Decode the standard part of an ISO insert designation, including shape, clearance, tolerance, size and nose radius. Brand-specific chipbreakers and grades are handled separately.
How the ISO code works
An ISO turning insert designation is read left to right: four letters for the geometry, then three groups of digits for size, thickness and nose radius.
The letters carry shape, clearance angle, tolerance class and mounting style. Everything after them describes the body and the edge, not the carbide grade or the coating.
How to read CNMG 120408
Why the same size code can mean different dimensions
The two digits after the four letters are not always the inscribed circle. For rhombic shapes such as C, D and V they give the inscribed circle, but for triangles (T), squares (S) and trigons (W) they give the cutting-edge length.
This is the single most common reason two inserts with the same number do not fit the same holder.
inscribed circle 12.7 mm
cutting-edge length 12.7 mm — a physically larger insert than the CNMG above despite the smaller number
cutting-edge length 22 mm
What the ISO code does not tell you
- Grade
- Chipbreaker
- Coating
- Recommended cutting speed
- Recommended feed range
These are manufacturer- and application-specific and must be checked separately.