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.
Standard center drill sizes — quick reference
Combined drill and countersink sizes follow an ANSI standard. Each number corresponds to fixed body and countersink diameters:
| Size | Body drill (in) | Countersink diameter (in) | Typical use |
|---|---|---|---|
| #1 | 3/64 (0.047) | 1/8 (0.125) | Small precision work, watchmaking |
| #2 | 5/64 (0.078) | 3/16 (0.188) | Light lathe work |
| #3 | 1/8 (0.125) | 1/4 (0.250) | General-purpose, most common |
| #4 | 3/16 (0.188) | 5/16 (0.313) | Medium shafts |
| #5 | 1/4 (0.250) | 7/16 (0.438) | Larger shafts and cylinder work |
| #6 | 5/16 (0.313) | 9/16 (0.563) | Heavy turning |
| #7 | 3/8 (0.375) | 5/8 (0.625) | Large shafts |
| #8 | 1/2 (0.500) | 7/8 (0.875) | Heavy industrial work |
Size #3 is the most commonly stocked size in general machine shops because it suits the majority of lathe and mill centering operations.
Practical machining tips
Run fast, feed light. Center drills have a narrow neck that snaps under side load or heavy feed. High spindle speed with a very light, deliberate feed is the correct technique — the cutting forces should be barely perceptible.
Use a spot drill for milling. On a milling machine, a 90° or 120° spot drill is often preferable to a center drill for starting holes, since it does not have the fragile pilot. Reserve center drills for lathe work where the 60° cone is actually needed.
Check the seat, not just the depth. After drilling, check that a dead center or live center sits on the cone, not bottoming out on the pilot flat. The correct seat depth is when the center touches the cone all the way around with no contact on the bottom of the pilot hole.