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Thread: Fixed Gantry Build

  1. #21
    Community Moderator ger21's Avatar
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    Default Re: Fixed Gantry Build

    Any ideas on how then to mount the carriages to the surface plate?
    Flanged blocks can be bolted from the bottom with the next smaller bolt size. Just drill and tap the plate from the top, and bolt them down.

    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  2. #22
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    Default Re: Fixed Gantry Build

    Quote Originally Posted by peteeng View Post
    Hello Jack - I used Izz so did compare torsional rigidity. Adding the internal brace stops the section from lozenging. This is a different approach then filling with epoxy. Adding the brace correctly in practice is quite a mission. I've done the same FEA work in the Maximus thread if you look that up. There is a critical tube thickness where this lozenging becomes acceptable. By acceptable I mean within the deflection target you set. So if you take the brace material and distribute it to the tube you may end up with an acceptable design. Plus adding a brace like you have makes the structure asymmetric so the deflection maybe different if you push in the other direction (so check this) . Since you are welding extra bits on for the rails I suggest you move the rails into the same plane as the top and bottom flanges. This will also minimise lozenging as the loads go directly into the flanges.
    ...
    As an afterthought the ends of your gantry are open, if you close them you will improve again maybe won't need the brace.

    UCB this is same for your design, close the ends of the gantry. Open ended sections are not very stiff. Cheers Peter
    Peter, thanks, "lozenging" was the word I was missing.
    The amount of engineering effort you put in your Maximus design is impressive, I'll read the whole thread later.

    In my simulations EG filling is even more effective than the partial bracing, and it also provides the much needed vibration damping - a hollow steel tube rings like a bell.
    Closing the ends is better than nothing, but much less effective than filling to prevent the lozenging.

    So I believe my initial point still stands: filling the gantry tube with EG is an easy way to stabilise the cross-section with the added advantage of vibration damping, especially in a fixed gantry setting where the extra weight does not matter.

    Last edited by jackjr-123; 09-16-2019 at 10:38 AM.


  3. #23
    Member UCBones's Avatar
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    Default Re: Fixed Gantry Build

    Thanks for the heads up on the flange blocks! As for Z height I'll have a look at increasing it. It should be as easy as making the uprights taller. For the box section, enclosing each end should be easy enough.



  4. #24
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    Default Re: Fixed Gantry Build

    Hi Jack - If weight does not matter then picking a much thicker tube is more economic than using EG. The size of the steel tube and its weight indicates it will vibrate at a freq outside the usual resonance range of the machine. You have FEA capability so doing modal analysis should resolve these issues without using EG. It seems to me EG is a patch up approach vs doing it within the steel or alum paradigm. Perhaps a better thought is to start somewhere else then using a std box section? I'm playing with inventor at the moment and things like Fusion 360 have form finding algorithms. Start with an arbitrary block and it will find the "ideal" form. Steel SHS and RHS like Al extrusions are used because they are convenient in some way. Perhaps there is a better topology for a gantry?? Does solidworks FE have a form finding feature? UCB keep building....Peter



  5. #25
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    Default Re: Fixed Gantry Build

    Hi UCB - To address the weld distortion issue you could soft solder the parts together. There are many solders in the 200-250degC range that would provide the required strength. At this temp there's hardly a colour change, a propane torch will do the trick easily. The loads in the structure are nowhere near the material strength due to the large stiffness requirement. We are looking for joint stiffness and efficiency with no distortion, metal to metal contact provides this. Silver solder definitely provides the answer, I do this alot. But softer solders will work and are cheaper and easier to do due to the lower temp. This also allows very thin things to be connected together easily... Cheers Peter

    https://en.wikipedia.org/wiki/Solder_alloys



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