CCMT vs CNMG vs DNMG vs VNMG: CNC Turning Insert Shapes Explained

If you're new to CNC lathe work or have been handed a toolholder someone else set up, the first confusing thing is the soup of 4-letter codes on every insert box. This post breaks down the five most-common turning insert shapes — what each one is designed to do, and why swapping them isn't always a free choice.

The first letter = shape

Every ISO turning insert starts with a shape letter. The shape determines the approach angle the cutting edge presents to the workpiece, and the included angle at the corner (which controls corner strength and chatter behaviour).

C — 80° diamond (CCMT, CNMG, CNMA)

The everyman shape. The 80° corner is a workable compromise between strength and access. One insert can face, turn, bore, and do shallow profile work without changing tools.

Best for: general-purpose turning where you don't want to swap inserts mid-job.
Typical use: shafts, flanges, faceplates, turning from chuck side.
Trade-off: 80° is not sharp enough for deep profile work — use D or V for that.

D — 55° diamond (DCMT, DNMG, DCGT)

55° corner — much pointier than C. Gives you access to profile contours with steeper angles without the corner walking into the work.

Best for: profile turning, deeper shoulders, copy turning.
Trade-off: pointier corner is weaker — don't push feed as hard as CCMT. Expect ~60% of CCMT's roughing capacity.

V — 35° diamond (VBMT, VNMG, VCGT)

The finishing specialist. 35° corner lets you get into very tight shoulders and profile work that CCMT or DCMT can't reach.

Best for: final finishing passes, tight internal/external profiles, shouldering.
Trade-off: fragile corner. Don't use for roughing. Chip control at medium feeds is trickier than C or D shapes.

W — 80° trigon (WNMG, WCMX, WNMX)

Six cutting edges per insert. Same 80° included angle as C-shape, but in a trigon (three-sided) body instead of a parallelogram — so you get double the edges and therefore double the mileage per insert.

Best for: roughing, production shops where cost-per-edge matters.
Trade-off: less versatile than CCMT; corner access is more limited. Excellent for what it does, not a replacement for a CCMT.

T — triangle (TNMG, TCMT, TCGT)

Three cutting edges, 60° included angle. Smaller approach angle than C-shape, giving better access to tight shoulders and long overhangs.

Best for: reaching shoulders, small-diameter work, medium turning.
Trade-off: fewer edges per insert (3 vs 6 on W-shape) — economics slightly worse on high-volume roughing.

S — square (SNMG, SNMA, SNMX)

The roughing bull. Four or eight cutting edges, strongest corner of any common shape, handles deep cuts and heavy feeds without complaining.

Best for: first-pass roughing, heavy stock removal, interrupted cuts.
Trade-off: poor access to profile features. Can't finish a shoulder cleanly — swap to C, D, or V after roughing with S.

R — round (RCMT, RCMX)

No corner, just a continuous radius. Copy turning, heavy roughing, and materials prone to chipping (cast iron, hardened steel).

Best for: copy turning, form turning, materials where corner chipping is the main failure mode.
Trade-off: generates a corner radius equal to the insert radius — not suited for sharp corners or narrow profiles.

Which shape do I actually need?

• "One tool does everything" → CCMT or CNMG (start here if unsure)
• Roughing-heavy production → WNMG or SNMG
• Profile turning, contours → DCMT or DNMG
• Tight shoulders, fine finishing → VBMT or VNMG
• Cast iron, copy work → RCMT

Any toolholder is designed around one specific insert shape — a CCMT holder won't take a DCMT, even if both inserts look similar. Always match holder and insert from the same ISO family.

Browse our full turning insert range →