CNMG and SNMG are the two most widely stocked negative turning inserts, and they are often interchangeable on the same part. The difference is entirely in the tip angle: CNMG is an 80° rhombus, SNMG is a 90° square. That one difference drives everything else.
Side-by-side
| CNMG | SNMG | |
|---|---|---|
| Shape | Rhombic | Square |
| Included (tip) angle | 80° | 90° |
| Usable cutting edges | 4 | 8 |
| Edge strength | Good | Best |
| Reach into corners / profiling | Better | Limited |
| Can produce a square shoulder | Approx. 80° — needs a second operation | Yes, true 90° |
| Cutting force | Lower | Higher |
| Cost per edge | Higher | Lower |
| Most common size | 120408 (12.7 mm IC, 4.76 mm, R0.8) | 120408 (same dimensions) |
Why SNMG is stronger
A 90° corner puts far more material behind the cutting point than an 80° corner, and it conducts heat away from the tip better. For the same reason, SNMG handles interrupted cuts, scale and hard skins better. It also has eight edges instead of four, so the cost per edge is roughly half.
Why CNMG is more popular
The 80° tip reaches further. It can turn along a shoulder, follow a contour, and get into places a square insert physically cannot. It generates lower radial force, so it is better on slender parts and weaker machines. For general-purpose turning where you do not need a true 90° shoulder, CNMG is the more useful tool — which is why it is the most stocked turning insert in the world.
The decision rule
Work through these in order:
- Do you need a true 90° shoulder or a square step? Yes → SNMG. No → continue.
- Is the part slender, thin-walled, or the machine small? Yes → CNMG (lower radial force).
- Is the cut interrupted, scaly, or heavy? Yes → SNMG.
- Is cost per edge the main driver, on simple cylindrical work? Yes → SNMG.
- Anything else — general profiling, mixed work? → CNMG.
A common middle ground
If you want more edges than CNMG but more reach than SNMG, consider WNMG (80° trigon, 6 edges) or DNMG (55° rhombic, 4 edges, much better reach). And if you need the strongest possible edge for roughing castings, RNMG round inserts outlast both, at the cost of a large radius left on the part.
Can you swap them in the same holder?
No. The shape letter in the holder designation must match the insert. A CNMG goes in a holder coded C (for example PCLNR 2020 K12); an SNMG needs an S-coded holder (for example PSBNR 2020 K12). The size digit also has to match — a 12-size insert needs a K12 holder. If you are unsure, send us the holder code and we will confirm the match.
Alternatives worth checking
- WNMG — 6 edges, similar reach to CNMG, better for profiling at higher feed
- DNMG — 55°, much better for contours and undercuts
- TNMG — 6 edges, cheapest per edge, weakest tip
- VNMG — 35°, for fine profiling and copying
- All turning inserts
Related
- Turning insert selection guide
- ISO insert code decoder
- Chipbreaker selection guide
- Carbide grades explained
Standards and further reading
The material group definitions used throughout this site follow ISO 513. The manufacturer links below are independent technical references.