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  1. #1
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    Jul 2008
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    5

    Granite base router

    Hi all
    After running my extrusion based router for the past year, I've decided to build a new frame using a granite surface plate as the base. I want it to cut aluminum. The one I built does a fair job, as long as you go slow. Very slow. I'm now in design paralysis when it comes to the gantry. The machine will be fixed gantry moving table layout. The base plate is a granite surface plate 24x36x4 inches. The gantry is on the long axis. I'm undecided as to the beam material, steel vs granite. If I go steel, it would be 8x8x0.5 inch square tubing. I think that would be stiff enough. I have most of the details worked out, but it will require some outside machining I would like to avoid, so I'm leaning toward granite. The thickest table top granite I can find locally is 30mm. What about laminating four slabs together with epoxy? Gantry support will be 5x5x0.375 inch sq steel tubing, with 0.5" plate welded each end. The unsupported length of the gantry will be 26 inches.

    I plan on moving all the motion parts from the current machine to the new one. I have Hiwin 25mm HGH rails and carriages, 2505 screws driven by clearPath servos controlled by a Masso G2. All these parts work great, with NO issues.

    ANY help you guys can give would be very much appreciated!

    Ben

  2. #2
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    Jan 2008
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    1047

    Re: Granite base router

    Have to you looked at the basic values for stiffness of the steel vs (laminated) granite beam?
    That's where you should start.

    How are you planning to mount the linear rails?
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

  3. #3
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    Re: Granite base router

    Hi pippin
    I have played around with online calculators using Al values for the granite, but that probably is not correct for a laminated beam. The beam would be
    215mmx120mm. Four slabs 30mm thick.

    The rails will be mounted using anchors set in drilled holes with an Al2o3 epoxy mix. I will use the surface plate to insure alignment. Rail alignment on the extrusion
    router was a nightmare!

    I did a crude stiffness test on the present router using a spring scale on the spindle with all axis centered. I got deflection values of 0.0007" on Y and 0.0026" on X. This was a 22Lb (100N) load.
    I would like to improve on this, a lot. Your suggestions are welcome. I have followed your writing for quite some time and have learned a lot.

    Ben

  4. #4
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    May 2016
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    189

    Re: Granite base router

    This is what I think you want.

    https://www.youtube.com/watch?v=gUFeoDf1YvM


    The other option is to have the gantry move up and down for Z and have the spindle fixed to the X-slide. In that case I'd suggest steel as the gantry beam and just bite the bullet and pay for machining. The beam needs to be lighter if it's moving and I think steel would be better than a laminated granite beam in this configuration.

    -Ralph

  5. #5
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    5

    Re: Granite base router

    Hi all
    I decided on using granite for the gantry and some progress has been made. I bought a drop from a local granite table top supply and had them cut four slabs 900x225x30mm. I picked the flattest one, using the surface plate, and used that as the face for the gantry. I put this side down on the surface plate and glued each plate one at a time to make it four layers thick. I used epoxy mixed with Al203. The mix was about 75% Al203, quite thick. After this cured, I tested the face for flatness using the surface plate, it was out by about 0.002 to 0.004". I then lapped the rail surfaces using a 2"x 4'" diamond lap. This took some time, and I quit when the worst error was about 0.0006". I may work on this again after the rails are mounted, and tested.
    I made anchors using coupling nuts that I turned round, leaving a small hex to prevent turning. I knurled the round section to help with grip. I did pullout tests with anchors glued in with epoxy/Al203 mixed about 60%. The anchors would hold to around 200 to 300 Inch Lbs. I was surprised they held this kind of force, about 4000lbs. The anchor always pulled through the epoxy. The epoxy bonds WELL to the granite.
    The gantry support tubes and plates are welded up.
    Their is one part of the design that I have not decided on. Do I need two screws on the Y axis? The Rails will be 500mm apart with the screw in the middle. I'm trying to make this thing stiff. The Y plate will be, most likely, steel T-slot about 300x600x20mm thick running on 25mm HGH rails with the long carriages. Do you think it will rack?
    Opinions welcome.

    Ben

  6. #6
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    May 2005
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    1496

    Re: Granite base router

    No drawings to share? I'm picturing something like this where X would be the direction of the moving table.
    You will need a stiff table as the blocks are the only support.
    Click image for larger version. 

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    Anyone who says "It only goes together one way" has no imagination.

  7. #7
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    1496

    Re: Granite base router

    Okay what I labelled 'X' you are calling 'Y'.
    The gantry spans the longest dimension, or maybe I misunderstand the meaning of "The gantry is on the long axis", that's why drawings help.
    The more the bearing spacing resembles a large square the more resistance to racking. Otoh the rails being used are hefty so a lot of resistance to racking is built-in.
    Anyone who says "It only goes together one way" has no imagination.

  8. #8
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    Jul 2018
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    1148

    Re: Granite base router

    Hi Mr Ben - Granite is only half the stiffness of aluminium but since you have made the gantry solid it will be very stiff and damp. Granite is about twice as damp as Al. Photos would be good, we like to watch. Tables are typically driven with one drive, no need for two unless you need more grunt. Peter

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