Hard turning of interrupted surfaces using CBN tools

Some of the advantages of using turning instead of grinding for the finish machining of hardened steel surfaces are: high flexibility, the ability to cut complex surfaces with a single machine set up, cost of the process, and the possibility of turning without cutting fluid. On the other hand, turning is not very suitable for cutting hardened interrupted surfaces, since most of the tools used in this operation are not very tough. This work aimed to broaden the use of hardened steel turning tools, comparing the use of two CBN tool grades (low CBN content, called 7020, and high CBN content, called 7050) and two cutting edge micro geometries (chamfered and rounded edge) in radial turning of three kinds of surfaces: continuous, semi interrupted and interrupted. The comparison was in terms of tool wear and tool life. The workpiece material was AISI 4340 steel with 56 HRC of hardness. Results showed that the longest tool life was obtained when the 7020 grade was used, regardless of surface type. However, in continuous cutting, tools with chamfered edges proved to have the longest life, while for interrupted cutting the best results were produced by tools with edge rounding.