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Carbide Insert Basics for Turning
Guide · Updated October 27, 2026
Turning inserts follow an ISO/ANSI code that describes the shape, clearance, tolerance, type, size, thickness and nose radius. Once you can read the code, picking an insert gets simple.
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Step by step
- Shape: C (80° diamond) is strong and general purpose. D (55°) is for profiling. T (60° triangle) is economical. V (35°) is for fine profiling. W (80° trigon) suits roughing.
- Clearance: N is negative (0°), double-sided and strong. C is 7° positive, for lighter cuts.
- Size code: In inch codes like CNMG 432: 4 = 1/2 in inscribed circle, 3 = 3/16 in thick, 2 = 1/32 in nose radius.
- Nose radius: A larger radius is stronger and gives a better finish, but it pushes harder. A smaller one suits light cuts and slender parts.
- Grade and chipbreaker: Match the coated grade to the ISO material group (P steel, M stainless, K cast iron, N non-ferrous) and the chipbreaker to the depth of cut.
Tips
- Start at the maker's mid-range speed and feed.
- Feed per rev must be enough for the chipbreaker to work.
- Set speeds: speeds and feeds calculator.
Related: speeds and feeds basics, coolant.
Worked example: picking an insert for a steel shaft
You need to turn a 1.5 in 1018 steel shaft on a manual lathe with a right-hand holder that takes CNMG inserts. Pick a CNMG 432 (1/32 in nose radius) in a P-grade (steel) coated carbide with a medium chipbreaker. Look up the maker’s starting surface speed for that grade in mild steel and convert it to RPM for the 1.5 in diameter with the speeds and feeds calculator. Set the feed per rev inside the chipbreaker’s listed range, not below it, so the chips curl and break instead of forming long strings. Set the tip exactly on center height. If you see stringy chips, raise the feed or depth slightly; if the edge chips, check center height and rigidity before slowing down. Index to a fresh corner when the finish or size starts to drift.
FAQ
What does CNMG 432 mean?
An 80° diamond, 0° clearance, M tolerance, with a hole and chipbreaker, 1/2 in IC, 3/16 in thick and 1/32 in nose radius.
Which insert shape is most common?
C (80° diamond) for general turning and facing.
What nose radius should I use?
1/32 in (0.8 mm) is a good general choice. Use 1/64 in for light finishing on small parts.
How do I check turned sizes?
Mics and calipers, checked on gage blocks. Build any reference size free with the Gage Block Calculator at gageblockcalc.com.
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General shop information only — follow your gage manufacturer's instructions and your quality system.