Achievement Unlocked: Drill a Square Hole With a Rotary Broach

square hole rotary broachThere are times in a man’s life when he needs to drill a square hole through an 8mm thick piece of steel. If that man doesn’t have one or two thousand dollars to spend on commercial tooling to do this, he might just shrug his shoulders and make do with round holes. But if that man is [Chris], he rolls up his sleeves and makes his own tool to drill square holes with a rotary broach.

This tool that [Chris] has named the Wobble Drive drills a square hole by applying force to each of the corners of a square bit one a time. How, you might ask, did he achieve this? With a two-part tool and the power of offset driving. He took a cylindrical chunk of steel and bored a little cup for a ball bearing to move around in. He didn’t have one rolling around his tool box, so he liberated one from a 2209 double row self-aligning cylindrical bore with a screwdriver. Then he hammered a square rod of steel into a hole in the other end and made the rod’s bottom a little bit concave on the grinding wheel. He also took a little off the sides to aid the weeble wobble action. A second steel cylinder with a ball bearing cup sits in the chuck of his Bridgeport mill and wobbles the tool bit through the power of a 1/4″ offset.

[Chris] tested it on the same sacrificial plate he used to demonstrate the awesome power of Lil’ Screwy, his 100-ton homebrew press. He drilled a 3/8″ round pilot hole and then went to work with the Wobble Drive. The tool bit side proved to be too long to provide the requisite stroke, so he cut it down by about half. Once the tool has chewed through the steel, the tool bit decouples at the ball bearing and [Chris] has himself a square hole and that much more hacking cred.

[Delightfully coarse language ahead!]

Comments

  1. w says:

    Nicely done.

    For those interested in this kind of tooling, rotary broaches can also be used as a stationary tool in a lathe. Often useful for boring holes in things like allen bolts or pentalobe bolts etc…

    They key thing is to drill an undersized round hole first to let the chips escape.

    • w says:

      Clarification: when I said “stationary tool” above I mean that it is not actively turned by the lathe itself, the workpiece causes it to turn through interference friction.

  2. Waterjet says:

    I thought he broke Lil’ Screwy?

  3. Adobe/Flash hater says:

    (lol)Anyone who cavalierly pops a hardened bearing ball out of the retainer
    across a steel bench is a true optimist!
    ….either that or the top of his bench is more “fully utilized”
    and more of a part trap then it appears from the video.
    Until you’ve played workbench pinball with a small important part,
    you haven’t truly lived !!
    loved the offhand comment about feebaying the bearing remnant also (too true).

  4. Reg says:

    FWIW Two methods of drilling square holes are described by Alexander Weygers in The Recycling, Use and Repair of Tools reprinted in The Complete Modern Blacksmith volume which combines 3 of his books.

    The technique described here is quite different.

  5. P says:

    Two “fuck”s is “Delightfully coarse? Shit, y’all motherfuckers are gonna love me. :D

  6. SuperUnknown says:

    Great write up Kristina!

  7. static says:

    Somewhat *similar* to the drill press mortising attachment in my high school wood shop in the early ’70s. Most us will have to do with this method, if we don’t file away for the rest of our lives, even uses grinders many may not have.

  8. bob says:

    Only problem is that the vibration induced from the drill as it side cuts , will shag the motor bearings on the power tool

  9. Menga says:

    I love it, extremely simple yet effective. I will make one to test it. Probably i can use this metod in a lathe just offsetting the tailstock center and using it to push the broach. But i will end building this one anyway: http://www.youtube.com/watch?v=wpx76-_lPkM

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