The battle for rigidity with laser cut quad frames by enivid

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Posted: July 25, 2014  |  3,621 views
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I've been trying to design a 100% laser cut frame.  The goals were ease of build, low cost along with an optimal use of ponoko materials (which is my laser cut supplier).  As my designs progressed, I have never been satisfied with the rigidity of the booms/arms of the quads I built.  When I held a blackout (carbon fiber) in my hands at the local meetup I knew right away that I needed to find a way to improve rigidity.

Bamboo plywood

My first design had very narrow bamboo arms which were not rigid enough, to correct for the lack of rigidity I played with PIDs on low end KK (low frequency) flight controller board and it flew.

I then reworked it a little, simplified it but also tried to make the arms more rigid by widening the booms.

Again with the basic low frequency flight controllers I lowered the PIDs and it flew quite well.  But lack of rigidity also meant that these things were very fragile and not at all crashproof.

Bamboo and Delrin

I decided to move away from bamboo booms and try out delrin.  This time with a more modern multiwii FC.

I sandwiched the delrin booms (3mm) in between some bamboo center plates.  This flew, but the flex was still there (became worse in hot conditions).  The new FC didn't help, it has high frequency corrections that triggered crazy oscillations (felt like I was flying a little bee) on throttle up.  The only way I could eliminiate some of the oscillation was by enabling the low pass filter features in the multiwii software.

I went back to the drawing board.  Do I laminate or do I spar? I knew a spar would work but it would make my build more complex so I've tried lamination and here it is.

I decided to sandwich delrin in between a bamboo core.  Since gluing delrin is near-impossible, they're bolted together.

The rigidity has improved a lot from some manual tests.  I plan to maiden it soon without the low pass filters to see how I did.

 

 

 

 

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Comments

JACKFIRE July 26, 2014
Hi, besides the choose of the material you may improve the resistance of your frame with a little distance between layers, this will increase it's resistance, the more distance more resistance.
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enivid July 26, 2014
thanks for the tip... I'll take that into account in my next attempt!
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XFlyer July 26, 2014
Have you thought about using G10? I use it in my builds and it's stronger than anything! I don't know if it an be laser cut though... I just get my frames CNC milled by a friend I know
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enivid July 26, 2014
I haven't... ponoko doesn't offer that option but I will ask why not!
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eric July 29, 2014
G10 is very difficult to laser cut, and the finish is never satisfactory.
You might want to look at the problem as a tradeoff between rigidity (elastic modulus), toughness, and weight. Delrin is very tough, but is much heavier and bendier than plywood. Maybe you could try laminating more layers of plywood? Or perhaps Ponoko has lighter woods you could laminate as well. Remember there's a reason they still make planes out of balsa!
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enivid July 29, 2014
Yeah lamination seems to be the key here... I think it would have almost been smarter to drop delrin all together and go for all plywood.
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ukarmy04 July 25, 2014
Nice progression! I like those designs. I'm actually also experiementing with DIY composite frames that can be manufactured with a laser cutter. I'm trying out all sorts of stuff (wood/acrylic/cork/etc) to see what can give me the highest bending stiffness without too much of a mass penalty. Let me know how those bolts work out. I've been trying to avoid them because I feared the concentration of shear stress around the bolt hole would break the frame in a crash. But if they end up working out well for you, then I might give it a shot.
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enivid July 25, 2014
Stay away from Acrylic... it's too brittle. I think delrin is very resistant to this shear stress you're concerned about.
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enivid July 30, 2014
So I had my maiden, the bolts for lamination worked beyond my expectation. This quad is solid enough to run multiwii base SW. Going to do some FPV on it for the next few days.
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Davy Crockett July 27, 2014
Hi, try using Micarta its a high density material used in civil aviation for pulleys and floor boards etc.
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enivid July 29, 2014
Yeah I'll take a look. Seems like I need to mill this though.. more than laser cut.
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