Thread: Siff' Build Log

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  1. #21
    I've taken a break from getting the drivers to work to re-think (again!) my cnc layout, I came up with the design below. What do you guys think? The Y axis is more secure and lower down, the end plates are more substantial and will have 4 x m8 fixings each and the aluminium rail is 60 x 60 all around. I haven't added in the Y axis ballscrew yet but I think i'll put it along the top. I've also used one solid piece for the back-plate which will hopefully add a bit more strength. It needs it.. All my rails arrived yesterday and I didn't realise how heavy they all were combined :confused:

  2. #22
    Looks much better to me.

    Putting the Y-axis ballscrew along the top is not a good place since it's so far from the cutting force. The Y-axis bearings obviously don't provide any support parallel to the Y-axis, so they would be pushed slightly in that direction. If the screw is lower down it's less of a problem. The best place would be to put it on the opposite side, but that's a bit in the way...

    Not sure about the way you've mounted the X-axis linear bearings. Though the forces might by more even, it's much much harder to get them parallel. I'd keep the orientation you had previously. I think it's best to have them that way anyway as the forces are not even - you've got the weight of the gantry loading them in one direction.

  3. #23
    Ok thanks, think I'll stick with the previous design then, because that solves most of the problems.

  4. #24

    Hi guys, whilst i'm waiting for my extrusion to come, I've been faffing with my design and one think that has struck me is how off balance the z axis spindle is in relation to the linear bearings/center of gravity. Maybe i'm underestimating the strength of the rails/aluminium but would the design be stronger if I were to add some type of counterbalance or am I looking into this too much?


  5. #25
    That looks stronger than previously :)

    Put the rails on the Z-axis and bearing blocks on the Y axis box as that's stronger. With how you've drawn it the Z plate will bend wherever the Z-axis is. Third time I've posted that this week...wait you had it like that in post #21, why the change?

    The centre of mass is not ideal, but it depends how you look at it. Ideally you would have the contact force at each bearing equal. If you calculate the forces and compare to the ratings of the bearings ( I should find the spreadsheet I did for that) then you will find its not significant. If you're drilling something the forces will change and actually get more balanced.

    You could compensate by mounting the gantry further back between the bearings so that the spindle is closer to them. Ideally work out the centre of mass of the gantry assembly, minus X bearings, and mount it such that that point is directly between the two bearings. That's what I did, but couldn't get it that far back so ended up about 30mm off the centre of mass.

  6. #26
    Ignore the z axis, I've just been faffing (a product of impatience) and this is by no means my final design.

    Cheers, if you have that spreadsheet still, id appreciate it! 'If you're drilling something the forces will change and actually get more balanced' is something I never considered. I imagine you could go on forever in the pursuit of perfection but i'm happy in the knowledge that it will probably be negligible

    Cheers, Joe

  7. #27
    Just an quick update to let you guys know that this project is still alive...

    I have been mega busy with work lately. I work in the solar PV industry, and as some of you may know, they have slashed the FiT so the last month has been hectic getting installs done.

    Anyway, Not much progress has been made other than I have just bought a new stepper and driver so I can have dual steppers on the X axis.

    I'm off for three weeks soon, so I'm going to get stuck back into it!!

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