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  • How Tight to make a Drawbar

    Seems like a dumb question but not really. I have a Craftex mill/drill, the one with a 4 1/2" column. I also have two grandsons who use my shop. When it comes time for me to change bits or chucks, the fun begins. First, I use a 3/4" box end wrench, mumble to the machine gods and, with much grunting, loosen the draw bar a couple of turns. Then I use the lead hammer, (which I cunningly cast on the other end of the wrench,) to beat a few times on the draw bar. Then I mumble LOUDER to the gods and get a bigger hammer. I eventually win. This is a) not very efficient, b) more than a little frustrating, and c) might eventually break something.
    It would seem that I must offer a little training to my Grandgorillas, but I thought that I would solicit some opinions from you clever folks.
    I think that, to reliably install a chuck, arbor, or collet, I should turn the draw bar down until it snugs against the quill. Then using the wrench, take up any remaining slack, and then add 1/4 turn or thereabouts. I think that any more torque is just overkill.
    Comments please, and by the way, the quill accepts MT-3 collets etc.
    Duffy, Gatineau, Quebec

  • #2
    My Bridgeport has a 7/16-20 drawbar. The spec for tightening a grade 5 bolt of that size is 41 ft. lbs. I've never put a torque wrench on it, but I would guess that's about all the torque I put on it. I never have to use any kind of hammer to loosen or tighten it, and I've never had an end mill slip. I use a standard length 3/4" wrench and tighten about an additional 1/2 to 3/4 turn after snugging it up by hand.

    Comment


    • #3
      I think that it depends on the tool that you put in the spindle.

      A collet has to collapse enough to hold the tool in it. A face mill or boring bar, on the other hand, only needs to be tight enough to wedge the taper into the spindle.

      I've not had a tool slip in a long time, and I use the same concept when using a mill with 2MT spindle or a lathe or a mill with R8. Tighten till it seats and then a light tug beyond that.

      For your situation, I'd use a torque wrench to measure how much force is needed to wedge the collet into the spindle. Once you have that figured out, make the Grandgorillas use a torque wrench to tighten.

      Dan
      At the end of the project, there is a profound difference between spare parts and left over parts.

      Location: SF East Bay.

      Comment


      • #4
        It probably depends on what the collet is.

        R8 seem to take more pull to prevent slipping than an MT collet(which I have on most of the machines).

        You can make a self-ejecting drawbar, if there is a way to attach a cap to the end of the spindle. I have a decent length of threads showing at the end of the spindle on one mill, so I screwed a cap on the end, which allows self-ejecting.

        You make the drawbar as big as will fit through the spindle, put the cap on the drawbar, and under the cap you fasten a collar in position so it is just loose on the cap. Now when you unscrew the drawbar, the collar hits the bottom of the cap.

        As you unscrew the drawbar more, it pops the collet out the end of the spindle with no fuss, no muss, and no bother. I put wrench flats on the cap, so that all you really need to do is to squeeze two wrenches together. I find that a moderate squeeze does the job, where it previously took some clouting with a plastic faced hammer.



        Last edited by J Tiers; 05-01-2018, 06:31 PM.
        2730

        Keep eye on ball.
        Hashim Khan

        Everything not impossible is compulsory

        Comment


        • #5
          Hi,

          This happens to us all. I know I'm not nearly as manly as I was when I got up this morning. Just because I got older.

          Give the Grandsons a bit of class time on tightening drawbars. Then give them a cut down wrench to use. Frankly, they probably don't really know just how over tight they get it. I have often cut vise handles down to prevent 300lbs gorillas from breaking even Kurt vises.
          If you think you understand what is going on, you haven't been paying attention.

          Comment


          • #6
            Duffy, it's not a good thing to need to wail away on the drawbar. That impact is being transferred to the spindle bearings. You need to threaten the boys with excommunication from the shop for a "time out" if they don't smarten up.

            You did mention a 4 3/4" column. I take it that means a round column mill-drill? So quite likely it has a morse taper quill? If that's the case then I totally agree with the idea to make up a new wrench which is only about 4" long. And tell them if they use anything BUT that shortie wrench it's another Time Out from the shop. And that doesn't mean hitting the end of it with the lead hammer either.
            Chilliwack BC, Canada

            Comment


            • #7
              A Morse taper locks pretty well, even before you cinch it up with a drawbar. You're not going to be able to break the taper's grip without some kind of coaxial force. It is my opinion to just snug up the drawbar - probably less than 10 ft-lbs of torque.

              Comment


              • #8
                Hi
                Morse taper only needs to be held in place no excessive force needed I.E. Thumb on the nut end of the spanner and apply force with Pinky and ring finger right hand on the spanner. Supply a shortened spanner for the job and keep handy to the mill.
                Never had a failure so far with many years of use changing various chucks and boring bars.

                Eric

                Comment


                • #9
                  On my R-8 collets and everything else for that matter, just a good snug tightening with a very short box end is all I've ever needed to never having anything slip.
                  After I'm done releasing the collet it just takes a light tap with a 5/8" x 8" brass bar I keep on the mill for just that purpose.
                  I always make a point of making sure that both the inside and outside of the collet as well as the endmill going into it is clean and dry.
                  The taper locks up better and so do the collets without all the usual long term storage lube in there to grease things up. The MT3 taper on my tailstock is usually just driven in hard by hand as well and I can't remember having it let go either.

                  I'm surprised that with the torque the grand kids are putting on that drawbar that the threads are still on the end of it. Emphasize to them how the taper works and that they're not bolting a flywheel onto a ship's engine.
                  Home, down in the valley behind the Red Angus
                  Bad Decisions Make Good Stories​

                  Location: British Columbia

                  Comment


                  • #10
                    I really don't have much experience with R8. I currently have NMTB 30 and 40. I have been wondering how tight they should be torqued. I have searched the internet and forums and have not found any useful information. I probably tighten to about 20 to 30 ft lbs and have had none come loose.

                    Bob
                    Last edited by rwilliams; 05-02-2018, 08:50 AM. Reason: clarity
                    Bob, 71193, Central Arkansas

                    Comment


                    • #11
                      My MT generally hold with very little torque. But that does not mean they come out easily.... hence the self-ejecting drawbar.

                      Don't the NMTB also have drive lugs? So the taper is mostly for alignment and not critical for torque transfer?
                      2730

                      Keep eye on ball.
                      Hashim Khan

                      Everything not impossible is compulsory

                      Comment


                      • #12
                        Originally posted by J Tiers View Post
                        My MT generally hold with very little torque. But that does not mean they come out easily.... hence the self-ejecting drawbar.

                        Don't the NMTB also have drive lugs? So the taper is mostly for alignment and not critical for torque transfer?
                        The NMTB spindle does indeed have drive lugs. Most tooling has slots to engage those lugs, although I have at least one piece that does not have them.

                        Tapers are funny things. My mill is NMTB40, which is supposed to be a self-releasing taper. But, even though the spindle bore and toolholders are free from dings and burrs, and with oil on the taper and not much torque on the drawbar, it almost always takes a couple of good hits with a lead hammer to get the taper to release.

                        I've never had the taper slip. Even when starting a cut with an 8" milling cutter, and those can put quite a bit of torque on the taper.

                        John

                        Comment


                        • #13
                          Originally posted by rwilliams View Post
                          I really don't have much experience with R8. I currently have NMTB 30 and 40. I have been wondering how tight they should be torqued. I have searched the internet and forums and have not found any useful information. I probably tighten to about 20 to 30 ft and have had none come loose.
                          It's probably excessive, but the power draw bar that came with my mill (40 taper) claims 150 foot pound at 90 psi. Hasn't slipped yet!

                          Comment


                          • #14
                            Originally posted by Duffy View Post
                            Seems like a dumb question but not really. I have a Craftex mill/drill, the one with a 4 1/2" column. I also have two grandsons who use my shop. When it comes time for me to change bits or chucks, the fun begins. First, I use a 3/4" box end wrench, mumble to the machine gods and, with much grunting, loosen the draw bar a couple of turns. Then I use the lead hammer, (which I cunningly cast on the other end of the wrench,) to beat a few times on the draw bar. Then I mumble LOUDER to the gods and get a bigger hammer. I eventually win. This is a) not very efficient, b) more than a little frustrating, and c) might eventually break something.
                            It would seem that I must offer a little training to my Grandgorillas, but I thought that I would solicit some opinions from you clever folks.
                            I think that, to reliably install a chuck, arbor, or collet, I should turn the draw bar down until it snugs against the quill. Then using the wrench, take up any remaining slack, and then add 1/4 turn or thereabouts. I think that any more torque is just overkill.
                            Comments please, and by the way, the quill accepts MT-3 collets etc.
                            If your approach works for you, no slippage and releases easily, then thats the way to go. Just tell the little gorillas how to do it, and that you will nail them to the fence if you find them overtightening again. I've never been too happy about having to hit the end of my 3MT drawbar to get it to release, so I'm planning a captive drawbar conversion soon, so the drawbar will free the tooling as well as tighten it. Trouble is, having bought the tooling in bits and pieces over a lot of years, I've got about 3 or 4 different thread types and sizes of loose drawbars.
                            'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

                            Comment


                            • #15
                              I'd guess I tighten mine around that posted 40ft/lbs. My wrench as well has a brass slug on the opposite end. But all mine takes is a single light tap on the draw bar and the collet pops.


                              just a FYI, everyone mentioning "I only snug it up good" doesn't really give a good idea what "snugging up" is. Snug to some guys is 10ft/lbs, the same snug to other guys is 85ft/lbs.
                              Andy

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