From: frisbie@flying-disk.com (Alan Frisbie)
Newsgroups: rec.crafts.metalworking
Subject: Re: Drilling Aluminum
Date: 3 Jan 96 09:46:19 PST

In article <DKK4wn.DGG@adra.com>, 
mac <mac@adra.com> writes:

> After drilling a hole through a piece of Al. on my lathe with the drill
> chucked in the tail stock and the piece in a 3 jaw on the headstock, the
> hole is bigger than the drill.

Just in case you didn't already know it, a *drilled* hole will
always be larger than the drill size.   Usually it is only
a few thousandths unless something else want wrong (which
appears to have happened in your case).   Really accurate
holes must be either reamed or bored.

> It appears that the drill got moved of center after Al. adheared to
> one side of the tip, and caused the drill to have a boring effect and
> increased the size of the hole.

To drill reasonably accurate holes, it is necessary to follow
a few precautions:

1) Remember that drill bits cannot cut at the exact tip,
   therefore a center-punch or pilot hole is necessary, so
   that the initial cutting takes place at the edges.

2) The first bit should be either a center drill or, better
   yet, a spotting drill.   These are very rigid and tend
   to wander much less.   (In my opinion, once you have used
   a spotting drill you will swear by it.   They aren't
   cheap, however.)   Most center and spotting drills are
   either 60 or 90 degrees, ensuring that the actual drill
   bit will make contact at the edges first.

3) If the drill bit is not ground exactly symmetrical, one
   side will cut more than the other.   This will cause
   both oversize holes and drill wandering.   One of my
   friends even refuses to use a new drill bit until he has
   re-sharpened it with his Darex.

4) Use the shortest drill possible to minimise flexing.
   A set of stub drills can be a good investment.

--  Alan E. Frisbie               Frisbie@Flying-Disk.Com
--  Flying Disk Systems, Inc.
--  4759 Round Top Drive          (213) 256-2575 (voice)
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