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.
|Drill diameter mm||Hard cast iron||Cast tool steel, Stainless steel, High tensile steel, un-hardened Silver steel||Bronze, Cast iron, German metal, Gunmetal, Phosphor Bronze||Brass, Freecutting steel, Plastics (speed x 2)||Aluminium, Tufnol|
|0.75||4900||maximum speed||maximum speed||maximum speed||maximum speed|
|1.5||2455||4300||maximum speed||maximum speed||maximum speed|
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:
|Material||Plastics||Cast Iron (soft)||Cast Iron (close grain)||Aluminium / Copper||High Tensile Steel|
See also: Drill Bits.