Reading an insert code

Four letters, then the size

The code on a turning insert box follows ISO 1832. The first four letters are shape, clearance, tolerance and type. In CNMG: C is an 80° diamond, N means 0° clearance (a negative insert), M is the tolerance class, and G means a hole with chipbreakers on both sides.

A negative insert has no relief ground under the edge; the holder tilts it to make clearance. That lets both faces cut, so a CNMG has four usable corners instead of two, and it is stronger. Positive inserts (like CCMT, with 7° clearance) cut with less force, which suits boring, slender parts and soft materials.

Inch and metric sizes

Inch codes count in fractions. In CNMG 432, the 4 is the inscribed circle in eighths (4/8 = 1/2"), the 3 is the thickness in sixteenths (3/16"), and the 2 is the nose radius in sixty-fourths (2/64 = 1/32").

Metric codes give the same insert as CNMG 120408: 12 is the cutting edge length in mm, 04 the thickness code (4.76 mm), and 08 the nose radius in tenths of a mm (0.8 mm). After a dash comes the maker’s chipbreaker, then the grade.

Why the shape matters

The bigger the corner angle, the stronger the corner. Round and square inserts are the strongest, then 80° (C, W), then the triangle (T), then the 55° diamond (D), and the 35° diamond (V) is the weakest. Sharp corners reach into profiles and undercuts; blunt ones take the heavy roughing.