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  • Formulas

    Does anyone know the formula to set up the compound rest on a lathe where you can feed in .001 on the the dial and move the doc in by .0001 Mr Addy I'm sure that you know this one.

  • #2
    sin(theta) = .0001/.001

    therefore theta = arcsine(1/10)

    theta = 5.73 degrees. You can round to 6 degrees.

    It's just a basic trig function. You want to move in .0001 for every .001" you move along the hypotenuse. Draw a triangle, and figure out your ratios.



    • #3
      ok that's pretty close to how I figured it but now I have another question. Is it worth the time and will it give consistant results?


      • #4
        Will it give consistent results? As repeatable as your lathe is; the math won't change!

        Whether it's worth the trouble or not is up to you. More objectively, it depends on what the particular job is that you're doing.
        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


        • #5
          I know a lotta guys who've talked up the 1 10 to 1 slope gimmick as a means of securing finer infeed resolution but few continue its use after the first few trials. Problem is there are other factors to interfer with cutting exactly what you dial like edge build-up, heat, tool wear, deflection, and so on far into the night.

          Most canny machinists confronted with holding +0.0006 -0.0000 tolerence for a class 5 bearing fit stop a thou or two oversize and polish/file/stone to final size. Takes a few minutes but prevents that nasty 0.0002" overshoot. I prefer the stone and wet-or-dry method myself.

          Besides dialing off 0.010 on a slope to get 0.001" infeed means you have to tweak your end stop if there's a shoulder to contend with.

          My 0.0001" resolution DRO on my lathe has made me lazy. I just set the DRO from a mike size and go from there. I hit 'em most of the time but I still stop short and hand work the last thou on tight tolerences. I may be lazy but I know machine work, probablility, and Murphy's law.


          • #6
            I set my compound at 6* when using my tool post grinder. But for cutting, it's just splitting hairs too finely to be useful, for all the reasons Forrest mentioned.


            • #7
              That Murphy guy really has it in for me. sometimes I wish he would make things go wrong somewhere else and leave me alone for a week or two