Saturday, March 26, 2011

Machining Fixtures

One of the required tasks to move manufacturing from handcraft to mass production is the right fixtures.  Part of my manufacturing process involves drilling three holes 3/8" from one end of a nominal 2" O.D. aluminum tube, 120 degrees apart, then threading them--and making sure that they are exactly square--which is not trivial when you are going into a round surface.  Doing this isn't hard.  Doing it consistently is very slow--unless you build the right fixture.

What I did was buy a nominal 2" I.D. steel tube.  Then I squared the ends on the lathe, and bored the interior to be 2.02" I.D. and 1.00" deep so that it would slide onto the tube that needs the threaded holes.  (Then I polished it up with #50, #150, #1500, then #2500 sandpaper so it is all shiny and pretty.)

Next, I used the vertical mill to very precisely place the first hole .375" from the end of the steel tube. 

Then I used the protractor that I made a few days ago to mark and drill the next two holes.

Now, I needed to make sure that the tap went in exactly square.  The instructions for doing this with a drill press and a conventional tap wrench are here.  The only difference is that I did not have a spring center, but I found that a dead center from the lathe in the drill press chuck worked just fine.  You put slight pressure on the drill press, and the dead center going into the hole in the back of the tap wrench keeps everyone lined up as you turn the tap wrench by hand.  After the first few threads, you can let go of the drill press, and complete the process by hand.

Last step: drill and tap a fourth hole in the tube to accept a 1/4"-20 bolt.

Now I can slip the 2" O.D. aluminum tube into the sleeve, and tighten down the 1/4"-20 bolt.  I put the entire assembly in the drill press vise, and position the drill bit precisely in the center of each of the three holes, and drill down.  Now I put the tap wrench with the tap into the three threaded holes, and tap through the aluminum inside. 

Because the threaded holes in the sleeve are square, I don't have to worry too much about whether I am starting the tap square or not--because the first threads the tap touches are square.  By the time the tap reaches the aluminum, it is square as well.  I can now hand this task to my minions, and they can make perfectly square threaded holes at exactly the right angle and distance in very little time.


  1. What would be a good way for a wannabe hobbyist to learn about nd get started in metal machining?

  2. I didn't do it this way, but Home Shop Machinist magazine might be a good start.

  3. So, why does it need to be perfectly square, by the way?

    (From context I'm assuming they're for thumbscrews or the like to provide adjustment locking - and for that, "more or less square" would seem to be sufficient, even with a fairly heavy load...)

  4. These are thumbscrew holes, and yes, they do not need to be perfectly square, but they do need to be pretty close.

    In addition, my goal is not simply "good enough" but "delight the customer." When you open the box, if everything is absolutely perfect--or so perfect that you can't see any obvious flaws or imperfections, it makes you really happy. If problems develop later on, you have the good will that comes from those positive memories of delight at box opening.

  5. I can now hand this task to my minions...

    You have minions? Splendid! Onward to world domination! Mwahaha...

  6. Someone from church and my son are both doing some very part-time work for me. It is better pay than they can get elsewhere, works around my son's college classes, and they are learning a useful skill.