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How to Use a Radius Turning Attachment

Guide · Updated October 28, 2026

A radius turning attachment replaces the compound or tool post with a pivoting tool holder. Swing the handle and the tool travels in a true arc, so balls, convex and concave radii come out round without stepping and filing.

Check the ball diameter

Mic the ball in several directions, proved on gage blocks. The free Gage Block Calculator builds the stack.

Step by step

Mount the attachment: Fit it to the cross slide with the pivot axis vertical.
Set tool height: Tool tip on center.
Locate the pivot: Move the carriage and cross slide so the pivot axis is directly under the radius center on the work.
Set the radius: Adjust the tool out from the pivot by the radius; behind the pivot cuts convex, in front cuts concave.
Lock the carriage: Lock it so the pivot does not move.
Swing light cuts: Swing the handle through the arc, feeding the tool in a little each pass.
Measure: Check diameter in several directions.

Worked example

Cut a 1.000 in ball on 1.125 in stock. Leave a neck; neck diameter 0.375 in. Center the pivot on the ball center along Z. The tool starts at a radius of 0.5625 in (stock radius) and is fed in 0.010 in per swing until the radius is 0.500 in: (0.5625 − 0.500) ÷ 0.010 ≈ 6–7 passes. The arc you can swing to before hitting the neck is limited where the ball diameter equals the neck: half-angle = sin⁻¹(0.1875 ÷ 0.500) = 22°, so the ball wraps to within 22° of the axis on the neck side.

Convex vs concave

With the tool behind the pivot (toward the operator) it cuts a convex radius. Set it in front of the pivot for concave radii, such as a cup in the end of a part.

Common problems

An egg shape means the pivot is not under the ball center. Flat spots mean the tool is off center height. Chatter means too much tool overhang on the attachment.

Tips

By hand: cut a ball on a lathe by stepping.
Radius: cut a radius on a lathe.
Check: radius gage.

FAQ

What does a radius turning attachment do?

It pivots the tool in a true arc to cut balls and radii.

Where do I set the pivot?

Directly under the center of the radius.

Convex or concave?

Tool behind the pivot for convex, in front for concave.

Why is my ball egg-shaped?

The pivot is not under the ball center.

How do I check the ball?

Mic it in several directions.

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