Civil Engineering Reference
In-Depth Information
Fig. 11.4. A set of centre drills. Longer centre drills are available.
Most centre drills are double ended. You can get long series centre drills,
and I find if I cut these in half they are the ideal length for use in a chuck.
CNC centre drills
The second type of centre drill is called a CNC centre drill, so called because
they are most often used on CNC mills. Rather than a pilot, these drills have
a 90 degree inclusive angle so they can be used to centre to their full diamet-
er. A CNC centre drill 10mm diameter can produce a centre spot up to 10mm
diameter. This is very useful as, say, you want an 8.2mm diameter centre drill
point to countersink a hole for an M8 screw, you just centre drill 4.1mm deep
(half the centre diameter) and you get the correct size countersunk hole for
an M8 thread. This diameter may vary slightly between makes of CNC centre
drills, but 4.1mm would be a good start. Other sizes of countersink can be
made by varying the depth of cut.
