GlossaryKind of term: Standard and conventionISO material group (P, M, K, N, S, H)
What is an ISO material group?
The ISO material group sorts workpiece materials into six main groups, P, M, K, N, S and H, as the basis for choosing cutting material and cutting data.
What places a material in a group is how it behaves in cutting: P stands for steel, M for austenitic and duplex stainless steel, K for cast iron, N for non-ferrous metals such as aluminium and copper and for non-metallic materials, S for heat-resistant superalloys and titanium, and H for hardened steel and chilled cast iron. Tooling suppliers use the letters to match cutting materials, insert grades and starting cutting data to materials. A group is not a grade: within one letter, materials can machine very differently.
What does the standard say about the ISO material group?
The classification comes from ISO 513, which classifies hard cutting materials such as carbide, ceramics, CBN and diamond by the material they cut. Ferritic and martensitic stainless steels fall under P in the standard, not M. The standard gives each main group a colour: blue for P, yellow for M, red for K, green for N, brown for S and grey for H. Within a group, application groups such as P10 and P30 show whether a cutting material leans towards wear resistance or toughness: a lower number stands for more wear resistance, a higher number for more toughness. Further subdivisions, by hardness or alloy for example, are tooling suppliers' own classifications and differ between suppliers.
How does the ISO material group work in practice?
The group is a first step towards cutting data and insert choice, not a time standard. Within P, free-cutting steel and tool steel are far apart, and within N aluminium and copper machine differently. In general, cutting speeds are highest in N, P and K sit in a middle range, and M, S and H need lower cutting data, with more tool wear. How large the difference is for a given part depends on condition, hardness, operation, machine and setup. Estimate austenitic stainless as if it were non-alloy steel and the machining time usually comes out too short.
What is the ISO material group often confused with?
The ISO material group is not a material specification: P says steel, the drawing says 42CrMo4+QT. Equally, the group is not the application group of a cutting material: P25 says what a carbide grade within group P is meant for, not which steel you are cutting.
Frequently asked questions
Which ISO material group does titanium belong to?
Group S, together with the heat-resistant superalloys based on nickel, cobalt and iron. These materials conduct heat poorly and need low cutting speeds. Tooling suppliers give starting values for cutting data per group and per grade.
What does P25 on an insert box mean?
That the cutting material is meant for group P, steel, in the middle of the range between wear resistance and toughness. A lower number suits higher speeds and a stable cut, a higher number interrupted cuts and less stable conditions.
Does stainless steel machine more slowly than steel?
Usually, yes. Austenitic stainless in group M work hardens, sticks and conducts heat poorly, so it needs lower cutting data than non-alloy steel in group P. How much slower depends on grade, condition, operation and tooling.