Increasing the durability of cutting tools

Abstract

It is well known from the theory of cutting that slide friction plays the dominating role in the wear of the cutting tool. The shaving, running off the part, rubs continuously and with great force against the front edge of the cutting tool, the edges of the spiral grooves of drills, taps, and milling cutters, and heats the metal, thus reducing its resistance towards the cutting stresses and accelerating wear. Modern cutting is characterized by an increased automation of metal-cutting machines, by increased speeds, loads, and temperatures, andbyhigh requirements with regard to the precision and quality of the machine parts manufactured. For this reason the increase in productivity in the cutting of many metals is directly connected with an increase in the strength and wear resistance of the cutting tool. Wear-resistant and antifriction coatings have recently become widely used for the purpose of improving the durability of cutting tools. Thus, for instance, eiectrolytie chromium plating forms a surface layer with a high corrosion resistance, a low friction coefficient, high hardness, and wear resistance. The practice shows, however, that chromium plating is efficient only on cutting tools made of fast-cutting and carbon cutting steels (thread and gear cutting tools, etc.) where relatively low quantities of heat are created at the cutting edges, i.e., at relatively low cutting speeds (25-30 m/min) and pressures, and at cutting temperatures below 500 ~ since chromium begins to oxidize above 500 ~ and loses its hardness. It has been found [1] that the durability of a chromium-plated metal cutting tool increases as follows when working on steel and cast iron: in the case of drills by a factor of 3, of milling cutters by a factor of 2.53.5, of twist drills by a factor of 3, of thread cutters by a factor of 3-4, of cutting dies by a factor of 2.5, and in the case of slot hob cutters by a factor of 3. Significant developments have taken place in the chromium plating of tools (chromium plating in cold, tetrachromate, self-adjusting electrolytes, in an ultrasonic field, etc.). Nevertheless, the process is very laborious and expensive. For this reason, in an effort to improve the durability ofcuttingtools, great attention is paid to coatings of other metals and alloys. It has been found that the inclusion of other substances in the galvanic platings increases their hardness, thus increasing to some extent the wear resistance of the tool. It has been shown experimentally that the service life of hard-alloy blades can be increased significantly by coating their surface with a very thin layer of highly wear-resistant metal carbides [2]. Investigations of the durability of cutting tools with an electrochemicall y applied layer containing nickel and phosphorus, and of the effect of this coating on the cutting forces and the durability of cutters made of hard alloys, and of drills made of fast-cutting steels,have shown that the working properties of ha rd-alloy blades can be improved by the application of a nickel-- phosphorus coating on working surfaces that had been deeply etched. This pretreatment enhances the penetration of the coating material to greater depths, thus ensuring the presence of a lubricant on the toot surface for long periods of time. The durability of cutters with blades made of T15K6 alloy for the turning of steels that are difficult to machine, with a tensile strength of 82-84 kgf/mm 2, is increased by a factor of 1.6-2.2 and in the ease of drills made of R18 and I16M3 steels by a factor of 3-5 [3, 4]. With the aim to introduce cutting tools with anttfriction coatings in the fittings industry, the efficiency of the existing methods for the increase in durability of toots was analyzed at the VNIITarmaturoi. The following requirements were taken into consideration: The durability (service life) of the tool must be stable, the methods (of increasing the durability) must be efficient, relatively simple and inexpensive, and the tools should fit in the existing conditions of production with minimum capital expenses. Having in mind that the wear resistance of tools made of hard alloys is relatively high and that methods of further increasing the durability are complex, and that they require special equipment, tools made of