TttREE-COMPONENT

LATHE DYNAMOMETER

M. F, P o t e t i k a Translated from Izmeritel'naya Tekhnika, No. 3, pp. 18-20, March, 1962 Tests made by the Tomsk Polytechnical Institute have shown that the most stable results in measuring cuttingforce components are obtained with elastic dynamometers on stiff three-dimensional frame systems. One of these dynamometers was designed and made by us according to Loewen and Marshall's scheme [1, 2, 3]. The principle of its operation is based on the use of an elastically strained ring to which a normal and tangential force are applied. Figure 1 shows the schematic of the dynamometers, tts front portion is suspended from the casing on four symmetrically placed elastic elements, each of which consists of a semicircular ring with external faces. The radial component Py of the cutting force produces in the four elastic elements a compressing force normal to them. Strain gauges l z , 2z, 5z, 6z, ly, 2y, 5y and 6y which are glued to them are extended and gauges 3z, 4z, 7z, 8z, 3y, 4y, 7y and 8y are compressed. The main component Pz of the cutting force will impress on the upper pair of the elastic elements a tensile strain normal to them, and on the lower pair a compressing strain. The sign of the deformations in the strain gauges of the lower belt will not change, but in the upper belt gauges lz, 5z, ly and 5y will be compressed and 3z, 7z, 5y and 7y extended. The radial (ly - 8y) and vertical (lz - 8z) force gauges are connected differentially to the bridge circuit in such a manner as to compensate for the interaction between the cutting force components.

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Fig. 2. a) Transducers of Px; b) transducers of Py; c) transducers of Pz-

Figure 2 shows the electrical circuit of the dynamometer strain gauges. It will be seen from the circuit that gauges ly and 3y (and respectively 2y and 4y, l z and 3z, 5z and 7z, etc.), one of which is extended and the other compressed, are connected to opposite arms of the bridge. For the same reason the gauges in the upper and lower belts which record force Pz (3z and 4z, lz and 2z, etc.) are also connected in opposite arms of the bridge. In order to raise the sensitivity gauges measuring force Pz are glued at the maximum distance from plane xy, and gauges measuring force Py are placed, on the contrary, as closely as possible to plane xy in order to reduce their sensitivity to component Pz- For the same purpose the gauges of the upper and lower belts which record force Py are connected to the bridge in series (ly and 2y, 5y and 6y, etc.). The transducers of the axial component Px of the cutting force are glued as far as possible symmetrically with respect to the nodal points of each of the elastic elements. Hence, forces normal to the transducer of component Px will produce deformations in one-half of this transducer equal and opposite to the deformations in the other half. The transducer's total electrical resistance will not change and hence, transducers of component Px will be insensitive to components Pz and Py. Component Px will produce in each of the elastic half-rings a tangential force, with gauges 3x, 4x, 5x and 6x being extended and lx, 2x, 7x and 8x compressed. These gauges are connected respectively to opposite arms of the bridge. A series connection of gauges lx and 2x, 7x and 8x, 3x and 4x, 5x and 6x provides the compensation of component Px and Py.

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In practice, It is very difficult to provide an equal density in gluing the gauges. Nonuniformity of gluing, however, disturbs compensation of the cutting-force components in the above circuit. In order to avoid these effects it was necessary to abandon the symmetry of the circuit. The interaction of the components was reduced to a satisfactory value of 1% by displacing the transducers of Pz and Py in the vertical direction. The dynamometer consists of an elastic body of a sufficiently complex shape to preclude an accurate calculation of stresses and strains at any of its cross-sections. We have, therefore, contented ourselves with determining the linear displacement of the application points of forces (Px, Py and Pz). These displacements were measured along the three axes by means of a microindicator which is mounted on the dynamometer by means of a bracket. Smaller size dynamometer models were tested in order to determine their limiting loads. They were loaded consecutively in the directions of the three components Pz, Px and Py right up to the appearance of residual deformations. The dynamometer readings were recorded by means of two strain-gauge three-channel amplifiers. One of them was built to the design of the NIAT (Scientific Institute of Automobile Transport) and the other to that of the TsNIITMASh (Central Scientific Research Institute of Technology and Machinery) [4]. The calibration of the dynamometer in conjunction with the amplifier and a pointer indicating instrument at its output provided a linear relationship for all the components. The possibility of using the dynamometer for measuring forces during the cutting process was evaluated by testing brand 12KhN3A steel machined with a T15K6 cutter (~ = 0~ ~ = 70~ X = 0~ t = 3 rnm, S -- 0.3 mm/rev). The tested manometer was compared with other similar instruments (a double-component elastic dynamometer with wire strain gauges, a three-component elastic dynamometer with inductive transducers, a hydraulic, lever and other types of dynamometers). The readings were compared at a constant loss of turnings which, in our opinion, is the most accurate method, since this excludes the effect of possible instabilities in the cutting on the dynamometer readings. The test results thus obtained were in good agreement with those obtained by other instruments in measuring components Pz and Pxo All the points in the diagram fell within the range of experimental dispersion. In the measurements of the third components (Py) certain discrepancies were observed, with the tested dynamometer providing slightly higher values for this component. 1. 2. 3. 4.

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LITERATURE CITED E. Go Loewen, E. R. Marshall and M. C. Shaw, Electric strain-gauge tool dynamometers, Proceedings of the Society of Experimental Stress Analysis, Vol. 8, No~ 2 (1951). N.H. Cook, E. G. Loewen and M. C. Shaw, Machine-tool dynamometer, The Machinist, Vol. 98, No. 27 (1954). tDynamometers with octahedral annular sensing elements for metal-working lathes, t Technical Information Bulletin of the Ministry of Machine-Tool and Tool-Making Industry, No. 4 (1956). M.B. Georgievskii, S. V. Cheredov and M. D. Medvinskii, "Multichannel measuring devices for operation with resistance-wire strain-gauges (PET-3-V)." Collection of the Acad. Sci. USSR entitled Experimental Methods for Investigating Machines (t954).