"Robin S." wrote in
news

Do you find you have issues with drills being pulled into the work
when double drilling? Working in a tool shop, we have no need for
holes within a thou or two (either rough drilled or reamed only) so I
don't have to do it very often at all. When I do, I find the drill
frequently pulls into the hole making for a broken drill bit, spun
part, or both. We have gear feeds on our radials at work so it's not
necessarily an issue for us, but HSM's rarely have that luxury.

IMHO and experience, you only want the undersized hole to be about 1/2 or
less of the dia of the finish drill. What is important for the hole size
is that the pilot hole is just slightly larger than the web wedge on the
following drill.
The wedge is where you get in trouble with drilling. Since the wedge
displaces metal instead of cutting it, any variations in the
microstructure of the drilled material (clumps of alloys or other
materials) will pull the drill off-center. Yes, I said pull, since
physics plays a role here, the drill (or any cutting tool, for that
matter) is going to 'walk' along the path of least resistance.
When you center drill, you are not only centering the drill with the
cone, you are relieving the area under the wedge, so the drill will not
walk.

--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
better idiots.

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