Air Wrench Becomes a Milling Machine Power Drawbar

We sometimes wonder if designers ever actually use their own products, or even put them through some sort of human-factors testing before putting them on the market. Consider the mechanism that secures toolholders to the spindle of a milling machine: the drawbar. Some mills require you to lock the spindle with a spanner wrench, loosen the drawbar with another wrench, and catch the released collet and tool with – what exactly?

Unwilling to have the surgical modifications that would qualify him for the Galactic Presidency, [Physics Anonymous] chose instead to modify his mill with a power drawbar. The parts are cheap and easily available, with the power coming from a small butterfly-style pneumatic wrench. The drawbar on his mill has a nearly 3/8″ square drive – we’d guess it’s really 10 mm – which almost matches up with the 3/8″ drive on the air wrench, so he whipped up a female-to-female adapter from a couple of socket adapters. The wrench mounts to a cover above the drawbar in a 3D-printed holster. Pay close attention to the video below where he goes through the Fusion 360 design; we were intrigued by the way he imported three orthogonal photos on the wrench to design the holster around. That’s a tip to file away for a rainy day.

This is a great modification to a low-cost milling machine. If you’re in the process of buying machine tools, you should really check out our handy buyer’s guides for both milling machines and lathes. It’ll let you know what features to look out for, and which you’ll have to add later.

19 thoughts on “Air Wrench Becomes a Milling Machine Power Drawbar

  1. Why not an electric tool instead of an air tool? Then you wouldn’t have to run an air line to it. Also, is there no way to just hold the tool and start the mill in reverse at slow speed to remove it like you can do with electric drills to remove bits by holding the chuck?

    1. Impact drills that run on electricity are usually not that compact (and the ones that are are MUCH more expensive), and running the lathe motor and holding it also requires 3 arms.

      1. There are very short cordless models for example from Makita and Milwaukee. After cutting off the handle they are very compact.
        But they are very expensive. Looking longer on eBay for a used model without battery and charger. For sure this needs to be mounted as Physics Anonymous did.

    2. Electric does not “free wheel” like the air tool does, when you turn the machine on, and the spindle in in the up position spindle will be engaged to the power draw bar. Cheap butterfly impact is around $20.. you do need the air tho…

      Spinning up the machine with a wrench on the drawbar sounds like a bad idea.. 2-5 horsepower has a bit more torque than your hand drill. It has happened more than once… wrench is typically ejected/drawbar broke and collet falls out damaging your piece and tool.

  2. There are far more power drawbar versions on the internet using compressed air “Butterfly” drills instead of using a cordless electric impact drill (controlled from an arduino). That’s what I would prefer. And I would like to add an ATC.
    My SP2217-IV has a MT3 instead the R8 collet. Does every R8 push the tool out when losing the top screw? I need a hammer.

    1. R8 is generally self-releasing when used as a simple taper shank, e.g. a drill chuck shank. When it’s used as a collet, however, the angle is effectively halved, so it usually sticks a little and needs a rap to break the collet free when operated manually. Still can’t be stuck as fast as an idiot can get a Morse taper stuck.
      I’m not familiar with the hobbyist versions on the internet, but we have Kurt power drawbars on a couple bridgeports, and they’re basically just a 3/8″ butterfly impact, and a single-acting air cylinder to pull the impact wrench down into engagement with the drawbar. There’s a shoulder on the drawbar below the drive splines; if the collet sticks in the spindle taper, this shoulder rises as the drawbar unscrews, and impacts the mounting plate to break the collet free.

      1. I saw one of those expansive brand name power drawbars for a Bridgeport and was surprised to find under the cover was a Chicago Pneumatic 3/8″ butterfly impact wrench. Brand new right from the factory they’d bought a wrench from Harbor Freight and didn’t even bother to cover or peel off the labels. I guess they figured their cover over the assembly was good enough.

    1. Nice demonstration of the working principle!
      Yes thank you, I read about that thread that my machine does not have. I will have to modify my spindle.
      I bought already a m16x1,0 screw tap to add the needed tap on the upper insert of the drawbar. I will turn a custom draw bar as Arbalist did it: https://madmodder.net/index.php?topic=11023.0
      Hopefully that works for changing TTS holders in a 17mm Id MT3 collet.

      1. TTS holders also work in a 4C spindle with a 3/4″ collet. Of course there’s only two or three models of machine tool that were made that used 4C. One was the vertical head attachment for a Hardinge TM or UM horizontal mill.

  3. I feel on a machine like that it’s a bit of a “putting makeup on a pig”. It’s a decent enough machine, but are you really doing tool changes that often that it requires a power drawbar (which has it’s own fiddlyness to work out too probably). I’ve never had problems doing it manually on exactly the same mill (different manufacturer name-plate, exact same design). If it works for you though, more power to you.

  4. I have the exact same mill and because I am not using it as a production machine, I don’t find that a quick-change draw bar would be worth the trouble of rigging up a standing source of compressed air. However, this mill lacks a spindle brake (common on larger Bridgeport style mills) and loosening/tightening the drawbar is a PIA. Also, the square wrenching design of the drawbar sucks. I am in the process of designing a spindle brake and I have fabricated a drawbar that takes a 12-point wrench. Also, a power drive on the Z-axis would be a better place to invest some funds.

    BTW, if my small shop area was big enough for a Bridgeport, this bench top machine would be gone in an instant.

  5. I didn’t realize this was something new. The Bridgeport clone we have here at work has a very similar setup for the power drawbar from the factory. I just thought it was kinda normal for these smaller machines

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