufficient power of the machine. To compensate for this, a comparatively large feed value was chosen.

Tests of each type of cutter were carried out at three speeds. The cutting speeds for the corn-cob cutter were taken as 48.75 m/min;

Fig. 2

37.2 m/min; 30 m/min. The cutting speeds for the cylindrical cutter were 46.5 m/min; 43 m/min; 37.2 m/min.

During each experiment, up to the final blunting of the cutter, the cutting speed was maintained strictly constant.

During each experiment, at short intervals of time, the wear on the rear face of the cutter teeth was measured with a 20-power loupe. In Fig. 2, for the corn cutter, and in Fig. 3, for the cylindrical cutter, the change in the width of the wear land on the rear face \(\delta\) is presented as a function of the cutter operating time. A comparison of these two figures shows that, at the same cutting speeds, wear on the teeth of the corn cutter proceeds much more intensively than on the teeth of the cylindrical cutter.

The character of wear of the rear faces of the teeth of cylindrical and corn cutters is different. In cylindrical cutters, on the rear faces of the teeth there is observed, along the entire length of the cutting edge located under the chip, the appearance of a uniform wear land, which gradually widens as the cutter operates, while remaining of approximately the same width along its entire length.

Fig. 3

Only in the very last moments of cutter operation, preceding the final destruction of the cutting edge, do local sharp widenings of the wear land appear; once they have appeared, they increase rapidly and finally lead to the final destruction of the cutting edge at these locations.

In corn cutters the wear of the rear faces proceeds differently. For clarification, Fig. 4 shows the shape of the cross section of the chip removed by the cutting element of a tooth of the corn cutter. As Fig. 4 shows, the chip is removed along the perimeter 1234, with part of the chip having the thickness \(S_{e1}\), the same as on the tooth of the cylindrical cutter, while another part of the chip has a thickness \(S_{e2}\) increased severalfold. This increased thickness is removed by the part of the cutting edge located on the cutter cylinder over the length \(\frac{h}{z}\) (for the designation \(h\), see Fig. 1) and by the part of the side cutting edge 34. The length \(\frac{h}{z}\) is the main cutting blade, 34 the auxiliary one. Thus, on the cutting edges of the corn cutter there are sections of length \(\frac{h}{z}\) loaded with a thicker chip—

than the other sections of the cutting edges. The ratio \(\dfrac{S_{e2}}{S_{e1}}\) is very significant and, in general form, is expressed by equation (1)

\[ \frac{S_{e2}}{S_{e1}}=z-\frac{rz}{h}+1, \]

which, for the dimensions of the corn cob cutters tested, gives

\[ \frac{S_{e2}}{S_{e1}}=6.34. \]

Corner of tooth 3 is the apex of the blade, at which the section of the main blade, loaded with a thick chip \((S_{e2})\), and the auxiliary

Fig. 4

blade 34 meet. It is natural to expect that wear of the cutting edge of the corn cob cutter should begin at corner 3 and be localized mainly along the length of the edge \(\dfrac{h}{z}\). Observations of wear confirm this assumption and show that the greatest amount of wear on the flank face is always found at corner 3, where the final failure of the

Fig. 5

blade occurs. Figure 5 shows a tooth of a corn cob cutter with a wear land on the flank face, considerably widening at the corner. This widening should be regarded as the wear that limits the service life of the cutter.

On the basis of Figures 2 and 3, Figure 6 was constructed; in it, in logarithmic coordinates, \(V\) (cutting speed in meters per minute) and \(T\) (tool life in minutes) are used to plot the lines \(T=f(V)\) for the cylindrical and corn cob cutters. To construct these lines, we used the value of limiting wear on the flank face \(\delta=1\) mm. The position of the straight lines in Fig. 6

shows that the corn-cob cutter has a tool life 4–5 times lower than that of the cylindrical cutter within the range of speeds and tool lives at which the tests were carried out. It should be noted that the tool lives obtained by us in the tests (for the corn-cob cutter from 8.5 to 82 min, for the cylindrical cutter from 34 to 115 min) are not economical for cutters. But this in no way changes, in principle, the results of comparing the tool life of corn-cob and cylindrical cutters. At longer tool lives, their ratio will be still less favorable to corn-cob cutters, owing to the somewhat steeper position of the straight line for the cylindrical cutter. We also consider it quite permissible to construct the straight lines in Fig. 6 from only three points, since these points are the result of reliable experiments, during which wear changed in an entirely regular manner, as is seen from Figs. 2 and 3; all the more so because the experiments had the purpose only of making a qualitative comparison of the tool lives of corn-cob and cylindrical cutters and were not intended to give any precise quantitative characteristics, for the derivation of which, of course, a considerably broader investigation would have been necessary.

Fig. 6

Despite the small number of experiments carried out, on the basis of their results we can nevertheless draw certain quantitative conclusions as well, which will be of value in view of the fact that up to now no relationships have been obtained for corn-cob cutters.

For cylindrical cutters, according to the latest standard data, in the case of machining hard alloy steels the following relationships are available.

According to Orgaviaprom data for chromium-nickel steels [2],

\[ V=\frac{C_v\cdot D^{0.25}} {T^{0.22}\cdot S_z^{0.39}\cdot t^{0.27}\cdot b^{0.09}\cdot z^{0.1}\cdot \sigma^{0.27}} \]

According to the data of the Bureau of Technical Standards of the People’s Commissariat of the Tank Industry for steel Kh4N [3],

\[ V=\frac{C_v\cdot D^{0.5}} {T^{0.22}\cdot S_z^{0.4}\cdot t^{0.27}\cdot b^{0.09}z^{0.1}\cdot \sigma^{0.18}} \]

In these equations:

\(D\) — cutter diameter;
\(T\) — tool life;
\(S_z\) — feed per tooth;
\(t\) — milling depth;
\(z\) — number of teeth;
\(b\) — milling width;
\(\omega\) — helix angle.

It may be assumed that these dependencies may also be used for corncob cutters, provided that the following circumstances are taken into account.

The blunting of the cutting edges of a corncob cutter occurs on those sections that remove a thickened chip. These sections have a length \(\dfrac{h}{z}\) in the direction of the milling width. The quantity \(\dfrac{h}{z}\) should therefore be substituted in the equations instead of the milling width \(b\).

The chip on these sections of the cutting edges is \(\left(z-\dfrac{rz}{h}+1\right)\) times thicker than on the cutting edges of a cylindrical cutter. Therefore, instead of the feed per tooth \(S_z\), in the equations given above one must substitute a value increased \(\left(z-\dfrac{rz}{h}+1\right)\) times.

Then the equation for a corncob cutter will have the form

\[ V_k=\frac{C_v \cdot D^x} {T^m \cdot \left(z-\frac{rz}{h}+1\right)^y \cdot \frac{S^y}{z}\cdot t^n \cdot \left(\frac{h}{z}\right)^p \cdot z^g \cdot s^u}. \]

The ratio of speeds for the same tool life, for those cylindrical and corncob cutters that we tested at identical values of \(S_z\) and \(t\) and that had identical \(D\) and \(z\), but different \(\sigma\), will be

\[ \frac{V_k}{V_{\mathrm{ц}}} = \frac{b^p \cdot \sigma_{\mathrm{ц}}^u} {\left(z-\frac{rz}{h}+1\right)^y \cdot \left(\frac{h}{z}\right)^p \cdot \sigma_k^u}, \]

where letters with the subscript “\(k\)” refer to the corncob cutter, and with the subscript “ц” to the cylindrical cutter.

For our case:

\[ z-\frac{rz}{h}+1=6.34; \qquad \frac{h}{z}=1.5; \]

\[ \sigma_k=25^\circ; \qquad \sigma_{\mathrm{ц}}=30^\circ; \]

\[ b=30\ \text{mm}. \]

According to Orgviiaprom data,

\[ y=0.39; \qquad p=0.09; \qquad u=0.27. \]

According to Narkomtankoprom data,

\[ y=0.4; \qquad p=0.09; \qquad u=0.18. \]

Substituting these values, we obtain

a) according to Orgviiaprom data

\[ \frac{V_k}{V_{\mathrm{ц}}}=0.67 \]

b) according to Narkomtankoprom data

\[ \frac{V_k}{V_{\mathrm{ц}}}=0.646. \]

Such should be the ratio of cutting speeds for the corncob and cylindrical cutters, if our assumptions are correct. Figure 6 shows that for tool lives above 100 minutes (such tool lives are of interest ...

... and production during the operation of cutters), calculated from cutting-speed ratio equations, are confirmed quite accurately by our experiments, which indicates the possibility of using standard reference materials in calculating cutting conditions (speeds) for corn cob cutters, taking into account the remarks made above.

Conclusion

1. The investigation carried out showed that the tool life of corn cob cutters is lower than that of cylindrical cutters, owing to the increased wear of those sections of the cutting edges that remove the thickened chip.

2. The possibility has been demonstrated of using, for calculating cutting speeds in the operation of corn cob cutters, those relationships that were obtained for cylindrical cutters, taking into account the features of the geometry of the chip removed by the cutting elements of the corn cob cutter.

Cited Literature

1. Rosenberg, A. M. — Dynamics of the Corn Cob Roughing Cutter.

2. Orgaviaprom. — Guiding Materials on Cutting Conditions. Milling. Defense Industry, 1942.

3. Narkomtankoprom, Bureau of Technical Standards. — Handbook on Cutting Conditions on Milling Machines. Mashgiz, 1942.