Twist Drill

A commonly used metal-cutting drill, usually made with two flutes running around the body. They have helical grooves for the passage of chips and for admitting coolant to the cutting edges. Twist drills are used either for originating holes or for enlarging existing holes.

Twist drill with cutting point to the right and shank to the left.

Drilling Speeds

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
6.56001100153023003650
11.035061087013002100
12.531054075011501850
16.02504306109101450
25.5150270380570900

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


Lathes 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.

Drill Point Angle

The standard drill point has an included angle of 118°. This has been found to be the best for general use. However, the following table gives optimum values for the different groups of materials:

MaterialPlasticsCast Iron (soft)Cast Iron (close grain)Aluminium / CopperHigh Tensile Steel
Point angle90°90°118°100°130°
HelixStandardStandardStandardQuickSlow


See also: Drill Bits.

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Subjects: Tools