Principles of machining stainless steels

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 characterised by their high work hardening rates and poor chip breaking  properties during machining. This article covers the important issues that influence the successful machining of these steels.

Machine and tooling rigidity
When machining stainless steels, it is 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, and brasses are unlikely to be substantial enough  for the successful machining of stainless steels.

Machines should not be prone to excessive vibration in 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. Arbours for supporting barrel milling cutters should be stout as short as possible. The arbour supports should be as close as possible to the ends of the cutter to provide maximum support.

Some‘squealing’as the metal is being cut is not unusual, but can indicate that the tool may be wearing and need replacing.

Tool materials
Either high speed steel (HSS) (wrought or sintered) or cemented carbide tools can be used for machining stainless steels.

High-speed steels
Either tungsten or molybdenum HSS can be used. These are particularly useful in machining  operations involving  high feed and low speed machining operations where there are variable cutting edge stresses induced from complex tool shapes. The tungsten types (eg T15) can be useful for their good  abrasion resistance  and red hardness. The molybdenum HSS are more widely used, M42 being useful for applications such as milling cutters where a good combination of hardness and strength are required at lower cutting speeds. M42 has  better hardness than grades like themore common M2, but may not be as tough however.

If the tools are prone to edge chipping, use a tougher grade, eg M2, M10 If tools are burning, use a higher red hardness grade, eg M42, T15 If the tools are wearing, use a more abrasion resistant grade, eg T15

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. The‘straight’ tungsten carbides  grades are used for machining austenitic and duplex stainless steels and the‘complex’carbides are used for machining martensitic and ferritic family grades.

Coated carbides have the additional benefit of improved wear resistance and resistance to breakage. Consequently they are capable of higher cutting speeds compared to un-coated carbide tools.

Tool geometry and sharpness
It is essential to keep the cutting tools sharp when machining stainless steels. Careful  grinding and honing of the tool faces to give accurate and sharp face angles is important. This helps optimise: tool life; finish, accuracyNikeLab ACG.07.KMTR