CNMG vs WNMG vs CCMT: How to Choose the Right Insert Shape

Walk up to any turning shop and you will see three boxes open more than the rest: a double-sided 80° rhombic, a six-pointed trigon, and a lighter positive-clearance rhombic. In catalogue language those are CNMG, WNMG and CCMT. They look similar enough that people default to whichever is already in the turret, but each one trades something away — edges, access, strength or finish — to be good at a particular job. Choosing on purpose instead of habit is usually the difference between a stable process and one that chips, vibrates or costs more than it should.

This guide compares the three on the things that actually change at the machine: how many cutting edges you get, how the tool reaches the part, how rigid the setup is, and which operations each shape is built for. Every point links to a real model in the InsertBase database so you can move straight from shape to size to a catalogued grade. If you need to decode any of the letters first, the ISO 1832 walkthrough covers them position by position.

The three shapes at a glance

CNMG: the default you reach for first

The CNMG120408 is the workhorse of the indexable world. Its 80° rhombic point with 0° clearance is double-sided, so every insert gives you four usable edges, and the thick negative geometry absorbs interrupted cuts and scale without complaining. The reasonably sharp point still reaches into shoulders and shallow profiles better than a square insert can. If a job has no special reason to be anything else, it is a CNMG.

You pay for that generality in two ways. Four edges is good, but not the best. And because the 80° point is the most exposed part of the insert, very heavy roughing on rigid machines can favour a rounder or more blunt geometry. For a wide band of general steel and cast-iron work, though — and across grades from finishing to roughing — nothing beats it for flexibility. The size ladder runs from the small CNMG090304 for compact bars up through CNMG160612 and CNMG190616 for heavy stock.

WNMG: six edges when the geometry lets you in

The WNMG080408 is a trigon — effectively a triangle with its corners clipped into six usable points. That gives you six edges per insert instead of four, which lowers cost per edge on high-volume, repetitive outside-diameter and facing operations where the machine never stops. On a production run of simple shafts and collars, that edge count is a direct, measurable saving.

The catch is geometry. The clipped corners of a trigon have a wider included angle, so the tool nose cannot reach into tight shoulders, square-bottomed grooves or narrow profiles the way a CNMG point can. WNMG also needs enough room and rigidity to present its broader cutting zone cleanly; on a flimsy part or a slender bar it will tell you with chatter. Reach for WNMG when the part is open, the operation is repeated, and you genuinely consume edges — not because the insert is cheaper if half its points can never reach the cut. Sizes step from WNMG060404 up to the 12.7 mm WNMG080412.

CCMT: positive rake for the jobs negative inserts cannot do well

The CCMT060204 keeps the same 80° rhombic point but adds 7° of positive clearance and a single-sided, two-edge design. That is not a step down — it is a deliberate tool for lower-power and constrained work. Positive rake shears the material with far less cutting force, so a small CNC, a boring bar, a thin-walled tube or a slender shaft can cut without deflection. This is why almost every internal-boring catalogue is full of CCMT, DCMT and TCMT rather than their negative cousins.

The trade is blunt: you get two edges instead of four, and the lighter edge is not built for heavy interrupted roughing. Use CCMT where its strength is the point — internal boring, fine finishing on low-horsepower machines, long slender parts that deflect under a negative tool, and many stainless and high-temperature jobs where a freer cut reduces work-hardening. The family runs from the tiny CCMT060202 for small bores through CCMT09T304 to the CCMT120408 for larger bars.

How to decide on the shop floor

Run the decision in this order. First, can the tool reach the feature? A narrow shoulder or internal bore removes WNMG and often points you at positive CCMT before anything else matters. Second, is the operation open and repeated enough to spend six edges? If yes and the setup is rigid, WNMG earns its cost advantage. Third, what does the machine and workpiece tolerate? Low power, thin sections, boring bars or deflection-prone parts want positive CCMT; everything in the broad middle — general external turning of steel and cast iron on a normal machine — runs happily on CNMG.

Shape is only the first half of the choice. Once the geometry is fixed, the grade decides what workpiece material the edge actually survives: the same CNMG body comes in very different coatings and substrates. Pair this guide with the ISO 513 grade-selection guide to choose P/M/K/N/S/H and the CVD-versus-PVD coating for your material, then enter the code in the interactive decoder to confirm every dimension.

The short version

If you remember nothing else: CNMG is the four-edge default for general turning, WNMG is the six-edge money-saver for open, repeated, rigid work, and CCMT is the low-force positive insert for boring, finishing and slender setups. Pick by access first, productivity second, and machine/workpiece limits third — and only then choose the grade.

Want to see the catalogued options for a specific geometry? Browse the model index, compare brands on the cross-reference finder, and use the inquiry box on any model page for the size and grade you need.