Turning inserts are one of the most frequently purchased consumables in any machine shop but with dozens of grades, geometries, and coatings available, selecting the wrong one costs you time, money, and surface finish. This guide covers the key selection criteria so you can match the right insert to your material and application every time.
1. ISO Insert Shape
The shape of the insert determines the number of cutting edges and the approach angles available. Common shapes include:
- CNMG / CCMT (80° diamond) — general-purpose turning, good strength
- TNMG / TCMT (60° triangle) — versatile, three cutting edges, ideal for profiling
- VNMG (35° diamond) — excellent for profiling and finishing, tight corners
- WNMG (80° trigon) — strong, suited to heavy roughing
A larger included angle means a stronger cutting edge. Choose a smaller angle (35–55°) when you need tight profile access; choose a larger angle (80°+) for roughing and interrupted cuts.
2. Insert Grade (Substrate)
The grade refers to the carbide substrate and any coating applied:
- Uncoated carbide — best for non-ferrous materials (aluminium, copper, brass) where coating adhesion can cause built-up edge
- CVD-coated (TiCN / Al₂O₃) — high wear resistance for steel and cast iron at medium-to-high speeds
- PVD-coated (TiAlN / TiN) — sharper edges, better for stainless steel, hardened steels, and finishing cuts
3. Chip Breaker Geometry
Chip breaker codes (the middle two letters in the ISO designation) indicate the geometry:
- MF / MG — medium finishing, light-to-medium feeds
- PM / GM — medium roughing, versatile
- PR / GR — heavy roughing, high feed rates, interrupted cuts
Match the geometry to your depth of cut (DOC) and feed rate. Using a finishing geometry for heavy roughing will cause premature chipping.
4. Nose Radius
The nose radius affects surface finish and edge strength. A larger radius (0.8–1.2 mm) improves finish at high feeds and strengthens the edge for roughing. A smaller radius (0.2–0.4 mm) is better for profiling, undercutting, or working close to shoulders.
5. Insert Size and Fixing System
Insert size must match your toolholder. The ISO designation encodes the inscribed circle (IC) diameter, common IC sizes are 9.525 mm (3/8"), 12.7 mm (1/2"), and 15.875 mm (5/8"). Always check your toolholder's insert seat before ordering.
Quick Selection Reference
| Material | Recommended Grade | Geometry | Speed Tendency |
|---|---|---|---|
| Carbon steel | CVD TiCN/Al₂O₃ | PM/GM | Medium–high |
| Stainless steel | PVD TiAlN | MF | Medium |
| Cast iron | CVD Al₂O₃ | GM/GR | High |
| Aluminium | Uncoated / PCD | MF (sharp) | Very high |
| Hardened steel | CBN or fine-grain PVD | MF | Low–medium |
Shop Turning Inserts at Tooltec
Browse our full range of turning inserts and toolholders, including CNMG, TNMG, VNMG, and CCMT styles across a range of grades for steel, stainless, cast iron, and aluminium. All inserts are stocked in Perth and available for fast dispatch across Australia.