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Thread: CNC machine made out of granite

  1. #13
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    Default Re: CNC machine made out of granite

    Good luck scraping granite flat. If it's not flat when you get it, messing around with hand tools won't make it any flatter. Cast iron is hard to scrape, but at least it's possible. Granite - not so much...

    Andrew Werby
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    Default Re: CNC machine made out of granite

    Quote Originally Posted by awerby View Post
    Good luck scraping granite flat. If it's not flat when you get it, messing around with hand tools won't make it any flatter. Cast iron is hard to scrape, but at least it's possible. Granite - not so much...
    It's possible if you buy the right tools.... if you can remove material and you have (lots of) time and patience, you can make it flat but.... there is little point because granite is readily available already precision flat. Cast iron is not (although it can be found).

    Given the choice, and all else being equual, granite is the supperior material for this purpose. The only exception is if a person is making a dovetail slide or box ways as cast iron has a unique ability to slide.



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    Default Re: CNC machine made out of granite

    Definitely an impressive build, fox3d!

    I have several questions regarding your machine, but I'll start with the size and machineable envelope. I would really like to try to build something similar, but I wonder if I could get away with using a smaller surface plate, as not everything about a machine like this scales down very well.

    • Given the z-axis' plate width, what is the maximum travel along the width of the machine?
    • Given the thickness of the gantry, 2 times rail and bearing thicknesses, thickness of the two aluminium spacer plates on the z-axis, and the radius of the spindle housing, what is the maximum machineable depth?
    • What is the clearance between the gantry and the surface plate?




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    Default Re: CNC machine made out of granite

    Given the z-axis' plate width, what is the maximum travel along the width of the machine?
    granite surface plate is 900mm x 600mm Z axis plate with is 200mm so the travel is 700mm


    Given the thickness of the gantry, 2 times rail and bearing thicknesses, thickness of the two aluminium spacer plates on the z-axis, and the radius of the spindle housing, what is the maximum machineable depth?
    i guess it will depend on the length of your endmill. Z axis travel is around 160mm

    What is the clearance between the gantry and the surface plate?
    around 180mm.



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    Thanks for the reply!

    Quote Originally Posted by fox3d View Post
    Given the thickness of the gantry, 2 times rail and bearing thicknesses, thickness of the two aluminium spacer plates on the z-axis, and the radius of the spindle housing, what is the maximum machineable depth?
    i guess it will depend on the length of your endmill. Z axis travel is around 160mm
    I wasn't clear enough when I wrote that question. I was thinking of the depth along the y-axis. With gantry machines like yours there is usually some area in the back of the machine that the spindle can't reach. I guess if the gantry beam is 50mm thick and you use 20mm rails?(?) with a rail+block height of 30mm with two 16mm Alu plates the "lost" y-axis depth in the back is (50+2*20+2*16)mm=122mm. And that is before the spindle is attached.

    If I were to build something similar but with a smaller surface plate I probably wouldn't be able to shrink that loss very much. I guess it is possible to offset the two side slabs of granite so that they stick out/overhang some cm towards the back. Do you think that would destroy rigidity and/or create a significant cracking risk on the side slabs?



  6. #18
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    Default Re: CNC machine made out of granite

    Nothing really stopping a guy from putting a spindle on each side of that gantry beam. Not only could you machine the full bed length, but you could machine the ends of longer parts too.
    Also if a job only required two tools, you are all set.

    Lee


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    Default Re: CNC machine made out of granite

    In video #4 you described how you aligned the gantry rails parallel to the surface plate, but how did you align the rails on the side slabs? I guess it is fairly straight forward to get the two rails to match up laterally but there is no guarantee that they are straight.



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