METHOD FOR MEASURING DISPLACEMENTS OF DRILL CUTTING EDGES AND ELASTIC DEFORMATIONS IN THE SPID SYSTEM DURING VIBRATION DRILLING

D. V. Kozhevnikov, I. M. Konovalov, V. N. Yundunov

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

To ensure reliable chip breaking when drilling holes in heat-resistant and stainless steels, methods of kinematic fragmentation have recently been used, in particular the method of imposing low-frequency harmonic oscillations in the direction of drill feed from an external source.

In works [1, 2], the degree of influence of each vibration parameter on the kinematics of the vibration-drilling process is shown analytically. At the same time, these works assume that the cutting edges of the tool also perform harmonic oscillations with a certain amplitude, which is part of the amplitude specified by the vibrator, and with a frequency equal to the frequency of the disturbing force. The influence of the rigidity of the SPID system on the kinematics of the process is not taken into account.

The harmonic oscillations of the tool are set by a vibrator rigidly connected by a clamping device to the rear side of the drill shank. During operation, the SPID system experiences variable loads from the action of the disturbing force and, consequently, elastic deformations that vary in magnitude.

Under these conditions, the amplitude of the imposed oscillations is partially absorbed by elastic deformations and possible clearances in the mechanisms of the machine tool.

In addition, torsional oscillations of the drill shank influence the phase shift specified in the system and periodically change the actual cutting speed.

Without taking these factors into account, calculation of the parameters of forced vibrations necessary for normal chip fragmentation may lead to significant errors and fail to produce the expected effect.

Therefore, studying the influence of the rigidity of the SPID system on the process of deep vibration drilling is of great theoretical and practical importance.

For these purposes, in the metal-cutting laboratory of TPI, a method and the necessary equipment were developed that make it possible to carry out dynamic tests of the SPID system during deep vibration drilling with measurement of the magnitude of elastic deformations of its component parts.

The work was carried out on an installation for deep vibration drilling [3].

The arrangement of the displacement sensors is shown in Fig. 1. Vibration drilling was carried out with two-flute drills of 25 mm diameter, made of R18 steel, with internal chip removal. During operation the drill was guided by guide bushing 5, fitted with a sliding fit to the hole in oil receiver 6. Disk 9, carrying the moving parts of sensors 1 and 2, was fastened to drill 8. The fixed parts of the sensors were mounted on special platforms secured to the oil receiver. To eliminate the influence of feed on sensors 1 and 2, the oil receiver was connected to the carriage by rigid tie rod 7. This sensor arrangement makes it possible to measure the axial vibrations (sensor 1) and torsional vibrations (sensor 2) of the drill cutting edges. The axial vibrations of the shank at its connection with the hydraulic vibrator were measured by sensor 3, whose moving part is connected to the shank and whose fixed part is connected to the carriage of the setup.

Fig. 1

Sensor 4 recorded the trajectory of motion of the carriage; for this purpose its cores with coils were connected to the bed, and the armature to the carriage of the setup.

The oscillatory system under consideration has two degrees of freedom. The cutting edges are displaced by the vibrator in the axial direction and have torsional vibrations under the action of cutting forces. The design of sensor 1 must ensure reliable measurement of the axial vibrations and must not respond to torsional vibrations. In addition, at idle running the amplitude of vibrations of the system increases approximately threefold compared with the working amplitude; as a result, the working gaps of sensors 1 and 3 must be increased by the same amount, which leads to a sharp decrease in the sensitivity of the measuring system. Therefore, for measuring axial vibrations at the cutting edges and at the point where the shank is fastened, powerful transformer-type sensors were used, which make it possible to obtain a significant output voltage with an appropriate ratio between the number of turns of the measuring windings and the excitation windings [4, 5].

The schematic diagram and design of sensors 1 and 3 are shown in Fig. 2, a. Core 2 is made of Armco iron, is rigidly connected to the moving part of the system, and accurately reproduces the vibrations of the cutting edges or of the shank, depending on where it is installed. A slot is cut in the core, into which the excitation winding is placed; it is connected to a power source with frequency \(f = 500\) Hz and voltage \(U = 40\) V.

The measuring windings of sensor 1 are wound on magnetic cores of the PL 12.5×16×40 type, and those of sensor 3 on ShL 12×20 (two half-cores), and are connected into the circuit in opposition.

Fig. 2

In carrying out the experiment, only the linear portions of the sensor characteristics were used.

The slope of the characteristic depends largely on the parameters of the filter elements; therefore the values of the resistances and capacitances in them were selected on the basis of the maximum slope of the characteristic and satisfactory recording quality.

Fig. 3

The calibration curves, obtained with an MPO-2 loop oscillograph, for sensors 1 and 3 are shown in Fig. 3, where \(H\) is the height of the trace on the photographic film, and \(L\) is the linear displacement of the armature.

Sensor 2, intended for recording torsional oscillations, must not respond to axial oscillations. Known designs of angular-displacement sensors proved unsuitable under this condition because of the bulkiness of their magnetic systems. Therefore, for the...

Fig. 4.

To measure angular vibrations, a special sensor was developed on the basis of two toroidal magnetic cores, in whose slots an armature of Armco iron rigidly connected to the drill is displaced (Fig. 2, б).

The slots are made of such a size that the armature can move freely together with the drill in the axial direction within 1 mm. Since the total clearance in each magnetic core during axial vibrations will remain constant, the sensor does not respond to these vibrations. During angular displacements, the armature changes the inductive resistance of the bridge arms, as a result of which a current proportional to the angle of rotation appears in its diagonal.

The calibration curve of sensor 2 with the MPO-2 loop oscillograph is given in Fig. 3, where \(\alpha\) is the angle of rotation of the armature.

Sensor 4 must record the displacement of the support over a considerable length, have a high characteristic slope, and at the same time not overload the vibrator of the oscillograph over the entire measurement range. The sensor design shown in Fig. 2, в, is the most convenient for these purposes. The laminated magnetic cores are made of alternating layers of magnetic material (steel E-320) and nonmagnetic material (textolite B4). The length of the movable magnetic core \(l_1\) is chosen to be greater than the length of the fixed magnetic core \(l_2\) by the amount of the measured range of support displacement. The fixed parts of the magnetic cores are displaced so that the magnetic layers coincide with the nonmagnetic ones. The windings of the magnetic cores are connected in a bridge circuit. The schematic circuit for connecting all four sensors is shown in Fig. 4. The measurement method described made it possible to carry out experimental studies of oscillatory processes in the SPID system with high accuracy.

Conclusions

1. A method has been developed for measuring the parameters of oscillatory processes in the SPID system during vibration drilling.

2. The schematic circuit and design of a sensor have been created for measuring angular displacements of the drill cutting edges under the action of cutting forces.

3. Designs have been developed for sensors of small and large linear displacements.

4. The method and measuring means make it possible to study, with high accuracy, oscillatory processes in the SPID system during vibration drilling.

References

1. E. A. Satel, V. N. Poduraev et al. The vibration properties of holes in stainless and heat-resistant steels. Vestnik mashinostroeniya, No. 1, Moscow, 1962.

2. A. G. Tuktanov. Vibration drilling of small-diameter holes in stainless and heat-resistant steels and alloys. Author’s abstract of dissertation. Bauman Moscow Higher Technical School, 1964.

3. I. M. Kopovalov, A. V. Vodopyanov. A setup for studying the cutting process when drilling deep holes. Electromechanical devices and electronic instruments. “Energiya,” Moscow–Leningrad, 1969.

4. L. F. Kulikovskii. Inductive displacement meters. Gosenergoizdat, Moscow, 1954.

5. A. M. Turichin. Electrical measurements of nonelectrical quantities. Gosenergoizdat, Moscow, 1954.