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Re: [SherlineCNC] Lathe Turning Results in Larger Diameter Than Planned

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  • William Rutiser
    ... How long was the workpiece? How was it held in the lathe? Did you have tail-stock support? -- Bill
    Message 1 of 34 , Dec 13, 2010
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      KWC wrote:
      > I've seen mention of this issue posted here before but never saw a solution. Now I'm experiencing the problem myself.
      >
      > I'm using CNC to turn a fairly intricate profile with 6061 on the lathe. I use a single RH carbide tool and remove .005" per pass. When the job is complete, the diameters of some of the profile sections are several hundredths too large.
      >
      > At the end of the job I brought the tool back to the touchoff location and found that the DRO was several hundredths less than it should be. Curious, I touched off the tool a second time (to correct the error) and reran the job on the already turned piece. This time it cut a lot of air but when it got close to the end of the job it started cutting again and now the dimensions are dead on.
      >
      > I know I have the backlash properly compensated in Mach3, so I'm guessing the X cross slide has some give in it.
      >
      > Does this make sense? How do I adjust the machine to eliminate this problem?
      >
      > Thanks,
      > Ken
      >
      >
      >
      >
      How long was the workpiece? How was it held in the lathe? Did you have
      tail-stock support?

      -- Bill
    • imserv1
      ... Draw it in diameter and scale by 2X as your last operation. That way you only have one chance to make a mistake, instead of on every diameter you convert
      Message 34 of 34 , Dec 20, 2010
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        --- In SherlineCNC@yahoogroups.com, "Ken Condal" <kencondal@...> wrote:
        >
        > I see what you're saying about diameter mode if you're working from someone
        > else's drawings.
        >
        >
        >
        > As a hobbyist, I draw all my own parts. My CAD program only needs the
        > profile of the part (from the centerline up) so I'm accustomed to thinking
        > in terms of radius as I draw.

        Draw it in diameter and scale by 2X as your last operation.
        That way you only have one chance to make a mistake, instead of on every diameter you convert to radius.

        How do you measure your part? Do you use radius or diameter?
        If you measure a part at .251 and want .250, do you go back to your Cad program and change everything by .0005, or just adjust your lathe coordinate system by .001 to accomodate it. ( you probably divide the measurement by 2 and adjust by that amount, twice the work and twice the chance to make a mistake)

        If your part is tapered and goes from .251 at the front and .249 at the back, is it oversize or undersize? How do you compensate? The diameter G-code is
        G1 X.250 Z.1
        G1 Z-1.0

        The solution is to decrease the size by .001 and program out the taper with the following modification:

        G1 X.250 Z.1
        G1 X.252 Z-1.0

        If you don't have a concern about the tolerances in this example, you don't need the "improved" positioning that a 4 jaw chuck can offer. You may as well save yourself a bunch of wasted time spinning the chuck and indicating it in when all you have to do is tighten the Tommy bars on a very adequate self centering 3 or 4 jaw chuck.

        You measure your parts in Diameter. If you also program in Diameter, you will save time, effort and reduce mistakes compared to using radius mode.

        Just because an amateur provided you with a radius post processor with your cad cam is no reason for you to also jump over the same cliff.

        Fred Smith - IMService
        http://www.imsrv.com
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