FORCE RELATIONSHIPS IN DRILLING STEELS EN316, Kh25SN3D, and 1Kh18N9T

A. G. Dolodarenko

(Presented by the scientific seminar of the Department of Machine Tools and Metal Cutting)

The magnitudes of the forces arising in drilling heat-resistant and stainless steels are important characteristics of this process. Accurate determination of cutting forces makes it possible to select rational cutting conditions and is also necessary in the design of machine tools, fixtures, and tools.

The determination of force relationships in drilling cast heat-resistant steels EI316 and Kh25SN3D and rolled acid-resistant steel 1Kh18N9T was carried out on a modernized model 2150 vertical drilling machine with a high-rigidity drilling dynamometer. The dynamometer has measuring ranges: axial force \(P_0\) up to \(30\ \mathrm{kN}\) and torque \(M_{\mathrm{tor}}\) up to \(250\ \mathrm{N\,m}\).

The tests were carried out with standard high-speed-steel drills made of R18 steel with diameters of \(8.7;\ 13.7;\ 16.5;\ 19.4;\ 27.5;\) and \(35\ \mathrm{mm}\). Drill sharpening was performed on a model 3A64M universal sharpening machine along two planes. In all cases the sharpening angles were as follows: \(2\varphi = 118^\circ\), \(\alpha = 15^\circ\), and \(\Psi = 50^\circ\).

The tests were carried out with cooling by a 5% aqueous emulsol solution. Cutting conditions: feed \(S\) from \(0.0239\ \mathrm{mm/rev}\) to \(0.0755\ \mathrm{mm/rev}\), rotational speed \(n\) from \(4.34\ \mathrm{rev/sec}\) to \(34.8\ \mathrm{rev/sec}\).

During the experiments, the average cutting temperature was recorded by the natural thermocouple method. All readings of forces and temperature were recorded on an MPO-2 oscillograph and then decoded at tenfold magnification using the corresponding calibration graphs.

The temperature in the graphs is given in millimeters of beam deflection on the oscillograph film, since, for identifying general regularities, it is not necessary to know the absolute temperature values.

As a result of the investigations, relationships were obtained for the average cutting temperature, axial force, and torque as functions of speed and feed for each drill diameter on all materials. For drills of diameter \(\varnothing 27.5\ \mathrm{mm}\) in Kh25SN3D steel, these relationships are given in Fig. 1.

The change in the average cutting temperature with increasing speed follows a monotonically increasing curve; moreover, the temperature increase is at first more significant, and then the temperature curve becomes flatter. The change in the slope of the temperature curve with increasing speed is explained by improved heat removal due to the increase in the thermal conductivity of the chip material (as temperature rises) and by the greater mechanical removal of heat by the chip as a heat carrier (as the chip-flow speed increases).

Steel Kh25SNZD, drills made of steel R18, Ø 27.5 mm, sharpening along two planes, cooling by pouring

○ — \(S = 0.0239\ \mathrm{mm/rev}\) \((0.15\ \mathrm{mm/rev})\), △ — \(S = 0.0374\ \mathrm{mm/rev}\) \((0.235\ \mathrm{mm/rev})\)

• — \(S = 0.0478\ \mathrm{mm/rev}\) \((0.3\ \mathrm{mm/rev})\), □ — \(S = 0.0605\ \mathrm{mm/rev}\) \((0.38\ \mathrm{mm/rev})\)

× — \(S = 0.0755\ \mathrm{mm/rev}\) \((0.475\ \mathrm{mm/rev})\)

Fig. 1. Dependence of temperature, axial force, and torque on speed when drilling cast heat-resistant steel Kh25SNZD.

With a change in feed, the character of the dependence \(t^\circ=f(V)\) does not change; however, as the feed increases the curve is located higher, which is natural, since with increasing feed the total cutting work increases, and consequently so does that part of it which is converted into heat.

The axial force and torque vary with a change in cutting speed along a concave curve (Fig. 1), which, of course, is determined by the change in the geometry of the built-up edge under the given cutting conditions. The minimum of the dependences shifts to the left with increasing feed; this also agrees with the usual principles of cutting theory.

Processing of the dependences \(P_0=f_1(V)\) and \(M_{\mathrm{tor}}=f_2(V)\) made it possible to reveal the influence of feed on the axial force and torque under conditions of constant temperatures (Fig. 2). Using A. M. Rozenberg’s method for finding the forces on the flank face of the tool by extrapolation to zero chip thickness, we obtain the total normal force on the flank face of the drill \(P_0^{3.2}\) and the friction moment of the flank faces at a cutting temperature corresponding to a 3 mm beam deflection on the oscillogram.

For determining the share of the load falling during drilling on the chisel edge, special experiments were carried out on reaming holes whose diameter (6.5 mm) was equal to the length of the chisel edge in drills 27.5 mm in diameter. The tests showed that, in the steels studied, up to 68% of the axial force and up to 30% of the torque fall on the chisel edge. This ratio of axial force and torque on the chisel edge to the total load on the drill is determined by the large negative rake angles on the rake face of the chisel edge for \(P_0\) and by the small radius of application of the force for \(M_{\mathrm{tor}}\).

Labels in Fig. 2: axial force \(P_0\) (kgf); axial force \(P_0\) (kgf); torque \(M_{\mathrm{tor}}\) (kgf·m); torque \(M_{\mathrm{tor}}\) (kgf·m); \(S\), mm/rev; steel Kh25SN3D; drills of steel R18, \(d=27.5\) mm; sharpening by two planes; cooling by emulsion; temperature corresponds to a 3 mm beam deflection.

Fig. 2. Dependence of axial force and torque during drilling of Kh25SN3D steel on the magnitude of feed at constant temperature

Determination of the influence of the sharpening method on the axial force and torque was carried out for two-plane sharpening, conical sharpening (the Washburn method), and helical sharpening (on a model 3658 drill-sharpening semiautomatic machine). On the magnitude of the torque, the sharpening method...

had no effect; the results of the experiments for all steels are equivalent. In terms of the magnitude of the axial force, sharpening along a helical surface and along two planes are equivalent, whereas with sharpening along a conical surface the axial force on steel Kh25SNZD is 45% greater, and on steels ENZ16 and 1Kh18N9T almost twice as great. This is explained by the fact that, when sharpening along a helical surface and along two planes, the rake angles on the transverse cutting edge have a smaller negative value than when sharpening along a conical surface, and the steels investigated are highly prone to work hardening, especially steels ENZ16 and 1Kh18N9T.

The study to determine the effect of drill wear on the magnitude of the force and torque showed that flank wear up to 0.4 mm, measured at $\frac{r_x}{R}=0.6$ and critical for drills of 17.5 mm diameter, has practically no effect on $P_0$ and $M_{\mathrm{cr}}$.

The investigations carried out showed that the principal dynamic regularities identified in the drilling of carbon steels and described in the literature are also confirmed when drilling the investigated heat-resistant and stainless chromium-nickel-based steels. The materials on the drilling dynamics of the steels described are being published for the first time and are of interest to machine-building enterprises involved in the machining of heat-resistant and stainless steels.