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Why drill speed matters
Every drill bit has a speed where it cuts cleanly, and speeds above or below it cause trouble. Spin too fast and the bit overheats: the cutting edge dulls, wood scorches, and steel turns blue and loses its temper. Spin too slowly and the bit rubs instead of cutting, which also builds heat and leaves a rough, torn hole. The right speed is not a guess. It comes from two facts you already know: how wide the bit is, and what material you are drilling. This calculator turns those two facts into a revolutions-per-minute figure you can set on your drill press or match on a variable-speed hand drill.
The maths behind it is simple. A cutting edge travels around the bit once per revolution, so the distance it covers per minute is the bit’s circumference times its RPM. Materials have a safe cutting speed: the fastest that edge should travel. Softwood allows about 100 metres per minute, hardwood about 70, mild steel only about 25, and stainless steel just 15. Divide the safe cutting speed by the circumference and you get the RPM. That is why a 10 mm bit in mild steel runs at about 796 RPM while the same bit in softwood runs at about 3,183 RPM. The material, not the drill, sets the limit.
How to use the calculator
- Measure your bit diameter in millimetres. It is printed on the shank of most bits.
- Pick the material from the list. If your material sits between two entries, choose the slower one.
- Read the recommended RPM and the rough feed rate, which is how fast to push the bit forward.
- Set your drill to the nearest speed at or below the figure. On a drill without exact settings, choose the slower pulley or dial step.
A worked example
Suppose you need a 10 mm hole in mild steel. The safe cutting speed for mild steel is 25 metres per minute, which is 25,000 millimetres per minute. The circumference of a 10 mm bit is about 31.4 millimetres. Divide 25,000 by 31.4 and you get about 796 RPM. Now try the same bit in softwood at 100 metres per minute: 100,000 divided by 31.4 gives about 3,183 RPM. The fourfold difference surprises beginners, but it is real, and it explains why steel drilling feels slow. A 5 mm bit in mild steel doubles the steel figure to about 1,592 RPM, because halving the diameter halves the circumference. Small bits spin fast; large bits spin slow. That single rule prevents most drilling mistakes.
Feed rate matters almost as much as speed. Pushing too hard snaps small bits and stalls the drill. Pushing too gently lets the bit rub and overheat. The calculator’s rough feed figure gives a starting point: advance steadily, watch the chips, and back the bit out often to clear them, especially in deep holes. Long, curling chips mean the cut is healthy. Dust, smoke or squealing mean something is wrong: slow down, add cutting fluid for metal, or sharpen the bit.
Tips for clean holes
- Centre-punch metal before drilling so the bit does not wander across the surface.
- Clamp the workpiece. A spinning piece of wood or steel is dangerous, and it ruins the hole.
- Use cutting fluid on steel and stainless steel. It cools the edge and doubles bit life.
- Step up in stages for large holes in metal: a small pilot hole first, then the full size.
- Back a wood board with scrap timber to stop splintering where the bit exits.
Common mistakes
The most common mistake is drilling everything at full speed. A hand drill at maximum RPM feels powerful, but in steel it destroys bits in seconds. The second mistake is the opposite: crawling through softwood at a few hundred RPM, which tears fibres and leaves a ragged hole. The third mistake is ignoring heat. If a bit is too hot to touch, it is already damaged. Let it cool, slow down, and check whether it still cuts or just rubs. A dull bit is not fixed by more pressure; it is fixed by sharpening or replacement. Finally, remember that this tool is an educational estimator, not professional advice. It gives safe starting speeds for ordinary workshop jobs, not for production machining, hardened steel, or structural work, where a machinist’s tables and a professional’s judgement govern.
Who uses drill speed figures
Woodworkers use them when boring dowel holes and hinge cups, metalworkers use them on the drill press every day, and DIY builders use them whenever a project mixes wood and steel fixings. Keep this calculator open beside the drill press and the numbers stop being a mystery. Measure the bit, name the material, set the speed, and the holes come out cleaner, the bits last longer, and the work goes faster in the end, because you spend less time fighting burnt, wandering holes.
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