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How to Use a Boring Head

Guide · Updated October 28, 2026

A boring head holds a single-point bar on an adjustable slide so a mill can cut a round, accurate hole of any size. Most dials read on diameter, but check yours before you trust it.

Measure the bore

Check the bore with a telescoping or dial bore gage set against gage blocks. The free Gage Block Calculator builds the setting stack.

Step by step

Mount the head: Clean the shank and spindle taper and tighten the drawbar.
Pick the bar: Use the shortest, largest bar that fits the hole and clears the depth.
Align the tool: Point the cutting edge in line with the slide direction and lock it.
Set speed: Run about 60–70% of the turning speed; the head is unbalanced.
First pass: Bore a test cut, measure and note the difference.
Dial in: Unlock, dial out the needed amount and lock the slide.
Finish: Take two equal light passes at the same setting for a spring pass.

Worked example

A drilled 0.969 in hole must become 1.000 in. Rough to about 0.990 in, measure with a bore gage: 0.9895 in. Your dial reads on diameter, so dial 0.0105 in, take a pass, measure 0.9995 in, dial 0.0005 in and take a final pass and a spring pass at the same setting: 1.0001 in.

Taper and oversize

Taper usually comes from bar deflection; use a stiffer bar, lighter cuts and a spring pass. Oversize holes often come from a slide that was not locked, or misreading a radius dial as diameter.

Speeds and feeds

For a 1 in hole in mild steel with carbide, about 400–600 RPM and 0.002–0.004 in/rev works as a start. Keep RPM lower on large heads that vibrate.

Tips

On the mill: boring on a mill.
Gaging: dial bore gages.
Centering: indicating a bore true.

Quick shop check

Example: a bore measures 0.990 in and needs 1.000 in. On a boring head whose dial reads diameter, dial 0.010 in; on one reading radius, dial 0.005 in. Take a spring pass after the final cut to check for tool push-off, which can be 0.0005 in or more with a long boring bar.

FAQ

Does a boring head dial read radius or diameter?

Most read diameter, but some read radius; check the markings or test it.

What RPM for a boring head?

About 60–70% of normal turning speed, lower for large heads.

Why is my bore tapered?

Bar deflection; use a stiffer bar and a spring pass.

How small a hole can a boring head do?

Depends on the smallest bar, often around 1/4 in.

Do I need to lock the slide?

Yes, always lock it before cutting.

Related guides and calculators

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General shop information only — follow your machine and tooling manufacturer's instructions and your shop's safety rules.