A few suggestions, most which have been given in one form or another:

Layout 1/4 inch from edge... Go to local industrial supply house and
purchase
a 1/4 inch square HSS tool bit. Sharpen end (90 degrees to length) to make
as sharp an edge as possible. Then clamp your angle firmly on a flat
surface
so that your reference edge is against same surface. Then simply run the
tool
bit along the length you want the holes. Press hard enough to score deep
enough so that when you center punch it, you can "feel" the score before
hitting
the punch.

Locate the first hole using an accurate scale. (Might consider getting
what's called
a square head, which you can adjust the scale itself to the dimension, then
mark
at the end of the scale.) For evenly spaced holes, get a good set of
dividers
and set the distance accurately. (I like to let the points "rest" in the
graduations of
the scale) Then step off the distance and use one leg of the divider to
scribe
across the first line, again pressing hard enough to make a good score which
you
can later feel. A little practice and it goes pretty quick.

Invest (or make) what is known as a "prick punch", and lightly prick punch
the
locations you've just laid out. Here is where you develop the feel for being
within
both score marks! Then visually check the prick punch. Not quite there?
Slant
the punch and put just at edge of 1st mark and "move" it by hitting at an
angle.
Again, a little practice helps, but it can be done.

Once all of the prick punches are to your liking, follow up with a properly
pointed center punch, holding it as square with the work as possible. It
also
helps to use a good quality ball peen hammer, striking the the punch as
square
as you can.

Now consider that alum. is relatively soft, so if a drill is not exactly
over the punch
mark, it is likely to not follow the point you have made. Hence the idea of
a smaller
drill (I like to use 1/16 inch or less) which will flex enough to go into
the center punched
position. Drill about 1/16 inch deep. Now visually examine the positions
again, mostly
to see how good you are. (A badly misplaced hole can still be moved, but
that's anothe
post?)

Now follow up with a center drill, and drill deep enough that the
countersink dia.
is slightly larger than your drill (say about 1/64 larger), and finally
drill though with desired
drill size.

The above can yield holes within .005-.015 inches with reasonble care. Skip
some of the
steps if not this is not required. By the way, tolerances for holes
locations are usally to
the center lines from some reference surface/edge/other hole.



"Daven Thrice" wrote in message
news:9Ik1d.263527$sh.202086@fed1read06...

"Richard J Kinch" wrote


Straightedge and carbide scriber for the lines. ($20)

OK, I'm working with 1" extrusions, that are 90 deg. angles. I want to
mark a hole that is 1/4" in from one edge. The only "straight edge" I can
think of using is a metal ruler, which is an 1" wide (or so). I'm
wondering how to clamp the ruler to the work piece good enough for me to
be able to drag that scribe along the straight edge without the straight
edge shifting.

I have one of those laser straight line things that you use for a level.
I'm thinking about taping the work piece to the wall and using the laser
level to create a straight line. Hmmm. Don't know if that will work. If it
does, though, I can then tape a ruler just below the straight line and
then mark of X's, which can then be center punched. This sounds like a lot
to go through, though. I'd rather just clamp a straight edge to the
aluminum, but how is that properly done?