Key Takeaways
- ISO material groups use colour-coded letters: P (blue) = steel, M (yellow) = stainless/multimaterial, K (red) = cast iron, N (green) = non-ferrous, S (brown) = superalloys/titanium, H (grey) = hardened steel.
- Within each group, lower numbers (P01, K01) = harder grade, higher speed, light cuts. Higher numbers (P50, K40) = tougher grade, lower speed, interrupted cuts.
- The insert designation code (ISO 1832) encodes shape, clearance, tolerance, chipbreaker, size, thickness, nose radius, and cutting edge in a standardised 9-position string.
- Most production shops need only 3–4 grades: a general-purpose steel grade (P25–P35), a tough interrupted-cut grade (P40–P50), a stainless/aluminium grade (M25 or K10–K20), and an N grade for aluminium.
ISO Material Group Overview
| Group | Colour | Materials | Characteristics | Grade Range |
|---|---|---|---|---|
| P — Steel | Blue | Carbon steel, alloy steel, ferritic/martensitic stainless | Long chip forming; requires good crater wear resistance | P01–P50 |
| M — Multimaterial | Yellow | Austenitic stainless, duplex stainless, cast steel | Work hardens; built-up edge tendency; mixed chip | M10–M40 |
| K — Cast Iron | Red | Grey, nodular, compacted graphite iron | Short chip, abrasive; flank wear dominant | K01–K40 |
| N — Non-Ferrous | Green | Aluminium, copper, brass, plastics | Soft; demands sharp edge; BUE risk | N01–N30 |
| S — Superalloys | Brown | Titanium, Inconel, Hastelloy, cobalt alloys | Low thermal conductivity; work hardens severely; notch wear | S05–S30 |
| H — Hardened | Grey | Hardened steel 45–68 HRC, hardened cast iron | Abrasive and hard; CBN or very hard carbide required | H01–H25 |
Understanding the Number Scale
Within each ISO group, the number suffix indicates where the grade sits on the hardness–toughness spectrum:
| Number Range | Grade Character | Cutting Speed | Depth of Cut / Feed | Application |
|---|---|---|---|---|
| 01–10 | Very hard, wear resistant | Highest | Light | Finishing, continuous cut, tight tolerances |
| 15–30 | Medium-hard, general purpose | High–medium | Medium | Semi-finishing, moderate chip loads |
| 35–50 | Tough, impact resistant | Lower | Heavy | Roughing, interrupted cuts, unstable setups |
A P25 grade is the workhorse for general steel turning — fast enough for production, tough enough for minor interruptions. P01 suits precision finishing at very high speed on a rigid machine. P50 handles deep interrupted cuts, mill scale, and forging scale where a harder grade would fracture.
ISO 1832 Insert Designation Code
Every indexable insert is identified by a standardised code. A typical 9-character designation looks like: CNMG 12 04 08. Each position has a specific meaning:
| Position | Code | Meaning | Example |
|---|---|---|---|
| 1 | Shape | Insert shape | C = 80° rhombic, T = triangle, S = square, R = round, W = hexagonal, D = 55° rhombic |
| 2 | Clearance | Relief angle | N = 0° (negative), P = 11°, C = 7°, A = 3° |
| 3 | Tolerance | Dimensional tolerance class | M = medium (±0.08 mm IC), G = ground precision, U = unground |
| 4 | Cross-section | Chipbreaker / mounting hole | G = with hole + chipbreaker, W = with hole, A = no hole no chipbreaker |
| 5–6 | Size | Cutting edge length (mm) | 12 = 12 mm edge length |
| 7–8 | Thickness | Insert thickness (mm) | 04 = 4.76 mm |
| 9–10 | Corner radius | Nose radius (mm × 10) | 08 = 0.8 mm radius |
So CNMG 12 04 08 decodes as: C-shape (80° rhombic), N-clearance (0°/negative), M-tolerance, G-type (hole + chipbreaker), 12 mm edge, 4.76 mm thick, 0.8 mm nose radius.
Negative rake inserts (N clearance) are double-sided — each insert has 2× the number of cutting edges of positive rake inserts, halving per-edge tooling cost. Positive rake (P, C, A clearance) inserts have lower cutting forces — preferred for stainless, aluminium, and thin-wall parts.
Grade Selection by Material
| Work Material | Operation | ISO Grade | Substrate Notes |
|---|---|---|---|
| Mild / Carbon Steel (P group) | Finishing (ap <0.5 mm) | P10–P15 | CVD TiCN/Al₂O₃ coated; high speed |
| Semi-finish / general | P20–P30 | PVD TiAlN or CVD multilayer | |
| Roughing / interrupted | P40–P50 | Tougher substrate, thicker edge, TiN or uncoated acceptable | |
| Austenitic Stainless 304/316 (M group) | Finishing | M10–M15 | PVD TiAlN; positive rake geometry; sharp edge |
| General turning | M25–M30 | Avoid high CBN content; ceramic unsuitable for SS | |
| Grey Cast Iron (K group) | Finishing | K01–K10 | CVD Al₂O₃ dominant; flank wear resistant |
| Roughing / skin | K20–K30 | Harder substrate, thicker coating | |
| Aluminium <12% Si (N group) | All operations | N10–N20 | Uncoated sharp edge preferred; PCD for high volume |
| Aluminium >12% Si (hypereutectic, N group) | All operations | N05–N10 or PCD | Si particles very abrasive; PCD grade preferred |
| Titanium alloys (Ti-6Al-4V) (S group) | All operations | S10–S20 | PVD TiAlN; low speed, high feed; avoid Al₂O₃ (reacts) |
| Inconel 718 / Ni superalloys (S group) | Finishing | S05–S15 or ceramic | SiAlON or whisker-reinforced ceramic for high speed |
| Hardened steel 45–65 HRC (H group) | Finishing | H01–H10 (CBN) | CBN (cubic boron nitride) — hard turning; replaces grinding |
Major Brand Grade Systems
Each cutting tool manufacturer uses proprietary grade codes alongside the ISO designation. The table below maps common brand codes to the equivalent ISO application range:
| Brand | Grade | ISO Equiv. | Description |
|---|---|---|---|
| Sandvik Coromant | GC4325 | P25–P35 | Inveio coating, general steel turning |
| Sandvik Coromant | GC1115 | M15–M25 | Stainless steel, positive geometry |
| Kennametal | KC5010 | P10–P25 | CVD coated, steel finishing to semi-finish |
| Kennametal | KC9225 | P25–P40 | Tough steel grade, interrupted cuts |
| Iscar | IC907 | P20–P35 | TiAlN PVD, general steel and stainless |
| Iscar | IC328 | K05–K20 | Cast iron, Al₂O₃ CVD |
| Seco | TP2500 | P20–P40 | Duratomic coating, steel roughing/semi-finish |
| Walter | WPP20 | P20–P30 | Tiger·tec Silver, steel ISO P |
| Mitsubishi | MC6015 | M15–M25 | Stainless and exotic alloys |
Brand grades change with new product launches. Always verify against current manufacturer catalogs — this table reflects 2025–2026 product lines.
Practical Grade Selection Process
- Identify the ISO work material group (P/M/K/N/S/H) from the material specification.
- Determine the operation type: finishing, semi-finishing, or roughing. Finishing = lower number; roughing = higher number.
- Assess stability: interrupted cut, thin wall, long overhang, or unstable fixturing all push toward tougher (higher number) grades.
- Select geometry: positive rake for low-force materials (stainless, aluminium, thin wall); negative rake for heavy steel roughing.
- Choose nose radius: smaller radius (R0.4) for finishing and thin wall; larger radius (R1.2–R1.6) for roughing (better strength, worse surface finish).
- Start at manufacturer's recommended cutting data for that grade, then adjust.
Using a harder grade than necessary (e.g., P10 for interrupted cuts) causes micro-chipping and early edge failure. Using a tougher grade than necessary (e.g., P50 for a smooth finishing pass) allows excessive built-up edge and poor surface finish. Match the grade to the actual cutting conditions.