ANALYSIS OF THE DESIGN OF THE SUPPORT OF A CROSS-PLANING MACHINE AND PROPOSALS FOR ITS IMPROVEMENT

G. L. KUFAREV, V. I. LIVSHITS

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

Tool-life experiments on the planing of high-manganese steels made it possible to establish that the support assembly of the standard model 7M37 cross-planing machine is insufficiently adapted for machining hard-to-machine materials, especially in the case of tool wear on the flank face.

When planing carbon steels, cutters having a bent shank are widely used (Fig. 1). The shape of the cutter shank is made such that, when it bends under the action of cutting forces, the chip thickness does not increase but decreases. This eliminates the appearance of scratches and scoring on the machined surface and is considered an advantage of bent cutters over straight ones, which increase the chip thickness during deformation (Fig. 1). However, bent cutters always have a greater overhang than straight ones. This circumstance, as well as the low rigidity of the bent cutter shank, leads to the fact that bent cutters possess much lower vibration resistance than straight cutters. Attempts to use bent cutters in machining high-manganese steels ended in failure. Planing of high-manganese steels can be carried out only with straight cutters having a shank cross section of \(20 \times 30\) or \(30 \times 50\) with minimum overhang. In this case no scoring or scratches were observed on the machined surface even at large chip cross sections. The latter circumstance is apparently connected with the fact that, when high-manganese steels are machined, very considerable forces arise on the flank face of the tool, neutralizing the deflection of the cutter in the direction of increasing depth (Fig. 1).

This assumption is confirmed by an analysis of the operation of the entire tool-holder assembly during tool wear. As was noted more than once in the experiments, as wear progresses the ability of the tool to cut under the chip decreases; in other words, its deflection increases under the action of the forces \(P_x\) and \(P_y\) (Fig. 2, force \(P_x\) is shown in spatial position). When critical wear on the flank face is reached, the force \(P_y\) increases so much that the clockwise moment \(P_y \cdot L\) exceeds the counterclockwise moment \((P_z \cdot M +\) the action of springs 1), and causes rotation of the swing plate clockwise about axis \(A\), i.e., withdrawal of the cutter from under the chip. It is precisely this phenomenon that becomes limiting during cutter wear, and not the penetration of the cutter into the metal as a result of deflection. In other words, when planing high-manganese—

... manganese steels, one has to contend not with an increase in chip thickness, but with its decrease.

A. P. Sokolovskii, presenting the results of G. B. Fiks-Margolin [1], shows that under the action of cutting forces during planing there is possible (Fig. 3a, b) both penetration of the cutter into the metal (negative rigidity of the machine–fixture–tool–workpiece system) and deflection of the cutter (positive rigidity). It may be stated that in planing high-manganese steels there has always been positive rigidity of the machine–fixture–tool–workpiece system. This feature is a consequence of working with a small cutter overhang and of large forces on the flank face of the worn tool.

From the standpoint of counteracting positive deflections, the assembly consisting of the hinged board and the tool holder of the 7M37 machine is highly imperfect. One of the advantages of the machine—the withdrawal of the cutter from the cutting surface during the idle stroke—as a result of an unsuccessful design becomes a disadvantage when planing high-manganese steels. A considerable lever arm \(L\) (Fig. 2) and very weak springs 1 promote the appearance of positive deflections, up to the emergence of the cutter from under the chip onto the surface of the preceding cut.

Fig. 1.

Of course, all these shortcomings would be eliminated in the case of a one-piece tool holder without hinged board 2. However, as experiments have shown, operation without cutter lift-off on high-manganese steels is impossible—the tool fails extremely quickly. Therefore it is necessary to eliminate the shortcomings in the design with the hinged board. The first step may be the installation of springs 1 that are as strong as possible. An effective measure may be reliable locking of the hinged board during the working stroke. However, much greater results will be obtained by shifting the axis of the hinged board forward (Fig. 4), onto the same vertical line as the cutter tip. This measure immediately eliminates the moment \(P_y \cdot L\), so that now, in order to obtain a positive deflection, it is necessary to overcome the weight force of the slide; i.e., a considerable increase in the rigidity of the machine–fixture–tool–workpiece system can be stated.

The emergence of the cutter from under the chip onto the surface of the preceding cut occurs under the action not only of the force \(P_y\), but also of \(P_x\). In the dimensions of the workpiece being machined (its height), axial deflection is, of course, extremely insignificant; at the same time, it is quite commensurable with the chip thickness in semifinishing machining and may lead to ...

Fig. 3.

Fig. 4.

the cutter from under the chip. Therefore, measures are needed to increase the rigidity of the SPID system in this direction as well. The proposed improvements to the cross-planer support make it possible

Fig. 2.

to substantially increase the productivity of machining difficult-to-machine materials, including high-manganese steels.

REFERENCES

1. A. P. Sokolovskii. Calculation of Machining Accuracy on Metal-Cutting Machine Tools. Mashgiz, 1959.