Development of Endmills for Milling Hardened Steel Molds and Copper Electrodes

In recent years, mold manufacturers have had to meet growing demand for high precision molds, low cost and short delivery time due to the miniaturization of finished products, price competition with overseas companies, and reduced model cycles. In line with these market changes, users have been gradually inclined to shift their machining methods applied to key mold parts (made of hardened steel) from electrical discharge machining, using copper electrodes, to direct cutting. To meet those needs, we have enhanced the lineup of direct-cutting tools and developed new tools for copper electrode machining, still in use as a mainstream machining method. For direct-cutting endmills, cBN*1 has gradually been replacing conventional carbide alloys, since it is excellent in wear resistance for high-speed milling. In high-speed milling applications at 20,000 rpm or higher, cBN endmills are vastly superior to carbide endmills in terms of tool life and surface finish quality, though in low-speed milling at approximately 10,000 rpm they fail to fully exhibit the material characteristics of cBN: high hardness and high thermal conductivity. cBN, on the other hand, being less tough and more subject to chipping at the cutting edge than carbide, has been considered difficult to exploit its full performance. The radius BNBR type of highly chipping-resistant cBN endmills, shown in Photo 1, has been added to the Sumiboron Mold Finish Master series. Featuring application-specific grades and optimal cutting edge design, the newly added cBN endmills provide a solution to the aforementioned problem. Regarding copper electrode machining tools, tool manufacturers have launched micromachining tools that feature CrN-coated carbide. Many of them are multi-purpose tools designed with application to other workpiece materials in mind. Photo 2 shows SNB2-type Aurora Coat (DLC*2) long-neck, ball-nose endmills, developed as micromachining tools specifically for copper electrodes, and the BNBC-type Sumiboron Mold Finish Master. This paper reports on the features of these tools and application examples. 2. Direct-Cutting Tools “BNBR-Type Sumiboron Mold Finish Master”