Several months back, I made a jig to simplify drilling and tapping three 1/4"-20 holes in 2" OD aluminum tubes for ScopeRoller. There were several reasons for making the jig:
1. I wanted the holes quite perpendicular to the tube.
2. I wanted them consistently at 0.375" from the end of the tube, without having to remeasure the location for the hole each time.
3. I did not want to keep having to adjust the tube's location in the drill press vise.
4. I wanted the holes as close to 120 degrees apart as I could manage.
This actually worked out rather well. I started with a piece of aluminum tube, and bored it 2.00" inside diameter and 1.00" deep. Then I used a protractor to mark 120 degrees apart, and then marked where the holes should go in the exterior of the tube where the 120 degree marks crossed 1.000" - .375". I drilled and tapped these holes, plus one more into which I put a 1/4"-20 set screw.
Now I can slide the 2.00" tube into the end until it seats, turn down the set screw, and use the end of the collar as a stop when I put it in the drill press vise. With the holes in place, you just position a .203" drill above each hole, drill through the collar and the tube, rotate and repeat twice, then use a 1/4"-20 tap to thread each hole.
This all works, but I confess that I wanted something a bit more precise for the 120 degree angles than using a protractor. I bought a 4" rotary table from Sherline, which looks like this:
This attaches to the tilting table for the vertical mill, allowing me to rotate objects perpendicular to the vertical axis. It allows you to precisely turn an object in 0.1 degree increments--and a bit finer, if you are prepared to interpolate between the lines on the handwheel.
I needed to make a new one of these collars for a somewhat smaller diameter set of tubes (for a different production run of telescope casters), so my son and I spent some time making the smaller collar out of a piece of scrap acetal using the rotary table--and my, did this work well!
The art to any mass production activity (or even multiple production, because ScopeRoller's volume hardly qualifies as "mass") is to make everything as identical as the customer can afford--and that means creating tooling that lets you minimize the time that you are spending positioning workpieces and adjusting tools.