High-speed face and end milling of stainless steel grades
High speed machining was developed over sixty years ago and at present is still limited to just a few materials, these being aluminium and its alloys, hardened steels for the mould and die industry and titanium for aerospace applications. Recently there has been additional work in high speed machining on some of the more exotic aerospace alloys such as inconel and nimonic alloys. This thesis addresses the problems encountered when machining high chromium/nickel steel alloys that are part of the stainless steel family. Three grades of stainless steel were selected for this purpose, these being a martensitic grade (416) and two austenitic grades (303 & 316). These materials were high speed milled at cuttinspeeds of up to 3,000 m min-1, via the use of two machining methodologies; high torque-low speed and low torque-high speed milling. High torque-low speed milling was accomplished through the use of specially designed large diameter dual-plane balanced face mills, that when rotated at 6,000 rev mon-1 generated a cuting speed of 3,000 m min-1. These cutters were capable of machining three grades of stainless steel at approximately ten times that of the cutting speeds normally selected for materials that are cut conventionally. However, at the elevated cutting speeds machining of the two austenitic grades of stainless steel exhibited high tool wear rates which contributed to both poor surface finish and high cutting forces.The lo torque-high speed machining of these stainless steel grades was undertaken via specifically designed variable axial rake angle cutters in conjunction with a gear-driven speed increaser. This head gave a maxiumum cutting speed of 750 m min-1 when the cutters were rotated at 20,000 rev min-1. Experiments indicated that the three grades of stainless steel could be machined at high rotational speeds, with good surface finish produced with negative axial rake angled tools. This machining strategy caused tool 'ironing' of the machined surface. cutting forces were lower at all cutting speeds when a positive axial rake angled tool was employed, although at elevated cutting speeds tool wear was excessive regardless of any axia rake angle geometry.