GlossaryKind of term: Manufacturing termMachinability
What is machinability?
Machinability describes how a material behaves under the cutting edge: tool life, cutting forces, chip form and surface finish under given cutting conditions.
Machinability is not a single material property but an assessment of how a material behaves in cutting under given conditions. The usual criteria are tool life at a given cutting speed, cutting forces and power, chip form and chip breaking, and the surface finish that can be achieved. It is sometimes expressed as an index relative to a reference material; such a number depends on the chosen reference and test and is only comparable within one source.
What does machinability depend on?
Machinability depends on composition, microstructure and condition: the same alloy annealed, quenched and tempered or hardened machines like a different material. Sulphur, lead or bismuth in free-cutting steel, free-machining brass and EN AW-2011 promote short chips. Tool life is shortened by work hardening under the edge, as in austenitic stainless and nickel alloys, by poor thermal conductivity, as in titanium, and by hard particles such as silicon in cast aluminium or fibres in plastics. Tool makers therefore group materials into the ISO groups P, M, K, N, S and H and give starting values per group, which you adjust per machine, setup and tool.
How does machinability work in practice?
Machinability feeds into machining time, tool cost per part and process reliability. Lower cutting data make each feature take longer, and shorter tool life means more tool changes. Long chips can get in the way of unattended running; short, well-broken chips make it more feasible. How much a difficult material raises the cost depends on the share of machining time, the quantity and how you allocate tool cost. A time per feature per material group is a common estimating convention; the actual time only becomes known at the machine.
What is machinability often confused with?
Machinability is not hardness: annealed austenitic stainless is softer than quenched and tempered 42CrMo4 and is still often harder to machine, because work hardening and chip form weigh more than the hardness number. Nor is it the cutting data: cutting data are the choice you make based on machinability, tool and machine.
Frequently asked questions
How do I find out how well a material machines?
Start from the ISO material group and the condition on the drawing, and from your tool supplier's starting values. Then adjust them from your own experience of tool life, chip form and chatter on your machines.
Does machinability affect the price per quantity break?
Yes, through machining time and tool cost per part. In a common estimate these stay roughly constant per part across the breaks, while setup and programming per part fall. At larger quantities machinability therefore weighs relatively more.