Today, I find myself wanting desperately to feel as though I’m doing something right. I think it’s likely all the rain we’ve had that’s tainting my mood, but I’m discouraged. I’ve been adding things to my bill of materials and the proportions are seeming less than hopeful in some areas.
Now I need to start shaving cost off things that are needlessly spendy. I guess this is where I learn to be inventive and shrewd all at once.
It might just be my lack of experience in doing PR related stuff- but I’m stuck again with the getting the word out part. Maybe I’m not begging loudly enough. I’ve never been all that good at asking for attention. So this is another hurdle to overcome.
This morning I did mock ups of several different variations Mark came up with for my delta’s arms. We foresee the cups that hold onto the ball bearings wearing out over time, so we’re trying to come up with a solution. One idea was to have a slit down the center of the U-joint that some hardware would compress together so that the tension on the ball could be adjustable :
The other thing Mark suggested was that I ditch the cup entirely. He said it would be too difficult to mill if I decided to have my pieces machined. His idea to replace it was a simple square peg that had a “+” sign cut into it… leaving four small notches that would then grip the ball joint. I gave that a try too.
The compression design worked well. It’s a good idea, but if I go with this type of arm, I’ll have to add another 12 pieces of hardware, plus 12 washers and nuts per kit…
The replacement for the cup worked in theory- but the ball was apt to pop out left, right or any of the directions it didn’t have material forcing it in place. So maybe not such a good idea after all.
Other than that… I’m attending a hardware startup meetup tonight with Jeff and Mark. We should all learn something from it. Tomorrow is Friday and I’m back over at Mark’s to finish and send off Rev 2 of the delta brain to be fabbed at OSH Park. Small steps. Chin high.
Robotic Arts 
An idea for milling your cups (except you’ll need a 4-axis machine):
* The outside of the cup need not be cylindrical
* The small hole drilled in the cup can go straight through the whole arm pair
The larger the hole, the better the retention will be.
Still not as good as a true spherical surface as far as wear is concerned, though.
OTOH, a tool change to a spherical ball end mill would allow milling a true spherical cup.
A (possibly more common?) hemispherical ball end mill could also be used on a 4-axis machine.
In place of the screws, you could rely on the springiness of the material. You might have to beef up the arms though.
For both aspects, you might want to have two versions: one RP, and one conventional, that are each optimized for the strengths and weaknesses of the material and fabrication method.
Thank you for the feedback. I’m not sure if I’m going to need to have the pieces milled over seas- but I want to have an alternate design prepared incase I do. I’ll try making the changes you mentioned and see how it works out. =] Will post pictures!
Rather than using a screw to tension the u, could you use a collar to compress the slit? If you redesigned the arm to have ratcheting teeth along it’s length, and a very slow taper, a collar with matching ratchet teeth could provide very fine adjustment to the tension, and be as simple as a zip tie to release.
That is a good suggestion… definitely worth designing that out to see if it works when 3D printed!
Agree with Sam A – a simple compression collar. Put the “teeth” in the arms and just use a collar. I think the collar would need to be spring steel though, but would be MUCH lighter than bolts/washers/nuts. Be careful not to force too much into the 3D print side, although instead of steel, you could probably use laser-cut delrin.