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Stopping a threaded lathe chuck from unscrewing

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  • Stopping a threaded lathe chuck from unscrewing

    I have been trying to come up with a way to keep a threaded spindle lathe from unscrewing. I have a VFD on my lathe and would like to be able to use the breaking feature. I've avoided that because I don't really want the chuck coming off.

    One way to keep a part from loosening is to use a set screw, but a single one could push the chuck out of alignment. So I thought it might work to cross drill four holes in the spigot of the mounting plate of my four jaw chuck so that set screws would bear on the un-threaded part of the spindle. I'd use soft face screws or brass pads to keep from scaring the spindle. This should hold the chuck in place for pretty much any breaking, and it might work OK for light cuts in reverse.

    It seems that this won't really affect the accuracy of a four jaw - once it's solidly against the spindle register it's not going to move too much. Using padded set screws should protect the spindle so I don't think it's going to damage anything.

    I can't be the first one to think of doing this yet I haven't seen anyone else mention it, so what am I missing? Why is this a bad idea?

  • #2
    Unless you run it in reverse there should not be any way for the chuck to unscrew.

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    • #3
      Originally posted by Abaker View Post

      One way to keep a part from loosening is to use a set screw, but a single one could push the chuck out of alignment.idea?
      You just answered your own question. Chucks unscrewing is a long tome problem and if there was a good way to stop it, it probably would have been done. It is most likely why screwed mandrels have fallen out of favor.

      The best thing you can do is make sure that the threads and all mating parts are very clean and then make sure the chuck is seated properly.
      The shortest distance between two points is a circle of infinite diameter.

      Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

      Southwestern Ontario. Canada

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      • #4
        In the past there have been some examples of what you seek posted here. Probably in a thread relating to a rear tool post. My search ability is not very good.
        Byron Boucher
        Burnet, TX

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        • #5
          Originally posted by Dr Stan View Post
          Unless you run it in reverse there should not be any way for the chuck to unscrew.
          Oh yes there is! If your chuck is spinning fast and/or has a large work piece, and you stop the spindle quickly, the chuck can easily unscrew. Don't akk how I know.

          I've had an couple of Emco lathes. They have a locking split collar as part of the backing plate. That clamps onto a shoulder on the spindle. There are two models -one was integral with the backplate; the other (later) was just a collar that compressed the backing plate stub nose (it was split into 4 with a saw). This is preferable to the set screw idea - you will push your chuck around and I doubt they will lock well.
          Last edited by lakeside53; 07-27-2012, 12:50 PM.

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          • #6
            i have a machine w/ a threaded chuck and a vfd too, i also thought about the same thing. eventually decided that there was no reason i can think of(here at home) to stop that fast and just set the decel time down to a couple seconds.
            if i actually had to stop instantly i wouldn`t hesitate to go cam lock.

            2 pesos

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            • #7
              Does it acually stop in 2 seconds though from high speed? Lathes are tough to hit a number with a vfd as the inertial load varies tremendously. Some VFD will simply extend the decelleration if the DC buss rises to max; at low speed or lightly loaded they can stop quickly, but at higher it takes much longer. I also took aggressive braking off the Emco's, and also decided a foot brake addition wasn't a good idea either for my non-collared chucks! It was interesting to see the chuck "float out" a little from the spindle then seat again. lol..

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              • #8
                The Sherline was my first lathe, bought second hand. One of the first things I did was spin off the threaded chuck and ding the brass ways. It was in Forward, full speed, and I spun the speed control to zero. Ding!

                Every photo Itake, I can see it.
                JFLingg
                JFLingg @ 25 miles east of Colorado Springs, Colorado

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                • #9
                  Exactly what I want to avoid, except that 6" 4 jaw has a bit more mass. My luck the ding would be on me. I have a pretty robust cranium (runs in family) but still, that's gonna hurt.

                  Lakeside: Thanks for that idea about splitting the stub and then clamping it with a collar. Did you ever try to work in reverse with it?

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                  • #10
                    If you have sufficient room on the spindle this method is a very positive way of retaining a threaded backplate/chuck.
                    Spaced 180° apart two of these retainers are more than adequate to prevent spin-off.


                    Home, down in the valley behind the Red Angus
                    Bad Decisions Make Good Stories​

                    Location: British Columbia

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                    • #11
                      the above is how my "taiwanese traytop" does it.
                      as far as my vfd set-up, i`ve never really checked to see but i`d guess it stops pretty close to the set time- it`s on a small SB though and the biggest chuck i have is a 5", and i usually use a 4" on it so not a lot of weight.

                      true story- when i was playing with the vfd after initially getting it set up, i was using the 5" chuck turning a piece of about 1/4" material around 900-1000 rpm, i was using a live center in the tailstock.
                      found out the hard way i did not have enough clearance for the outboad spindle bearing. locked it right up. luckily no damage except my pride.
                      the chuck didn`t unscrew but it did break free. live center took the brunt.
                      it`s scary to think about it actually coming off and whipping around with that skinny little piece sticking out.

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                      • #12
                        The German Solution

                        Here is a German lathe manufacturers' solution to locking the chuck on a threaded lathe spindle using pins on the register:







                        It is basically three bronze pins that bear on the spindle register. The internal taper of the threaded ring applies a very high mechanical advantage to the pins, much more than can be achieve from a threaded screw bearing directly on the register. The arrangement has the additional advantage that the force on each pin is self equalising (due to the play in the thread in the clamping ring), eliminating any tendency to misalign the chuck. It's German so of course the design is complicated, difficult to say if it's over engineered, I've never tried to spin the chuck off the spindle on purpose It does imply that screws acting directly against the register is not enough.

                        Phil

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                        • #13
                          Originally posted by lakeside53 View Post
                          Oh yes there is! If your chuck is spinning fast and/or has a large work piece, and you stop the spindle quickly, the chuck can easily unscrew. Don't akk how I know.
                          Guess I've just been lucky, but then I've never tried to instantly stop a screwed on 3 jaw. All the lathes I've used that were equipped with a brake also had cam lock spindles.

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                          • #14
                            I drilled & tapped the spigot on my 8" 3-jaw chuck and inserted 3/8" setscrews that bear on the spindle threads through pads made of crushed small-diameter brass wire. I don't have a VFD, but I do sometimes start my spindle in reverse. The chuck weighs at least 30 lbs, and the motor has 5 real, old fashioned HP, so the acceleration is virtually instantaneous. It hasn't unscrewed yet. But I re-tighten the setscrews EVERY time before tempting fate.

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                            • #15
                              I'm not familiar enough with VFDs but believe you can adjust the braking amount so you don't experience the dead stop which could cause the chuck to unscrew.

                              Another thing you could try though not obviously useful for all operations is to use the tailstock with a centre onto the job to prevent any movement of the chuck.

                              I use a tailstock where possible if ever I have to run my lathe in reverse.
                              Last edited by DickDastardly40; 07-27-2012, 07:54 PM.

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