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Power draw bar for step pulley mill

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  • Power draw bar for step pulley mill

    After watching a couple of videos on DIY power drawbars, it seems to me that of them address how the draw bar is made to bump the collet out of it's taper. Have I missed something obvious? Is it that the unscrewing pressure on it is enough to pop the collet loose? If so that would seem to be hard on the drawbar's threads. Then again, maybe not as bad as hammering on it is. Just trying to wrap my head around the concept.
    “I know lots of people who are educated far beyond their intelligence”

    Lewis Grizzard

  • #2
    The wedging force from pushing the arbor out of the lock is never going to be anything near as bad as the hammering force.

    After all do you worry about the draw IN forces on the thread? No, of course not. They are made for that, right? So yes there's some extra locking force to deal with but pushing the arbor out of the lock isn't all that much worse than drawing it tight in the first place.

    From my very first mill/drill my main concern was the spindle bearings. When I hit the end of the draw bar if the arbor or collet does not break loose then all that shock goes into the bearings. Which is why I've always used a slug of lead that used to be a scuba diving weight belt "bullet" weight back in a previous life. And why if I didn't have that slug of lead as my milling machine buddy I'd be using a dead blow hammer as a replacement. We need to move the drawbar. But I want to move it in the most push like manner I can. And actually a drawbar cap that let's me push the arbor or collet out would be a lot more kindly to the bearings than any sort of beating device.
    Chilliwack BC, Canada

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    • #3
      Thanks BC, that makes perfect sense.
      “I know lots of people who are educated far beyond their intelligence”

      Lewis Grizzard

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      • #4
        So for those out there with captured drawbars what does the capture collar look like? I'm looking at my own smaller knee mill and there's no room at all to mount anything that would hold down the drawbar. I sort of doubt that I can fit one in my case.
        Chilliwack BC, Canada

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        • #5
          I suppose I should have mentioned that my mill is a Bridgeport clone I hate when someone forgets to tell what machine they are talking about, and here I go doing it myself.
          “I know lots of people who are educated far beyond their intelligence”

          Lewis Grizzard

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          • #6
            I fitted a captive drawbar to the R8 spindle of the Tom Senior light vertical mill. It was made possible by a step in the bore between the end of the tool and the start of the smaller bore for the drawbar. The length of the cavity was important. I feel that having the thread pushing the tool out is less traumatic for the thread than the traditional whacking with a hammer. This is not powered, but would make a powered drawbar easier to engineer. I made sure there was a good length of thread engaged, and have used moly grease on it. If your Bridgeport clone has the R8 spindle, you should consider whether the locating pin should be left out or not.


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            Last edited by old mart; 12-24-2020, 05:00 PM.

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            • #7
              There are a few spring retained R8 drawbar systems, these are the fastest tool change systems available.
              They use a spring loaded retention knob system at the top of the spindle to draw the collet, if you only do very light work these can up your game.
              They do not hold tools well however.

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              • #8
                Old mart, I like that idea. I'm going to see if my mill is a possible candidate for a similar thing.

                I'm thinking that it would thread onto the excess threads and that I'd make up a two pin style driver to install the collar and allow for removal through the collet end. Heaven help the person that finds the 2 pin driver and tosses it thinking it has no application though... Come to think of it I'd better label it since my own memory isn't all that great any more either....

                EDIT- that's not going to work. It would simply unthread over time. And using pins means I'd have to install it up from below. And there's not enough room for that.... More thought required.

                Last edited by BCRider; 12-24-2020, 06:47 PM.
                Chilliwack BC, Canada

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                • #9
                  Originally posted by Dave C View Post
                  After watching a couple of videos on DIY power drawbars, it seems to me that of them address how the draw bar is made to bump the collet out of it's taper. Have I missed something obvious? Is it that the unscrewing pressure on it is enough to pop the collet loose? If so that would seem to be hard on the drawbar's threads. Then again, maybe not as bad as hammering on it is. Just trying to wrap my head around the concept.
                  it depends on the taper. i have an iso30 quickchange and the tool mostly just falls out.

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                  • #10
                    I have a typical Kurt "impact" style power draw bar. It does tend wear (you can see) the threads on the bar but no failure. I find it's important not to "over tighten" the bar - so easy to do; that just makes it work way harder to undo. Good news, it's lubed very operation - likely way more often I would ever have done manually.
                    Last edited by lakeside53; 12-25-2020, 12:03 PM.

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                    • #11
                      Before I installed my pneumatic power draw bar, I had to whack the drawbar with a mallet after loosening it. But now with the impact tool doing the loosening, the collet drops out automatically.

                      Ed
                      For just a little more, you can do it yourself!

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                      • #12
                        I've "learned" the hard way a few times .. put hand or rag beneath the tooling. Carbide drops out and chips (or worse) often before the collet if not!

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                        • #13
                          With an R8 taper, you can get a good idea whether a captive drawbar similar to mine in post #6 could fit by looking with a mirror. If that looks feasible, then be warned that the drawbar will have to be fitted from below as that extractor nut is threaded on and loctited in place. I made it out of bronze. At the top, I have three nuts, half nuts either side of a larger hex one for a short ring spanner. The spindle is held with a spanner at the bottom, manual only. I have adjusted the drawbar to break the taper after 1/4 turn of slack.

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                          • #14
                            I wondered the same thing when I built my power drawbar unit. Nothing is ever said about that potential problem in any of the DIY articles and videos that I've seen. I figured that the vibration from the impact wrench must break the collet loose. I've found, like Ed mentioned, that that's usually the case. But I do have a sticky collet and a sticky drill arbor. Usually I can free them with a few sharp pulls of the wrench, getting as much of a vertical impact stroke as I can. But not always. Occasionally I have to remove the impact wrench so that I can get a brass hammer in there, which is really annoying. The 40 year old spindle taper is no longer pristine, and I plan to someday regrind it with my toolpost grinder mounted to the table. Maybe that will help. I hope it doesn't make it worse. I suppose I could replace the offending collet and arbor and hope for better performance.

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