MEASURING

THE DEPTH

IN THE COURSE A.

M.

OF S K I M M I N G

OF G R I N D I N G

Makhanov

UDC531.717

An important criterion in evaluating the quality of a diamond wheel consists of its efficiency. A wheel which removes a m a x i m u m volume of the ground m a t e r i a l for its m i n i m u m wear is considered to be best. Diamond grinding is used, as a rule, as a finishing process under light operating conditions. It is difficult to guarantee, for small displacements of the grinding stock, that the actual depth of cutting will correspond to the feed indicated on the vernier scale of the machine. Moreover, the rigidity of circular grinding machines used for diamond finishing is such that deformations of its separate units are of the same order as its transverse feed. Therefore, in order to evaluate correctly the efficiency of diamond wheels, it is necessary to measure with great precision the true depth of grinding. The scientific and production tool laboratory of the Kuibyshev Polytechnical Institute has developed a device which can be used for determining the true amount of skinned m a t e r i a l for each operation. The measuring system should gualantee: 1) a measurement error not exceeding 0.25-0.3 p, 2) a recording of measurements on a chart, 3) an e l i m i n a t i o n of the effect of grinding forces on the instrument's readings. These requirements were dictated by the following conditions. A transverse feed of 0.002-0.006 m m is permitted in circular external grinding of alloyed steels with a diamond wheel and a measurement error not exceeding 510a/o of the feed. Since high-precision circular grinding of products is an operation entailing repeated passing, it is necessary for analyzing the operation of the wheel to know the true depth of cutting for each pass. Therefore, the true skimming of the allowance from the products must be indicated on the diagram of the recording instrument, The recording on the diagram serves to observe the course of grinding and facilitates considerably the processing of measurement results. The instrument (Fig. 1) is placed on the table of machine 312M. The external gauge which consists of upper plate 4, lever 5, and plate 6 with a strain-gauge transducer is fixed by means of nuts 2 and 3 on stand 1. Variations in the diameter of the ground article turn lever 5, since weight 7 presses continuously measuring tip 8 against the surface of the article. The other end of lever 5 bends plate 6. The signals from the transducer are fed to the instrument's e l e c t r i c a l circuit. The use of lever 5 made it possible to solve several problems. First, the instrument's sensitivity was raised, since the ratio of the lever arms was set at 1 : 3. Thus the deformation of plate 6 isthree times larger than variations in the diameter of the article. Second, the plate with the transducers has been taken to a considerable distance away from the machining area, thus eliminating any cooling liquid dropping on it.

7 ~-

! O

2

33

Fig. 1

7~

Fig. 2

Translated from I z m e r i t e l ' n a y a Tekhnika, No. 4, pp. 86-8% April, 1968; Original article submitted July 21, 1966.

546

Slide wire EPD-12

Fig. 3 The measuring part of the instrument is suspended on flat springs which act as a hinge. Therefore, the bending of the article under the circular grinding force does not affect measurement results. The accuracy of the instrument's operation is determined to a great extent by the stiffness of plate 6 and the pressure exerted by the measuring tip on the article. In order to solve this problem we have analyzed the forces acting on various components of the system. Figure 2 represents the force diagram. By solving an equation of the moment of forces with respect to point M, it becomes possible to determine the compressing force as =

Pcmp

P. 75 - - P*. 95 33

N.

(1)

However, force P' is constant and depends on the sagging of plate 6 (see Fig. 1), i.e., on the value of its preliminary tension. Plate 6 has the shape of a beam of uniform resistance. The bending of such a beam at any cross section can be calculated from the formula

[ =

Pla 2EI

I -- 2 ~

+

10--s ram.

(2)

l

where x is the distance in mm from the point at which the force is applied to the cross section under consideration; g is the length in mm of the plate with the transducer; E is the elasticity modulus of the plate in N/m2; I is the amount of inertia of the plate in m4; P is the weight of the load in N. We are interested in the sagging at the point where the force is applied, i.e., for x = 0. Therefore, the expression for evalua'ting the relationship between the sagging and the force assumes the form p,la f_

2El

i0 -G ram.

(3)

From (1) and (3) it is possible to calculate the compressing force Pcmp with respect to the value of the preliminary tension. The compressing force attains its m a x i m u m of 14.0 N for f = 0.1 mm, a n d i t is equal to zero for f = 0.36 ram. Under our operating conditions the value of the weight and the cross section of the plate with the transducers were calculated for a stable operation of the instrument and a skimming from the article of 0.05-0.07 m m of its allowance, It is possible to compute by the method suggested above also for other operating conditions. The transducer signals were recorded on an electronic potentiometer ~PD-12 whose electrical, circuit was slightly altered (Fig. 3). The strain-gauge transducers are included in the bridge whose unbalance rotates the indicating pointer and displaces the recording pen on a circular diagram. For setting the instrument to zero the potentiometer is replaced by device A.

547

The circular diagram of an EPD-12 instrument rotates at the speed of one revolution per 24 h, whereas we require a rotation speed of one revolution per 5 min. In order to obtain this speed the electric motor was connected directly to the last wheel of the reduction gear, omitting the first two stages. The above device is highly r e l i a b l e and simple to adjust. In the above form it is suitable for measuring d i a m eters of 30 to 70 mm with an error of 0.25 #. It should be noted that the instrument's sensitivity can be adjusted by selecting a suitable shunt resistance Rsh (see Fig. 3) and can be varied over a large range attaining values below 0.25/~. The measuring range on the scale is then also changed. The application of this device for measuring the true depth of grinding served to apply considerable corrections to the evaluation of the efficiency of grinding wheels for various operating conditions.

548