1. #1
    I've put a lot of thought into the force and travel requirements of a CNC machine and what I have come up with differs from the usual design. My machine is mostly based on milling softwood at very high speed with a clean planed finish. Cost is very much an issue but quality within tolerance is more important.

    Diagram is of cross section of Y-axis including X-Axis rail. Requirements are X-axis 1000mm, Y-axis 1600mm, Z-axis 20-50mm (Yes, Y-Axis is larger than X-Axis!)

    Quick diagram key:
    Dark grey: X axis rail, supported above the bed and approximately level with cutting surface.
    Green: Carriages
    Yellow: Aluminium plate - the width of the carriage to support the Blue C-sections.
    Blue: Aluminium C-section providing the basic structure of the Y-axis. Dotted lines represent possible I-Beam shape, however I have not been able to locate suitable parts.
    Light Grey: Y-axis rails, four times cost but they are a structural item and of minimum size.
    Orange: Aluminium plate to support Z-Axis. Structurally screwed to Y-Axis carriages.
    Central Dotted region: Z-Axis and spindle attached to orange plate
    Holes in Orange Plate: Proposed ball screws for Y-axis.

    The idea is that the single carriage on the X-axis maximises travel.
    The blue C-sections (approx 6" high) act as a single very large beam when combined with the rigid orange plate (which moves on the Y-axis with the spindle)
    The usual 30mm travel of the Z-axis creates very little torque although it can travel more depending upon design.
    The Y-axis rails add structural support to the blue C-Section.
    The blue C-sections will overhang beyond the X-Axis' allowing for stepper control beyond axis and full travel.
    The ends of the blue C-Section will be structurally capped with a larger 'Orange Plate'

    I'm not certain if the blue C-Sections would benefit from being I-Beams as shown by the doted lines. There is little effected on the design, however I cannot find any I-beams made for non architectural purposes (which are massively over engineered for a CNC machine).
    The orange plate should prevent almost all torque twisting the blue C-sections.
    The Y-axis supported rails should add significant strength to the design.
    The X-axis carriage should be fine with a short Z-axis but may need to be doubled with further travel.
    The blue C-sections should be of minimal thickness, the strength coming from the light-grey rails and the Orange Plate.

    Sorry for the bitmap drawing but I have not yet found a CAD program which I find intuitive and creative to use.

    I very much require input and criticism about this design before I progress to the next stage - anythng will help because I often overlook the obvious! :)

    Click image for larger version. 

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  2. #2
    Still trying to get my head around this one, could do with a few more drawings.

    Have you got X and Y axis labelled correct? you wouldn't normally have the Y longer than the X as it places unnecessary force on the Y beam and X bearings.

    which leads to the problem of the single bearing on the x Axis, I'm assuming supported rail as that is what you have in the y Axis and that is the a big no no. They cant resist the moment load from the Y axis and also the large distance across the X axis will cause racking problems. you definitely need two bearings on the X axis as wide as the Y carriage.

    There is no need the make the C channels in to I beams as it is not needed. the I beam would only increase bending strength which is lower than most designs as you have reduced it by having a beam either side of the load. You pretty much have a resultant I beam from the two C channels.

    Personally I would go for profile rails instead of 4 supported rails and 2 ballscrews, the cost would be nearly the same but the strength is much better and can simplify the design so saving money on the other bits.

    How are going to do the z axis?

  3. #3
    Light Grey: Y-axis rails, four times cost but they are a structural item and of minimum size.
    Are you bolting through the top flange into the rail? it will not be that structural as the top and bottom sections of the C channel can flex. there is no fixity between the beams (as the bearings rotate) so they wont combine as one structure, each beam will work separately but split the load between them. the torque from the cutter will rotate the y axis beam so one will bend up and the other down.

    What size? the really small open bearings have fewer rows of ball bearings so have a poorer radial load distribution, you really need 4 or 5 rows.

    The orange plate should prevent almost all torque twisting the blue C-sections.
    Unless I'm looking at it wrong the orange plate is the Z axis carriage and if so that will be what is causing the beams to twist!

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