CNMG vs DNMG vs VNMG: Which Turning Insert Shape?

Walk into any turning shop and you will find the same three rhombic insert shapes in the drawer: CNMG, DNMG and VNMG. They look similar at a glance — all four-sided, all negative rake, all available in the same size ranges — yet they are engineered for different jobs. The difference is the included angle: 80° for CNMG, 55° for DNMG and 35° for VNMG. That single number decides how heavy a cut the corner can survive, how deep the tool can reach into a profile, and how the edge behaves when the workpiece starts to vibrate.

This guide compares the three shapes side by side, explains the strength-versus-reachability tradeoff behind them, shows where each one earns its keep, and ends with a selection table you can use on the shop floor. If you only need the product details, jump to our turning inserts overview or the dedicated CNMG, DNMG and VNMG pages.

Why shape matters before grade

When machinists talk about picking an insert, the conversation usually starts with grade — the carbide substrate and coating that suit a given workpiece material. That instinct is understandable, but it puts the cart before the horse. Grade tells you how the cutting edge will wear; shape tells you whether the edge can physically do the job at all. A perfect P25 steel grade in a VNMG geometry will not survive a heavy roughing pass, and a CNMG will not reach into the corner of a narrow undercut profile no matter how sharp its coating is.

Shape controls four things that grade cannot fix:

• Corner strength — how much cutting force the nose can absorb before chipping or breaking.
• Access — how far the tool can reach into grooves, shoulders and complex profiles.
• Edge count and cost per edge — how many usable cutting edges you get per insert.
• Force direction and stability — how the cutting forces push against the workpiece and the machine.

In the ISO 1832 system, the first letter of the code is the shape. C means an 80° rhombic insert, D a 55° rhombic, V a 35° diamond. The letter is the first decision you make, and it is the hardest one to reverse later: you cannot change the shape of an insert to fit a holder, and holders are built around one shape family. Choosing the wrong shape means re-tooling, not re-ordering.

The three shapes decoded: 80°, 55°, 35°

All three inserts are rhombi — four-sided shapes with two acute and two obtuse corners. What differs is how sharp the acute corners are.

CNMG — 80° rhombic. The acute corners measure 80°, the obtuse corners 100°. As a negative insert (the N in the code), it is double-sided, giving four corners per side and eight cutting edges per insert. The 80° corner is strong enough for heavy roughing on steel and cast iron, which is why CNMG is the default recommendation for general external and internal turning.

DNMG — 55° rhombic. The acute corners measure 55°, the obtuse corners 125°. Also negative and double-sided, so eight edges again. The sharper corner gives noticeably better clearance to the workpiece, letting the tool trace steeper profile walls and shoulders that an 80° corner would hit. The corner is still strong enough for moderate cutting loads.

VNMG — 35° diamond. The acute corners measure 35°, the obtuse corners 145°. Negative and double-sided, eight edges. This is the narrowest profile of the three: the tool can enter spaces the others cannot, but the 35° corner is the most fragile. It is a finishing and profiling geometry, not a roughing geometry.

All three share the same letter sequence beyond the shape: N = 0° clearance angle (negative), M = tolerance class, G = hole with chip breaker on both sides. Sizes, thicknesses and nose radii follow the same ISO numbering — a CNMG120408 and a VNMG160404 differ in shape but are read the same way. If the code system itself is unfamiliar, our guide on reading a carbide insert identification chart walks through every position.

Strength vs. reachability: the core tradeoff

Every rhombic insert family sits somewhere on a single spectrum, and the spectrum is the whole story: the wider the corner angle, the stronger the corner; the narrower the corner angle, the farther the tool can reach. You never get both from the same shape. The choice between CNMG, DNMG and VNMG is really a choice about where on that spectrum your job lives.

Think of the corner as a cantilever. An 80° corner has a thick, blunt triangle of carbide behind the cutting point. Cutting forces spread through a wide cross-section, so the edge absorbs shock from interrupted cuts, scale and variable depth of cut. A 35° corner has a slender wedge of carbide behind the point; the same force concentrates in a much smaller volume, and the corner is far more likely to chip or fracture the moment the load gets rough.

Reachability works the other way. The angle between the insert side and the workpiece face determines how steep a wall the tool can cut without the holder or the insert body colliding with the material. A CNMG at 80° leaves little room between the insert flank and the workpiece — fine for straight cylindrical turning, limiting for steep shoulders. A VNMG at 35° can work against a near-vertical wall and still keep clearance, which is exactly what copy turning and contouring demand.

This tradeoff also shows up in the numbers you actually program. Because the corner is weaker, a VNMG needs lighter depth of cut and feed than a CNMG doing the same material — the machine does less work per pass, but the tool does the work it could not reach. The economic question is never "which shape is better" but "which shape can complete this feature at an acceptable cost per edge."

What the included angle changes in practice

Beyond strength and access, the included angle shifts several practical behaviors that affect the job from setup to surface finish.

Maximum depth of cut. The cutting edge on a rhombic insert runs from the nose along two sides. On an 80° insert, a large portion of the edge stays engaged at a given depth of cut, so the load spreads along a long edge. On a 35° insert, the same depth of cut forces the load onto a short edge segment close to the weak corner. The practical result: CNMG handles deep passes, VNMG is limited to light passes, and DNMG sits in between.

Feed rate ceiling. Feed is limited by the nose radius and by the corner's ability to take the chip load. A VNMG corner at 35° flexes and heats up faster than an 80° corner at the same feed. In finishing, where feeds are light anyway, this matters little; in roughing it is the difference between a productive pass and a broken edge.

Profile geometry. The obtuse corners matter too. When you index a rhombic insert you can also cut with the 100°, 125° or 145° corner in some holders, which effectively gives you a different approach angle for the same insert. In practice most turning holders present the acute corner, but the option exists, and it is one reason the same CNMG can appear in both external and internal toolholders.

Tool deflection. A slender insert body and long overhang deflect more under load. A VNMG in a boring bar reaches deeper into a bore than a CNMG could, but it also bends more, so it rewards light, consistent passes. The sharper geometry and the longer reach usually come as a pair in real jobs — deep internal profiling is a VNMG specialty precisely because nothing else fits.

Nose angle, cutting forces and vibration

Vibration — chatter — is one of the most common reasons a turning job goes wrong, and the insert shape plays a real part in it. The mechanism runs through the cutting forces.

When the cutting edge engages the work, the force splits into components: a cutting force in the direction of motion, a feed force along the axis, and a radial (depth) force pushing the tool away from the workpiece. The radial component is the one that excites vibration, because it acts against the stiffness of the toolholder and the workpiece. Sharper insert corners and smaller nose radii produce lower radial forces; wide corners and large nose radii push the tool harder against the work and are more prone to chatter on flexible setups.

That is why the same job can run smooth with a VNMG and chatter with a CNMG — and the opposite on a rigid machine. A VNMG's narrow included angle keeps the force vector closer to the feed direction, which helps on long overhangs, thin workpieces and small diameters. A CNMG's wide corner drives a larger radial component, which is fine on a sturdy lathe with a short tool overhang and a rigid workpiece, and problematic on slender parts.

Nose radius interacts with shape here. Within any shape family, a larger nose radius (e.g. 08 or 12 in the code) increases the radial force and improves surface finish; a smaller radius (02 or 04) reduces force and vibration but leaves a rougher finish. When chatter appears, the sequence worth testing is: reduce nose radius first, then reduce depth of cut, then consider a sharper shape family. And the sharper shape family gives you headroom — a VNMG can often run a finishing pass chatter-free where a CNMG of the same nose radius cannot, purely because of the force direction.

One caution: chatter resistance from a sharp shape only applies within its load envelope. Push a VNMG past its depth-of-cut limit and the corner deflects, the edge breaks, and the chatter gets worse, not better. The stability of a sharp corner is a light-cut stability; it does not replace machine rigidity.

CNMG: the general-purpose workhorse

The CNMG is the shape most shops reach for first, and for good reason: its 80° corner combines the highest strength of the three rhombic families with enough versatility for everything from roughing to semi-finishing. On a rigid lathe turning a solid steel shaft, a CNMG with the right grade will out-produce both of its sharper cousins at equal depth of cut and feed.

Typical CNMG territory:

• Roughing and medium turning of carbon steel, alloy steel and cast iron — interrupted cuts, forged surfaces and scale are survivable because the corner absorbs the shocks.
• General external turning of shafts, flanges and stepped diameters where the profile is simple.
• Internal turning of bores with adequate diameter — an 80° corner in a boring bar covers most straight-bore work.
• Heavy feeds on machines with good rigidity and short overhangs, where the long engaged edge spreads the load.
• Both roughing and finishing in job-shop work where one insert code must do everything — the 80° corner is the most forgiving shape when you do not know what the next job will be.

The limits appear when the profile tightens. A steep shoulder, a narrow groove wall or a small-diameter bore will stop a CNMG cold, because the 80° corner runs into the material before the intended feature is complete. That is not a defect — it is the shape doing what it is designed to do. The CNMG turning inserts range at our factory covers the common size and radius combinations, with chip breakers suited to steel, stainless and cast iron.

DNMG: the profiler's choice

The DNMG's 55° corner is the middle of the spectrum, and the middle is where most profiling work actually lives. It keeps enough corner strength for moderate roughing and semi-finishing while gaining the clearance to cut profile walls that an 80° insert cannot reach. If you only stock two rhombic shapes, the pair is usually CNMG plus DNMG.

Typical DNMG territory:

• Copy turning and contouring where the tool follows a changing profile — the 55° corner clears the previously cut wall without rubbing.
• Stepped shoulders and undercuts with wall angles up to roughly 30° or more from the axis, depending on the holder.
• Moderate depth-of-cut profiling on steel and stainless, where VNMG would be too fragile and CNMG would not fit.
• Semi-finishing passes that blend a roughing geometry with a finishing requirement on the same feature.
• Smaller diameters and bores where an 80° holder is too bulky — the 55° shape fits into tighter internal clearances.

On stainless steel and other work-hardening alloys, the DNMG earns extra points: the sharper corner cuts with lower pressure and presents less edge to the work-hardened layer, which helps control built-up edge and keeps the cut stable. It is also a common choice for finishing passes that follow a CNMG roughing pass on the same part, since the two shapes share tooling philosophy — negative rake, eight edges, same chip breaker families. Our DNMG turning inserts are stocked in the standard sizes that cover most profiling work.

VNMG: maximum access for finishing

The VNMG is a specialist, and it is usually the last of the three to enter a shop's inventory — but once a job requires it, nothing else will do. Its 35° corner provides the best access of any common turning insert, at the cost of the lowest corner strength. It is not a roughing shape; it is the shape you switch to when the feature, not the material, is the constraint.

Typical VNMG territory:

• Narrow and deep profiling — tight grooves, steep flanks and complex contours that an 80° or even 55° corner cannot trace without collision.
• Finishing passes on complex turned parts where surface finish and profile accuracy matter more than metal removal rate.
• Small-diameter internal work — a VNMG in a slim boring bar reaches deep bores that wider shapes physically cannot enter.
• Light cuts on slender or thin-walled workpieces where low cutting forces keep the part stable and chatter-free.
• Lathe work on parts with clearance restrictions, such as valve components, fitting profiles and precision shafts with multi-step contours.

Because the corner is fragile, VNMG work should be planned: leave material for the finishing pass, keep depth of cut and feed conservative, and use the sharpest grade geometry the material allows. In return, the 35° shape delivers profile fidelity that the wider families cannot match. The VNMG turning inserts range covers the common 1604 and 1103 sizes used for this class of work.

How WNMG and CCMT fit in

No comparison of rhombic inserts is complete without the two shapes that flank them in the catalog: WNMG and CCMT.

WNMG — 80° trigon. The WNMG is not a rhombus; it is a trigon, a three-cornered shape with 80° included angles, negative rake and six cutting edges (three corners, double-sided). Its 80° corners give it strength comparable to a CNMG, while the trigon form presents a more oblique cutting edge that reduces cutting forces and improves chip flow in many steel applications. The tradeoff is six edges instead of eight, and the same limited access as any 80° shape. If you compare a CNMG and a WNMG on the same job, the WNMG often wins on force and chip control, and the CNMG wins on edges-per-insert and corner robustness. Both are general-purpose shapes; WNMG is arguably the more popular general-purpose line in many markets because of its price-per-edge and the slightly easier cut.

CCMT — 80° rhombic, positive. The CCMT is an 80° rhombus with a positive clearance angle (7°), single-sided, giving four cutting edges. Positive geometry means the edge is presented to the work with a sharper, freer cut: cutting forces are lower, so CCMT is the classic choice for light finishing, aluminum and non-ferrous materials, thin-walled parts, and machines with limited rigidity. The price is fewer edges and less corner strength than a negative insert. Where a CNMG and a CCMT share the same 80° geometry, the choice is really negative-versus-positive: strength and edge economy versus low cutting forces and sharpness.

For the comparison at hand, think of the shape families as two separate axes. Along the access axis, the order from most accessible to least is VNMG (35°) → DNMG (55°) → CNMG/WNMG (80°). Along the rake axis, positive shapes (CCMT) sit alongside the negative shapes at their own strength point. If your problem is access, move down the angle axis; if your problem is cutting force on a flexible setup, consider the positive CCMT before you give up on rhombic shapes entirely.

Selection decision table by application

The table below summarizes the decision for the most common turning scenarios. Use it as a starting point, not a substitute for testing on your own machine — holder geometry, nose radius and grade all shift the outcome.

Common mistakes when choosing

After years of seeing buyers and machinists pick between these three shapes, the same mistakes keep recurring. Avoiding them saves money and scrap.

1. Choosing shape before checking holder compatibility. The shape letter must match the toolholder pocket. A CNMG holder cannot take a DNMG or VNMG insert, and forcing it damages both. Check the code on the holder or the pocket before ordering — then read the ISO code of your current insert box if in doubt.

2. Roughing with a VNMG because "it cuts smoother." The 35° corner is stable only within its light-cut envelope. Run it at CNMG-style depths and the corner chips, often mid-pass. If the job needs metal removal, start with the CNMG and switch to VNMG only for the finishing pass.

3. Assuming 80° means "roughing only." A CNMG with a small nose radius (04) and a finishing-grade chip breaker is a perfectly good finishing insert on rigid machines. Shape and nose radius are two separate decisions; do not let one override the other.

4. Ignoring the holder's approach angle. The holder sets the lead angle, which changes the effective cutting edge angle and chip thickness. The same VNMG in a 93° holder and a 45° holder behaves differently. The insert shape is only half the geometry — the holder is the other half.

5. Blaming chatter on the insert when the setup is the problem. Long overhang, worn bearings, a flexing workpiece — no shape fixes those. Sharpening the corner angle helps, but it masks the real limit. Check the setup first, then the shape.

6. Comparing inserts across different size classes. A CNMG120408 and a VNMG160404 are different sizes with different nose radii; a direct "which is stronger" comparison is meaningless unless the size class and radius are matched. Compare like with like.

7. Buying one shape for the whole shop "to keep inventory simple." Inventory simplicity is real, but it costs far more when a CNMG-only shop faces a profiling job and a VNMG-only shop faces a roughing job. The standard trio — CNMG, DNMG, VNMG — covers the widest range with only three codes.

A quick checklist before you order

When you are ready to order, run through this checklist and include the answers with your inquiry. It takes two minutes and prevents most mismatches.

• Shape: CNMG (80°, general/roughing), DNMG (55°, profiling) or VNMG (35°, access/finishing) — confirmed against your holder.
• Size: the inscribed circle and thickness from your holder's pocket code (e.g. 1204 = 12.7 mm × 4.76 mm).
• Nose radius: 04 for light cuts and low vibration, 08 for general use, 12 for heavy feed and finish on rigid setups.
• Material and operation: workpiece material, and whether the pass is roughing, semi-finishing or finishing — this drives the grade and chip breaker.
• Machine: rigidity, spindle range and tool overhang, especially for VNMG and boring applications.
• Quantity and packaging: per-box count, bulk or OEM branding if you distribute.

The short version of this whole guide: CNMG when the cut is heavy, VNMG when the access is tight, DNMG when it is somewhere in between — which turns out to be most profiling work. Match the shape to the feature and the grade to the material, verify the holder code, and test at conservative parameters before you push. If you are unsure which of the three fits your application, send us the code on your current insert box or your holder pocket — we will match it or recommend a better shape for the job.