Re: drilling aluminum, imperfect circle [TANKS]

[Frank Pittelli <frank-at-rctankcombat.com>]

Amir Tahvildaran wrote:
> I'm drilling through a 1/8" walled 1" aluminum square tube using a S&D 
> 5/8" black oxide bit.  I've found that the holes aren't perfect circles 
> in some cases.  Anyone have tips?

Use a "step-drill" like the Unibit when drilling holes in flat stock or 
tubing up to 1/8" thickness.  They make versions that have a 5/8" step. 
  You'll also like the unibit because it will naturally clean up the 
shavings around the hole and even chamfer the hole if you press in a 
little and allow the next step to partially cut into the material. 
Leaves a nice professional looking hole.  You'll still need a pilot hole 
to help the bit get started properly.  So, use your favorite and most 
reliable center-tip twist drill (1/4" in my workshop) to start and then 
step-it-up from there to the final size required.

A twist drill tries to grab too much (especially in soft materials like 
aluminum, copper and brass) which either causes it to bind up or wander 
off center near the end of the cut.  Prior to discovering the wonders of 
unibits, I used to have a set of drill bits that I deliberately dulled 
for use in brass (which we used a lot of on the warships).  That almost 
eliminated the grabbing problem (assuming you fed it in at the proper 
rate), but twist drills still don't result in a hole as round as that 
produced by the unibit.

CAUTION: Just like any bit, don't run the bit faster than designed for a 
given material or you'll dull one of the steps, making it harder to use 
for any step beyond that one.  And, of course, don't advance the bit too 
slowly either, because that can dull it also.  Naturally, always use the 
proper amount and type of oil when cutting as well, or, guess what, 
you'll dull the bit.  Finally, and most importantly, don't lend your 
best bits to friends or family .... they will always come back dull :-) 
  Have a spare set of dull bits for them to use!!!!

        Frank P.