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Build stiffer with triangles

Discussion in 'General Talk' started by stevempotter, May 10, 2018.

  1. stevempotter

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    I am looking at a lot of builds of CNCs (routers, laser cutters, 3D printers) and am surprised that there are only right angles between extrusion beams. Any structure is much stronger and less flexible if it incorporates triangular elements. Stiffness (flexing, resonant frequencies) determine the effective resolution we can get from our CNCs. What solutions do you have for joining extrusions at angles other than 90 degrees?
     
  2. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    In most applications the main reason for using trusses is to span a distance with the lowest possible weight, not for strength or stiffness. Trusses flex just like every other framing system. And with the lower weight they also have the potential to vibrate more. But the main reason for not using them in CNC is the connections will eat you alive both financially and time-wise. Trusses also take up more space resulting in loss of usable working area on the cutting table. About the only way trusses would really make sense would be a truss cut web on a flitch beam. It's doable but as most people won't have a machine to cut them on until they get their system built it gets to be kind of a non-starter.

    If you wish to show us what you have in mind though, feel free. Hopefully you've figured out something the rest of us have overlooked. As far as available connections about all there is available would be the Makerlink connectors which won't be of much value in a truss. You'll pretty much need to go with custom cut side plates. And that's where it starts getting expensive.
     
  3. stevempotter

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    Thanks, Rick, for confirming my observation that there does not seem to be a cheap corner brace or connector designed to connect extrusions at non-right angles, except Makerlink (which does not seem to include a 45° bracket). I was not referring to a beam _replacement_ by a truss, but a way just to attach two extrusions at, say 45 degrees, so you could easily add diagonal members to a cube-shaped design. The example that I have been pondering most recently is due to the problem that laser cutters made from extrusions bolted together only at right angles go out of alignment if the laser cutter is moved even just a few feet. These devices are _very_ sensitive to any deformation of the structure, and re-alignment of the optics can take a whole day. A few diagonal extrusions would go a long way to fix that problem. But the few designs I see in your gallery that incorporate diagonal beams or braces used custom 3D printed plastic unions to accomplish this. Compared to joints made from aluminum castings or even steel die-cuts, those are not very strong and are slow to make and complex to design. I bet you guys could come up with some simple, strong, metal connectors in a variety of angles.
     
  4. Kyo

    Kyo Master
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    For 20x20 extrusion I have found adjustable corner brackets handy like these "Here". Bolts to the end of one extrusion and the side of another allowing excellent freedom in angle adjustability.
     
  5. stevempotter

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    Thanks, Kyo! Those hinges are nice especially if you don't know exactly what angle you will need until you have the parts in front of you.
    But for making something that you want to be really stiff, I would not want to use a movable hinge, especially if only fastened by one cross-head screw. There would be some play in it, and some risk of it coming loose. I suppose you could pack it with epoxy putty once installed. But it would be better to have a variety of brackets like these in that are just one piece (not hinges), available in a few set angles, made of sturdy castings. Is there such a thing?
     
  6. Kyo

    Kyo Master
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    You also have a number of options from places like Misumi. Example part numbers would be HBL45TS5 and HBL45TD5. Similar can be found at other online retailers as well. ebay, amazon, aliexpress.. Most common angles can be had 45, 60, 90, 135, ect. But the more obscure/custom the angle the higher the cost or the lack of availability. The cheapest option would be to cut the extrusion end at the angle you need and then drill / counter bore and solidly bolt the two extrusion together using a bolt like this.

    [​IMG]
     
  7. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    I received a shipment yesterday from the parts store that included some of the hinges/adjustable angle connectors so I knocked together a quick mock-up.

    IMG_0621.JPG IMG_0622.JPG

    The nice thing about them is they fit cleanly within the framework, not extending outward like plates bolted to the face would. They are also very cost effective, coming in at half what using face plate connections would cost (based on using a pair of 90 degree joining plates with the vertical leg extending up to grab the diagonal).

    And they're a lot stiffer than you might think. Just as a test I tightened down the set screw on a dogleg and was able to support 5 pounds at 5" off the hinge. Not extremely strong but more than stiff enough to keep a framework square especially since there would be no moment involved as the load would be truly axial on the framework. And if you still have worries, you could easily drill out set screw holes and install a larger, snugger screw to where there would be no potential for play.

    IMG_0625.JPG

    As an aside, I also modified one with a cam clamp and a 10mm spacer between the inner prongs and was able to hold 10 pounds at 6" away from the hinge and still had capacity to spare.

    IMG_0627.JPG

    But that's an issue for another day and a different thread. Anyway the OpenBuilds part store has these connectors should you decide to give them a try. (The parts store is also half the price of what Adafruit wants for them.)
     
  8. Kyo

    Kyo Master
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    Very cool :thumbsup: Nice to see them at the parts store I did not notice they added them. I have purchased the Adafruit ones in the past. The more options we have direct from the OB parts store the better, and cheaper to boot :D
     
    Mark Carew likes this.
  9. Mark Carew

    Mark Carew OpenBuilds Team
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    Great ideas Rick! thanks for sharing your experiments and ideas.
     
  10. GrayUK

    GrayUK Openbuilds Team Elder
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    Rick. Where do you find the time for all these test pieces and and designs??? :D
     

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