Why does a center drill make a 60-degree hole?

Standard center drills are ground with a 60-degree included countersink angle to match the 60-degree point on lathe live and dead centers. The matching angle gives full conical contact, which centers the work and spreads thrust load evenly.

How deep should I plunge a center drill?

Plunge until the countersunk cone reaches the seat diameter you need, but stop before the wide rim of the countersink reaches the surface. The tool computes that geometric depth so the bell mouth is never cut past the body diameter.

What is the seat diameter?

The seat diameter is the diameter of the 60-degree cone at the part surface. It should be slightly larger than the small flat on your center's point so the center seats on the cone, not on the bottom of the pilot hole.

What happens if I plunge too deep?

If you plunge past the point where the countersink rim reaches the body diameter, the thin neck behind the pilot tip can snap, and the bell mouth grows oversize. The tool flags the maximum safe plunge for the size you selected.

Do these numbers match Bell-type center drills too?

The plunge geometry is the same, but bell (radius) center drills add a protective 120-degree chamfer above the 60-degree cone. Use the plain combined-drill dimensions here for the 60-degree seat, then add the bell pass separately.

Center Drill Depth Calculator

A center drill (combined drill and countersink) cuts a small pilot hole and a 60-degree cone in one tool. Plunge it too shallow and your live center bottoms out on the pilot; plunge too deep and the countersink rim breaks the body diameter and snaps the fragile neck. This calculator works out the exact depth to reach a chosen seat diameter for any standard #1 to #8 center drill.

How it works

A center drill has two diameters: the small pilot (body) drill diameter d, and the larger countersink (body) diameter D. The 60-degree countersink starts where the pilot ends. Because the cone has a 60-degree included angle, its half-angle is 30 degrees, so the radius grows with depth at the rate tan(30 deg).

half angle      = 30 deg  (60 deg included)
seat radius      = seat diameter / 2
pilot radius     = d / 2
countersink depth (below where cone reaches pilot dia.)
                 = (seat radius - pilot radius) / tan(30 deg)
total plunge     = pilot drill depth + countersink depth
max seat radius  = D / 2   (rim reaches body diameter)

The maximum safe plunge is the depth at which the cone reaches the full countersink diameter D. Going beyond that cuts the bell mouth oversize and weakens the pilot neck.

Example and notes

A #3 center drill has a 1/8 in (0.125 in) body drill and a 1/4 in (0.250 in) countersink diameter. To reach a 0.180 in seat diameter, the countersink must grow from the 0.125 in pilot to 0.180 in, a radial increase of 0.0275 in, which at tan(30 deg) needs about 0.0476 in of countersink depth. Keep the seat diameter between the pilot and body diameters so the cone is well-formed but the rim never breaks through. Always run a center drill at high RPM with cutting oil and very light, deliberate feed.