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Bridgeport Knee binding

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  • Bridgeport Knee binding

    Well out of the blue my brideport knee won't raise past a certain
    point anymore.

    And Murphy's law is in full effect: With all my strength I can get it
    to within about 0.1" of the cutter -- with the spindle fully extended.

    It's about 1/2 way up the column more or less.. I can lower it just fine..
    when I raise to that certain point it starts to bind. Like the knee is
    locked. If I reverse I can get maybe a full turn or two and then the
    knee "drops" a bit.. like when you lower the table a hair and release the
    lock you know that "thump"?

    Before I tear into this any wagers what it might be?

    I have way guards on it the ways look clear (ie no big chips in there) and
    it always happens at the same spot.

    Lock seems to work fine everywhere.

    Could this be a broken gib? (tightening as I raise it?)
    I really hope not. I have X and Y gibs (waiting for that special day
    when I can break the BP down and refit them) -- but I never
    bought a replacement knee gib.

    Tony

  • #2
    There could be a number of reasons......... first if you had a broken gib you would be able to lower the knee, but raising it from any point would cause it to bind as the broken piece of gib would instantly start to bind on the way up. I would check it anyway. Also check the lock pin. Did you make any adj. to the gib at a different point in the colume??? Sometimes adjusting the gib at a loose point will cause binding at point in the colume with less wear.
    Lastly there coiuld be a piece of swarf stuck in the thread of the screw.
    Also look for any external things, such as misc readout brackes etc.

    JL................

    Comment


    • #3
      Thanks JoeLee, yeah I'll be digging through it tomorrow
      and no, no recent adjustments, its been fine for 5 years now like
      I said pretty much out of the blue. Lets hope its just a chip.

      Tony

      Comment


      • #4
        Check the elevating screw. It's telescopic. One piece screws into the other. Ones in a great while, one piece will lock up and you have about 1/2 the travel you need.

        Never ever apply "all your stregth" to an axis motoin. Any unexpected unexplainable resistance (clamp dragging etc) and that's your cue to stop and investigate.
        Last edited by Forrest Addy; 01-25-2012, 03:51 PM.

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        • #5
          Thanks Forrest.. how do I go about checking that? I'll break out the drawings
          to see if I can learn something there. Thought it was one piece.

          Tony

          Comment


          • #6
            before you take the knee gib out ..make sure the whole of the knee is supported from the ram .



            the adjustment screw ..the head of it is in a slot in the gib.




            you can just see the slot to the far right of the picture

            Last edited by aboard_epsilon; 01-25-2012, 03:49 PM.

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            • #7
              after its out ..looks like this




              i spent a couple of hours sawing bits of string up and down the gap ..whilst rinseing with kero .

              while you are there, make sure the oiler holes are clear .

              all the best.mark
              Last edited by aboard_epsilon; 01-25-2012, 03:51 PM.

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              • #8
                There is a screw in the column to prevent you from raising the knee too high.
                It may have come out and hit the top of the knee, and you can't see it with way covers.

                Rich

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                • #9
                  Epsilon, The shop vac is your friend. It will suck up all those beards of chips as they emerge from the disassembly. .

                  Comment


                  • #10
                    Originally posted by Forrest Addy
                    Never ever apply "all your stregth" to an axis motoin. Any unexpected unexplainable resistance (clamp dragging etc) and that's your cue to stop and investigate.
                    Agreed! For a good reason why, Try pushing on your table and use the handle to move the axis. Then realise that a light turn on the handle = way more force then you could ever exert. after disengaging the leadscrew I hardly even move my table against the resistance of the gibs by pushing on the table.

                    "All your strength" = Something will bend and/or snap and it won't be something cheap.
                    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

                    Comment


                    • #11
                      My main advice is to under no circumstances let the knee get all the way down when something is wrong. If the gib wedges tight it can generally be freed by lowering the knee. If it wedges with the knee at the bottom it is likely to be a major operation to get it loose. Like laying the mill on its back and cutting a hole thru the base to drive the gib out, or something similar!

                      My first step would be to remove the gib with the knee as high as possible without binding. Don't try to force it if the screw does not want to lift it. That is almost sure to break the notch out and make it much harder to remove.
                      Don Young

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                      • #12
                        Originally posted by Forrest Addy
                        Check the elevating screw. It's telescopic. One piece screws into the other. Ones in a great while, one piece will lock up and you have about 1/2 the travel you need.

                        Never seen one of those on a BP. Just a single jack screw that has a top bearing to push the knee working against a brass nut on the bottom post.

                        Carefully check the gib... Pull it out or use a pin to drive from the bottom (knee raised a bit), and see if it's on one piece, or two.

                        Comment


                        • #13
                          If youve never had the table off, now is a good time to pull it and clean out the knee. Personally, I would go through all parts to all three axes and clean/check them thoroughly. You can save your time/effort now, but you will pay greatly for it later.
                          "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


                          • #14
                            Broke the head off of the gib:





                            Not as bad as I'd feared, I suppose. I imagine I can just weld/braze this
                            back on for now?

                            The bigger problem is that the gib looks like it hasn't seen oil in quite
                            some time. The wear area looks to be about 0.020" or so.. at least by
                            feel anyway.

                            Comment


                            • #15
                              lots of wear there ..you can clearly see it ...the same wear will be on the knee ways ..

                              if it has that sort of wear /...in what i think is a low wear part of the machine..

                              then the rest may be worse

                              i say put it back together ..and put it on Craig's list .

                              find i nicer one for hopefully the same price as yours sells for

                              another pic of mine ..

                              it is a little bit worn in that area ..but not as much as yours ..mines a 1965 machine



                              all the best.markj
                              Last edited by aboard_epsilon; 01-26-2012, 09:40 AM.

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