Thursday, June 24, 2010

Square Hole Drill

Yes, there is such a thing--it's not a prank you play on apprentice machinists. "Mike, could you go retrieve the square hole drill?" It turns out that a combination of a triangular cutter turning on an offset will actually cut a square. There's a demonstration here of how this works--apparently a practical application of pure math. This company sells a variant that produces a variety of polygonal shapes using this approach.

I have a request for a new ScopeRoller variant that requires me to cut a rectangular blind hole in acetal--and the more direct approach (using a small diameter end mill to square a round hole) doesn't produce a particularly attractive finish. I'm still mulling over some other way to accomplish the same ends.


  1. How hard is this plastic? In woodworking we using a Mortising machine, or an attachment that does the same thing for a drill press.

    Could the hole be made all the way through, so you can cut it with the appropriate broach, and then plug it with another piece?

    If the hole absolutely must be stopped, that could be a problem. There are blind Broaches, but

  2. The hollow chisel mortising machine is probably not going to work for this. Acetal is about 90% of the hardness of aluminum. I'll wear out those chisels in a hurry!

    It can't go all the way through, unfortunately.

    I'm working on another strategy that clamps two pieces together instead.

  3. If you used PVC/ABS instead of Acetal/Delrin, then you could use a solvent based glue to weld the two halves together instead of clamping them.
    a Woodworking mortise tool should work fine for Acetal, it is not as hard on tools as you make it out to be because it is self-lubricating.

    Another method is to cloverleaf it. drill a small diameter hole centered on the theoretical sharp of each corner. then drill a large pilot hole that is just shy of the flat to flat distance required, then use an end mill just a tad smaller than the four corner drills to square up the four sides. This has the advantage of not leaving a sharp internal corner to act as a stress riser/crack initiation point as well as allowing you to make multiple passes to ensure a tight fit.

  4. Tell you what, I'll sketch out what I mean and email it to you.

  5. Unfortunately, PVC/ABS isn't hard enough for me to have confidence in tapped threads. I tried the cloverleaf approach; the problem is that a small enough end mill to do the trick in the corners is too short to go deep enough. A long enough end mill that diameter is probably going to be too flexible.

    I have another approach that involves milling a channel, then attaching a top surface with four 8-32 screws. This has the additional advantage of providing room for adjustment. (The tripod legs in this case are maple, and natural variation might cause enough tolerance that letting the user adjust the clamping force has some virtues.)

    The same general approach will work for the new Losmandy tripod, too.

  6. Yeah, I think I'd have to see a sketch or picture to be able to make more useful suggestions.

    (Ordinary twist drill to make the corner holes?)