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How to Counterbore on a Mill
Guide · Updated October 28, 2026
A counterbore is a flat-bottomed recess that lets a socket head cap screw sit flush or below the surface. On a mill you can use a piloted counterbore in the drilled hole, or an end mill on position.
Check the depth
Measure counterbore depth with a depth mic you have checked against gage blocks. The free Gage Block Calculator builds the stack.
Step by step
- Size the counterbore: Look up the head diameter and height for the screw; counterbore slightly larger and deeper.
- Drill the clearance hole: Spot, then drill the clearance hole on position.
- Pick the tool: Use a piloted counterbore matching the hole, or an end mill the counterbore diameter.
- Set speed: Run about half to two-thirds of drilling speed for a counterbore of that diameter.
- Touch off: Bring the tool to the surface and zero the quill or Z.
- Cut to depth: Feed steadily to depth using the quill stop or DRO, with oil.
- Dwell and check: Dwell a moment for a flat bottom, retract, deburr and test with the screw.
Common sizes (inch)
Typical socket head cap screw counterbores (normal fit): #10: 0.201 in clearance drill, 0.406 in counterbore (13/32), 0.200 in deep. 1/4 in: 0.266 in drill (H), 0.438 in counterbore, 0.260 in deep. 5/16 in: 0.332 in drill (Q), 0.531 in counterbore, 0.323 in deep. 3/8 in: 0.397 in drill (X), 0.625 in counterbore, 0.385 in deep. Add a few thousandths of depth for flush-below. Check your own standard or drawing.
Worked example
A 1/4-20 SHCS flush in steel: drill 0.266 in through. Use a 7/16 in piloted counterbore with a 0.266 in pilot. A 7/16 in tool at about 60 SFM in mild steel runs near 3.82 × 60 ÷ 0.4375 ≈ 520 RPM. Zero on the top and feed to 0.265 in deep, then test the screw sits just below the surface.
Chatter and rough bottoms
Chatter comes from too much speed, too little feed or a loose quill. Lock the head, slow down and feed firmly. With an end mill instead of a piloted tool, plunge with a center-cutting end mill or helical interpolate on CNC.
Tips
- Basics: countersink and counterbore basics.
- Drilling: drilling on a mill.
- Seats: spotfacing.
FAQ
What size counterbore for a 1/4 SHCS?
About 0.438 in diameter and 0.260 in deep, with a 0.266 in clearance hole.
Can I counterbore with an end mill?
Yes, with a center-cutting end mill on position, after drilling the hole.
What speed for counterboring?
About half to two-thirds of the drilling speed for that diameter.
Counterbore vs spotface?
A spotface is shallow and only cleans a seat; a counterbore sinks the screw head.
Why is my counterbore oversize?
The pilot is loose in the hole or the tool is out of round; use the right pilot.
Related guides and calculators
- Countersink and Counterbore Basics
- How to Spotface a Hole
- How to Drill on a Mill
- How to Center Drill and Spot Drill
- How to Use a Depth Micrometer
- Drilling Speeds and Feeds
- How to Use a Keyway Broach
- How to Use Parallels in a Vise
- All free machinist calculators
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