Dimensional Wear of VK8 and VK3 Alloys and TsM-332 Inserts during Turning of Gray Cast Iron

A. K. Baikalov

(Presented by Prof., Doctor of Technical Sciences A. M. Rozenberg)

In the course of operation, the cutting edge of a tool wears continuously, and its position does not remain stable with respect to the workpiece being machined. Displacement of the cutter tip in the direction perpendicular to the machined surface produces a dimensional error and leads to the formation of taper in the machined part.

For dimensional finish turning of parts, it is necessary to know the magnitude of the radial wear of cutters as a function of operating time, or as a function of the cutter path traversed in the metal. In other words, it is necessary to calculate the dimensional tool life of the tool. Taking into account the magnitude of the tolerance for inaccuracy in manufacturing the part, the dimensional life of cutters may be represented as:

\[ T_r=\frac{1}{2}\frac{\delta}{U_0'},\ \text{min} \tag{1} \]

or

\[ L=\frac{1}{2}\frac{\delta}{U_0},\ m. \tag{2} \]

Here:

\[ \begin{aligned} T_r \text{ and } L &\text{— dimensional life, in minutes or in meters of path traversed} \\ &\text{by the cutter in the metal;}\\ U_0' \text{ and } U_0 &\text{— relative magnitude of tool wear, in } \mu\text{, attributable to one minute} \\ &\text{of cutting or to one meter of cutter path in the metal;}\\ \delta\ (\mu\text{) }&\text{— the part of the tolerance field for inaccuracy in manufacturing} \\ &\text{the part that is allocated to compensation of the dimensional error} \\ &\text{of the part due to tool wear.}^{1} \end{aligned} \]

Calculation of dimensional life by formulas (1) and (2) is possible because the intensity of dimensional wear, for a given cutting regime, remains constant up to a certain amount of wear (Fig. 1). It is evident that \(T_r\) or \(L\) are the maximum permissible lives when working without resetting the tool to size.

In those cases where it is expedient to work with resetting of the tool to size, the total life may be calculated by the formula:

\[ \sum T_r=\frac{h_r}{U_0'} \tag{3} \]

\({}^{1}\) The other part of the tolerance field provides compensation for errors associated with force and temperature deformations of the machine–cutter–part system, errors of static adjustment, etc.

or

\[ \Sigma L=\frac{h_r}{U_0}, \tag{4} \]

where: \(h_r\) is the blunting criterion, defined as the amount of radial wear in \(\mu\), up to which the condition \(U_0'\) is maintained, and \(U_0=\mathrm{const}\).

The main drawback of calculating dimensional life by formulas (1) and (2) is the difficulty of calculating the value \(\delta\), i.e., the tolerance field for compensating the dimensional error associated with tool wear.

Fig. 1

The magnitude of the tolerance field \(\delta\), in turn, depends on force and temperature deformations, the allowance for which presents great difficulties. Under shop conditions, the value of the dimensional life \(T_r\) or \(L\) can be established directly, by measuring the dimensional errors of the first batch of parts machined without adjusting the tool to size. Then the total life of the tool, after the use of which the cutters must be resharpened, can be determined by formula (3) or (4), and the number of adjustments is equal to:

\[ K=\frac{\Sigma T_r}{T_r}-1 \tag{5} \]

or

\[ K=\frac{\Sigma L}{L}-1. \tag{6} \]

For the practical use of formulas (3) and (4), it is necessary to know the variation of \(U_0'\) and \(U_0\) with the cutting conditions and the blunting criteria of various tool materials.

This article presents the results of experiments on dimensional turning of gray cast iron of hardness \(H_B=180\text{–}190\ \mathrm{kg/mm^2}\) with cutters fitted with hard-alloy inserts VK8, VK3, and the ceramic insert TsM-332.

Method for measuring radial wear

The magnitude of radial wear of cutters must first of all be related to the accuracy of manufacture of the parts. The loss of cutter dimensions will have the most substantial effect in the manufacture of parts of high accuracy classes and in machining surfaces with a long pass length.

For accuracy class III according to GOST, the tolerance-field value for the error in manufacturing parts with diameters of 50–500 mm, for various fits, lies within 0.06–0.19 mm. If, as a guideline, \(\delta\) is taken as equal to half the tolerance field for manufacturing inaccuracy, then the radial wear should not exceed 15–45 \(\mu\). For higher accuracy classes and smaller machining diameters, the permissible value of radial wear when working without adjustment to size will be still smaller.

Obviously, the method for measuring radial wear must ensure high accuracy in recording wear, preferably with an accuracy of \(\pm 1.0\,\mu\). From this point of view, measurement of radial wear with microscopes that provide a reading accuracy of 0.01–0.005 mm must be considered crude.

In our experiments, dimensional wear was recorded by means of a vertical optimeter with a scale division value of 1.0 \(\mu\). Before measurement, the optimeter table was carefully aligned for parallelism relative to the plane of the measuring tip. Constancy of the initial position of the scale (the “zero” of the scale) was checked by means of a block of gauge blocks.

To eliminate measurement errors associated with inaccurate seating of the tool during measurement and when installing the tool on the machine, a detachable design of the tool holder was created. The carbide cutting insert was mechanically fastened in a special holder having, on its end face, three hardened pins by which the holder was installed on the optimeter table. The three-point installation of the holder ensured a constant position of the cutting-insert tip relative to the measuring axis of the instrument.

Before turning, the holder was inserted into a longitudinal slot in the shank (parallel to its axis) and secured in it by screws. The assembled tool was fastened in the tool holder of the machine perpendicular to the machining axis by means of a universal protractor, which ensured an installation accuracy of \(\pm 15'\).

Throughout the entire series of experiments, the shank remained fixed in the tool head. For measurements, only the holder was removed. The stationary position of the shank on the machine ensured constancy in the location of the holder and cutting insert relative to the workpiece.

Thus, the wear-measurement procedure ensured a constant position of the cutting insert both relative to the measuring axis of the instrument and relative to the direction of radial wear, which eliminated measurement errors associated with nonuniform seating.

Wear measurement was carried out after the cutting insert had cooled to room temperature. The tool tip and the locating pins of the holder were washed with alcohol to remove adhering particles and dirt. Under some cutting conditions, the formation of a fairly strong built-up edge was observed on the tool tip; its presence could give an erroneous measurement result. The built-up edge was carefully removed with fine-grained abrasive paper, while the cleaning was checked with a microscope at 25× magnification.

Measurement with the optimeter made it possible reliably to record the increase in wear at intervals of 1–2 \(\mu\) and to obtain a complete picture of the dimensional wear of the tools within a narrow tolerance field.

Effect of depth of cut and feed on the dimensional wear of VK3 carbide

The study was carried out for cast iron of medium hardness \((H_B = 190\ \text{kg/mm}^2)\), i.e., the hardness most widely encountered in production.

As the cutting material, the hard alloy VK3 was selected; the advisability of using it in finishing operations may be considered established. The sharpening geometry of the cutters in the experiments was as follows:

\[ \gamma = 10^\circ,\quad \alpha = 10^\circ,\quad \alpha_1 = 10^\circ;\quad \varphi = 45^\circ;\quad \varphi_1 = 15^\circ;\quad r = 0.5 \div 1.0\ \mu\text{m}. \]

As the blunting criterion, wear \(h_r = 30\ \mu\text{m}\) was adopted, within the limits of which a proportional change of \(h_r\) with time is preserved.

The experimental results are presented in Figs. 1 and 2.

The depth of cut (Fig. 1), when varied from \(t = 0.2\ \text{mm}\) to \(t = 0.5\ \text{mm}\), has an insignificant effect on the dimensional wear of VK3 (the change in tool life is less than 50%), while the magnitude of the relative wear (per \(1000\ \text{m}\) of path in the metal) fluctuates within \(U_0 = 3\text{–}5\ \mu\text{m}\). Especially small is the difference in dimensional tool life for small depths \(t = 0.35\ \text{mm}\) and \(t = 0.2\ \text{mm}\), whose wear points up to \(h_r = 20\ \mu\text{m}\) practically coincide.

Fig. 2

It is evident from Fig. 1 that a more or less proportional change in radial wear may be assumed up to \(h_r = 30\ \mu\text{m}\). This value is therefore adopted as the blunting criterion. In addition, the wear curve has no running-in zone, as is often observed in studies of tool blunting on the main flank face; this makes it possible to replace the wear curve by an averaging straight line and to calculate the averaged specific wear \(U_0\) with accuracy sufficient for practice.

The influence of feed on dimensional wear (Fig. 2) is manifested much more strongly. With increasing feed, tool life decreases almost in proportion to the increase in feed (Fig. 3). The magnitude of the relative wear also varies over wide limits (from \(3.8\) to \(20\ \mu\text{m}\) per \(1000\ \text{m}\) of path in the metal at \(v = 135\ \text{m/min}\) and \(t = 0.35\ \text{mm}\)).

Fig. 3

The blunting criterion for different feeds (Fig. 2) may likewise be taken as equal to 30–40 microns.

Processing of the wear curves presented in Figs. 1, 2, 3 gives the following empirical relationships:

1. For cast iron of hardness \(H_B = 180—190\ \text{kg}/\text{mm}^2\), \(v = 120\ \text{m}/\text{min}\) and \(S = 0.2\ \text{mm}/\text{rev}\):

\[ T=\frac{C_1}{t^{0.55}}=\frac{34.4}{t^{0.55}},\ \text{min}. \tag{7} \]

1. For cast iron of hardness \(H_B = 180—190\ \text{kg}/\text{mm}^2\), \(v = 135\ \text{m}/\text{min}\) and \(t = 0.35\ \text{mm}\)

\[ T=\frac{C_2}{S^{0.84}}=\frac{5.2}{S^{0.84}},\ \text{min}. \tag{8} \]

Influence of Cutting Speed on the Dimensional Wear of VK8, VK3 Alloys and the TsM-332 Ceramic Insert

The character of dimensional wear of the VK3 hard alloy at various cutting speeds (\(v = 20—150\ \text{m}/\text{min}\)) is presented in Fig. 4. The least wear was recorded in the region of cutting speeds \(v = 80—100\ \text{m}/\text{min}\). A characteristic feature of cutter wear in this region is the uniformity of the course of wear with time.

At cutting speeds \(v < 80\ \text{m}/\text{min}\), the uniformity of the increase in wear is disturbed; the wear becomes unstable and continuously increases

Fig. 4

with decreasing cutting speed. At higher speeds the proportionality of the increase in wear is more or less preserved, but the intensity of wear rises sharply with increasing cutting speed. Thus, in machining cast iron, as in machining steel [1], there is an optimum zone of cutting speeds characterized by the smallest value of dimensional wear.

The character of the change in radial wear for various cutting speeds is shown most clearly in Fig. 5. Here, along the ordinate axis,

the relative wear \(U_0\), microns (wear referred to 1000 m of the cutter path in the metal), is plotted; along the abscissa is the cutting speed. The curves of the dependences \(U_0=f(V_{\text{rez}})\) are plotted for the hard alloys VK3 and VK8.

From Fig. 5 it is evident that the cutting qualities of the hard alloys VK8 and VK3 for the optimum cutting speed \(V=80\ \text{m/min}\) are equivalent, since the magnitude of the least wear for both alloys is the same and amounts to about \(1.0\ \mu\) for every 1000 m of cutter path in the metal. The advantage of alloy VK3 appears when working at higher cutting speeds. Thus, for \(V=120\ \text{m/min}\) the relative wear of alloy VK3 is about \(4\ \mu\), and of alloy VK8 about \(10\ \mu\), i.e., 2.5 times greater. In other words, at the same wear, or at one and the same dimensional life, alloy VK3 permits operation at higher cutting speeds.

Fig. 5

On the whole, the range of cutting speeds that can be used for dimensional turning of cast iron is small and considerably narrower than in the machining of steel. For hard alloy VK8 it lies within the limits of 80–100 m/min; for alloy VK3 it can be extended to 120 m/min. The reason for this is the great sensitivity of the dimensional wear of VK-type hard alloys to changes in cutting speed. At \(V_{\text{rez}}>V_{\text{opt}}\) the wear intensity, especially for alloy VK8, increases sharply owing to the rise in cutting temperature. Conversely, at \(V_{\text{rez}}<V_{\text{opt}}\) the cutting temperature is low; the impact toughness of the hard alloy is then insufficient, and tool wear proceeds very intensively by microchipping of the cutting edge. So-called brittle wear of the hard alloy occurs, characterized by an unsteady increase of wear with time and by an indeterminate tool life under the same cutting conditions (the hatched zone in Fig. 5).

The optimum cutting speed is characterized by such a value of the temperature on the rake face of the tool at which the strength of the cutting edge is sufficient to eliminate brittle wear, while the wear intensity is small owing to the relatively low temperature on the rake face. Under these conditions, wear proceeds uniformly with time by relatively slow abrasion of the hard-alloy material. Naturally, as the cutting speed is increased, the intensity of this abrasion will increase, which is what is observed in the range of speeds greater than \(V_{\text{opt}}\).

The wear intensity at a given cutting regime and for the material being machined is determined entirely by the wear resistance of the tool material. Thus, for a definite tool material and given cutting conditions \((H_B, t, S=\text{const})\), the dimensional life of the tool cannot be increased by reducing the cutting speed. This leads to the fact that in production the finishing dimensional machining of cast iron has to be carried out at cutting speeds close to the optimum \((V=80\ \text{m/min})\), which does not always satisfy production from the standpoint of machining productivity. In view of the absence of recommendations, these speeds are established in practice by trial and ...

naturally, is not always expedient. Finishing of a whole series of parts (for example, under the industrial conditions of the city of Tomsk, motor housings and end shields) is carried out at cutting speeds on the order of 60–90 m/min; moreover, the machine time for finishing operations is several times greater than for roughing operations. It is obvious that, in order to increase the productivity of dimensional turning, it is first of all necessary to set the cutting speed correctly, on the basis of the prescribed machining accuracy, and to use more wear-resistant cutting materials. Thus, by using the VK3 hard alloy instead of the VK8 alloy, the cutting speed can be increased by 15–20%.

More promising from this point of view are mineral-ceramic tools, which possess considerably greater wear resistance than hard alloys. To clarify the advantages of ceramic tools, a study was carried out of the dimensional wear of TsM-332 inserts¹.

The grinding geometry of the TsM-332 inserts was adopted as follows:

\[ \varphi = 45^\circ,\quad \gamma = 0^\circ,\quad \varphi_1 = 10^\circ,\quad \alpha = \alpha_1 = 10^\circ;\quad r = 1.0\ \text{mm}. \]

To strengthen the cutting edge, a land 0.2 mm wide (equal to the feed) was ground on the rake face, with a negative rake angle \(\gamma_0 = -5^\circ\).

Dimensional wear was studied under the same conditions as for hard alloys, i.e., at the regime \(t = 0.35\) mm, \(S = 0.2\) mm/rev on cast iron of hardness \(H_B = 180\text{–}190\ \text{kg/mm}^2\).

The experimental results are presented in Fig. 6. The curve of change in relative dimensional wear with cutting speed for ceramic inserts is similar to the wear curve for hard alloys and convincingly demonstrates the advantage of ceramic tools. Possessing high wear resistance, TsM-332 inserts have a range of optimum cutting speeds within 160–220 m/min, i.e., 2–2.5 times higher than for hard alloys.

Fig. 6

Increasing the cutting speed causes a relatively smaller increase in the intensity of wear (the wear curve for speeds \(V > 160\) m/min), while the absolute value of the relative wear at the optimum cutting speed (\(V = 200\) m/min) is 2 times lower than for hard alloys. Both of these circumstances make it possible to turn parts at cutting speeds 2.5 times higher than for hard alloys; in this case the tools will have double the life in comparison with the VK8 and VK3 alloys. Brittle wear of TsM-332 inserts is observed at cutting speeds below 160 m/min and proceeds analogously to the wear of hard alloys.

A substantial drawback of ceramic inserts is their considerably lower bending strength limit in comparison with hard alloys. Therefore, when using TsM-332 inserts it is necessary to use clamping of increased rigidity and to apply mainly

under steady operating conditions (without impacts and vibrations) [2]. In addition, the quality of sharpening and brazing of the inserts has a great influence on the life of ceramic cutters. Difficulties with brazing have now been overcome by the use of preliminary metallization of ceramic inserts [3]. The absence of proper control over the quality of cutter manufacture often nullifies the advantage of mineral-ceramic cutting materials.

For calculating the cutting speed in dimensional turning of cast iron with VK3 hard alloy (for an accepted tool life), the formula obtained by graphical processing of the data of Figs. 1, 2, 3, 4 may be used:

\[ V=\frac{C_v}{T^m \cdot t^{x_v}\cdot S^{y_v}} =\frac{170}{T^{0.17}\cdot t^{0.09}\cdot S^{0.145}}\ \text{m/min}. \tag{9} \]

The formula is valid for calculating cutting speeds within the limits \(V=80\text{--}150\ \text{m/min}\) for machining cast iron of hardness \(H_B=180\text{--}190\ \text{kg/mm}^2\).

For alloy VK8, for the particular case (\(t=0.35\ \text{mm}\), \(S=0.2\ \text{mm/rev}\), \(H_B=180\text{--}190\ \text{kg/mm}^2\)), the cutting speed for the selected tool life is equal to:

\[ V=\frac{C_z}{T^m}=\frac{220}{T^{0.2}}\ \text{m/min}. \tag{10} \]

The data from the graphs (Figs. 5 and 6) may be used for practical calculations when machining cast iron under conditions close to the experimental ones.

The material presented in this article is comparative in character and, naturally, cannot satisfy practice from the standpoint of calculating cutting conditions over wide ranges.

Conclusions

1. In studying the regularities of dimensional wear, it is necessary to ensure high accuracy in measuring wear. Experiments have established that recording wear with an accuracy of \(\pm 1.0\ \mu\) is quite achievable.

2. The dimensional wear of cutters is most significantly affected by cutting speed and feed. With increasing feed, the relative wear continuously increases. When the cutting speed is varied, the wear \(U_0\) changes along a curve with a minimum, which indicates the existence of an optimum cutting speed at which the tool has the highest dimensional life. The practical range of cutting speeds providing acceptable tool life when machining cast iron with hard alloys is considerably narrower than when machining steel.

3. The cutting qualities of the hard alloys VK8 and VK3 when operating at optimum cutting speeds are equivalent. The advantage of the hard alloy VK3 appears at higher cutting speeds (\(V>V_{\text{opt}}\)). With equal dimensional life of the cutters, the cutting speed for alloy VK3 in this case can be increased by 15–20%.

4. The regularity of dimensional wear of TsM-332 ceramic inserts is analogous to that of hard alloys. The use of mineral ceramics for dimensional machining of cast iron is promising, since it provides an increase in cutting speeds by 2–2.5 times (with double tool life) in comparison with hard alloys.

References

1. Accuracy of mechanical machining and ways of improving it. Collection of works of the LPI, edited by Prof. A. P. Sokolovskii. Mashgiz, 1951.

2. Isaev A. I., Zorev N. N., Kuchma L. K. Cutting of metals with a ceramic tool. Mashgiz, 1952.

3. Ioffe M. M. and Nekhlin S. I. Metallization of ceramic plates. Journal “Technology of Transport Machine Building,” No. 4, 1956.

4. Baikalov A. K. Optimal geometry of cutters in high-speed turning of gray cast iron. Dissertation, Tomsk, 1952.

1. Members of the student research circle M. E. Kazantsev and R. D. Leont’eva took part in the work. ↩