Holemaking and tapping
Center drill sizes and depth chart
Combined drill and countersink, plain type, sizes #00 to #8, 60° countersink.
Technical review pending. Formulas and values are cited. This notice comes down after a machinist review.
| Size | Body, fraction (in) | Body, decimal (in) | Body (mm) | Pilot, fraction (in) | Pilot, decimal (in) | Pilot (mm) |
|---|---|---|---|---|---|---|
| #00 | 1/8 | 0.125 | 3.18 | none (decimal size) | 0.025 | 0.64 |
| #0 | 1/8 | 0.125 | 3.18 | 1/32 | 0.031 | 0.79 |
| #1 | 1/8 | 0.125 | 3.18 | 3/64 | 0.047 | 1.19 |
| #2 | 3/16 | 0.188 | 4.76 | 5/64 | 0.078 | 1.98 |
| #3 | 1/4 | 0.250 | 6.35 | 7/64 | 0.109 | 2.78 |
| #4 | 5/16 | 0.313 | 7.94 | 1/8 | 0.125 | 3.18 |
| #5 | 7/16 | 0.438 | 11.11 | 3/16 | 0.188 | 4.76 |
| #6 | 1/2 | 0.500 | 12.70 | 7/32 | 0.219 | 5.56 |
| #7 | 5/8 | 0.625 | 15.88 | 1/4 | 0.250 | 6.35 |
| #8 | 3/4 | 0.750 | 19.05 | 5/16 | 0.313 | 7.94 |
Decimal inch is rounded to 3 places, as drill sizes are written; the fraction is the nominal size, and the calculator uses it. Pilot length and overall length vary; measure the tool or check the maker's data.
Source: ANSI/ASME B94.11M (twist drills: combined drills and countersinks, plain type); Machinery's Handbook
How to read the table
The size number is stamped on the tool. The body is the shank you chuck; the pilot is the small drill on the end. The 60° countersink joins them, and its diameter at the part face is what you control with depth.
Center drill depth for a countersink diameter
Cone depth past the pilot
in
h = (C - d) / (2 x tan(30°)) = (0.1875 - 0.109375) / (2 x tan(30°)) = 0.0677 in
0.0677 in (1.72 mm). Add the pilot length for the tip depth below the face.
How the depth works
h = (C - d) / (2 x tan(30°))
| Symbol | Meaning | Unit |
|---|---|---|
| h | Axial length of the 60° cone, from the pilot diameter up to the countersink diameter | in or mm |
| C | Countersink diameter at the part face | in or mm |
| d | Pilot diameter | in or mm |
| 2 x tan(30°) | 1.1547; the inverse is 0.8660, so h = (C - d) x 0.8660 | none |
Source: Cone geometry for a 60° included angle; center drill sizes from ANSI/ASME B94.11M
Tip depth below the face = pilot length + h. The formula treats the countersink as a sharp cone meeting the pilot, which is how the tool is drawn.
Worked example: #3 center drill, 3/16 in countersink
A #3 center drill and a 3/16 in (0.1875 in) countersink diameter at the face.
- Read the pilot diameter from the table: 7/64 in, which is 0.109375 in (0.109 in in the table).
- Subtract: 0.1875 - 0.109375 = 0.078125 in.
- Divide by 2 x tan(30°) = 1.1547: 0.078125 / 1.1547 = 0.0677 in. The same as 0.078125 x 0.8660.
- Add the pilot length of your tool to get the tip depth below the face.
Result: 0.0677 in (1.72 mm) of cone past the pilot. A starting point; check the diameter on the first part.
Worked example: #2 center drill, 1/8 in countersink
A #2 center drill (pilot 5/64 in, 0.078125 in) and a 1/8 in (0.1250 in) countersink diameter.
- Subtract: 0.1250 - 0.078125 = 0.046875 in.
- Multiply by 0.8660: 0.046875 x 0.8660 = 0.0406 in.
Result: 0.0406 in (1.03 mm) of cone past the pilot.
Shop notes
- Shop rule of thumb, not part of the standard: keep the countersink diameter at or under about 2/3 of the body diameter. For a #3, 2/3 x 0.250 in = 0.167 in. Check the tool maker's chart.
- The 60° countersink is made to seat a 60° lathe center. The pilot hole gives the center point clearance.
- The pilot is the weak point, most of all on #00 to #1. On a light lathe or a drill press, feed by feel and back out to clear chips.
- On a rigid CNC machine, program the tip depth (pilot length + h) from the face and confirm the countersink diameter on the first part.
- Center drill makers do not all grind the same pilot length. Measure the tool before you set a depth stop.
FAQ
What angle is a center drill?
A plain type center drill (combined drill and countersink) has a 60° included countersink, so the hole seats a 60° lathe center.
What are the dimensions of a #3 center drill?
Body 1/4 in (0.250 in, 6.35 mm) and pilot 7/64 in (0.109 in, 2.78 mm), per ANSI/ASME B94.11M.
How deep do you feed a #3 center drill for a 3/16 in countersink?
The cone takes 0.0677 in past the start of the countersink: (0.1875 - 0.109375) x 0.8660 = 0.0677 in. Add the pilot length of your tool to get the tip depth below the face.
How big should the countersink on a center hole be?
Shop rule of thumb, not part of the standard: keep the countersink diameter at or under about 2/3 of the body diameter, so the cone stays off the full body. For a #3 that is up to about 0.167 in. Check the tool maker's chart for the size you run.
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