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How to Spotface a Hole

Guide · Updated October 28, 2026

A spotface is a shallow, flat seat cut around a hole so a bolt head, nut or washer sits square on a rough or sloped surface. It is like a very shallow counterbore, cut only deep enough to clean up.

Check the seat

Check the seat depth and flatness with a depth mic verified against gage blocks. Check it with a mic you've verified against gage blocks; the free Gage Block Calculator builds the stack.

Step by step

Read the callout: Get the spotface diameter and depth from the drawing; it is often written as SF or with a counterbore symbol and depth.
Pick the tool: Use a piloted counterbore matching the hole, or a flat end mill on the hole center.
Locate: Center the spindle on the hole using the DRO or a pin in the hole.
Set RPM: Run slower than drilling, about 1/2 to 2/3 of drilling speed for the tool diameter.
Feed in: Feed in gently until the seat cleans up all the way round.
Dwell: Pause briefly at depth so the bottom is flat and smooth.
Check: Check the seat is fully cleaned up and deburr the hole edge.

Worked example

A 3/8 in bolt with a SAE flat washer (about 0.73 in OD) on a rough cast boss: use a 7/8 in end mill. In cast iron with HSS at about 60 SFM: RPM = 60 × 3.82 / 0.875 ≈ 260. Feed in until the seat cleans up, about 0.030 in deep here, and dwell for a turn or two.

Spotface vs counterbore

A counterbore is deep enough to sink a cap screw head below the surface. A spotface only makes a flat seat; if no depth is given, cut just deep enough to clean up.

Back spotfacing

When the seat is on the far side of a part, a back spotface tool passes through the hole and is pulled back into the face. Run slowly and stop the spindle before passing the tool back out.

Tips

Counterbores: countersink and counterbore basics.
Drill the hole first: drilling on a mill.
Measure depth: using a depth micrometer.
Speeds: milling speeds and feeds.

FAQ

What is a spotface?

A shallow flat seat around a hole for a bolt head, nut or washer.

How deep is a spotface?

Usually just deep enough to clean up, unless the drawing gives a depth.

What size spotface for a bolt?

Slightly larger than the washer or head diameter; follow the drawing.

Can I spotface with an end mill?

Yes, a flat end mill centered on the hole works well.

What is back spotfacing?

Cutting a seat on the far side of a part by pulling the tool back through the hole.

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