PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE NAMED AFTER S. M. KIROV,
ORDER OF THE OCTOBER REVOLUTION AND ORDER OF THE RED BANNER OF LABOR

Volume 241 1975

IMPROVING THE SURFACE FINISH OF PARTS BY VIBRO-ABRASIVE PROCESSING

N. T. LYALIKOVA

(Submitted by the scientific seminar of the Department of Applied Mechanics)

Vibratory grinding, depending on the nature of the working medium used, is a mechanical or chemical-mechanical process of removing the finest particles of metal or metal oxides from the surface being processed, or of smoothing microroughnesses by means of their plastic deformation by particles of the working medium, which during operation perform oscillatory and rotational motion.

As is known [1], metal removal and surface finish during processing in a vibratory unit depend on the mechanical properties of the parts being processed; for example, as the hardness of the metal being processed increases, removal decreases, while the surface finish improves. With increasing grain size of the abrasive medium, metal removal increases, while the finish of the surface being processed deteriorates. The finish of the surface layer during processing is affected by the loading volume of the vibratory bunker and by the operating regime, such as the oscillation frequency, amplitude of vibro-displacements, processing time, etc. All these relationships were derived for working media consisting of abrasive chips of grades EV63, ST3, K, and KCh6-8VTK of various grain sizes, with continuous washing by a soda solution.

The present work gives data on the investigation of the finish of the surface layer of metals during their processing in an abrasive medium consisting of high-speed chips from under a milling cutter, sanding grit, and metallic sand made of white cast iron, which were mixed in a definite ratio [2].

The processing regime adopted was as follows: oscillation frequency of the vibratory bunker, 1900 per minute; amplitude of vibro-displacements, 4 mm; processing time, 75 min. In preliminary tests, under this regime, optimal values of the surface finish of the parts being processed were achieved. Finish measurements were made after every 15 min of processing. Measurement of microroughnesses and recording of profilograms were carried out using a profilometer-profilograph of block design manufactured by the Kalibr plant (GOST 2789-59). The specimens were made in cylindrical form, 20 mm in diameter and 40 mm long, from steel 45 and R18, from one and the same bar, in the annealed and hardened states. Five specimens of each type of metal and condition were tested simultaneously. Annealing and hardening of the specimens were carried out by the standard method. Steel 45 was hardened to a hardness of 40 HRC, and high-speed steel R18 to a hardness of 67 HRC. The specimens were preliminarily machined on a lathe. The loading volume of the vibratory bunker was taken as 17.5 kg.

Fig. 1 presents a graph of the change in the surface finish of steel specimens as a function of processing time at a regime of 1900 oscillations/min. Each point on the graph is the result of the arithmetic mean of five measurements along the cylindrical part of the specimen. As can be seen from the graph, the change in surface finish over time is curvilinear in character. The most noticeable change in surface finish takes place in the first period of vibroprocessing, i.e., during cleaning of the specimens for 15–30 min; thereafter the surface finish either remains unchanged or deteriorates somewhat. This can be explained as follows: after machining, the micro-irregularities consist of scratches with sharp peaks and depressions. In the first period of processing, over 15–30 min, intensive crushing, smoothing, and chipping of the sharp peaks of the micro-irregularities occur; then, after processing for the next 45 min, uniform abrasion and smoothing of the micro-irregularities take place. A certain deterioration of the finish after 45 min of processing, as, for example, in hardened P18 steel, can be explained by the fact that hardened steel has greater brittleness; therefore the chipping of the sharp peaks of the micro-irregularities occurs more intensively, which leads to additional roughness. In addition, the scratches, which are thinner at the base, break off upon collision with other specimens and create additional waviness of the surface. The most intensive change in surface finish toward improvement, in comparison with the initial condition, is observed in annealed steels 45 and P18 during the first 30 min; with further processing the surface finish changes almost not at all. In Fig. 1, to the right of the graph, a scale of finish classes is given. For all steels, improvement of surface finish, especially in the first period of processing, occurs by one to two classes.

Fig. 1. Change in surface finish as a function of vibroprocessing time

In analyzing the results obtained, the following conclusions may be drawn:

1. When parts are processed in the adopted abrasive, the finish of the surface layer can be significantly increased in comparison with the initial one.

2. To obtain a good commercial appearance of parts and to improve surface finish, it is sufficient to carry out processing for 15–45 min, since further processing may lead to a reduction in the quality of the surface of the part being processed.

LITERATURE

1. V. I. Dyachenko. Investigation of the vibro-abrasive processing process. Collection of the NIITM, issue 12, Rostov-on-Don, 1968.

2. N. T. Lyashikova. On the question of the characteristics of wear and of the condition of the surface layer of metals processed in a vibrating abrasive medium. Collection “Vibrational Grinding, Finishing, Strengthening.” Rostov-on-Don, 1969.