Drilling holes (was Re: Dumb question on the drilling of holes

> >Drill bits must be ground off axis or
> >they will burn themselves up quickly. Most grinders do not have the
> >capability for perfect symmetry, and most people don't have the
> >patience to make that happen anyway.
>
> You did real well down to here .. it is the aim of all grinders to make the
> flutes equal, to prevent the drill cutting ovesize. That is also part and
> parcel of all instructional materials on drill pointing. The reason they
> don't burn up is the reduced land behind the edge.
>
>
> Regards, Hoyt McKagen

<snip>

Hi Hoyt,

Drills MUST drill an oversize hole or they would be press fit and the
forces to spin the thing would rise well above the machine hp or drill
shank material strenght.

It is desirable to approach perfect symetry, but tools like Drill
doctor cannot acheive it with production tolerances of the Drill Doctor
assembly line.

If you use any manual drill sharpening devices, the weight of your hand
will allow a few tenthousandths to be ground off one flute or the
other. If you take a very high end grinder, the potential is there,
but in practice the results are typically not there.

I found out about this little know phenonemon when I made perfectly
symetrical drill points and the shop I was grinding for burnt up a lot
of drills. it is a life lesson when a guy hands you a 2.5" cobalt
drill with the edge burnt up along the first 2" and your shop just
spent a few hundred dollars for it.

I was the only guy who would sharpen till the drill was sharp, then
keep the drill going until it just barly sparked on the wheel, then did
the other flute until the sparked with just a few on either side. you
easily get a shower if you rested your hand on the drill, so it was
only with light finger pressure and lots of patience that one could
make it symetrical. And then only because the drill shparerner had the
capability.

Like I said, grind a drill in your surface grinder then put in the
relief, then drill. IF your grinder was better than a tenth your drill
with self destruct. But even high quality collets will have some run-
out. once our machines approach the zero error we will have to add the
offset much like we do with backlash. I imagine there were years when
backlash wasn't even considdered a problem in machines. Of course,
that was before DRO's. Since so many people view drills as
utilitarian and the drill manufactures don't release specifics of
manufacture, the problem of exact symetry is not often found in shops.

In additon, some people are under the false impression that the drill
actually tapers from end to end and that as the drill cuts, the drill
body tapers to allow clearance.

Dave

ps: I know Bill asked us to take this off the list as it is off topic,
so if you want to discuss this further, please e-mail me direct.