A method of sharpening standard jobber drills that has frequently been featured in
magazines is the four facet method. As the name implies it is a drill point system
that has four faces, two on each cutting edge.
This would appear to be the equivalent of an end mill with one narrow face at a shallow
angle to produce the cutting edge and a steeper face behind it just to provide extra
clearance. This is not so, the steeper angle is an important part of the drills cutting
ability as I will explain.
Either facet can be ground first but I will go with producing the cutting edge first.
The primary angle should be in the order of 10°, but no more, say 8° to 10°, and
ground on each cutting edge as a flat surface. That is, there is no increasing clearance
as the face moves away from the cutting edge as on the more normal drill. The drill
will still have a chisel point but this will be less than 130°.
The secondary clearance face will have the effect of slewing round the chisel in
the same way as the increasing clearance did with a drill sharpened normally. However,
the amount that the chisel rotates will be dependant on the angle of the secondary
face, this angle cannot therefore be chosen arbitrarily. For a chisel angle of about
130° a secondary clearance of 25° should be suitable, but in this case, no less,
say 25° to 30°.
Unfortunately, grinding the secondary clearance is rather more complex than just
grinding on a clearance of 25°. With too little ground off there will still be some
of the original chisel left resulting in it having a jagged formation, see Sk. 7
If on the other hand, too much is ground away, then the line between the two secondary
clearances will pass each other and the drill will end up with a forked chisel as
seen in Sk. 8. What is required, ideally, is that the two lines meet in the middle
as Sk. 9 shows and illustrating that the result is for the chisel to be converted
into a shallow point. This all adds up to the whole process being a critical one.
As a concession to accuracy, it is normal for the two lines to stop just short of
lining up leaving just a small portion of the initial chisel, say 0.1 to 0.2mm long,
a magnifying glass will help.