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RE: [SherlineCNC] cheap DRO from caliper?

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  • John Dammeyer
    They don t compensate for backlash. Nothing attached to a lead screw can. Only a preloaded ball screw might work well enough. Take a look at the Shumatech
    Message 1 of 7 , Oct 15, 2008
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      They don't compensate for backlash. Nothing attached to a lead screw can.
      Only a preloaded ball screw might work well enough. Take a look at the
      Shumatech DRO 350 if you want to use the Chinese scales with a nice large
      LED readout for a DRO. The kit is not too expensive and has a good user
      group.

      John Dammeyer


      >
      > Has anyone drilled some holes into a caliper and used it as a DRO for
      > the sherline? Seems like a cheap way to get 3 axis DRO. The sherline
      > DRO that they sell is rotary so I'm not sure how they compensate for
      > backlash.
      >
      > ------------------------------------
      >
      > Yahoo! Groups Links
      >
      >
      >
      >
      >
    • Henry Liu
      Thanks for the heads up. Backlash can be compensated pretty easily in my own driver routine. I plan on running my cnc through matlab or labview but Mach3 or
      Message 2 of 7 , Oct 15, 2008
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        Thanks for the heads up.

        Backlash can be compensated pretty easily in my own driver routine. I
        plan on running my cnc through matlab or labview but Mach3 or
        something might not have compensation.

        The basic algorithm is:

        while(error<epsilon)
        {
        error=DRO_position-assumed_position;
        assumed_position=error;
        call move_stepper_function(assumed_position)
        }

        The DRO will say what position you are at and output an error. If you
        overshoot, just move back the other direction.

        Thanks for the heads up on the shumatech DRO. They reference cheap
        chinese linear scales but I can't find these on ebay. Anyone know
        where to buy these "cheap chinese scales" Thanks.

        Henry

        On Wed, Oct 15, 2008 at 4:08 PM, John Dammeyer <johnd@...> wrote:
        > They don't compensate for backlash. Nothing attached to a lead screw can.
        > Only a preloaded ball screw might work well enough. Take a look at the
        > Shumatech DRO 350 if you want to use the Chinese scales with a nice large
        > LED readout for a DRO. The kit is not too expensive and has a good user
        > group.
        >
        > John Dammeyer
        >
        >>
        >> Has anyone drilled some holes into a caliper and used it as a DRO for
        >> the sherline? Seems like a cheap way to get 3 axis DRO. The sherline
        >> DRO that they sell is rotary so I'm not sure how they compensate for
        >> backlash.
        >>
        >> ------------------------------------
        >>
        >> Yahoo! Groups Links
        >>
        >>
        >>
        >>
        >>
        >
        >
      • Ron Ginger
        ... Your algorithm for backlash is fine, as long as you have the ability to move the axis with INSTANT acceleration. If less than instant move there will be a
        Message 3 of 7 , Oct 16, 2008
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          >
          > Thanks for the heads up.
          >
          > Backlash can be compensated pretty easily in my own driver routine. I
          > plan on running my cnc through matlab or labview but Mach3 or
          > something might not have compensation.
          >
          >

          Your algorithm for backlash is fine, as long as you have the ability to
          move the axis with INSTANT acceleration. If less than instant move there
          will be a time when the axis is unrestrained and the tool can drag it.
          You will at best leave a divot in the part, at worst you will grab and
          break a tool.

          It is not hard to write an algorithm for backlash, it is not possible to
          make one work correctly. Mathematically easy, just the laws of physics
          that make it hard. This has been tried, and discussed here and on every
          CNC forum or list for several years. Good luck with your algorithm.

          ron ginger
        • alenz2002
          It is not hard to write an algorithm for backlash, it is not possible to make one work correctly. ron ginger Hi Ron, Succinctly stated with a
          Message 4 of 7 , Oct 18, 2008
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            <snip>
            It is not hard to write an algorithm for backlash, it is not possible
            to make one work correctly.
            ron ginger
            <snip>

            Hi Ron,
            Succinctly stated with a keen eye for the obvious!
            Al Lenz
          • Fred Smith
            ... can. The Sherline digital readouts do compensate for backlash: http://www.sherline.com/8100inst.htm The same as most of the currently popular CNC
            Message 5 of 7 , Oct 18, 2008
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              --- In SherlineCNC@yahoogroups.com, "John Dammeyer" <johnd@...> wrote:
              >
              > They don't compensate for backlash. Nothing attached to a lead screw
              can.

              The Sherline digital readouts do compensate for backlash:

              http://www.sherline.com/8100inst.htm

              The same as most of the currently popular CNC controllers compensate
              for backlash.

              We are working on an accessory for our servo systems that will tap into
              the rotary encoders and provide 4 decimal inch readout, screen
              displays. The backlash tracking is part of the process, because, just
              like a CNC'd Bridgeport with a digital readout, which DRO do you
              believe, the one on the controller or the one on the scales?

              Fred Smith - IMService
              http://www.imsrv.com
            • John Dammeyer
              My apologies. The Sherline I played with didn t have the instruction manual for the scales and the backlash compensation was obviously not enabled. That it
              Message 6 of 7 , Oct 20, 2008
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                My apologies. The Sherline I played with didn't have the instruction
                manual for the scales and the backlash compensation was obviously not
                enabled. That it didn't compensate led me to an invalid conclusion.

                John Dammeyer


                Automation Artisans Inc.
                http://www.autoartisans.com/ELS/
                Ph. 1 250 544 4950


                > -----Original Message-----
                > From: SherlineCNC@yahoogroups.com
                > [mailto:SherlineCNC@yahoogroups.com] On Behalf Of Fred Smith
                > Sent: Saturday, October 18, 2008 8:29 AM
                > To: SherlineCNC@yahoogroups.com
                > Subject: [SherlineCNC] Re: cheap DRO from caliper?
                >
                >
                > --- In SherlineCNC@yahoogroups.com, "John Dammeyer" <johnd@...> wrote:
                > >
                > > They don't compensate for backlash. Nothing attached to a
                > lead screw
                > can.
                >
                > The Sherline digital readouts do compensate for backlash:
                >
                > http://www.sherline.com/8100inst.htm
                >
                > The same as most of the currently popular CNC controllers compensate
                > for backlash.
                >
                > We are working on an accessory for our servo systems that
                > will tap into
                > the rotary encoders and provide 4 decimal inch readout, screen
                > displays. The backlash tracking is part of the process,
                > because, just
                > like a CNC'd Bridgeport with a digital readout, which DRO do you
                > believe, the one on the controller or the one on the scales?
                >
                > Fred Smith - IMService
                > http://www.imsrv.com
                >
                >
                > ------------------------------------
                >
                > Yahoo! Groups Links
                >
                >
                >
                >
                >
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