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Carbide Insert Designation Chart: How to Read an Insert Code

Guide · Updated October 28, 2026

Every indexable insert carries a code like CNMG 432 or CNMG 120408 that describes its shape, relief, tolerance, holding style, size, thickness and corner radius. The letters and digits follow ISO 1832 (metric) and ANSI B212.4 (inch), and the first four letters are the same in both systems. Once you can decode the code you can swap brands, choose a holder, and enter the right nose radius in your offset table. The letter tables below are taken from ISO 1832:2017 and checked against the Cutting Tool Engineering summary of ANSI B212.4.

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Step by step

  1. Shape letter: Position 1 gives the shape and included angle: C is an 80° rhombic, D 55°, V 35°, T triangle, S square, W 80° trigon, R round.
  2. Clearance letter: Position 2 gives the normal clearance: N is 0° (negative insert), C 7°, P 11°, B 5° and so on.
  3. Tolerance letter: Position 3 gives the tolerance class; M is the common pressed tolerance, G is ground on the periphery.
  4. Type letter: Position 4 tells whether there is a hole, a countersink and chipbreakers: M, G, T, W, N, R and A are the common ones.
  5. Size digits: In inch codes the first digit is the inscribed circle in 1/8 in; in metric codes the first two digits are the cutting edge length in mm.
  6. Thickness and radius: Inch: thickness in 1/16 in, radius in 1/64 in. Metric: thickness in mm (truncated), radius in tenths of a mm.

Position 1: insert shape (ISO 1832 table 1)

A larger included angle gives a stronger corner and more edges per insert for roughing; a smaller angle (D, V) reaches into profiles and undercuts but is weaker. An 80° C insert is the usual compromise for turning and facing on one holder.

Position 2: normal clearance

An N insert is a negative insert: the holder tips it to make clearance, and both faces can be used, doubling the edges. Positive inserts (C, P, B) cut with less force and suit small parts, boring bars and less rigid setups, but are single-sided.

Position 3: tolerance class

The tolerance letter controls how closely the inscribed circle, thickness and corner location are held. M is the everyday pressed-and-sintered class, with corner location around ±0.08 to ±0.18 mm depending on size, so the corner can shift after indexing. G and E are ground classes held to roughly ±0.025 mm, used where a part must size without re-touching the tool after each index. Check the maker's catalog for the exact values in each class.

Position 4: insert type (hole and chipbreaker)

Countersunk T and W inserts are screwed down and are typical on positive inserts such as DCMT and CCMT. Straight-hole M and G inserts are held by a lever or pin, typical on negative inserts such as CNMG and WNMG.

Size, thickness and corner radius digits

Some small inch inserts use half digits such as 21.51 (IC 1/4 in, thickness 3/32 in, radius 1/64 in). Those follow the same units.

Common inch to metric equivalents

The metric size digits differ by shape because they measure edge length, not the inscribed circle. A 1/2 in IC is a 12 mm edge on a C insert, 15 mm on a D, 16 mm on a T and 08 mm on a W trigon. 1/32 in (0.031 in) is close to, not exactly, 0.8 mm (0.0315 in).

Worked example: reading CNMG 120408

C: 80° rhombic. N: 0° clearance, a negative double-sided insert. M: pressed tolerance class. G: hole with chipbreakers on both faces, A C insert has two 80° corners per face, so a double-sided CNMG gives four usable 80° corners. 12: 12 mm edge, 12.7 mm (1/2 in) IC. 04: 4.76 mm (3/16 in) thick. 08: 0.8 mm nose radius. Enter 0.8 mm (0.0315 in) in the tool nose radius offset. Anything after the dash, such as -PM or -MF, is the maker's chipbreaker code and is not standardized.

Worked example: will this finish meet the print?

Theoretical peak-to-valley is f² ÷ (8 × r). With CNMG 432 (r = 0.0313 in) at 0.006 ipr: 0.006² ÷ 0.25 = 0.000144 in (144 µin peak-to-valley). Ra is roughly a quarter of that for a pure feed-mark profile, about 36 µin, so a 63 µin Ra callout is comfortable and a 32 µin callout needs a lower feed, a larger radius or a wiper insert.

Common problems

Mixing an inch holder with a metric insert of the wrong thickness, forgetting to change the nose radius offset when switching from a 431 to a 432, using a positive insert in a negative holder, and assuming two brands' chipbreaker suffixes mean the same thing.

FAQ

What does CNMG stand for?

C 80° rhombic shape, N 0° clearance, M tolerance class, G hole with chipbreakers on both faces.

What is CNMG 432 in metric?

CNMG 120408.

What does the 2 in CNMG 432 mean?

A 2/64 = 1/32 in corner radius.

What does the 08 in CNMG 120408 mean?

A 0.8 mm corner radius.

Is the insert code the same for every brand?

The first seven positions follow ISO 1832 and ANSI B212.4; chipbreaker suffixes are brand specific.

What is the difference between a negative and positive insert?

A negative insert has 0° clearance (N) and can be flipped; a positive insert has built-in clearance and is single sided.

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