ACCURACY IN SMOOTHING BROACHING OF PARTS WITH NON-UNIFORM STIFFNESS

M. G. GOLDSHMIDT, D. E. ZAITSEV, B. F. SKVORTSOV

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

Machining holes with a smoothing carbide tool is one of the simplest and most productive methods for obtaining accurate and very accurate holes. This, however, cannot be extended to the machining of parts non-uniform in stiffness in the axial or radial directions, the dimensional accuracy of which must depend substantially on the degree of “non-uniform stiffness” and on the machining conditions.

The purpose of the present study was to investigate the influence of smoothing-broaching conditions on dimensional accuracy and surface quality of parts having different stiffness in a section perpendicular to the axis.

Experimental bushings (sketch in Fig. 1) were made of steel 45. The final dimensions of the bushings were made proportional to the corresponding dimensions of the sleeve of an MO6K-OA2 percussion hammer. The outside diameter of the bushings was 35 mm, the size between flats (section A—A) was \(30.7_{-0.05}\) mm, and the length was 100 mm. The inside diameter of the bushings prepared for broaching varied from 27.2 to 27.95 mm, which, with a diameter of the sizing teeth of the smoothing broach equal to 28.15 mm, produced a total interference within the range from 0.2 to 0.95 mm.

Fig. 1

One batch of bushings was broached after the flats had been milled, the other in the opposite sequence of operations. On the finally machined bushings, a roundness chart of the machined hole was recorded using an instrument of the TELIROND type, and the surface roughness was evaluated with a profilograph-profilometer.

As was to be expected, the surface roughness after broaching bushings with flats proved to be nonuniform in sections A—A.

and B—B (Fig. 1), which is explained by the difference in the magnitude of the contact stresses acting during smoothing broaching, caused by the different stiffness of the bushings in these sections. Increasing the interference reduces the height of the microroughnesses; however, the difference in roughness of the machined surface in sections A—A and B—B increases even more. To ensure sufficient surface finish, it is advisable to broach the blanks before the machining operation that creates nonuniform stiffness of the part; i.e., for the part being analyzed, the flats should be machined after smoothing broaching of the hole.

Figure 2a shows a copy of the roundness diagram recorded during broaching of a bushing with flats; the total interference was 0.55 mm. The scale division value of the roundness diagram is 5 µm.

Fig. 2

The out-of-roundness of the machined hole proved to be 150 microns.

Analysis of the out-of-roundness of samples broached with flats as a function of interference shows that the latter has no substantial effect on the geometric accuracy of the hole. Thus, even with interferences of 0.2 mm, the out-of-roundness of the hole proved to be 80–100 microns.

The form error of the hole is found to be somewhat lower when the flats are removed after broaching (Fig. 2a).

The asymmetry of the hole shape in the machined part is explained as follows. When the first two parallel flats are removed, elastic-plastic deformation occurs, leading to a substantial increase (on the order of 0.3 mm) in the hole diameter at the locations where the flats are removed and to a decrease (about 0.1 mm) in the diameter in the perpendicular direction. Machining the next pair of flats causes an increase in the diameter in that direction and a decrease in the perpendicular direction. If the recorded form distortions arising as a result of the action of residual stresses were purely elastic in character, the shape of the hole in the machined part would be symmetric.

Sequential machining of the flats in 3 passes led to a reduction of out-of-roundness to 50 microns. The shape of the hole in the machined part proved to be practically symmetric.

Because the absolute majority of precision parts used in mechanical engineering are of non-uniform stiffness, further investigations are necessary in order to obtain analytical relationships for estimating the form error of a hole as a function of the dimensions of the part being machined and the broaching conditions.

The investigations carried out make it possible to outline certain technological methods for reducing the form error of the machined hole: where possible, annealing should be performed to relieve residual stresses; machining of surfaces that produce non-uniform stiffness should expediently be carried out after broaching and in several passes, etc. The high efficiency of smoothing broaching in producing precision holes justifies, in some cases, design changes to machine parts in order to impart to them an admissible non-uniform stiffness.