Spindle weight, Y-lenght and combining lowrider with MPCNC?

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This topic contains 11 replies, has 3 voices, and was last updated by  Alex 6 months, 1 week ago.

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  • #88479

    Alex
    Participant

    Hi ,

    great project(s). Thanks a lot for sharing!

    I’m in the ‘first phase’ of becoming clear about which design to build, and maybe you can give me some advice.

    My background is (kite) surf / hydrofoil boards and wings. So far I hand shape the boards (XPS) and print (CR10), sand/filler the moulds for the foil-wings which then get done as composite. I also have a quite large BOSCH gof2000ce professional spindle with which I milled composite contours and coarse-shape XPS. (Note: I do not plan to mill composite on the MPCNC; only XPS foam and possibly some MDF for moulds).

    My first idea was to use that spindle, but it is quite large and might be to heavy (6kg).

    The other point is I really like the design of the MPCNC with the stainless steel tubes. However, I’d like to go for Y axis of about 1.8m (~59 feet) to be able to mill the boards. Working through MPCNC and lowrider, I came to the conclusion that what I actually would like to do is a combination of both:

    Y-axis like the MPCNC, but supported on the bottom – will there be enough space to do so?
    X-axis inspired from the lowrider with 2 tubes in parallel, and the spindle in between. For x I’d need about 0.6m effective toolpath.

    So what do you think about such a combination? And how about the tolerable spindle weight (I guess the inertia might lead to problems with the stepper motors?)?

    Or should I just go for a ‘default spindle, build a large Y MPCNC and support it?

    Thanks a lot!

    Alex

     

    #88500

    Barry
    Participant

    You’re looking at roughly 1 by 2 meter mpcnc to get your cutting area.  I’m pretty sure that’s doable with just center span supports for the long side.  Your router is probably a little too heavy though.  The real question is how tall do you need it to be?

    #88594

    Alex
    Participant

    Ok.

    What do you mean by

    The real question is how tall do you need it to be?

    The router? The complete machine?

    Alex

    #88620

    Barry
    Participant

    Sorry, the tallest thing you’ll be milling, z depth.

    #88651

    Alex
    Participant

    Hi,

    ah, Ok. I think I should not need more than 20cm in Z. However I would like to be prepared for more.

    As far as I understand, larger z-height means longer feet and the shear force will bend the feet/top-frame connection.
    What about adding a plane of e.g. plywood to all  sides. This would support y  and stiffen against the shear forces – not?

    #88686

    Barry
    Participant

    That would help.  The z assembly will also start flexing at that height too.

    #88809

    Alex
    Participant

    Another thought is: keep it small and use a default spindle (allows to stick to the common design).

    The background is that I need only 100cm x 50cm x 5cm for the mould (probably done in MDF), but would like to have a good precision on these.

    The reduced (usable) Y-length to about 1m would mean that I need to reposition the foam for the boards once per side.

    One could e.g. mill 3 reference points which could be used for repositioning.

    Would this be worth the fuzz?

     

    And regarding Z-height: I did not really get it: I read that a larger Z is unproblematic at the bottom, bending/precision gets worse when going up. Is this true? By this, one could mill not so tall things (like the moulds) at ‘high’ precision while having the necessary Z height for the foam, where precision is uncritical.
    Looking at the pictures (of the mpcnc) I think that going all the way down for flat objects would also lead to bending in the Z assembly – not?

     

    #88818

    BT
    Participant

    If I understand you correctly, yes, the MPCNC is stiffer the closer the router bit is to the height of the X and Y rails.  The more the gantry gets inserted, the longer the lever arm and the more possible play in the bit positioning.  The LowRider, on the other hand, is stiffer at lower z heights.

    2 users thanked author for this post.
    #88821

    Barry
    Participant

    If I understand you correctly, yes, the MPCNC is stiffer the closer the router bit is to the height of the X and Y rails. The more the gantry gets inserted, the longer the lever arm and the more possible play in the bit positioning. The LowRider, on the other hand, is stiffer at lower z heights.

    BT nailed it.

    1 user thanked author for this post.
    #88823

    Alex
    Participant

    Oh, ok.
    So having larger Z-height (longer feet on the frame) we have two effects:

    • bending of the frame which could be counteracted (partly) by adding some planes of plywood as discussed above
    • the lever arm is longer and bit positioning will get worse when moving away from the x y rails height -> not so tall workpieces should be elevated on the table, such that the working area gets close to the ‘sweetspot’ Z-height

    Did I now get it?

    Thanks a lot!

    Alex

    #88826

    BT
    Participant

    One other thing to consider is that even though you stiffen the outer frame rails with plywood, you still have the X and Y rails used to hold the gantry which you cannot easily stiffen and over distances longer than 2-3 feet can cause more deflection down (sagging).

    1 user thanked author for this post.
    #88840

    Alex
    Participant

    One other thing to consider is that even though you stiffen the outer frame rails with plywood, you still have the X and Y rails used to hold the gantry which you cannot easily stiffen and over distances longer than 2-3 feet can cause more deflection down (sagging).

    Good point!

    One idea would be to use 2 tubes in parallel at a certain distance (comparable to the lowrider’s x). But adding all these adaptions might make it much more expensive.

    So what do you think about building smaller and reposition the larger (foam) workpieces as suggested above? Sounds like the better option to me

    Alex

    edit:

    From one of the builds using carbon tubes (not a good idea i think) I had the idea: why not insert carbon tubes in the stainless steel. I plan to build the 25mm version, and one can get 23 with 2mm wall carbon tubes. Use some epoxy to glue the in should result in quite stiff gantry rails – what do you think?

    Edit2: One usually overestimates the stiffness of carbon based composite. Its the stiffness/weight ration which is good. But the coefficient of elasticity of about 88 GPa is much lower than that of steel 210 GPa. So using a thicker wall for the steel tubes makes more sense.

    However doubling the length of the tube gives us l³ =8x bend down for a given weight. Doubling the wall diameter is about linear at our sizes. So we would still end up with a factor of 4 more bend down.
    I should have known it, but seeing the numbers makes again clear: shorter axis have an over-proportional effect on precision.

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