ISO inserts: read the designation

How to read an ISO insert code

Turning insert codes follow ISO 1832: four letters and three pairs of digits, the same for every manufacturer. Take CNMG 120408: C is the 80° rhombic shape, N the 0° clearance angle (negative insert), M the tolerance class, G the type with a hole and chipbreaker on both faces. Then 12 is the cutting edge length in mm, 04 the thickness and 08 the nose radius, 0.8 mm.

After the sizes, manufacturers often add a chipbreaker code such as -PF or -MR. That part is not standardised: read it in their catalogue.

Insert shapes

• S, square 90°, and R, round: the strongest, for heavy roughing.
• C, rhombic 80°, and W, trigon 80°: versatile, turning and facing with the same insert.
• T, triangular 60°: general turning.
• D, rhombic 55°: copying and finishing, reaches into shoulders.
• V, rhombic 35°: complex profiles and grooves, with a more fragile edge.

Positive vs negative inserts

A negative insert (second letter N) has edges on both faces: it is stronger and cheaper per edge, but needs stable machines and clamping. A positive insert (C, P, B and the others) cuts with less force and less vibration: the choice for bores, thin parts, finishing and less rigid machines.

Nose radius and surface finish

The nose radius, the last two digits, sets finish and strength. Small radii (0.2–0.4 mm) finish with low forces, 0.8 mm is the all-rounder, 1.2 mm and up rough. As a rule of thumb the feed per revolution stays below half the radius; the working feed depends on material, grade and chipbreaker and comes from the catalogue.

How to choose a turning insert

Start from the job. For roughing: a strong shape (C, W, S), a negative insert, a large radius. For finishing or profiling: a sharper shape (D, V), a positive insert, a small radius. Grade and cutting data then come from the catalogue, based on the workpiece material.

The most common codes, one page each