Reference to Sk. 3 shows the basic drill point having a chisel angle of 130°. From
this, it can be seen that the chisel has to remove the centre of the hole and has
a quite different cutting action to the two main edges. Actually, the chisel can
hardly be called a cutting action it is rather more a case of scraping out the centre
of the hole, obviously much less efficient than the cutting edges. Because of this,
should the chisel length be increased as illustrate by Sk. 4, the increased angle,
and therefore the longer chisel, creates a larger centre section to be removed. A
result of this will be that more pressure is required to force the drill through
the material being drilled. An additional,though less critical, problem with this
is that the centre punch mark used to position the drill for drilling will have to
be larger to enable the chisel to locate into it.
What then effects the chisel angle and therefore its length. Photograph 5 showed
earlier how the clearance angle can vary, from this is should be obvious that a larger
clearance angle will have the effect of slewing round the chisel and increasing its
size. Photograph 6 shows the end view of the same two drills and clearly indicates
the effect on the chisel angle, that on the left having a greater chisel angle due
to too much clearance with the reverse being the case for that on the right. It has
been learnt therefore that the correct clearance is when the chisel angle is correct.
Having established the correct chisel angle requirements, and therefore the clearance,
there still remains the angle between the two cutting edges. For the vast majority
of the drills sold to the home workshop owner, or even in industry, the angle required
is 118°. This could be measured using an engineer's protractor of some form but as
it is not critical, within a few degrees, a simple visual check will suffice as follows.
There are three situations that can result through the angle ground on the drill
being varied. If at the correct angle (118° in almost all cases)the cutting edge
will be straight, but if greater (flatter)the edge will be convex and if less (more
pointed) then concave. The reason for these situations can be easily seen if a rule
is placed in a large drill's flute and moved from a flat point position to a very
It can be said therefore that if the cutting edge is straight and the chisel angle
correct then all other factors will automatically fall into place.
Returning to photograph 6 showing the end of two drills after sharpening, in this
it can be seen that the one on the left has a convex cutting edge. This, even though
it had been sharpened at the usual angle of 118°.