ON FORCES AT THE HEADS OF SINGLE-LIP DEEP-HOLE DRILLS

D. V. KOZHEVNIKOV, A. V. VODOPYANOV

(Presented by the joint scientific seminar of the Department of Machine Tools and Metal Cutting and the Department of Mechanical-Engineering Technology)

In recent years, carbide single-lip drills with external supply of cutting fluid and internal chip removal have found wide application in production for drilling deep holes (Fig. 1). The formation of the machined surface when working with these drills is accomplished through two closely interrelated processes: cutting and plastic deformation.

The nature of these processes depends on the cutting forces at the cutting edges of the drill and the friction forces on the guide pads. Therefore, the study of these forces is of great importance both for analyzing the quality of the hole surface and for setting up the operation. In view of the complexity of this process, it is impossible to determine separately, directly during drilling, the forces on the cutting edges and the guide pads of the drill. A dynamometer makes it possible to record only the total forces, torque, and axial force, which include both the cutting forces and the friction forces.

A method was developed that made it possible to separate the cutting forces from the total forces in deep-hole drilling and thereby determine the friction forces on the drill guide pads.

The total forces were measured when drilling holes of diameter 35 mm to a depth of 400 mm in steel 45 at cutting speeds \(V = 88\)—137 m/min and feeds \(S = 0.1 \div 0.15\) mm/rev.

The cutting forces were determined separately during free drilling of bushings and bars by different sections of the drill cutting edges with the aid of a special dynamometric setup (Fig. 2), installed on a 1K62 lathe. This also made it possible to determine the regularities in the variation of the cutting forces on each section of the cutting

Fig. 1

Fig. 2

edge and their distribution along its length as a function of cutting conditions, sharpening geometry, and drill wear. The cutting forces were measured under the same conditions as in deep drilling. As an example, Fig. 3 shows the distribution of forces along the edge at different numbers of revolutions.

Fig. 3

The results of measuring the total forces in deep drilling are presented in Fig. 4. As can be seen from Fig. 4a, which gives data on the axial force and torque, the force values at \(n = 800\) rpm and \(S = 0.1\) mm/rev differ from the forces at \(n = 1000,\ 1250\) rpm. Thus, at \(n = 800\) rpm there is a significant fluctuation of the forces. Recording on an oscillograph showed that these oscillations are of a low-frequency character. With an increase in cutting speed, the fluctuation of the forces disappears, and the values of the axial force and torque practically do not differ from their minimum values at \(n = 800\) rpm. In drilling with a larger feed, \(S = 0.15\) mm/rev (Fig. 4b), force fluctuation is likewise absent, and the forces at \(n = 800\) and \(1000\) rpm are practically identical.

Since the cutting forces during drilling remained practically unchanged, as was determined in free drilling of bushings with a sharp and worn drill, it may therefore be considered that the fluctuation of forces in deep drilling is associated with the process of friction on the drill lands. The process of friction on the lands takes place under complex conditions, namely: at high normal loads, speeds, and temperatures. The process of friction under such conditions has much in common with sliding friction. In the latter case, as established in works [1, 2], at high loads and temperatures destruction of the boundary layer of lubricant occurs

Fig. 4

at individual points, and the lubricant, in this way, does not prevent seizure and built-up-edge formation. Obviously, these processes also occur during friction of the drill lands and are the cause of force fluctuations under certain operating conditions.

After separating the cutting forces from the total forces in deep drilling, data were obtained on the friction forces on the drill lands. As can be seen from the diagram in Fig. 5, the friction forces in the overall force balance are quite significant and in some cases reach 40%. With increasing speed, the friction forces decrease. The decrease in friction forces is evidently associated with a decrease in the coefficient of friction, and not with a decrease in the normal load on the drill lands, because these loads, on the contrary, even increased somewhat owing to the increase in cutting forces with increasing speed. The latter is evident from the same diagram (Fig. 5).

Fig. 5. Diagram of torque components: friction torque and cutting torque. Vertical axis: \(M_{\mathrm{cr}}\), kgm; horizontal axis: rev/min; \(S = 0.1\) mm/rev. Values shown for 800, 1000, and 1250 rev/min.

At present, however, when calculating the friction forces on drill lands for deep drilling, the coefficient of friction is taken to be the same at different cutting speeds, equal to 0.2 [3, 4]. As can be seen from the data presented, this does not correspond to reality.

It is quite obvious that in another feed range the degree of change in friction forces with increasing speed will be different. Moreover, as is clear from Fig. 5, the character of their change in the general case may not coincide with the character of the change in cutting forces. On this basis, it may be considered that the regularities in the change of the total forces when the cutting-condition factors are varied in deep drilling are quite complex.

The influence of the design of the lands on the forces in deep drilling was also investigated. The experiments showed that with a decrease in the length and an increase in the back taper of the lands, the forces practically do not decrease. On the contrary, in this case, owing to intensive wear of the lands after drilling several parts, a noticeable increase in forces is observed, which is not found in drills with long lands and a small back taper.

Some authors [5, 6] recommend reducing the diameter over the lands relative to the diameter over the sizing chamfer of the cutting edge, which supposedly provides a significant reduction in torque.

Experiments with drills having different diametral positions of the lands relative to the sizing chamfer showed that when the diameter over the lands was reduced by up to 0.14 mm, no decrease in forces was observed. The results obtained are natural, since in this case practically neither the cutting forces on the cutting edges nor the friction forces on the lands changed. The great importance of the forces when the lands are located on the same diameter as the sizing chamfer, which is described in works [5, 6], is evidently associated with other phenomena.

First of all, this is associated with chip removal by the drill lands. Indeed, as our experiments showed, the lands can perform the functions of cutting edges and remove chips if their ends are not rounded. This is confirmed by inspection of the bottoms of the holes.

If the guide pads cut the chip, then the axial force and torque increase sharply (by almost 70%). In this case the quality of the machined surface also deteriorates, and the service life of the drills decreases.

REFERENCES

1. K. V. Savitskii. Studies of plastic deformations and properties of the outer layers of metallic bodies under various friction conditions. Doctoral dissertation. Tomsk, 1959.

2. N. I. Reznikov, I. G. Zharkov, V. M. Zaitsev, A. S. Kazarin, B. A. Kravchenko, F. P. Uryvskii. Production machining of stainless and heat-resistant materials. Mashgiz, 1960.

3. N. D. Troitskii. Machining of deep holes. In the collection: “Progressive Technology of Mechanical Engineering.” Book 41. Mashgiz, 1956.

4. Yu. M. Fridman. Production and operation of tools. Experience of Ural plants. Issue 7. Mashgiz, 1955.

5. V. Yu. Katsnelson, B. G. Chizhov. A production method for drilling deep holes. TsINTI, Metal Cutting, issue 12, topic 8, M—60—46/12, Moscow, 1960.

6. M. A. Minkov. Technology for manufacturing deep precision holes. Moscow—Leningrad, “Mashinostroenie,” 1965.