← AskMachinist · Guides · Calculator · 🔎 Search

How to Bore on a Mill

Guide · Updated October 28, 2026

Boring on a mill opens a drilled hole to an exact size and position with an offset boring head. Drilling gets you close; the boring head is what puts the hole on size, round and where the print says.

Hit the bore size

Measure with a bore gage or inside mic set against a ring or gage blocks. Check it with a mic you've verified against gage blocks; the free Gage Block Calculator builds the stack.

Step by step

Locate the hole: Pick up the part edges with an edge finder and move to the hole position; lock the axes you are not using.
Drill and open up: Spot, drill and step drill to within about 0.030–0.060 in of size so the bar is not roughing.
Pick the bar: Use the largest, shortest bar that fits the hole and reaches the depth.
Set the head: Set the cutting edge on center height in line with the slide, then lock the bar.
Know the dial: Check whether your head reads on diameter or on radius; many read 0.001 in of diameter per graduation, but not all do.
Take passes: Bore at a steady quill or knee feed, return with the spindle stopped or bar backed off, then measure.
Finish: Take a spring pass at the final setting and measure in two directions and at the top and bottom.

Worked example

1.250 in bore in 1018 steel from a 1.188 in drilled hole, carbide bar: RPM ≈ 300 SFM × 3.82 / 1.25 ≈ 920; with a heavy head run about 600. At 0.002 in/rev the quill feed is about 1.2 in/min. Leave 0.010 in on diameter for the finish pass: if the hole reads 1.2400, dial 0.0100 in on diameter (10 graduations on a diameter-reading head, 5 on a radius-reading one).

Taper or bell mouth

A hole bigger at the top is usually bar deflection or a dull tool. Take a spring pass, reduce stickout, and check the quill is locked out of play. A tapered hole can also come from a head out of tram.

Chatter

Shorten the bar, drop the RPM, increase feed slightly, and use a sharp insert with a small nose radius. Balance matters on big offsets.

Tips

Tram the head first: tramming a mill.
For an existing hole, indicate the bore true before you cut.
Measure with a dial bore gage set on a ring or gage blocks.
Use milling speeds and feeds as a starting point.

FAQ

Does a boring head dial read diameter or radius?

It depends on the head; many read diameter, so check the manual or test-cut and measure.

How much should I leave for boring?

About 0.030 to 0.060 in on diameter after drilling, with 0.005 to 0.010 in for the finish pass.

Why is my bored hole tapered?

Bar deflection, a dull tool, a loose quill or a head out of tram.

What RPM for a boring head?

Base it on the finished diameter, then slow down for big offsets or an unbalanced head.

Can I bore with the quill?

Yes for shallow holes; lock the knee and use the quill feed for a steady finish.

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.