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How to Cut a Radius on a Lathe
Guide · Updated October 28, 2026
Corner radii and full ball ends show up on handles, shafts and fillets. Small radii are cut with a ground form tool or insert nose radius; larger ones use a ball turner or step-cutting with X and Z coordinates and a file to blend. A radius gage checks the result.
Check diameters with blocks
Confirm your mic on gage blocks, then check the radius profile with a radius gage. The free Gage Block Calculator builds the stack.
Step by step
- Pick the method: Up to about 1/8 in: form tool or insert nose radius. Larger: ball turner or step-cutting.
- Grind or select a form tool: Grind the radius on HSS and check it against a radius gage.
- Set the tool on center: A form tool off center cuts the wrong shape.
- Plunge the form tool slowly: Low RPM and light feed with oil; form tools chatter easily.
- For step-cutting, compute coordinates: For each Z step, the X offset from the corner uses the circle equation.
- Blend: File and polish between steps with a fine file and abrasive.
- Check the radius: Use a radius gage against a light to see gaps.
Worked example
Cut a 0.250 in convex corner radius on a shoulder by stepping. With the radius center at (0,0), for each Z distance z from the center, the X depth to remove from the corner is x = R − √(R² − z²): at z = 0.050, x = 0.250 − √(0.0625 − 0.0025) = 0.250 − 0.2449 = 0.0051 in; at z = 0.100, 0.250 − 0.2291 = 0.0209 in; at z = 0.150, 0.250 − 0.2000 = 0.0500 in; at z = 0.200, 0.250 − 0.1500 = 0.1000 in; at z = 0.250, 0.250 in. Cut these steps on the cross slide (doubling for diameter), then file to blend.
Ball turners
A ball turner pivots the tool around a vertical axis set under the ball center, so it cuts a true sphere. It is the best manual method for full balls.
Common problems
A flat spot means the steps are too coarse; add more. Chatter on a form tool means too much width engaged; slow down and use oil. Wrong shape means the tool is off center.
Tips
- Check: use a radius gage.
- Tool height: set tool height.
- Taper: cut a taper on a lathe.
FAQ
How do I cut a small radius?
Use a form tool or an insert with the matching nose radius.
What is a ball turner?
A pivoting tool holder that cuts true spheres and radii.
How do I check a radius?
Hold a radius gage on the part against a light.
Can I cut a radius with the compound?
Not directly; it only feeds in a straight line, but stepping works.
Why does my form tool chatter?
Too much engagement and speed; slow down, use oil and keep the tool on center.
Related guides and calculators
- How to Use a Radius Gage
- How to Check Radius Gages
- How to Set Tool Height on a Lathe
- How to Cut a Taper on a Lathe
- How to Face on a Lathe
- How to Cut Square Threads on a Lathe
- How to Use a Tool Post Grinder
- How to Cut a Radius with a Rotary Table
- How to Cut a Ball on a Lathe
- Filing Basics for Machinists
- How to Use a Radius Turning Attachment
- How to Use a Form Tool on a Lathe
- All free machinist calculators
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