Using my Form2 to create functional, 1 off motor mounts, landing gear fixtures, and sensor mounts for an vtol drone running arduplane on a pixhawk. I’m currently fighting with material failures as my yield strength with tough appears to be closer to 20 MPa than the advertised 55 MPa. The next print will have 3k carbon fiber epoxied to the failure points.https://drive.google.com/open?id=0B9egjsa9aEXWVXZWM09NaVhkUlU
https://drive.google.com/open?id=0B9egjsa9aEXWaWlCVk9Tdl93SDg
Where are your parts breaking?
Looking at what I can see in the picture, I’m not surprised that you’re having some trouble. Tough doesn’t do well if it heats up. It gets very soft. Depending on how many watts you’re dissipating in those motors, part of the problem might be heat. But having the motor cantilevered like that doesn’t look robust to me, either. A little prop imbalance and the motors are going to start twisting the mount and beam.
I would modify your design a bit before giving up and going to the trouble of laminating CF on top of the prints. But what I’m thinking is hard to describe. If you want to upload the STL, though, I’ll tweak it the way I’m thinking and send it back to you…
Something else to consider is to make sure your parts are UV protected. If your tough resin parts are not painted or UV protected in some way it will they will become brittle in short orders. You might want to try the durable resin, though you won’t get as many parts per resin tray.
Here you go. The mount is designed to keep tilt servo load to a minimum. One broke at the connection point to the carbon fiber tube, the other broke at the hole for the motor mount. They fractured before I was able to break ground, so I don’t think there was an appreciable amount of heat buildup yet. i can export in other formats if you like.
vtol_front_mount.stl (256.9 KB)
I had thought about using durable for the next iteration. I was wondering if the vibration from the motors caused my failure. I don’t have enough information to do an accurate modal analysis at the moment, so I’m still leaning towards using cf.
Excellent! I’m very interested in a brushless mini quad. If you ever have use for a tiny whoop, I can send you an stl for one that is nearly indestructible when printed in durable
I forgot before I posted, but I added material to that mount and rotated the motor mount holes 45 degrees. So it’s not the same part that failed.
Thank you for that advice, I’ll be sure to paint the final product. The drone has a standalone gps/gsm locater but I can use all the help I can get if I have to go looking for it.
The props have also been balanced on a magnetic fixture. You can see the nail polish on the tips
Being a engineer for a time, it seems that the props produce a stress against the joints in tension and flexure instead of shear.
As a result the lifting force pulls the joints apart. A better design would place tie plates top and bottom across the prop tubes at the joints and bolt thru the plates. This would place the joints in shear across the bolts as the lifting force is applied.
I confess I can’t see the whole design from the photos.
My experience with durable is that it won’t work for those mounts. It works great for parts that can be soft and won’t see heat.
Really thick cross sections are what is needed here. Thin parts will break much more quickly.
Short of molding these in a material like ABS (preferably something glass filled) you’ll have issues because of the thin walls.
I would modify your part like this…
I just hacked this for illustrative purposes. I didn’t bother with the back side you can’t see in this image. But I would flare the tubular section and fare it into the backside of the “brace” thing I added so there are no sharp right angles at what will be the highest stress point.
Wouldn’t be a bad idea to add radii on the inside corners. Would help stress fractures on the corners.
Thats genius. I wish i had thought of that earlier