MECHANICAL MEASUREMENTS

DYNAMOMETER FOR MEASURING C U T T I N G FORCES ON A DRILL

A. Ya. Baer UDC 531.781 : 621.95

Dynamometers used for measuring cutting forces during drilling are complex in design, since they have in- dividual systems of elastic elements-supports for measuring each of the cutting forces. Therefore they are difficult to manufacture and use under plant conditions.

Free of these shortcomings is the proposed dynamometer intended for measuring cutting forces, viz., axial forces up to 2" 104 N and torque up to 250 N/m, during any tupe of operation on the drill (drilling, countersinking, reaming, tapping, etc.).

It consists of three main parts: plate (Fig. 1), which is installed on the drill bench, instrument housing, which is measuring element 3 mounted on plate 1, and clamp 3, which is attached to housing 2. The test specimen is in- stalled in the clamp, and during calibration a beam is installed for applying forces from standard dynamometers. Strain-gauge transducers 4 and 5 with a 5-ram base and 100 f~ resistance are cemented to the appropriate surfaces of the housing 2. The wires from the transducers are extended to socket 6, which serves for connecting the dynamo- meter to the recording instruments.

The elastic element of the dynamometer (Fig. 2) represents a cylindrical steel housing in which openings are cut out so that the top and bottom of the housing are joined by tetrahedral props. Since the rigidity of these props is much less than the rigidity of the other parts of the dynamometer, the clamp and plate are connected elastically with each other and the forces arising during cutting deform mainly the props. Therefore, the cutting forces are measured by determining the defon-nation of the props by means of the strain-gauge transducers cemented on them. The axial force P0 compresses the props and the torque Mt simultaneously bends them. To measure each of these forces separately, the strain-gauge transducers are connected into two separate systems and positioned on the props such that the effect of one force on the instrument readings is minimized with respect to the other force. In this

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Translated from Izmeritel'naya Tekhnika, No. 9, pp. 39-40, September, 1971. Original article submitted July 15, 1969.

�9 1972 Consul tants Bureau, a d iv is ion o f Plenum Pub l i sh ing Corporation, 227 West 17th Street, New

York, N. Y. 10011. Al l rights reserved. This art icle cannot be reproduced for any purpose whatsoever

without permiss ion o f the publisher. A copy o f this article is available from the publ isher for $15.00.

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case the effect of a random lateral force which can arise as a result of improper sharpening of the tool also becomes virtually imperceptible.

Strain-gauge transducers R1, R2, R3, R4, R'I, R~, R'3, R~, intended for measuring the axial force, are cemented on the inside and outside surfaces of the props in the mid- die with respect to their height and width, i.e., at the place where the bending moments (from the effect of torque) are equal to zero. These transducers are connected as a bridge circuit (fig. 3) and therefore measure only compressive

strain.

Strain-gauge transducers R51' R6, RT, R8, R'5, R'6, R~, R's, which record the magnitude of torque, are cemented near the base of the props on the side surfaces in the mid- dle with respect to the thickness of the props, i.e., on their neutral axis during bending from a random lateral force. These transducers are connected as a bridge circuit (Fig. 4)

for measuring only bending strains under the effect of torque. Compression of the props causes a like change of re- sistance in them, the effect of which is practically excluded electrically by the way it is wired (see Pig. 4). The effect of bending from a random lateral force is also thereby eliminated.

Compensation of the temperature changes of the resistance of the transducers for measuring the axial force is achieved by connecting into the circuit (see Fig. 3) eight compensating transducers 5 (see Fig. 1) cemented on the base of the housing. The circuit diagram of the transducers for measuring the torque (see Fig. 4) automatically gives temperature compensation.

Various measuring instruments can be used for recording the cutting force, such as microammeters or oscillo- graphs. It is more convenient to use N373 recording microammerers for this purpose. However, when using these in- struments any departure of the supply voltage of the circuit of the strain-gauge transducers causes a change of the instrument readings, which introduces large errors. Since the aforementioned instruments are dc instruments, a dc source is needed for supplying the dynamometer circuit, and if an alternating current is used the voltage must be carefully stabilized and, in addition, a rectifier must be connected ahead of the instrument, which increases the er-

rors of measuring the forces.

To eliminate these shortcomings, MS1-03 automatic Wheatstone bridges having a scale with 100 divisions are used as the indicating and recording instruments. One of the instruments indicates and records on a paper tape the magnitude of the axial force and the second one the magnitude of the torque. The main advantage of these instru- ments is that they react only to a change of resistance of the transducers. A change of voltage in the circuit of the strain-gauge transducers has practically no effect on the accuracy of the instrument readings. Therefore they can be connected directly into the ac main. For use for the indicated purposes it is necessary to introduce slight changes into the circuit of the instruments, which amount to a change of the measuring range of the instrument (toward an increase of sensitivity) and to connection of the transducers of the dynamometer in place of the resistors of the m e a - suring bridge of the instrument.

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The dynamometer was calibrated to establish the scale value of the instrument and to check the accuracy of measuring forces.

The following conclusions were made as a result of calibrating and testing the dynamometer during drilling.

1. The dynamometer provides a linear dependence of the instrument readings on the applied load (both the axial force and the torque).

2. The standard deviation of the measurement of axial force is 0.6% of the finite value of the working part of the scale of the indicating instrument and of measurements of torque, 0.3% of the finite value of the working part of the scale.

3. The effect of the application of torque and the effect of a lateral load on the instrument readings are im- perceptible with respect to axial force; the effect of application of an axial or lateral force on the instrument read- ings with respect to torque does not exceed 1% in each case.

4. The dynamometer has high rigidity: in the direction of the axial force it was 1000 N/m, and in the direc- tion of torque 34 N/m.

Owing to the high rigidity, vibrations did not arise during drilling even at high speeds (e.g., during drilling of cast iron by a 26-ram-diameter hard-alloy drill at a cutting speed of 81.7 m/rain and feed rate of 0.4 mm/rev).

5. The virtues of the dynamometer are simplicity of design and reliability in operation, and also its low cost in comparison with dynamometers of other designs. Therefore the described dynamometer can be used successfully in technological laboratories of machine-building plants.

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