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  • Half-nut

    Not sure if I have the terminology correct in the subject. I'm trying to revive this old (super duper expensive in its day) microscope. I am 99% there but ran into a major problem. The Y axis has a major issue. I was super bummed to find that out. The Y axis knob turns but the stage doesn't move. I forget what it's called, probably a half-nut, and it's assembly is missing. It's spring loaded and engages the Y axis stage to the threaded shaft so when you turn the knob the stage moves. The X axis works fine (shown on the left below). Extremely doubtful that parts are available or could be machined (by me). I'm not sure the X and Y 'components' are designed symmetrically but assuming they work in a similar way. All suggestions welcome.
    You may only view thumbnails in this gallery. This gallery has 3 photos.
    Last edited by Smokedaddy; 02-21-2020, 03:39 PM.

  • #2
    shouldn't be too hard to make if you have a mill - make or buy a tap to match the thread size and pitch, drill and tap hole in chunk of material, cut half the hole off, then machine the rest of the piece to match the other one. You could probably do it with hand tools (drill, saw, file) if you can find the right tap or a sacrificial nut to graft onto your version.

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    • #3
      An EvaNut would probably work. You could also cast a polyurethane nut for it.

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      • #4
        Originally posted by elf View Post
        An EvaNut would probably work. You could also cast a polyurethane nut for it.
        Hi elf, not sure what a EvaNut is. That's a long thread. Thanks for the suggestion.

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        • #5
          Originally posted by mattthemuppet View Post
          shouldn't be too hard to make if you have a mill - make or buy a tap to match the thread size and pitch, drill and tap hole in chunk of material, cut half the hole off, then machine the rest of the piece to match the other one. You could probably do it with hand tools (drill, saw, file) if you can find the right tap or a sacrificial nut to graft onto your version.
          Out of my league but I'm willing to try anything. I have an old Bridgeport. BTW, what sort of thread gauge measures threads like that?

          -JW:

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          • #6
            Originally posted by Smokedaddy View Post

            Out of my league but I'm willing to try anything. I have an old Bridgeport. BTW, what sort of thread gauge measures threads like that?

            -JW:
            That is a very small ACME thread. What a halfnut does is to engage the threads so that it pushes or pulls the stage as the screw turns. If the position does not have to be precisely matched to the position of the knobs (as in .36 degrees of turn moves it .010 inches) you can kludge together a usable substitute. You measure it with a ruler and magnifier, and then duplicate the general pattern of grooves on a piece of delrin, brass or cast iron with a small endmill in your bridgeport.

            You can also simply cut a piece of material to fit the mechanism and then use some bluing on the teeth of the screw to transfer the tooth spacing to the block. A dremel or file will cut out the rough shape of the teeth. Believe it or not, some locksmithing is done using a similar technique.

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

            Location: SF East Bay.

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            • #7
              Originally posted by danlb View Post

              That is a very small ACME thread. What a halfnut does is to engage the threads so that it pushes or pulls the stage as the screw turns. If the position does not have to be precisely matched to the position of the knobs (as in .36 degrees of turn moves it .010 inches) you can kludge together a usable substitute. You measure it with a ruler and magnifier, and then duplicate the general pattern of grooves on a piece of delrin, brass or cast iron with a small endmill in your bridgeport.

              You can also simply cut a piece of material to fit the mechanism and then use some bluing on the teeth of the screw to transfer the tooth spacing to the block. A dremel or file will cut out the rough shape of the teeth. Believe it or not, some locksmithing is done using a similar technique.

              Dan
              Kinda looks like acme thread, cannot imagine a lot of load on it. Might try gently heating one end enough to heatform a thread in a halfnut blank of HDPE, then hand trim to fit, copying the existing working unit.

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              • #8
                Originally posted by Smokedaddy View Post

                Hi elf, not sure what a EvaNut is. That's a long thread. Thanks for the suggestion.
                EvaNut was the term coined for the nut produced by heating and pressing acetal (Delrin) around a leadscrew. They are pretty easy to make with a heat gun and a vise. I've made several for my camera macro panorama setups. The process creates a backlash free nut.

                There are other epoxies like Moglice which can be used to cast nuts.

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                • #9
                  If you can measure the diameter and pitch of the thread, please post the figures. Also, because the forces are not very high, you could make a much simpler device to take the place of the nut. Imagine holding a piece of thin metal plate which just fits in the thread. Turn the thread and the thin plate will move along the thread just like a nut. Copy the hinged lever from the other axis and glue in a thin piece of steel with a semi-circular cutout to fit the thread into a saw cut in the lever. Angle the saw cut slightly to match the angle of the thread.
                  Last edited by old mart; 02-21-2020, 06:13 PM.

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                  • #10
                    Some times you can buy long acme nuts.. or someone here may have a tap..
                    we need diameter and pitch..

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                    • #11
                      Have you tried contacting the company to see if they can offer any info or even parts ?
                      https://www.semprex.com/index.html

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                      • #12
                        There should be so little force required that a heat-formed nut should be perfectly good indefinitely.
                        CNC machines only go through the motions.

                        Ideas expressed may be mine, or from anyone else in the universe.
                        Not responsible for clerical errors. Or those made by lay people either.
                        Number formats and units may be chosen at random depending on what day it is.
                        I reserve the right to use a number system with any integer base without prior notice.
                        Generalizations are understood to be "often" true, but not true in every case.

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                        • #13
                          Thanks for all the suggestions. I will measure it this weekend. Yes I have contacted the manufacture via email. No response yet.

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                          • #14
                            What you could do is cast one using epoxy putty. Use a mold release on the screw and use a holder of sorts to contain the putty. You'll end up with about a 1/3 nut, which should be lots of thread engagement for that use. You can work that stuff with your fingers and use water not only as a clean-up but as an additional anti-stick agent as you press the screw into the putty and press the putty into the threads to the height you want. Kind of like working with silly putty. At some point as the putty is beginning to harden, you make sure you can break the screw free, then place it back in the formed threads until full cure. I've done this, it works.
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                            • #15
                              As I mentioned before, I haven't heard back from the manufacture. I am not totally thread savvy but I did read up on the nomenclature. Assuming it is NOT metric, it appears to be 16 TPI, with a major diameter of 0.25". Here is a close up view of the working side.
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