Scott wrote:

Please be patient wiht me, Im new.

I have 2 1"x1/2"x 48" aluminum bars

I have a small vertical mill. I need to place 2 1/" dia holes in the
same exact location on both bars. How do i drill the forst ane and
ensure 100% placement on the second one?

Define "exact".

The ways I know (absent ATP's transfer punch suggestion):

1. Put a stop on your milling table, but the bar up against it, and
drill with a spotting drill or other variety of short, rigid drills made
for milling machines. Accuracy will depend on the finish of the end of
the bar, the fence, the repeatability of your vice, the rigidity of your
machine and the tendency of the bit to walk. This'll probably be
repeatable to within a few thousandths.

2. Find the end of the bar with an edge finder, do the math, go in just
the right amount and drill. Accuracy will be everything in #1 except
for the finish of the fence, and repeatability of the vice, but will now
include the accuracy of the edge finder on whatever surface you have on
the end of your bar. Unless you have a really crappy end stop and vice
this'll probably have a couple of thou more error than method #1.

3. Since you didn't define "exact", just eyeball it and use a hand
drill. Depending on the accuracy of your eyeball you can expect to be
within 100 to 50 thou, this is more than "exact" enough for a great
number of jobs -- but that's probably not what you meant by "exact".

3a. Clamp the bars together and proceed with 3. The match should be as
close as your ability to drill a straight hole at right angles to the
material.

Disclaimer: I'm a beginner and a hobbyist. Someone who's actually done
this for pay will have an answer that's both faster and more accurate
than the ones I gave above.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com