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I have some 2m long aluminium tubes of 15mm and 20mm outside diameter.

50 mm from the end of each tube I need to drill a hole to take a bolt and the two holes should line up along the length of the tube.

I have a drill press and machinists vice so drilling the holes is no problem. I just cant figure out a way of ensuring that, after sliding the tube along, it hasn't twisted.

Any ideas?

Keith Miller
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Immobilize the tube on a flat surface, then elevate a pencil on a block to a fixed height above that same surface (the writing tip should be at about half the tube's diameter). Slide the block and pencil down the tube length to draw a line parallel to the longitudinal axis. Drill both holes on this line.

popham
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  • Thank you! I was fixated on drawing along the top of the tube somehow but drawing on the side is much easier and has, of course, the same effect – Keith Miller Oct 20 '23 at 07:13
  • @Keith Miller, there are dedicated tools called "surface gages" for implementing the pattern. The machinists use a blue dye to stain the surface and scratch away the dye for their layout lines. – popham Oct 20 '23 at 07:30
  • I remember doing this when studying engineering 50 years ago. We had large, flat, cast iron plates to use the gauges on. – Keith Miller Oct 20 '23 at 07:54
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    Presuming the tubing is round (as is indicated by "diameter")... If the surface is flat and smooth, you could just mark the tube where it meets the surface. Or, just run the pencil flat along the surface. No need to elevate the pencil to the center of the tube - that's too much work! – FreeMan Oct 20 '23 at 13:24
  • @FreeMan, my mind didn't even bother converting the 20 mm to American. You're right on these tubes, but I'll leave the block for generalizing to larger tubes. – popham Oct 20 '23 at 13:39
  • Pipe size doesn't matter. If you need to drill holes lined up in a 6" PVC DWV pipe (~270mm), you can still lay it on a flat, smooth surface then run your pencil/pen along the surface and get a straight line. Still no need to elevate the marking tool. – FreeMan Oct 20 '23 at 13:51
  • Now, if it's square or rectangular tubing, then yes, absolutely you need to find the center of the face being marked. But then, a combination square will work well as the "block" and simply mark on the top face. – FreeMan Oct 20 '23 at 13:53
  • @FreeMan, I would agree if my marker tip was perfectly thin, flat, and sufficiently long. In the limit as pipe diameter increases, the sloped edge of a pencil's wood will interfere with the pipe and prevent the tip from contacting the pipe at all. At a more practical scale, as pipe diameter grows, the sloped edge of a pencil's lead contacts the pipe at an increasingly oblique angle, leaving fatter and fatter, more imprecise marks. – popham Oct 20 '23 at 15:32
  • Fair points, @popham! Use a metal scribe. :D (We've both made our points now, I think we can stop. ;) – FreeMan Oct 20 '23 at 15:40
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Hold the tube to the floor so it doesn't roll, and mark on each end the spot that touches the floor, and that gives you a reference on each end to know where the same location around the tube is located on each end.

Milwrdfan
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Clamp the tube down and mark both hole locations at the same time, then unclamp and drill.

Huesmann
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