On the Relationship Between the Average Contact Pressure on the Rake Face and the Length of Chip Contact with the Tool
Abstract Generated abstract
This paper examines how the average normal contact pressure on a cutting tool rake face relates to the relative length of chip contact during metal cutting. Using experimental measurements for several work materials with widely varying hardness, together with data from Zorev, it argues that most cutting factors affect average contact pressure chiefly through changes in the ratio of contact length to chip thickness, while rake angle also has an independent effect through deformation constraint. The results indicate an approximately single-valued inverse power relation between average contact pressure and relative contact length, expressed for a rake angle of 10 degrees as qN = 180/(c/a)^0.95, and generalized with rake-angle-dependent parameters.
Full Text
PROCEEDINGS
of the Tomsk Polytechnic Institute named after S. M. Kirov,
holder of the Order of the October Revolution and the Order of the Red Banner of Labor
Volume 209 1976
ON THE RELATIONSHIP BETWEEN THE AVERAGE CONTACT PRESSURE ON THE RAKE FACE AND THE CHIP–TOOL CONTACT LENGTH
M. F. POLETIKA
The average contact pressure \(q_N\) on the rake face is an important contact characteristic. The average coefficient of friction of the chip on the tool depends on it, and this coefficient largely determines the intensity of deformation in the chip-formation zone. This same pressure plays an essential role in evaluating the strength of the cutting part of the tool and the intensity of the wearing action of the chip.
According to experimental investigations [1, 2], the average contact pressure depends on a large number of acting factors, including the properties of the work and tool materials, the rake angle of the cutter, the chip thickness, the cutting speed, etc. With an increase in the strength of the work material, the average contact pressure increases considerably. It increases with increasing cutting speed and decreases as the rake angle of the cutter increases.
Analysis of the experimental data shows that, of the factors named, only the rake angle of the cutter has an independent influence on the contact pressure. The remaining factors affect the average contact pressure not directly, but through a change in the relative contact length.
Fig. 1 shows the results of measuring the value of \(q_N\) when turning four work materials having very wide ranges of variation in mechanical properties \((\mathrm{HB}=33 \div 320)\). Experimental points calculated from the experiments of N. N. Zorev [1] are plotted on the same curve. We are convinced that the dependence of the average contact pressure on the relative contact length is close to single-valued.
The existence of such an almost single-valued dependence is explained by the fact that a change in the relative contact length \(c/a\), irrespective of the cutting conditions, leads to a change in the concentration of normal stresses. As the value of \(c/a\) decreases, the concentration of normal stresses increases and the average contact pressure rises. Thus the average contact pressure is a function—and consequently also a characteristic—of the stressed state in the contact zone, and depends on the relative contact length to the extent to which the latter affects the stressed state in the contact zone.
With a change in the rake angle at a constant value of \(c/a\), the degree of constraint of deformation in the plastic region changes.
As the rake angle decreases, the degree of constraint of the deformation increases, and so does the mean hydrostatic pressure in the plastic region. This leads to an increase in the normal contact stresses and, as a consequence, to an increase in the average contact pressure.
Fig. 1. Dependence of the average contact pressure on the relative contact length at rake angle \(\gamma = 10^\circ\): \(\lambda\)—BrB2 bronze, HB = 320, tool—VK8; \(\square\)—annealed U12 steel, tool T15K6 (according to our data and those of N. N. Zorev); \(\blacktriangle\)—BrB2 bronze, HB = 200, tool—VK8; \(\triangle\)—BrB2 bronze, HB = 110, tool—VK8; \(\times\)—annealed 12KhN3 steel, tool T15K6; \(\blacksquare\)—copper, tool—R9; \(+\), \(\bullet\), \(\circ\)—experiments of N. N. Zorev (steels 10, 20, 18KhNV).
The dependence shown in Fig. 1 is expressed with sufficient accuracy by the formula:
\[ q_N = \frac{180}{(c/a)^{0.95}} . \]
In the general case, this dependence may be represented in the form:
\[ q_N = \frac{A_\gamma}{(c/a)^m}, \]
where the coefficient \(A_\gamma\) and the exponent \(m\) are determined by the value of the rake angle, with usually \(m < 1\). As the rake angle increases, the coefficient \(A_\gamma\) decreases.
REFERENCES
- N. N. Zorev. Problems in the Mechanics of the Cutting Process. Moscow, Mashgiz, 1956.
- M. F. Poletika. Contact phenomena in metal cutting. Izv. TPI, vol. 133, 1965.