Turning insert codes explained: how to read CNMG, DNMG, WNMG and choose the right insert

Updated: 2026-10-05 · 5 min read

Every turning insert carries a code such as CNMG120408 that describes its shape, clearance angle, size and nose radius. This guide shows how to read that code and how to choose shape, nose radius, chipbreaker and grade for everyday CNC lathe work.

1. Why the code matters

In many workshops, inserts are reordered by copying the label from the last box. That works until the material changes, the surface finish is no longer good enough, or a supplier offers "something equivalent". Being able to read the ISO designation (ISO 1832) lets a technician compare brands, speak precisely with suppliers and avoid buying inserts that do not fit the holder.

2. Reading CNMG120408 step by step

After the ISO part, each maker adds its own codes for chipbreaker and grade. These are not standardized, so they must be checked in that maker's catalogue.

3. Shape: strength versus access

The first letter gives the shape. The common ones are C (80° rhombic), W (80° trigon), S (square), T (triangle), D (55° rhombic), V (35° rhombic) and R (round).

The rule is simple: a larger point angle gives a stronger edge that can take heavier cuts but reaches fewer features. A smaller point angle reaches profiles and undercuts but is weaker and needs lighter cuts.

• C and W – general external turning and facing. A double-sided W insert offers six cutting edges, which is attractive on cost per edge.
• D – a good all-rounder for profiling and shoulders.
• V – fine profiling with light cuts only.
• S and R – the strongest edges, for heavy roughing.

4. Negative or positive?

The second letter is the clearance angle, and it decides more than most buyers expect.

Negative inserts (N, 0°) are tilted in the holder to create clearance. They are usually double-sided, have a strong edge and generate higher cutting forces. They suit rigid machines, external roughing and general turning.

Positive inserts (for example C = 7°, P = 11°, as in CCMT, DCMT or VBMT) are single-sided and cut more freely with lower forces. They are the usual choice for boring bars, small diameters, slender shafts, thin-walled parts and Swiss-type lathes.

If a part chatters or bends under a negative insert, switching to a positive geometry is often more effective than reducing speed.

5. Nose radius: finish, strength and vibration

A larger nose radius makes the corner stronger and, at the same feed, gives a better surface finish. It also increases radial force, which causes vibration on slender parts and in boring.

The theoretical roughness is approximately:

Rmax ≈ f² ÷ (8 × r)

where f is the feed per revolution and r is the nose radius, both in mm. Because feed is squared, halving the feed improves the theoretical finish four times, while doubling the radius improves it only twice.

In practice, 0.8 mm is the general-purpose choice, 0.4 mm suits finishing and slender work, and 1.2 mm suits roughing. Two checks are worth making:

• The nose radius must not be larger than the smallest corner radius allowed on the drawing.
• If the finishing depth of cut is much smaller than the nose radius, the insert tends to push rather than cut, and vibration and poor chip control follow.

Wiper inserts can hold the same finish at a higher feed, but they need a stable setup.

6. Chipbreaker: keep the chip inside its window

Every chipbreaker works within a window of feed and depth of cut, normally shown as a small chart on the box or in the catalogue. Makers group them broadly into finishing, medium and roughing geometries.

• Long, stringy chips mean the feed or depth is below the window, or the geometry is too heavy for the cut. This is common in low-carbon steel and stainless steel.
• Tightly crushed chips, noise and edge chipping mean the feed is too high for that geometry.

Long chips are not only a quality problem. On an unattended lathe they wrap around the chuck and the part and stop production.

7. Grade: start with the ISO colour group

Within a group, the coating matters. CVD coatings are thicker and more wear-resistant, suited to continuous cuts in steel and cast iron. PVD coatings are thinner and keep a sharper edge, which helps in stainless steel, interrupted cuts and small parts. Cermet gives a fine finish on steel, CBN is for hardened parts, and PCD is for high-volume aluminium.

A frequent mistake is running a steel grade on stainless steel. The result is built-up edge, notch wear and short tool life.

8. Checklist before you order

1. Holder designation – the holder accepts only one insert shape and size.
2. Workpiece material and hardness.
3. Operation – roughing, medium or finishing; continuous or interrupted cut.
4. Required surface finish and the smallest corner radius on the drawing.
5. Rigidity – machine size, part slenderness, boring bar overhang.
6. Coolant – flood, high pressure or dry.

Once the insert is chosen, check speed and feed with the cutting speed calculator. Read more: types of CNC lathes and CNC lathe workholding. Need a lathe or tooling setup recommended for your part? Request a quote and we reply within 2 working days.