What positive rake actually changes
A negative insert sits in the holder with a negative inclination, so its effective rake is close to zero and the cutting edge is backed by a thick section of carbide. A positive insert has its clearance ground into the insert itself, so the edge meets the material at a genuinely positive angle and slices rather than pushes.
The practical consequences follow from that single difference, and they explain every symptom a machinist notices when swapping between the two.
| Characteristic | Positive (CCMT, DCMT) | Negative (CNMG, DNMG) |
|---|---|---|
| Usable edges | One per insert | Two, four or six depending on shape |
| Effective rake | Genuinely positive | Close to zero |
| Cutting force | Substantially lower | High, particularly radial force |
| Edge strength | Lower; the edge is thinner | High; suited to interrupted cuts |
| Best for | Finishing, slender parts, small diameters, light machines | Roughing, interrupted cuts, rigid machines |
| Cost per edge | Higher, because there is one edge | Lower, because edges are multiplied |
When a positive insert is the only answer
There are four situations where a negative insert will not work regardless of how the parameters are adjusted, and each of them points at the same underlying cause: too much cutting force for the stiffness available.
- Slender shafts and thin-walled parts. A long shaft deflects under radial force, so a negative insert pushes it away and the result is a taper and chatter. A positive insert cuts freely enough that the deflection stays within tolerance.
- Small diameters. Below about 20 mm the workpiece itself is not stiff, and the radial force of a negative insert becomes the dominant error rather than tool wear.
- Low-rigidity machines and worn turrets. A turret with play or a light lathe cannot absorb the force a negative insert generates, and no amount of speed reduction fixes a mechanical limitation.
- Finishing passes where the finish is the specification. A positive edge shears the surface rather than burnishing it, which produces a finer finish at the same feed rate.
Choosing between CCMT, DCMT and VCMT
All three are positive with a 7-degree clearance, so the difference is the shape of the point and therefore the access and the strength.
| Type | Point angle | Best for | Limitation |
|---|---|---|---|
| CCMT | 80 degrees rhombic | General finishing and medium turning; the strongest of the three | Cannot reach into tight shoulders |
| DCMT | 55 degrees rhombic | Profiling, copy turning and work with shoulders | Weaker point than CCMT |
| VCMT | 35 degrees rhombic | Very tight profiles and delicate finishing work | The weakest point; light cuts only |
| TCMT | 60 degrees triangle | Internal boring and small diameters | Less rigid than the rhombic shapes |
Grades and coatings for positive inserts
Positive inserts are used at lighter depths of cut and often on finishing passes, so the thermal load is lower than in roughing and a PVD coating with a sharp edge is usually the right choice.
For steel, a PVD-coated P grade with a fine edge preparation holds up well. For stainless and other work-hardening materials the sharp edge matters more than the coating, because a positive geometry that rubs will harden the surface and make the next pass harder. For aluminium, a polished uncoated insert or a DLC coating prevents the built-up edge that a hard coating would encourage.
What positive inserts cannot do
Being honest about the limits is more useful than claiming a universal insert. A positive insert has one usable edge and a thinner one, so it is the wrong choice when the operation is interrupted, when the depth of cut is heavy, or when the material carries a hard skin.
The cost per edge is also higher, because a negative insert offers four to six usable corners against one. Where the machine is rigid enough to take the force, the negative insert is simply more economical, and we will say so rather than sell the geometry that suits us better.
Frequently asked questions
What does the C in CCMT mean?
The second letter is the clearance angle. C means 7 degrees of relief ground into the insert, which is what makes it a positive insert. An N in the same position means zero clearance, which is a negative insert used in a negative holder and turned over to use the other face.
Why does my part chatter and improve with a positive insert?
Because a positive insert generates substantially less radial force, and radial force is what deflects a slender workpiece or a lightly held one. If changing the parameters did not fix chatter but changing the insert geometry did, the limit was machine or workpiece stiffness rather than the cutting data.
Are positive inserts more expensive to run?
The cost per edge is higher because a positive insert offers one usable edge against four or six on a negative insert. Whether that matters depends on the operation: where the machine is rigid, the negative insert is more economical, and where it is not, the positive insert is the only one that produces an acceptable part.
Can I run a positive insert in a negative holder?
No. A negative holder inclines the insert so that its zero-clearance flank provides the relief. Putting a positive insert in it would give the insert far too much clearance, so it would rub below the cut and fail immediately.
What corner radius should I choose for finishing?
0.2 mm for the finest finish and the lowest radial force, 0.4 mm as the general finishing choice and 0.8 mm where the surface finish requirement is moderate but the insert needs more edge strength. A larger radius improves the theoretical finish and increases radial force at the same time.