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  1. #1
    I'm having some difficulty getting to grips with lateral resistance and torsional forces and deflection, particularly on a Y-axis gantry of about 800mm unsupported, so am hoping that a discussion comparing the coloured cross-section pics I've attached will help me and probably others too.

    Only now that I've drawn these up and can picture them properly, I think the varying degrees of ease-of-construction, alignment, ease-of-changing-pulleys, etc., are fairly self evident from the pictures, and know that the impact of the respective weights on acceleration needs to be considered, but the gist of my confusion is around rigidity and stiffness.

    GREEN -- 45x90 ------- 3.9kg/m extrusion
    PURPLE - 120x60x4 ---- 3.7kg/m rectangular tube
    BLUE --- 152x51x6.35 - 6.5kg/m rectangular tube
    RED ---- 152x51x9.53 - 4.9kg/m angle + 45x45 extrusion 1.9kg/m

    If cost were to be ignored for now, how do these cross-sections compare to one another in terms of resisting the forces experienced during cutting aluminium?

    Thanks,
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by WandrinAndy; 06-08-2012 at 09:11 PM.

  2. #2
    How come you didn't make a drawing of how JAZZCNC quoted in your build log.

    Quote Originally Posted by JAZZCNC
    The 45x90 will be better but I'd put it top & bottom and have the wide bit flat.
    This way it ties into the sides better and gives more resistance to bending in lateral cutting directions.,
    I've knocked up a drawing of this and also think this is a great way to go.

    1. It gives a nice straight guide to fix the profiled rails onto.
    2. Using the BK/BF blocks as spacers between the top and bottom extrusions, will help the profile rails being parallel with each other.
    3. Makes a nice void for the ball screw to hide in and keeping the z axis close to the extrusion.
    4. Better fixing to the gantry side plates by having 4 bolts in both extrusion ends, 8 for both ends.

    Cheers

    Click image for larger version. 

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  3. #3
    Quote Originally Posted by Iwant1 View Post
    How come you didn't make a drawing of how JAZZCNC quoted in your build log. I've knocked up a drawing of this and also think this is a great way to go.
    I don't think that's what Jazz was suggesting Adil... If anything I think having the two 45x90's flat like you've drawn would be lots weaker towards the middle of the gantry than the 45x45 which I was going to go for... similar to your original design... because the profile rails are so far removed from the back plate!

    I think Jazz favours the GREEN option for an initial DIY build which I'm now thinking is a far better option that what I designed.... I was thinking of using the GREEN at one stage but somehow dropped it and went off on a tangent after seeing your design pics. lol

    My thoughts are constantly evolving, and while I am looking at changing things, may as well look at the box-sections too, but I really need to understand things, hence the reason for this thread.

    I suspect that the PURPLE is too thin, so have added another option which I've attached below.

    ORANGE - 127x51x6.35mm
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  4. #4
    Quote Originally Posted by WandrinAndy View Post
    I don't think that's what Jazz was suggesting Adil...
    Adil's correct that is what I meant in that post. It will be hardly any weaker in the middle over this width but will add far better strength and rigidity to the gantry than 45x45 would in the lateral directions.

    Your also correct that I favor the green design but didn't want to get into that in your previous post because it's slight different design.

    I don't like box section for gantry's has it can produce resonance and vibration unless thick walled.? The bolted together extrusions are far denser therefore absorb vibrations far better.!! . . .Any resonance will transfer thru to the finish.

  5. #5
    Quote Originally Posted by JAZZCNC View Post
    Adil's correct that is what I meant in that post. It will be hardly any weaker in the middle over this width but will add far better strength and rigidity to the gantry than 45x45 would in the lateral directions.

    Your also correct that I favor the green design but didn't want to get into that in your previous post because it's slight different design.

    I don't like box section for gantry's has it can produce resonance and vibration unless thick walled.? The bolted together extrusions are far denser therefore absorb vibrations far better.!! . . .Any resonance will transfer thru to the finish.
    Thanks Jazz and apologies for doubting your interpretation Adil. Afraid I still have nagging doubts though about the vertical strength of those sections lying flat like that, and reckon there must be extra sag in the middle when compared to the GREEN design.

    Getting back to this thread, how would the design Adil listed compare to the GREEN design in terms of rigidity?

    I wonder what size box section Jonathan's gantry is and particularly what wall thickness?

  6. #6
    How about using components from my yellow and your green, tada.
    Click image for larger version. 

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    I added a 60x45mm extrusion in between the top and bottom extrusions. These can all be bolted through from the top and also screwed into the 10mm back plate. Now your worry about the weaker wider section is dealt with, plus that extra piece for 800mm length is only 12, sweet.

  7. Its quite hard to model these different designs without being able to calculate the moment of area. But to answer your last question, the L-shape created by two sections of extrusion, assuming they are rigidly joined will be slightly stiffer than the single piece in the X direction but two 45 x 90 in the X direction will be stiffer again.. without doing the math, I'm guessing the two 45 W extrusions bolted to a piece of plate in Jazz's design will be slightly more rigid in the vertical than the 45 x 45 bolted to a 45 x 90 horizontally. So on balance the yellow one I'd say is stiffer then the green one, certainly for static forces vertically or horizontally on the gantry, but it weighs more. Torsional stiffness is harder to assess. Box section is far better than extrusion in resisting torsional forces, but my gut feel is the green design is likely to be better than the yellow in this respect...

  8. #8
    Quote Originally Posted by WandrinAndy View Post

    I wonder what size box section Jonathan's gantry is and particularly what wall thickness?
    Jonathan's gantry uses 80x80 box section probably 3-4mm wall thickness. But as he says in other posts, the ball screw is not in an ideal location, the distance from the ball screw to the cutter is too far.

  9. #9
    Quote Originally Posted by WandrinAndy View Post
    Thanks Jazz and apologies for doubting your interpretation Adil. Afraid I still have nagging doubts though about the vertical strength of those sections lying flat like that, and reckon there must be extra sag in the middle when compared to the GREEN design.
    Obviously there will be some tiny tiny difference but when you have some profile in your hands you'll realise why it's not a problem.!! . . . . The difference you would struggle to measure with a dial gauge and I'm 99.9% sure you have far more pressing problems ahead to overcome than if the profile sag's 0.001mm more than 45x45.??
    The extra lateral strength and rigidity the extra bolting into gantry sides allows will far far outway the very very minute difference any sag will make.

  10. #10
    I've built machines using the green design and please believe me you have nothing to fear the strength is easily over the top for anything but steel or harder.

    If you use Bk/BF blocks then they line straight up with slots so easy fitting. It offers good strong attachment to gantry sides.
    Yes it places the ball-screw slightly further back than what's considered ideal but again it's nothing and doesn't make any difference to how machine performs. The positive is the screw is away from the firing line and protected.

    Not trying to sell it to you but I know it works with no detrimental side affects and plenty of positives.!!

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