Depending on how precise you have to be, if you are talking about drilling a
row of holes in line along the length of the tube, (linear alignment), a
small piece of angle iron crotched along the length of the tube will give
you an alignment method for scribing a layout and such, relative to the
existing hole. Another way to do this is to lay the tube on a flat surface
next to a flat piece of stock of appropriate thickness, and use this to do
the same process. This assumes a straight tube. If the existing hole goes
through both walls of the tube, a piece of properly sized round stock or
drill blank protruding through the hole, and aligned with a square off the
table surface or the drill press spindle will get your rotational relatively
correct for you. Of course, if the work is not clamped to a fixed reference
during drilling of all the holes, the alignment is lost. Now, if you're
talking about continuing the drilling of holes in a radial manner around the
tube, an indexing device would be helpful. The drill blank method used along
with alignment blocks of the proper angular cut for the degrees of rotation
needed, or the use of an angle protractor will get you in the ballpark. Two
or four holes can be done by attaching a square block to one end of your
tube for an alignment device, laying it onto your table as you drill. A hex
nut will work for three or six equally spaced holes. The V block itself can
be used to clock for two or four holes by laying it on it's sides on the
table. If clamping devices get in the way of this, use parallels under the V
block to raise the clamps above the table. I hope this answers your question
correctly for you.

RJ

"Miki" wrote in message
oups.com...
I tried searching, but couldn't seem to come up with the correct search
term. I'm quite sure this has been discussed though.

If you have a tube with a hole drilled in it radially, and want to
drill more holes exactly in-line with the existing hole, how would you
do this on a drill press? Finding the center of the tube is the easy
part, but how do you know you have the tube "clocked" correctly on the
V-block?