Linear Scales

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    Default Linear Scales

    I have a Bolton ZX45 mill onto which I have retrofit a three axis DRO. I am considering doing a CNC conversion. I have glass scales on all three axis. I read that you can use these scales in place of the encoders on the motors. Is it true that the glass scales are actually more accurate than the encoders for this use?
    Thanks,
    Papajoeslc

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    Member Jim Dawson's Avatar
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    Default Re: Linear Scales

    Quote Originally Posted by Papajoeslc View Post
    Is it true that the glass scales are actually more accurate than the encoders for this use?
    Thanks,
    Papajoeslc

    In my opinion, yes. The linear scales read the actual table position, rather than where it should be based on the lead screw.

    Jim Dawson
    Sandy, Oregon, USA


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    Default

    Thanks Jim,
    That's just what I thought. So, do the scales provide the same type of positional data output to the CNC controller as the encoders do? Or do you have to get another type of controller for those scales?
    Papajoeslc



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    Default Re: Linear Scales

    The advantage of linear scales is that they totally ignore backlash becasue they only change if the axis moves. And yes, they can be used instead of a rotary encoder. Linuxcnc supports them but I'm not sure how to configure for them. I think they just output the same waveform a rotary encoder does. I would look closely at the Mesa ecosystem and Linuxcnc. It depends on whether you want to connect your PC ethernet or build it in to a control panel and if you are going touse steppers or servos. I would suggest you post some questions on the linuxcnc forum and others with much more knowledge than me will be sure to help. See https://forum.linuxcnc.org/

    Rod Webster
    www.vehiclemods.net.au


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    Default Re: Linear Scales

    Quote Originally Posted by Papajoeslc View Post
    Thanks Jim,
    That's just what I thought. So, do the scales provide the same type of positional data output to the CNC controller as the encoders do? Or do you have to get another type of controller for those scales?
    Papajoeslc
    Most modern linear scales are available with a standard quadrature signal, exactly like a rotary encoder. My personal preference is magnetic linear scales. I have both Renishaw and Ditron scales on my machines.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Linear Scales

    Thanks Jim. I used Ditron on a lathe but I sold it! It was nice gear and very well priced.
    The other advantage of using your DRO's is that you don't need to replace any of the lead screws with ballscrews or use linear rails as your machine will be backlash free from the get go.
    LinuxCNC will allow interfacing with encoders via software and a parallel port but with limited bandwidth.
    My preference would be to use Mesa cards. I use their Ethernet cards so if using steppers a 7i76e (which I use) will do everything including managing your spindle and then a 7i85 4 encoder input board to read the DRO's (But you could do it via a pci or pcie slot on your PC using a 5i25 or 6125 and a 7i76 and 7i85).
    If you decided to use servos, the 7i77 includes the servo drive interfaces and the encoders but its not ethernet. You could connect it via a PCI or PCie board such as the 5i25 or 6i25 or use an ethernet 7i92.

    Rod Webster
    www.vehiclemods.net.au


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