← AskMachinist · Guides · Calculator · 🔎 Search

How to Use a Boring Bar

Guide · Updated October 28, 2026

A boring bar opens up an existing hole to an exact size and makes it round and straight, which a drill can't do. The same ideas apply on a lathe and in an offset boring head on a mill: keep the bar rigid, cut light, and measure often.

Measure the bore with confidence

Set your bore gage or telescoping gage from a ring or a gage block stack built with the free Gage Block Calculator.

Step by step

Drill a starting hole: Drill or rough the hole to leave about 0.020–0.060 in on diameter for boring.
Choose the biggest bar: Pick the largest bar that fits the hole with chip clearance, and the shortest stickout that reaches full depth.
Set center height: Set the cutting edge on center (or a hair above on small bores) so the heel doesn't rub.
Set depth stops: Mark or set the carriage stop for depth so you don't hit the bottom or a shoulder.
Rough passes: Take moderate cuts at a feed that makes a curled chip and keeps the bar loaded.
Measure: Back out, stop the spindle, and measure with a telescoping gage, small hole gage or dial bore gage.
Finish pass: Take a light finish pass, then a spring pass at the same setting to take out bar deflection.

Stickout rules of thumb

Steel bars chatter beyond about 4× diameter of overhang; heavy metal and carbide bars handle 6–8×, and damped bars more. Halving the stickout makes a bar about eight times stiffer.

Boring on a mill

In an offset boring head, the dial moves the tool on radius on some heads and on diameter on others. Check yours by taking a cut and measuring. Lock the gib screw after every adjustment, and run slower than you would for an end mill of the same size.

Worked example

Target 1.000 in bore. After roughing it measures 0.982 in. You need 0.018 in on diameter, 0.009 in per side. Take 0.007 per side, measure (0.996), then 0.002 per side and a spring pass, and check again with a dial bore gage.

Tips

Chatter? Cut stickout, slow the RPM, raise the feed and use a sharper insert with a small nose radius.
Indicate the bore first if it must be concentric; see indicating a bore.
Use air or coolant to clear chips in blind holes; see cutting fluid basics.
Check final roundness and taper at two depths.

FAQ

How far can a boring bar stick out?

As a rule of thumb, about 4 times bar diameter for steel, 6 to 8 times for carbide or heavy metal, and more with damped bars.

Why does my boring bar chatter?

Usually too much stickout, too much RPM, too light a feed, a large nose radius, or a tool off center.

Should a boring bar be on center?

Yes, on center or very slightly above for small bores so the heel of the tool clears the wall.

What is a spring pass?

A repeat pass at the same setting that removes material left by tool deflection, improving size and finish.

How do I measure a bored hole?

Use a telescoping gage with a micrometer, small hole gages for small bores, or a dial bore gage set from a ring.

Related guides and calculators

Build your stack in seconds

The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.

Questions? Ask a machinist.

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