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  • 5 tpi

    I have a Mcdougal lathe must be close to a century old, with 5 TPI crosslide feed screw, square thread, would this TPI be typical of other lathes? has about 100 thou backlash, can make a new brass nut, but was thinking about a split nut with a pinch bolt, so the backlash could be setup later on . any thoughts would be appreciated.

  • #2
    Try it with a piece of aluminium , and grind the tool to aproximate the less used section of the original. split the nut and use a couple of pipe clamps to test it.
    Michael

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    • #3
      Originally posted by muskegsmith View Post
      I have a Mcdougal lathe must be close to a century old, with 5 TPI crosslide feed screw, square thread, would this TPI be typical of other lathes? has about 100 thou backlash, can make a new brass nut, but was thinking about a split nut with a pinch bolt, so the backlash could be setup later on . any thoughts would be appreciated.
      5tpi or 0.200" lead.

      Yep, common as muck.
      Paul Compton
      www.morini-mania.co.uk
      http://www.youtube.com/user/EVguru

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      • #4
        I would consider purchasing a precision acme thread rod and nut to match. If the nut wears out replace it.

        Take a look:
        http://www.mscdirect.com/FlyerView?p...alogs/big-book

        Jim
        So much to learn, so little time

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        • #5
          Maybe convert the cross slide screw to 10tpi for improved control of the feed depth? Just a thought.
          ----------
          Try to make a living, not a killing. -- Utah Phillips
          Don't believe everything you know. -- Bumper sticker
          Everybody is ignorant, only on different subjects. -- Will Rogers
          There are lots of people who mistake their imagination for their memory. - Josh Billings
          Law of Logical Argument - Anything is possible if you don't know what you are talking about.
          Don't own anything you have to feed or paint. - Hood River Blackie

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          • #6
            Don't pick the solution until you know the problem.

            Yes, it COULD be wear, but NOT ALL OF IT, most likely.

            Your "advance" is 0.2 per turn. The "land" of the thread is 0.1" wide, roughly.

            If all your 100 thou was wear, you'd see it for sure, because that's the full width of the thread. One thread worn smooth, or both the nut and screw each worn to half thickness..... not likely unless you can see it.

            Causes of backlash....

            1) Obviously, wear. But as noted that is a limited amount before the thing ceases to function. The wear may be distributed between teh screw and nut. Unless you can see the screw is worn to a nub, a lot of it would have to be on the nut, IF it is all wear, and a new nut might make a big difference. I doubt it's all wear, though.

            2) Loose feed nut.... if the feed nut is not solidly pinned to the crosslide, it will move before any movement of the slide occurs. Be sure the nut is solidly mounted. Any movement adds to the backlash

            3) Slop in the feedscrew, endwise movement. You SHOULD see this. Obviously if the screw moves before it comes up against something solid, then that will add just that much to the backlash.

            You will ALWAYS HAVE backlash. But no reason to have more than you need to have, so check out the other causes before sourcing a new screw, etc, etc.
            Last edited by J Tiers; 02-07-2014, 09:44 AM.
            1601

            Keep eye on ball.
            Hashim Khan

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            • #7
              Dont forget the 'Evanut'!!

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              • #8
                I have to go with J. It may not all be backlash.

                Rig a DI to indicate the cross slide position. Pull the cross slide toward you as hard as you can: that should take up all looseness, not just the backlash. Now push the shaft/handle/dial forward as hard as possible: this will take up all other forms of looseness, just leaving the backlash. Set the DI to 0. Now push the cross slide away from you as hard as possible. The DI reading will show the backlash only.

                You may want to do this at several places along the screw as the backlash may, probably will, be different.
                Paul A.

                Make it fit.
                You can't win and there is a penalty for trying!

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