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Drill Bits

Normally made of hardened or high speed steel with two cutting edges on the conical end with two helical flutes that act to move material away from the cutting edges.

As well as metric and imperial drill sizes Number and Letter sizes have been used in the past.

Drill diameter mmHard cast ironCast tool steel, Stainless steel, High tensile steel, un-hardened Silver steelBronze, Cast iron, German metal, Gunmetal, Phosphor BronzeBrass, Freecutting steel, Plastics (speed x 2)Aluminium, Tufnol
0.754900maximum speedmaximum speedmaximum speedmaximum speed
1.524554300maximum speedmaximum speedmaximum speed
3.01220214031004500maximum speed

Drilling speeds in rpm for a range of drill diameters and material types.

Care should be taken when drilling plastics as they may tend to melt, in this situation the speed should be reduced. Clearing the flutes on a regular basis may also help reduce the problem of chippings welding to the item being drill or the item over-heating.

When drilling brass it helps to reduce the rake on the cutting edge of the drill to zero. Otherwise the drill may have a tendency to pull itself through the item.


If drilling in the lathe reduce the speeds in the above table by a quarter and take care to clear the flutes. Removing the drill from the work piece on a regular basis and applying cutting fluid to the drill bit will improve the penetration of cutting fluid to the cutting tip.

See also: Lathes.