On 22 May 2006 07:01:44 -0700, "nondem" wrote:

First: I know I need to sand/finish the cuts and holes I make on the
6061-T6 stock I use. The question is to what level. I know rough edges
are points of stress and should be removed. The question is what is
considered "rough?"
Like to what grit size sandpaper to work my way up to? Steel wool at
the end? I ask because the thousands of tiny scratches that even fine
sandpaper leaves could still be called a rough edge in some context.

In MY opinion, a fine file and a radiused edge. Depending on the
thickness, the hand deburring tools leave GREAT finishes.

Second: From what I read - the aluminum I use will be anodized which is
a process I know a little about - enough to know that any place it gets
drilled or cut will leave an un-anodized surface exposed to the
elements. What is the "Proper" way to protect these places?

Anodizing is a great non-structural finish. When it cracks, and it is
brittle, those tiny scratches you mention above are mild in
comparison. I don't like to anodize for structural appliations.
Definitely do not "hard cote" or hard anodize; the thickness really
will cause stress risers in whatever cracks occur.

Lastly: What is the 'ideal' 3/8" drill bit and cutting lube to use for
drilling 6061-T6 in .125" plate and 2" square x .125" tubing? There are
only a few sizes that I'll be using on this project so I'm interested
in a "cost is no object" brand/type that would be best.

There will be some who laugh here, but if you are rigid (stiff drill
press or mill), I like solid carbide, positive rake, at HIGH speeds. I
did some work some years back at 4000 rpm, 1/2" diameter, power feed
to pull a 0.003" or 0.005" chip per tooth. At your thickness, no lube
is required. We used high velocity air as the coolant on drill-mill
applications. Watching the table move was scary; note that we were
making 8000 cuts (0.005") per minute, so 40 inch per minute feed. We'd
have been through your plate in less than a quarter-second.

At lower speeds and feeds, our prototype shop machinist loved to spray
isopropal alcholol as the coolant/lube. I was surprised it worked as
well as it did; he gave glass-smooth finishes, as did the production
tooling above.

Note: CHIPS FLY!.