ISO 1832 Shape, Clearance, Tolerance and Fixing Codes

Letters are lookups, digits are mostly lookups too

In CNMG120408 the four letters are position 1 (shape, C), 2 (clearance, N), 3 (tolerance class, M) and 4 (fixing and chipbreaker, G). None of them is arithmetic: C does not mean 80° in the way 08 means 0.8 mm, it means the rhombic 80° body whose exact edge geometry the standard defines. The third letter is the one most often misread, because it carries two facts at once — how tightly the insert is toleranced and whether it is single- or double-sided.

The 367 designations published on InsertBase use 7 shape letters, 6 clearance letters, 4 tolerance classes and 8 fixing letters. Rows with a count of 0 are defined by the standard but not yet published by a catalogued insert here; no catalogue row in this set publishes a hand letter, so the position 8 table is included for completeness.

Position 1 — insert shape

The corner angle is what decides edge strength and accessibility: an 80° rhombic (C, W) is robust, a 55° rhombic (D) reaches into shoulders but is more fragile, a 35° rhombic (V) is the most fragile and is fed lighter. Shape choice is worked through in CNMG vs WNMG vs CCMT.

Shape gallery

Top view of the 7 shapes that appear in the catalogued designations, drawn from the same generator the decoder uses. Schematics, not to scale.

Position 2 — clearance angle

The N insert has no clearance of its own and relies on the holder's negative geometry, which is what lets it be used on both faces. Positive clearances (A, B, C, P) need a matching positive holder — fitting a C insert into an N holder changes the rake and clearance the workpiece actually sees.

Position 3 — tolerance class

Tolerance classes set the permitted deviation of inscribed circle, thickness and corner radius. The limits themselves vary with insert size, so the exact numbers belong to the ISO 1832 tolerance tables for that IC range rather than to this page. M — as-sintered, moderate — is the economical default for general turning; the ground classes suit precision boring and finishing.

Position 4 — fixing and chipbreaker

Both M and G are common on modern turning inserts; which one a catalogue uses is driven by its chip-breaker family rather than by ISO 1832. The hole/faces split matters when you count usable edges: a two-face insert can be flipped and indexed twice as often.

Position 8 — cutting hand (optional)

When the eighth position is omitted the insert is neutral — true of every designation in this database so far. A hand letter appears only where a catalogue publishes a true right- or left-hand variant of the same geometry, and none of the catalogued rows here does.

Reading the other positions

The numeric blocks are tabulated separately because they do not follow one rule. Position 5 selects an inscribed circle from a standard size table — see the ISO 1832 size code chart. Position 6 selects a standardised thickness while position 7 counts tenths of a millimetre — both are decoded in the thickness and nose radius code chart.

To decode a complete designation — letters and digits together — use the ISO 1832 decoder, or read the position-by-position explanation in the designation walkthrough.