Degree and depth of work hardening of the hole surface during deep drilling with carbide single-lip drills
Abstract Generated abstract
This paper investigates work hardening of hole surfaces produced by deep drilling with single-lip carbide drills, focusing on whether the plastic deformation caused by guide pads can provide surface properties comparable to heat treatment. Experiments on steel 45 jackhammer-barrel blanks measured microhardness on oblique sections after drilling under varied cutting speeds, feeds, drill-edge angles, and guide-pad geometries. The results indicate that surface-layer hardness increased by about 2 to 2.5 times relative to the initial hardness, with feed and guide-pad back taper influencing the effect, while cutting speed, drill-edge angle, and drilling depth showed little change within the tested ranges. The study suggests that this drilling process may make it realistic to omit subsequent heat treatment for some barrel-type parts.
Full Text
Degree and Depth of Work Hardening of the Hole Surface during Deep Drilling with Single-Lip Carbide Drills
A. V. Vodopyanov
(Presented by the scientific seminar of the Department of Equipment and Technology of Welding Production)
In recent years, single-lip carbide drills with external supply of cutting fluid and internal chip removal have found wide application for drilling deep precision holes (Fig. 1). Formation of the hole surface when working with these drills, unlike double-lip drills, is carried out through two processes—cutting and plastic deformation by the guide pads a and b (Fig. 1).
Fig. 1
Fig. 2
The deformation process contributes to a significant reduction in surface roughness and to an increase in its hardness and wear resistance. The latter circumstance is of great importance for many parts, in particular for the barrels of jackhammers, riveters, and chipping hammers. The required hardness of the surface of the central channel on these parts is achieved by heat treatment; the presence of this treatment substantially increases the labor intensity of barrel manufacture, since, according to the existing technological process, grinding and honing of the hardened surface follow. After these operations the hole is obtained with an accuracy up to the third class and a surface roughness corresponding to the eighth class of finish.
Practically the same accuracy and roughness are achieved directly after drilling with single-lip carbide drills (Fig. 1). It was therefore of considerable interest to determine how much the hardness of the surface layer of the hole increases after the drill passes, compared with the initial hardness.
This article presents the results of an investigation of the depth and degree of work hardening of the hole surface.
Deep holes of diameter 35 mm and length up to 400 mm were drilled in blanks (steel 45) of the jackhammer-barrel type on a special setup built on the basis of a model 163 engine lathe. The experiments were carried out at cutting speeds \(V = 88\text{–}137\) m/min and feeds \(S = 0.11 \div 0.15\) mm/rev. The degree and depth of work hardening of the surface layer were determined by measuring microhardness on specimens cut from the drilled parts. Microhardness was measured on the surface of an oblique section located at an angle of \(2^\circ\) to the hole axis, using a PMT-3 instrument with a 100 g load on the pyramid.
The main results of the investigation are presented in Figs. 2, 3, and 4.
As follows from Fig. 2, the depth and degree of work hardening of the surface layer of the hole are quite significant: thus, the microhardness of the surface layer when drilling with a feed of \(S = 0.10\) mm/rev exceeds the initial value by almost a factor of two, and at a feed of \(S = 0.15\) mm/rev by a factor of 2.5.
From Fig. 3 it may be noted that the pattern of change in the degree of work hardening with depth is practically the same within the range of cutting-speed variation from 88 to 137 m/min. This should be regarded as quite natural, since in these experiments no increase in the normal load on the supporting guide with increasing speed was observed. The normal load was determined by calculation on the basis of experimental data from measurements of the resultant main component of the cutting force \(P_z\) and the radial component of the cutting force \(P_y\) (Fig. 1) at different cutting speeds.
Fig. 3
Fig. 4
The investigations carried out showed that, when the angle at the tip of the outer portion of the drill’s main cutting edge (Fig. 1), \(\varphi_1\), was reduced from 10 to \(20^\circ\), the degree and depth of work hardening practically did not change. This can be explained by the fact that, as the angle \(\varphi_1\) increased, mainly only the radial component of the cutting force, \(P_y\), increased; in the overall balance of forces acting on the supporting guide, this component is relatively small.
Changing the design of guides a, b (Fig. 1), namely their length from 5 to 25 mm and the back taper from 0.05 to 0.8 mm per 100 mm,
showed that work hardening increases somewhat as the back taper of the guide pads decreases (Fig. 4).
When measuring the microhardness of the surface layer in different cross sections of the workpiece, no change was found in the degree or depth of work hardening along the drilling depth. This is explained by the fact that the cutting forces likewise practically did not increase toward the end of drilling, owing to the insignificant wear of the drill cutting edges.
Thus, the hardness of the surface layer of the hole exceeds the initial hardness by a factor of 2–2.5. Considering that, during heat treatment of the barrels of drop hammers, the hardness increases by no more than a factor of 2.8, it appears realistic to eliminate heat treatment when using the deep-drilling method with single-lip carbide drills.