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  • Question about Bridgeport leadscrew nuts

    Hello Everyone,

    I have a question about replacing the nuts on a Bridgeport mill.

    I just repaired the lube lines on my Kent mill as posted in a separate thread.

    Things went so well that I decided to pull the table off from my B'port and replace the nut on the x-axis. When I bought the mill, the seller included a pair of nuts. The leadscrew had about .060 backlash and the adjustment appeared to be used up.

    I got the table off and found that the nuts that I had were LH thread. The X-axis nut is RH thread. Come to find out, I had y-axis nuts.

    What to do? Of course the answer was to pull the saddle off and replace the y-axis nuts too. (and clean LOTS of crud out of the gib area, etc.)

    When I got it apart, I found that the nut that was in the machine was a one piece nut. The nut that I have is a 2 piece nut. See below.



    The new nuts don't have a undercut for the adjustment screw like the old ones do. Is the undercut just wear from the excessive backlash? If so, can I just reverse the nut? If I use the new nuts, do I leave one thread of clearance between them? And do I need the undercut?

    Thanks in advance for any advice on this.

    Brian
    OPEN EYES, OPEN EARS, OPEN MIND

    THINK HARDER

    BETTER TO HAVE TOOLS YOU DON'T NEED THAN TO NEED TOOLS YOU DON'T HAVE

    MY NAME IS BRIAN AND I AM A TOOLOHOLIC

  • #2
    B'port Nut

    The one piece one has the c'bore for the screw head to keep it from rotating with the leadscrew. The 2 piece has keyways for keys to keep them from rotating. You want 1/8 inch or more gap between them to keep from running out of adjustment.
    You may be able to still use the original by drilling holes 180 degrees apart outside of the major diam. of the leadscrew through the short side into the long side. Then cut the two pieces apart and face off the cut ends on the lathe. Then press fit pins into the long side and have them a slip fit in the short side. The pins will keep the short side from turning just as being connected did. Then you could reinstall the old nut as before and have more adjustment available. You would probably have to use small pins like 1/8 or 3/16.

    Comment


    • #3
      Bridgeport had solid nuts for replacement.
      This allowed someone to either slot them, which allows you to tighten up the backlash, or cut them in to two pieces.

      Slotting the nut ( opposite the C'bore Adjusting screw location) allows the nut to compress without worrying about backlash in the keyways. Some guys do a little saw cut half way through, and some will do a 80 % cut.
      Some replacement nuts had a slot put in already ( after market ?). These are done with a slitting saw ( not a bandsaw) and look very nice. The half way cut could then be made to 80 % for a later rebuild.

      Cutting the nut in half, means that that the adjusting screw become VERY sensitive and can loosen itself later as the bronze wears. Becareful here !
      I am not a fan of this common approach, particularly when the leadscrew is worn in the middle. You can get tight movement when extending the table and the loosness in the middle will allow the screw to back off. As mentioned, any keyway slop here ia agrivated with two nut pieces

      Also
      Make sure your oil passages are open. The nut you have is plugged.
      It also does not have the oil groove ?
      Bridgeport nuts always had a keyway cut in them and a oil groove around the OD of the nut. Besure this will allign with the oil supply, if the oil port does not
      When cutting a slit in the nut, most everyone cut in the oil groove to give a little flex, even though it is not in the center of the nut.

      Rich
      Last edited by Rich Carlstedt; 01-23-2012, 02:17 PM.

      Comment


      • #4
        Originally posted by Rich Carlstedt
        Cutting the nut in half, means that that the adjusting screw become VERY sensitive and can loosen itself later as the bronze wears. Becareful here !
        I am not a fan of this common approach, particularly when the leadscrew is worn in the middle. You can get tight movement when extending the table and the loosness in the middle will allow the screw to back off.
        Im a bit confused on this one. How would an adjustment screw that has a locking screw on top of it back off?
        "I am, and ever will be, a white-socks, pocket-protector, nerdy engineer -- born under the second law of thermodynamics, steeped in the steam tables, in love with free-body diagrams, transformed by Laplace, and propelled by compressible flow."

        Comment


        • #5
          The locking screw puts pressure on the adjusting screw which is loaded against the nut/Leadscrew.
          When the load disappears because of the split nut/ leadscrew wear neither screw is locked anymore. The adjusting screw can move away , thus releasing the preload from the lock screw

          With a slotted nut, there is always back pressure on the adjusting screw due to the section of nut that remains, reguardless of nut/leadscrew wear.

          Rich

          Comment


          • #6
            Hi Guys,

            Thanks for the quick replies.

            Rich, the lube ports were pretty full of crud and I will be doing a thorough cleaning of them when I get back to it in a day or so.

            I think I will be using the new nuts for the y axis. Got em, might as well use em.

            If I run into any more issues, I will let you know.

            Thanks again,
            Brian
            OPEN EYES, OPEN EARS, OPEN MIND

            THINK HARDER

            BETTER TO HAVE TOOLS YOU DON'T NEED THAN TO NEED TOOLS YOU DON'T HAVE

            MY NAME IS BRIAN AND I AM A TOOLOHOLIC

            Comment


            • #7
              Did my first real cutting the BP. As I started my cut the X jumped as the cutter started into the work. To me it seemed like it jumped 2 feet. In reality it jumped in to the work about 1/8”. Know this I preloaded the table by pushing it into the work (I am a big guy) and it feed nice. The rest of the cuts (cutter pushing on the work) were smooth and without jumping.
              Next step??? By a nut for the “X”? I don’t know if I have a split nut? Or what one would look like.

              I guess I looking for some thoughts. The ways are tight.

              Comment


              • #8
                how much backlash is there?

                sounds like you could need to adjust the nuts up, if it's out of adjustment; more than say 20thou on a machine thats seen some action, they can be tightened, there is a manual here if you don't have one already http://www.truetex.com/bridgeport-manual.pdf see section 4.1 / 4.2

                if there isn't enough adjustment then as above in this thread you will need to remove the nut(s), this can be done without removing the table, just the end bracket and a long screwdriver to remove the adjustment screws if needed. then pull the leadscrew out if you are lucky the nuts will come out of the housing still on the screw

                If you have the later pair of nuts (to the right on top picture above ) then you can just seperate them by an extra turn and refit, if it's the one-piece nut you may need to get creative as per the comments above and split them

                according to the manual the adjustment should be about 5 thou backlash if the screws are good, in practice with an older machine you have to set it at the ends of travel and live with what you get elsewhere, unless you can manage with restricted travel at the ends and set the adjustment where you'll use the machine the most.

                hope some of this helps
                Brian

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