The most common and hence most frequently machined stainless steels are the austenitic types, such as grades 304 (1.4301) and 316 (1.4401). These are characterized by their high work hardening rates and poor chip breaking properties during machining. The information below covers the important issues that influence successful stainless steel machining.
Machine and Tooling
When machining stainless steels it important to ensure that there is no dwell or rubbing caused by machine vibration or tool chatter. Machines must be ‘substantial’ and capable of making the deep cuts needed in machining austenitic stainless steel without slowing down the set feed or surface speeds. Small training or ‘hobbies’ lathes and milling machines intended for machining mild steel, brasses, etc. are unlikely to be substantial enough for successful stainless steel machining.
Machine Function
Machines should not be prone to excessive vibration on the machine bed, drives and gear boxes or at the cutting tool or its mountings. Large overhangs of tool shank out of the tool box should be avoided. The distance between the cutting tip and toolbox support should be as short as practicable and the shank cross section as substantial as possible. This can also help in dissipating heat away from the cutting faces.
Tool Materials
Either high-speed steel (HSS) (wrought or sintered) or cemented carbide tools can be used for stainless steel machining. Tungsten or molybdenum HSS can be used. These are particularly useful in machining operations involving high feed and low-speed machining operations where there is variable cutting edge stresses induced from complex tool shapes.
The tungsten types can be useful for their good abrasion resistance and red hardness. The molybdenum HSS are more widely used, M42 being useful for applications.
Cemented Carbides
Cemented carbides are normally used for machining stainless steels where higher speeds or higher feeds than those that can be produced using HSS are required. Either disposable insert or brazed-on tips (where lower cutting speeds can be tolerated) can be used and are composed of either tungsten carbides or a blend of tungsten and other metal carbides, including titanium, niobium, and chromium. The carbides are bonded with cobalt.
