Understanding ISO Turning Insert Designation: CNMG, WNMG & CCMT Explained -

Collection of CNC turning carbide inserts showing different ISO shapes and coatings — A selection of ISO-coded turning inserts in different shapes and coatings by the end of this article, you’ll be able to read codes like these yourself.

If you’ve ever picked up a box of carbide inserts and stared at a code like CNMG120412 wondering what it actually means, you’re not alone. The ISO 1832 standard gives every turning insert a code built from position-by-position rules shape, angle, tolerance, type, and size — all packed into 9-10 characters. Once you understand the system, you can read any insert code from any manufacturer and know exactly what you’re holding, without needing a catalog in hand.

In this article, we’ll break down three commonly used inserts — CNMG120412, WNMG080412, and CCMT09.— position by position, using real photos from our own tooling inventory.

How the ISO Code Is Structured

Every ISO insert code follows the same 10-position structure:

Position 1: Insert Shape
Position 2: Clearance (Relief) Angle
Position 3: Tolerance Class
Position 4: Insert Type (hole, chipbreaker style)
Positions 5-6: Size (inscribed circle)
Positions 7-8: Thickness
Positions 9-10: Corner Radius

Decoding CNMG120412

Let’s break down our first insert, CNMG120412, position by position — with real photos from our own inventory.

C — Shape: 80° Rhombic

The first letter defines the insert’s shape. “C” indicates an 80° rhombic (diamond) shape — a versatile, strong geometry commonly used for general-purpose external turning.

**Left:** the CNMG120412 insert as-is. **Right:** the same insert with its 80° included angle marked the defining feature of the “C” shape code.

N — Clearance Angle: 0° (Negative Insert)

The second letter, “N,” means this insert has a 0° clearance angle — making it a negative insert. The side wall of a negative insert sits perfectly vertical (perpendicular to the top face), rather than angled inward like a positive insert. This flat wall means both the top and bottom faces can be used as cutting faces, effectively doubling the number of usable edges you get from a single insert.

**Left:** side profile of the insert. **Right:** the same photo marked to show the wall’s 0° angle — perpendicular to the top and bottom faces, confirming this is a negative insert with two usable cutting faces.

M — Tolerance Class M

The third letter, “M,” specifies the insert’s tolerance class — essentially how tightly controlled the insert’s physical dimensions are during manufacturing. Tolerance class M is a widely used standard grade, offering good dimensional consistency suitable for general turning operations without requiring the tighter (and more expensive) tolerances found in precision-grade inserts.

G — With Hole, Chipbreaker on Both Faces

The fourth character, “G,” tells us this insert has a center hole and a molded chipbreaker groove on both faces. Below, the same insert is shown three ways — as-is, with the center hole marked, and with the chipbreaker groove marked.

**Left to right:** the CNMG120412 insert as-is, the center hole highlighted (used to clamp the insert into the toolholder), and the chipbreaker groove highlighted (shapes how the chip curls and breaks away during cutting) — both features together are what the “G” in the code represents.

The fourth character isn’t just one feature — it’s a combined code representing both the hole type and the chipbreaker style together. “G” specifically means: a cylindrical hole (for clamping into the toolholder) combined with a chipbreaker groove molded on both the top and bottom faces. Manufacturers use a single letter here because hole style and chipbreaker style are standardized together as a pair, rather than specified as two separate codes.

12-04-12 — Size, Thickness, and Corner Radius

The final three number pairs specify the insert’s physical dimensions. The first pair, “12,” refers to the size — specifically, the diameter of a circle that would perfectly touch all four edges of the diamond shape, known as the inscribed circle. In this case, that’s 12.7mm (½ inch).

**Left:** the CNMG120412 insert as-is. **Right:** the inscribed circle marked in green — this circle’s diameter is what the first size digit pair (“12”) in the code refers to.

A common point of confusion: the size code refers to the inscribed circle diameter, not the length of the cutting edge itself. For a diamond-shaped insert like this one, the actual edge length is somewhat longer than the inscribed circle — the diamond’s sharp points extend beyond where the circle touches. This distinction matters most for square-shaped inserts, where the inscribed circle diameter and edge length happen to be equal.

The second pair, “04,” specifies the insert’s thickness — the distance from the top face to the bottom face — which in this case is 4.76mm.

**Left:** side profile of the insert. **Right:** the thickness marked in purple — the top-to-bottom height that corresponds to the “04” in the code.

The last pair, “12,” specifies the corner radius — 1.2mm in this case. This is the curvature at each tip of the insert, visible in the close-up below.

**Left:** close-up of one corner of the CNMG120412 insert. **Right:** the corner tip marked, showing the radius referenced by the final “12” in the code.

Coming Up Next

We’ve covered CNMG120412 in full detail here. In our next article, we’ll break down WNMG080412 — the trigon-shaped insert — position by position, the same way, with real photos from our own tooling inventory. Stay tuned.