Burnishing of Holes with a Ball Mandrel as a Finishing Method

V. D. Varlakov

Burnishing of external surfaces is a comparatively long-known method of machining. In the railway transport of our country this method, as a finishing method, has already found wide application for the finishing machining of the journals of car and locomotive axles, crank pins, and other critical parts subjected to considerable loads of variable character and to increased wear. Despite the fairly extensive use of burnishing of both external and internal surfaces, until recently there have been no at all satisfactory data that would make it possible to use this method confidently for machining parts. The data needed for selecting rational machining conditions are extremely scant. This applies especially to the burnishing of holes. In the present article the author sets himself the aim of providing certain materials by means of which the necessary preliminary machining can be confidently specified, and gives a quantitative assessment of the smoothness and wear resistance of burnished surfaces.

The Essence of Burnishing Processes

In contrast to cutting methods of machining, burnishing is not accompanied by removal of the surface layer from the workpiece and its conversion into chips. During burnishing the surface layer of the workpiece is subjected to elastic and plastic deformation. Plastic deformation causes the metal to flow on the surface and, as a result, the depressions are filled by the displaced projections or ridges. As a result of burnishing the surface becomes smoother, acquiring under certain conditions a mirror-like gloss.

In addition to increasing the smoothness of the surface, during burnishing there is observed a certain strengthening of the surface layer—its work hardening. Work hardening, in combination with increased smoothness, gives a substantial change in the wear resistance of parts in the direction of its increase. However, the burnishing process must be carried out very carefully, since excessive deformation of the surface layer may cause its destruction and, consequently, instead of a positive effect a negative one will be obtained. The so-called “flaking” of the surface will begin. The wear of a flaking surface is significantly higher than the wear of an unburnished surface. In addition to strengthening of the surface layer, burnishing is also accompanied by general strengthening of the part. Its fatigue strength is considerably increased. The reason for the increase in fatigue strength lies, above all, in the fact that the flow of the metal of the workpiece is accompanied by the filling of depressions on the surface and, consequently, by the elimination of stress-concentration foci that cause fatigue failure of parts.

The process of burnishing with a ball mandrel can be carried out on turret, lathe, and drilling machines. The operating scheme of the ball mandrel is shown in Fig. 1. Here \(d\) denotes the size of the hole before machining, \(d_1\)—after machining, and \(d_y\)—the size of the mandrel. As can be seen from the figure, \(d_y\) is greater than \(d\) and \(d_1\). This is explained by the fact that the surface of the hole, during the burnishing process, is elastically and plastically deformed. As a consequence of the burnishing process, a certain increase in the diameter of the hole is obtained. The magnitude of this change depends on the difference between the diameters of the mandrel and the hole and on the material being machined.

Fig. 1. Scheme of burnishing holes with a ball mandrel

Change in the Diameter of a Hole When Burnishing It with a Ball Mandrel

The change in the dimensions of the part is an extremely important factor that must be known in order to set up a productive and good-quality burnishing process. This question arises especially sharply when it is necessary to carry out the process of burnishing holes. When burnishing holes, in addition to the magnitude of the change in the diameter of the hole after machining, it is also necessary to know the size of the tool capable of producing this change. In the process of burnishing holes, deformation of the surface layer is obtained because the tool has dimensions somewhat larger than the dimensions of the previously machined hole.

The magnitude of the residual deformation, or the change in the diameter of the hole during machining \((\Delta d)\), depends to a considerable degree on the difference between the dimensions of the tool and the hole before machining \((\delta d)\).

Our investigations were carried out on carbon structural steels with Brinell hardness from 134 to 207 units. The holes before machining and after it were carefully measured with the aid of a horizontal optimeter. Measurements of the holes before and after burnishing were made in the same sections and planes. From the readings thus obtained, arithmetic means were taken. The difference between the dimensions of the holes before and after burnishing gave us the change in the diameter of the hole during the burnishing process.

First of all, we investigated the influence of the difference between the diameters of the mandrel and of the hole before processing on the magnitude of the change in the hole diameter during burnishing. The results of these investigations, carried out on three steels, are presented in the graph of Fig. 2.

Fig. 2. Change in the diameter of a hole after burnishing as a function of the hardness of the steel and of the interference diameter of the mandrel

It is clear from the graph that, with an increase in \(\delta d\), the change in diameter increases. This is quite natural, since as the difference between the diameters of the tool and the hole increases, the forces arising on the balls increase. With an increase in the forces, deformation also always increases. The same graph shows that the magnitude of the change in the hole diameter, in microns, depends on the hardness of the steel being worked. The softer the steel, the greater the change in the hole diameter.

In the graph of Fig. 2, the region between the dashed lines \(a—a'\) and \(b—b'\) corresponds to a fully good-quality surface of the machined hole. Below the line \(a—a'\), traces of the preceding machining were observed, while above the line \(b—b'\) there was a peeling phenomenon, i.e., the surface proved to be damaged. From this we may draw a quite definite conclusion that the difference between the diameters of the tool and the hole before processing must be quite definite, and for steels it lies within the limits from \(0.15\) to \(0.28\) mm.

Figs. 3 and 4 show that the speed and feed during burnishing have no influence on the magnitude of the change in diameter, and likewise the size of the hole has no influence. With a constant difference between the diameters of the hole and the tool, however we vary the speed, the feed, and the diameters of the holes being machined, the change in the hole diameter remains constant. At the optimum difference in diameters, adopted in our investigations as equal to \(0.18\) mm, it lies within the limits from 17 to 29 microns. The optimum difference in diameters, equal to \(0.18\) mm, was adopted by us on the basis of the maximum hardness of the surface layer and its finish. The value obtained here for the change in the hole diameter gives us the right to establish that the allowance for burnishing should be taken within the limits from \(0.015\) to \(0.030\) mm.

All these investigations are valid for steel with a Brinell hardness of 197 units. In order to extend our conclusions, we checked them on steels of various hardnesses.

All of them were fully confirmed for other steels as well. In order to be able to determine the allowance in the case of machining steel of any hardness, we constructed the graph in Fig. 5, which gives the dependence of the change in hole diameter on the initial hardness of the steel. This graph is plotted in logarithmic coordinates, and for it we may write the following equation:

Fig. 3. Change in hole diameter after burnishing as a function of speed and feed

$$ \Delta d = C H_B^{-x}, $$

where \(C\) is a constant coefficient depending on the type of material. In our case it proved to be equal to 402600,

\(x\) is an exponent, also depending on the type of material and equal to 1.89,

\(H_B\) is Brinell hardness,

\(\Delta d\) is the change in hole diameter in microns.

Fig. 4. Change in hole diameter after burnishing as a function of speed, feed, and machining diameter

Hence the equation expressing the regularity of the change in hole diameter, at a constant difference of diameters, as a function of the initial hardness of the material will have the following form:

$$ \Delta d = \frac{402600}{H_B^{1.89}} . $$

This equation is valid for various materials (steels), for a given difference in diameters, different feeds, speeds, and machined diameters, since, as was shown above, their influence on the change in the diameter of the hole during machining is absent.

Often, using a mandrel of our design, it is impossible to machine holes in a single pass (blind, stepped, etc., holes). In this case the mandrel has to be withdrawn from the hole with the same feed as in the first pass. Undoubtedly, the second pass of the tool will cause some additional increase in the diameter of the hole. In connection with this, we investigated the effect of the number of passes on the change in the diameter of the hole during burnishing. The results of these investigations are presented in Fig. 6.

Fig. 5. Change in hole diameter as a function of material hardness

Fig. 6. Effect of the number of passes and hardness on the change in hole diameter after burnishing

From the graph it is evident that, as the number of passes over the layer increases, the change in the hole diameter also increases. At the same time, as is not difficult to see from the graph, with an increase in the number of passes over the layer, the relative magnitude of the change in diameter becomes smaller. This is fully explained by the fact that, as the number of passes increases, the current difference in diameters becomes smaller and, consequently, the pressures arising on the balls of the mandrel are also smaller.

Surface finish after burnishing and its wear

To obtain a complete picture of hole burnishing with a ball mandrel as a method of surface finishing, we conducted an investigation of surface finish.

Profilograms of the irregularities obtained during burnishing were recorded by us with a profilograph of the system of Docent A. N. Eremin. As the basis for evaluating surface quality by finish, we used GOST 2789–45, which recommends evaluating surface finish on the basis of the root-mean-square height of the irregularities. Using the profilograms, we estab-

... that the root-mean-square height of irregularities when burnishing holes with a ball mandrel lies within the range of 1.0–1.7 microns, which corresponds to grades 6–7 of surface finish. It should be said, however, that this surface finish in burnishing holes with a ball mandrel is not the limit. It can be substantially improved by increasing the diameter of the balls of the mandrel.

A check of the wear of the burnished specimens showed that it is four times less than the wear of carefully ground specimens. This significant increase in the wear resistance of burnished surfaces, as compared with unburnished ones, is explained primarily by the greater surface hardness of the former, acquired by them as a result of work hardening. The increase in surface hardness during burnishing is quite considerable and amounts to 28–48% of the initial hardness of the steel being machined.

Conclusions

1. Burnishing holes with a ball mandrel may be classed as a finishing method.

2. Because it produces only a very small change in the diameter of the hole, burnishing does not change, or changes only very slightly, the accuracy of the preceding machining operation.

3. The surface finish after burnishing with a ball mandrel, as measured by the height of irregularities, is not inferior to that of a ground surface.

4. A significant increase in the hardness of the surface layer and an improvement in its finish lead to a sharp reduction in wear.

5. The machining methods preceding burnishing must provide the required machining accuracy and, in terms of surface quality, must have a height of irregularities lying within the range of change of the hole diameter during burnishing. Such methods include finish counterboring, finish boring, and reaming.

Literature

1. Berman, S. R. The Unit of Surface Quality. Journal “Mashinostroitel’,” No. 7, 1938.

2. Berman, S. R. Standardization of Surface Quality. Journal “Vestnik standartizatsii,” No. 12, 1938.

3. Boas, V., and Schmid, E. Plasticity of Crystals—Especially Metallic Crystals. GONTI, 1938.

4. Gubkin, S. I. Plastic Deformation of Metals. ONTI, 1935.

5. Eremin, A. N. Profilographing of Surfaces by the Method of Intermittent Sensing. Izvestiya TPI, vol. 59.

6. Zaitsev, A. K. The Relation of Wear to Work Hardening. Collection of the Academy of Sciences of the USSR, “Friction and Wear in Machines,” 1939.

7. Konvisarov, D. V. Wear of Metals. ONTI, 1938.

8. Odin, G. Strength of Metals. ONTI, 1937.

9. Khudyakov, K. V., and Lesokhin, S. F. Surface Finish. Journal “Stanki i instrument,” Nos. 7–8, 1945.