Pick a drill press speed for a Forstner bit. Enter the bit diameter and a surface speed (or use the hardwood and softwood presets) and the calculator returns a starting RPM you can round down to the nearest belt or dial setting.
Forstner bits cut with a rim and two chisel-like lips, so most of the cutting happens at the outside edge. The bigger the bit, the faster that rim travels at any given spindle speed, and the faster it heats up. Run a 2 in bit at the speed you would use for a ¼ in brad point and you get smoke, a blued rim and a dull bit after a few holes.
The calculator holds the rim speed constant and works out the spindle speed for each diameter. The presets are deliberately conservative starting points: about 350 surface feet per minute in hardwood and 500 in softwood.
| Bit | Hardwood (350 SFM) | Softwood (500 SFM) |
|---|---|---|
| ¼ in | 5,348 | 7,640 |
| ⅜ in | 3,565 | 5,093 |
| ½ in | 2,674 | 3,820 |
| ¾ in | 1,783 | 2,547 |
| 1 in | 1,337 | 1,910 |
| 1¼ in | 1,070 | 1,528 |
| 1½ in | 891 | 1,273 |
| 2 in | 669 | 955 |
| 2½ in | 535 | 764 |
| 3 in | 446 | 637 |
Values are rounded to the nearest RPM. On a step-pulley drill press, choose the nearest speed below the number.
RPM = (SFM × 3.82) ÷ diameter in inches. The 3.82 is 12 ÷ π, which converts surface feet per minute at the rim to revolutions per minute. It is the same identity machinists use for twist drills and end mills; only the surface speed changes with the material.
RPM = SFM × 3.82 ÷ D
35 mm hinge cups in maple. 35 mm is 1.378 in. At 350 SFM: 350 × 3.82 ÷ 1.378 = 970.2467 RPM. Use the nearest drill press step at or below that figure; a 1,300 RPM step would be on the fast side.
A 2 in hole in oak for a lamp base. 350 × 3.82 ÷ 2 = 668.5 RPM. Many drill presses bottom out between about 250 and 600 RPM, so take the lowest step that is at or under the result.
A 1 in counterbore in pine. Choose the softwood preset: 500 × 3.82 ÷ 1 = 1910 RPM. Softwood tolerates more speed, but resin can still glaze the rim if you dwell.
Surface speed is a starting point, not a specification. Saw-tooth Forstner bits, carbide-tipped bits and cheap bits with poor heat treatment all behave differently, and the bit maker’s chart wins when it exists. In very dense or resinous species, start lower. In MDF, the glue in the board dulls edges quickly, so lower speeds and frequent chip clearing pay off.
Small ones, yes, with care. Above about 1 in the bit tends to wander without a guide and can wrench the drill. Use a drill press or a guide block.
Small bits cut cleanly at higher speeds; at very low speed they tend to tear fibres and wander. Matching speed to diameter gives cleaner holes and longer bit life.
Divide millimetres by 25.4 and enter the result. A 25 mm bit is 0.984 in.
Yes, if your bit maker publishes a recommended speed for a given diameter. Enter it once to back out their SFM, then reuse it for other sizes.