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  1. #1
    Hello :)

    I figured I best make a thread, it's good a place as any to start asking the myriad of questions and getting lots of criticism of my random ideas. This is going to be a bit of a living document (I'm going to change my mind when I feel like it ). You can click the pictures for bigger images.

    • To build a rig that can CNC aluminium designs

    • Cut a 500mm x 500mm size (Ok, so I picked these sizes at random :P )
    • Cut aluminium
    • Cut with accuracy
    • Cut with repeatability
    • Take a beating

    Entry 1: (26 Feb 2013)
    I have been working my way through the build threads and the huge amount of info that's around, trying to learn the basics. While I've been doing this I decided to kick off some CAD just to get back in the swing of it as it's been a few years since I last used it in anger. So I drew up some parts from data sheets as practice:

    NEMA 34, 15mm Rails

    And then I started doodling in CAD, I started with 30x30 mild steel box section with 3mm side wall, soon binned that deciding it wouldn't be strong enough.

    I've got to here so far, it's using 50x50 mild steel box section with 5mm side wall.

    • Bear in mind this is more of a concept than a design at the moment, but feel free to comment on it :)
    • The idea is to have the screw in the channel between the rails.
    • At the moment it looks like everything is welded, only because I haven't designed in any bolted joints yet, they will be there though.
    • I am not a welder, nor do i frequently work with steel, so if it looks like I have drawn an impossible weld or cut, let me know.
    • I made the bed 20mm Alu and there is another rail under it supporting.


    Husky the noob

    Entry 2: (27th Feb 2013)

    Ok, had a play with some more CAD :)

    Deleted the side rails & related
    Got rid of the aluminium bed
    Added more bars for a new bed
    changed the rails for slightly longer ones
    Added bolted section to allow fine tuning of angles

    Some pics:

    Entry 3: (3rd March 2013)

    Shortened the rails.
    Changed the bed from being welded to bolted, not quite finished yet.
    Changed the angles from box section to plate.
    Added bolted gantry cross beams
    Added bolts to attach gantry to rails. Had to enlarge box section on top of sliders to 60mm, 5mm wall.

    I need to get round to sorting the materials. I have the box section all set to mild steel, but all the fixings and rails are just set to a generic high strength steel.

    Current weight of what's on the screen ~55kg

    Entry 4: (3rd March 2013)

    Ok, drawings changed.

    Added bolts etc
    Changed gantry runners
    Changed gantry sides

    Last edited by Husky; 04-03-2013 at 04:01 PM.

  2. #2
    Like your drawings. For serious metal cutting I still think a fixed gantry would be better, and a much stiffer (double) gantry cross beam. G.

  3. The Following User Says Thank You to GEOFFREY For This Useful Post:

  4. #3
    I'll shoot first. .Lol

    Don't need Nema 34's 23's will be more than enough.

    Don't need 2x rails per side they won't add much to the machine other than expense. Linear rails can handle massive loads so one rail with 2 bearings will be fine.

    The Single piece Box on Gantry is NO where near strong enough for cutting Aluminium and will need massively beefing up.

    The 20mm Plate for Bed is Good but will still need a Decent frame building under it and 1 cross piece won't be enough.
    Don't under estimate the forces for cutting Aluminium.! . . To achieve decent quality of finish the machine needs to be very very ridged.

    What you have drawn so far won't cut Aluminium overly well and full fill your criteria.!! . . . . . More beef required, Esp on the gantry and don't be afraid of the weight for cutting hard materials it helps.

  5. The Following User Says Thank You to JAZZCNC For This Useful Post:

  6. #4
    Good thing I haven't drawn the gantry yet I'm still scratching my head on that one, the bar was kind of a place holder, don't panic haha.

    Ok, I was worried as I have never used the runners before, I shall go forth and delete the outside ones! Lets me get rid of those angles too, that would have been a pain to line up right in the build.

    Yeah I'm going back down to the 23's. I drew those before I really read into it and got some advice. They still look rather pretty though, so I put them up anyway.

    On the subject of the bed... Yeah, I'm not sure why this happened with only a 20kg load, but...

    I pressed a wrong button somewhere I think But still, I'll put more bars in as you say!

    I'll do a fixed gantry version next, I just can't get past the fact it'll be double the size of the cutting bed, minimum.

    Thanks guys!
    Last edited by Husky; 26-02-2013 at 11:13 PM.
    Just... One more project...

  7. #5
    I,m sure you pressed a wrong button. Not quite double the frame (not bed) length, but much easier to build a really strong machine IMHO. G.

  8. #6
    Too many Desperado's me thinks!
    Just... One more project...

  9. #7
    I admit to being biased!!! G

  10. #8
    Quote Originally Posted by GEOFFREY View Post
    I admit to being biased!!! G
    I agree 100% Geoff fixed gantry is Much Much better option for cutting hard materials if you have the space.

  11. #9
    OK, talked to a mate at work regarding my results shown above. Basically he said to look at the results like a scaled graph, it's scaled up so you can see it more easily but the actual numbers are the important thing. So, it could be that it's only deflecting a tiny bit, but scaled up it looks horrific. Also, I used a distributed load rather than a point load, I kind of knew this at the time but couldn't find the correct option, just wanted to see some results :P

    Looking forward to chopping it apart tonight for the next version.
    Just... One more project...

  12. #10
    You've set it to display stress, which is useful when determining if the material will fail, but that's clearly not the case here as the maximum stress is small. Also, you need to be careful at corners or near bolts as the stress outputted by the simulation at these boundaries can be a lot higher than reality, due to some of the assumptions in the calculation. We're interested in the deflection so set the display to displacement and find the maximum points. The results you get will depend greatly on the boundary conditions, so how you attach the bed to the frame in the simulation is important. The mesh quality on the simulation is very poor too, but there's not much you can do about that. If you're going to try and use FEA, then at least start with applying forces to the gantry as the gantry is going to be far less stiff than the machine bed even without additional supports. For the current simulation you needn't include the gantry and X-rails, so you might as well remove them as a significant number of the nodes in the mesh are consumed in these parts.

    The extra two rails wont make much difference because the vast majority of the force will be apparent on the other rail, with two blocks, since the structure in between is not particularly stiff. Just use one rail on each side and space the blocks out as far as you can, since the stiffness is proportional to the block spacing which means the deflection is inversely proportional to the bearing spacing squared, so increasing the spacing a little can make a big difference. That's assuming the structure they're mounted to is very stiff, which currently it isn't.
    Last edited by Jonathan; 27-02-2013 at 12:44 PM.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

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