On 2024-05-11 03:32, andy pugh wrote:
That's it. really. Why? A large-diameter one with a small drill point
could
make all the sizes. as far as I am aware the drilled hole is
irrelevant. I
suppose it might have mattered as a reservoir for the white lead in the
days of solid centres.
Interesting question!
The pilot (parallel) drill creates a hole for the very tip of the mating
centre. If the drill had to cut the exact shape of the conical tip of
the female hole, it would struggle to have proper cutting edges and
still make the true cone.
You are right that the conical end on the body of a large centre drill
(ignoring the parallel tip) could cut all sizes of cone. But there might
be some reasons for the different sizes:
1. very small centres would enter the parallel drilled hole, but miss
the conical part. I have just been trying to clean some small center
holes in a clockmaking tool designed to support the ends of small
diameter clock shaft pivots which run between two female centres. Those
pivots would completely miss the conical part of the female hole which
had anything but the smallest parallel section at the end.
2. for male centres which will take a decent load, the end needs to bear
inside a large (deep) female hole, so a large drill can be used here,
especially when pushing drilling feeds and speeds. The flip-side of
that is that allowing the body of the centre drill to set the final
diameter provides a small amount of parallel recess at the outer end of
the centre, which will foul a centre and hold it off the female cone.
Centre drills do exist to create 'protected' centres with a larger
parallel recess just at the entry to the female cone. Those drills have
a short stepped-out section of cutting edge of larger diameter than the
largest end of the female cone.
3. In pre-CNC days, the best way to set the size of a hole would be to
have a drill of the correct diameter(s) mounted in a turret with stops.
The operator then would not need to think, but could just pull the
lever. Aside from the problem in (1), you could, of course, set the stop
to make a large drill create a small diameter centre. Which makes CNC
an obvious advantage, of course.
Marcus
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