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  • stupid math!

    I have a 16tpi lead screw that I want to put a hand wheel and dial on. My 5th grade math tells me a dial with 80 divisions will give me 0.05" per mark. 160 divisions 0.005. Is my math good?
    I'm not a Machinist, but I did stay at a Holiday Inn Express once.

  • #2
    No. You will move .0625 per revolution, 1/16". You will move .125 per 2 revolutions, 1/8". To move .001 per mark you would need 62-1/2 divisions on the dial. To move .005 per mark you would need 12-1/2 divisions on the dial.
    Kansas City area

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    • #3
      If I were a math teacher I'd probably say, "show your work".



      My back-of-envelope calc says that if a lead screw is 16 tpi then one turn gives 1/16 of an inch travel (or 0.0625").

      Dividing 0.0625 by 80 I get 0.00078125 per division.

      And dividing 0.0625 by 160 I get 0.000390625 per division.

      Yes, I know that I have too many significant digits in those answers, so please ignore the excessive precision.

      But maybe I've overlooked something or misunderstood something.

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      • #4
        Maybe, you should have stayed a couple more years.Bob.

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        • #5
          This is a good question. I have a 6 tpi leadscrew that I need to translate into understandable divisions. My plan is to put the indicator dial separately and drive it through a gear ratio- if I'm not mistaken, it would take a 100 tooth gear on the leadscrew and a 60 tooth gear on the indicator dial. That way I can get a direct reading of thous if I mark out the dial properly.

          Your procedure could be the same- except that you mount a 100 tooth gear on the leadscrew and a 160 tooth on the separate dial.

          What I had in mind was to put a toothed belt around the 'gears' instead of engaging them as normal gears. That way I believe you get a smoother transition- a more linear relative rotation of the second, indicator, gear. If your gears mesh really well with little to no play, then perhaps that's just as good.

          I'm open to correction if I did the math wrong, and I welcome comments on this idea.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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          • #6
            If you put 60 divisions on the dial each will be .001041" - Is that close enough to what you're looking for?

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            • #7
              You would be out 2-1/2 thou per rotation of the dial. That's a lot closer than the dials I've seen on at least one combo machine , but you'd still have to make sense of having 60 thou per turn, and then count them up if you're moving a few inches- then add in the error. I don't think it's worth the hassle- set it up right the first time. MHO
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                Bob, if I stayed in school I would not had the opportunity to experience chronic knee pain from refinishing floors

                1/16 = 0.0625 (random #'s I should have wrote down\0.0625)*something=80
                dang it! It made sense a week ago when I signed up for the other forum, that still hasn't "approved" me, clearly I was wrong. Thanks for the helpful replies.

                darryl, maybe if you used a pulley with a tight rubber band or some such, there wouldn't be any slop in the gears. If the hand wheel drove the screw directly with the belt driving the dial you wouldn't have to worry about slippage.
                I'm not a Machinist, but I did stay at a Holiday Inn Express once.

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                • #9
                  Originally posted by epanzella View Post
                  If you put 60 divisions on the dial each will be .001041" - Is that close enough to what you're looking for?
                  62 divisions is .00100806" error of .00048387” per revolution (under 5 tenths), .00774192” per inch or 16 turns, and .0920304” per foot . . . You will never see the error unless moveing quite a way then just set a new index point or do the math to correct using the error numbers I just gave you.
                  Last edited by C_lazy_F_Guns; 01-16-2014, 09:42 PM.
                  Anybody that thinks they know it all doesn’t even know enough to understand they know nothing!
                  Andy

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                  • #10
                    I love your handle I miss my atlas.
                    I has a 10" F model Atlas lathe with babbit bearings.
                    It was kinda shltty for a lathe, but I learned a lot on it.
                    I miss my Atlas too.

                    --Doozer
                    DZER

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                    • #11
                      The simple answer here is a 16 TPI lead screw is not a good choice if you want 0.001" divisions. Nor are any other TPI numbers where the exact answer to 1/TPI has more than three decimal places. Thus; 5, 8, or 10 TPI would be good choices as 1/5 = 0.2", 1/8 = 0.125" and 1/10 = 0.1". All of these have three or fewer decimal places.

                      If you want to put a thousandths dial on a 16 TPI lead screw you will need to use some kind of trick. For instance, you could put 125 divisions on the dial and use alternate divisions on each rotation. That would be very confusing.

                      Another way would be to use either gears or a timing belt to a second, parallel shaft with the divided dial. The gear or pulley on that second shaft would twice as many teeth as the one on the lead screw. Then 125 divisions on the secondary shaft would product two rotations of the lead screw and each of the 125 divisions on it's dial would be exactly 0.001".

                      A ring and planet gear system could probably put this gearing on the same axis, but would be more complex.

                      The good TPI choices for getting 0.001" divisions are: 1, 2, 4, 5, 8, 10, 20, 25, 40, 50, 100. Outside of that range there are others, but it gets ridiculous. Any others in that range would require gearing or some other trick.
                      Last edited by Paul Alciatore; 01-16-2014, 11:35 PM.
                      Paul A.
                      SE Texas

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

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                      • #12
                        Math is not stupid. Math works. Math is the most logical thing you will ever encounter.

                        Math is your friend! Embrace it.
                        Paul A.
                        SE Texas

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

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                        • #13
                          Starts to make a Digital Read Out look very attractive doesn't it? They have come down in price, and there are some quick and dirty ways to use a 6" vernier calipers with magnets and/or clamps to give you a very useful "travel dial" of sorts.

                          doug

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                          • #14
                            You could buy a 20 tpi lead-screw for about 10 bucks a foot, thereby saving the cost of a pack of paracetamol

                            Phil

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                            • #15
                              Using a 16tpi lead screw 125 divisions on the dial will give you a resolution of 0.0005/division. In my mind the fact that the lead screw moves 0.0625/rev is a major drawback. When traversing having to count by 16ths of a inch is pain when working to decimal inches. I'd switch to a 10tpi lead screw with 100 divisions on the dial (0.001/div, 0.01/rev).
                              Last edited by jlevie; 01-17-2014, 01:48 PM.

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