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How to Use a Ball End Mill

Guide · Updated October 28, 2026

A ball end mill has a full radius on its tip, used for 3D contours, fillets and radiused slots. Because it cuts on a curve, the diameter actually cutting is often much smaller than the tool diameter, which changes your speeds and your finish.

Check radiused features

Check radii and fillet sizes against gages and blocks; build stacks with the free Gage Block Calculator.

Step by step

Pick the size: Use the largest ball that fits the smallest concave radius.
Find effective diameter: At shallow depth, effective diameter = 2 × √(D × ap − ap²), where ap is the axial depth.
Adjust RPM: Raise RPM so surface speed at the effective diameter is right.
Set stepover: Choose stepover from the scallop height you can accept.
Tilt if possible: On 5-axis or with a tilted part, avoid cutting with the dead center of the tool.
Rough with other tools: Rough with flat or bull nose end mills, then finish with the ball.
Check finish: Check scallops and size.

Worked example

1/2 in ball, 0.020 in depth: effective diameter = 2 × √(0.5 × 0.02 − 0.0004) = 2 × √0.0096 ≈ 0.196 in. RPM should be about 2.5× higher than for the full 1/2 in diameter.

Scallop height

Scallop height h ≈ stepover² / (4 × D). With a 1/2 in ball and 0.020 in stepover, h ≈ 0.0004 / 2 = 0.0002 in. Halving stepover cuts scallops to about a quarter.

Chip thinning

At light depth the chip gets thin, so feed per tooth can go up. Start conservative and listen.

Tips

Base numbers on milling speeds and feeds.
Grade results with surface finish basics.
Check radii with radius gages.
Set tool length right: tool length offsets.

FAQ

What is effective diameter on a ball end mill?

The diameter actually cutting at a given depth, found with 2 times the square root of D times depth minus depth squared.

What stepover should I use with a ball end mill?

Pick stepover from scallop height, scallop is about stepover squared over 4 times diameter.

Why is the finish bad at the tip of a ball mill?

The center of the ball has near-zero surface speed; tilt the tool or the part to avoid cutting on center.

Can I slot with a ball end mill?

Yes, for a radiused slot, but take lighter cuts because chip evacuation is poor.

Ball end mill vs bull nose?

A bull nose has a corner radius and a flat bottom, which is better for roughing and flat floors.

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