Turning Insert Designations Explained: How to Read CNMG, TNMG & Every Code on the Shelf

CNMG 432. Four letters, three numbers — and they tell you everything: the shape, how it sits in the holder, how it was made, how it breaks chips, and its exact size. Once you can read the code, you can walk up to any lathe job and know which insert it needs. Here’s the whole system, letter by letter, with every shape linked to what’s in stock.

Quick Decoder: Type Your Insert Code

Letter 1 — Shape

The first letter is the outline of the insert. The trade-off is always the same: a bigger tip angle is stronger and cheaper to run; a smaller tip angle reaches into tighter profiles.

Letter 2 — Relief (Clearance) Angle

N = 0° — the flanks are square to the top. That’s what makes a “negative” insert flippable: both faces cut, so a CNMG gives you 4 edges and a WNMG gives you 6. C = 7° — the flanks taper in, only the top face cuts, but the edge slices instead of plowing: less cutting force, less deflection. Other letters exist (A = 3°, B = 5°, P = 11°, E = 20°), but N and C cover almost everything in a turning crib.

Letter 3 — Tolerance Class

How precisely the insert was manufactured. M is the standard molded tolerance (roughly ±0.002–0.004" on the inscribed circle depending on size) — fine for general turning, since you touch off anyway. G means periphery-ground to about ±0.001" — that’s why finishing and aluminum-polish inserts are CCGT/DCGT/VCGT: when the code’s 3rd letter is G, corner-to-corner indexing repeats tight enough to hold size without re-touching.

Letter 4 — Hole & Chipbreaker

G = hole plus molded chipbreakers on both faces (the standard double-sided workhorse: CNMG, TNMG…). A = hole with flat top, no chipbreaker — that’s the cast-iron and interrupted-cut style (CNMA, SNMA, TNMA, WNMA). T = countersunk screw-down hole with a top chipbreaker — the positive screw-clamp family (CCMT, DCMT, TCMT). M = hole with a single-face chipbreaker.

The Numbers — Size in Three Digits

Inch (ANSI) sizing is beautifully simple: 1st digit = inscribed circle in 1/8ths, 2nd = thickness in 1/16ths, 3rd = nose radius in 1/64ths.

So CNMG 432 = 4/8 = 1/2" IC · 3/16" thick · 2/64 = 1/32" nose radius. The half-thickness in positive inserts is why you see codes like CCMT 32.51 — 3/8" IC, 2.5/16" thick, 1/64" radius.

ANSI ↔ ISO: The Same Insert, Two Names

A print from Europe says CNMG 120408; the crib label says CNMG 432. Same insert. ISO uses millimeters — edge length, thickness, and radius in tenths of a millimeter. Here are the pairings you’ll actually meet:

The Letters Don’t Pick the Grade

Two more choices finish the job, and neither lives in the ANSI code. The carbide grade and coating (our FM125, FM220…) sets which material the insert is happiest cutting, and the chipbreaker geometry molded into the top sets how it curls the chip. We built a separate visual guide for both: FM Carbide Grades & Chip Breakers decoder.

And the insert is only half the system — the 2nd letter of your tool holder’s code is the shape it accepts (an MCLNR takes C-shape inserts, an MTJNR takes T-shape). If holders are the part you’re shaky on, read the Complete Guide to CNC Tool Holders, then dial in your cutting parameters with the free Speeds & Feeds Calculators. And once the right insert is in the pocket, make every edge last: our TechTips guide Maximizing Insert Tool Life covers the five wear patterns, indexing discipline, and the cost-per-edge math.

FAQ

CNMG or DNMG — which one?

Same negative double-sided family. CNMG’s 80° tip is stronger and handles heavier feeds; DNMG’s 55° tip turns closer to shoulders and follows profiles. Many shops rough with CNMG and profile with DNMG in the same setup.

Is CNMG 120408 interchangeable with CNMG 432?

Yes — they’re the same insert named in ISO (metric) and ANSI (inch). Check the grade and chipbreaker suffix separately; those differ by manufacturer.

Can I run a CNMG 433 where the program used a 432?

It fits the same pocket — only the nose radius changes (3/64" vs 1/32"). Update the tool nose radius compensation in the control, or profiled dimensions will be off.

Why do my positive inserts have a dot in the size (32.51)?

Positive single-sided inserts are thinner than their negative cousins, so the thickness lands on a half-sixteenth — the “.5” is literally half a 1/16" step.