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How to Machine Hardened Steel
Guide · Updated October 28, 2026
Hardened steel (roughly 45–65 HRC) can be turned and milled instead of ground, if the setup is rigid and the tool is right. Hard turning can hold tight sizes and good finishes when the depth of cut is light and the machine is stiff.
Size to tenths
Hard parts are often finished to tenths. Prove the mic on gage blocks. The free Gage Block Calculator builds the stack.
Step by step
- Check the hardness: Test it; under about 45 HRC use normal carbide, above that use hard-turning grades, ceramic or CBN.
- Make it rigid: Short overhang, tight gibs, minimal stickout.
- Pick the tool: Negative rake with a honed or chamfered (T-land) edge.
- Set speed: Use the tool maker's chart; start in the middle of the range.
- Light cuts: Depth of cut about 0.004–0.010 in; feed about 0.002–0.006 in/rev for turning.
- Run dry or steady coolant: Do not use intermittent coolant on ceramic or CBN; thermal shock cracks them.
- Measure cool: Let the part reach room temperature before final size.
Worked example: RPM for hard turning
Hard turning 1.000 in diameter 58 HRC steel with a CBN insert at a chart starting value of 400 SFM. RPM = 3.82 × SFM ÷ D = 3.82 × 400 ÷ 1.000 = 1528 rpm. Leave 0.010 in on diameter after heat treat; finish in two passes of 0.004 in and 0.001 in depth (0.008 and 0.002 in on diameter). Always use the speed from your insert maker's chart; this is just the math.
Tool choices
Coated carbide: up to about 45–50 HRC, and interrupted cuts. Ceramic: fast and cheap for continuous hard cuts. CBN: best for 50–65 HRC and finishing, but brittle and expensive. Solid carbide end mills rated for hard milling for the mill.
Turning vs grinding
Hard turning removes stock faster and does profiles in one setup. Grinding still wins for long thin parts, the best finishes and the tightest sizes.
Common problems
Edge chipping: interrupted cut or too much feed for the edge prep. Chatter: rigidity. Poor finish: worn tool or feed too high for the nose radius. Size drift: heat.
Tips
- Hardness: hardness tester basics.
- Inserts: carbide insert basics.
- Grind: tool post grinder when turning is not enough.
FAQ
Can you machine hardened steel?
Yes, with rigid setups and carbide, ceramic or CBN tools.
What tool for 60 HRC?
CBN is the usual choice.
How deep a cut?
About 0.004–0.010 in.
Coolant or dry?
Often dry with ceramic and CBN; never intermittent coolant.
How do I find RPM?
3.82 × SFM ÷ diameter, using the tool maker's SFM.
Related guides and calculators
- Hardness Tester Basics for Machinists
- Carbide Insert Basics for Turning
- How to Use a Tool Post Grinder
- Turning Speeds and Feeds
- Grinding Speeds and Feeds
- How to Use a Cathead on a Lathe
- How to Cut a Taper with a Taper Attachment
- All free machinist calculators
- Annealing, normalizing and tempering steel
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