DIYbanter - View Single Post - Drilling straight through aluminum square tube

wrote in news:1106983013.521991.220000
@z14g2000cwz.googlegroups.com:

I've worked with wood most of my life, but I'm not as experienced with
metals... I recently found that I have to do a project for work that
involves drilling multiple holes through both sides of 3/4"x 3/4" and
1/16 thick square aluminum tube to make antennas. Imagine a TV antenna
basically... There needs to be multiple matched holes along each tube,
and into those holes, 1/4" solid round will be inserted through both
sides. In other words, the holes on both sides of the tubing have to be
aligned pretty darn well, otherwise a slight misalignment will
translate to the 1/4" round being way off center by the time it gets to
the end... Which is about 1-2 feet long. There will be a tad bit of
wiggle room since the round will be attached by threads on the end, and
a nuts on both sides of the square tube, but not a whole lot.

I had no idea this would be so difficult. I went and got a Sears 9"
bench drill press and proceeded by clamping the tube into a press vise.
I figured once I had it lined up and centered, I could just slide the
tube down for each hole... Wrong. There seems to be no rhyme or reason
to how it turns out in the end. If both holes end up being centered on
both sides, it seems to be luck more then anything. There seemed to be
a lot of slop in the spindle on the drill, so I took that back and
tried a Ryobi... Worse. The run-out on it was so bad, I could see it
with the naked eye on just a 1/8" bit. When I rotated the bit 180
degrees and made an indentation, there was almost 1/32 of an inch space
between the two. Ridiculous. I can't believe there isn't even a decent
small bench press available nowadays, but that's a different story.

I've tried using short bits, center punching first, not center
punching, 1/16" pilot holes, a countersink for a starting point...
Nothing works. I've actually had better luck just drilling with a hand
drill, but that only works for the top hole... There's no chance at
getting the alignment right with bottom hole by had. I tried one of
those portable drill press stands, but it just wasn't precise enough.
Is there some kind of a portable drill guide I could use with a bushing
or something?

I'm kind of lost at this point... Am I really trying to do something
that can't be done without a mill? That's hard to believe. The actual
physical aspects of the holes is good... Nice and clean. It's just the
alignment I seem to have a problem with. I had no idea something that
seemed so simple would give me such grief. I have a lot more respect
for you metal heads now. Any ideas are welcome.

If you're pretty good at wood working, this should be a pretty easy fix.
Build a jig out of wood. Use a good quality hardwood board to make a base
to which you will attach a fence. Next drill a 1/4" hole so that the center
is precisely 3/8" from the fence, drill another that is spaced the same
distance that you want the holes spaced on the tube., this hole will be
used for a dowel pin that will determine the linear spacing of the holes. I
would drill the holes first, then locate the fence by inserting a 1/4 inch
pin in the holes then placing a 1/4" piece of flat stock between the pins
and the fence. Instead of just using 3/8", I would measure the tubing and
divide by two and locate the center of the hole at that dimension. Now
clamp the jig to the table of the drill press. Locate the inside hole in
line with the spindle of the drill press. The easiest way to do this is to
insert the pin in the hole and then chuck on it with the drill chuck. Allow
the jig to float to the center of the drill chuck. After you drill your
first hole you are going to put a pin in the second hole and put the
previously drilled hole over it to give you the linear spacing. Now drill
the hole in the tube as follows: Locate the first hole by measuring, clamp
the tube to the base and the fence. You should be able to use "Quick
Clamps". Get a combination drill and countersink otherwise known as a
center drill. Start the hole by drilling with this bit so that the
countersink is just slightly larger than 1/4". Change bits to the 1/4"
drill and drill through one side only. Now unclamp the tube and insert the
pin in the hole located under the spindle. Flip the tube over putting the
previously drilled hole over the pin. Repeat, drilling with the center
drill and drill. If the mis-alignment is no good either adjust the jig or
if being exactly in the center of the tube isn't very important, flip the
tube and rotate it end for end. For the second and subsequent holes insert
the second pin and use it to locate for precise spacing between holes. You
need to minimize the drill runout if you can. If you can't, try using a
small trepanning tool, or a two flute end mill instead of the drills.

Hope you can understand what I'm trying to describe.
Good luck,

Dan