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Assembling the shoulder

This is an almost finished shoulder assembly. The part that's not there yet is the gears.

A few notes, with more pictures to follow.

The hardest part of this assembly is getting the tensioners right. The main challenge has to do with the tolerances, and slightly shorter than needed M4 screws.


  • The challenges with the tensioner assembly began with my very first prints, before I had any mechanical parts. But, even before that, this is how I got organized.

    It was nice having the plastic compartment box with all the parts as part of the kit. I'm not very good at fishing tiny screws and nuts out of those tiny little compartments though, so I organized my stuff in this way. I bought a 39 compartment deal, which just so happens to fit all the parts comfortably, with larger items such as the smooth rods and cable wraps being big and obvious enough that I could double them up.

    This makes it real handy when looking at the assembly drawings, and it says "M4 20mm", just a glance and you've got the parts at hand.

  • On my first run through printing parts out, I was kind of sloppy, using PLA/PLA+, and not paying much attention to tolerances. I didn't calibrate for over/under flow because I've printed lots of stuff that was 'ok' when it fit together. with this robot, things need to be a bit tighter. Here was my first set of tensioners.

    There's a couple of problems there. First, the gaps that are seen between the perimeter and the holes at the bottom are due to the selection of perimeter shells, infill percent, and first layer squish. Here I was probably using 2 shells and low infill (20%).

    The next problem probably had to do with my printer, and that particular filament. The first few layers seem fine, and then suddenly it becomes an over extruded mess. The overextrusion could be due to the filament suddenly changing diameter at that point, or this particular nozzle letting loose something that was partially clogging it before, or the nozzle itself getting worn out. subsequent prints didn't exhibit the same problem, so I'm assuming it's something temporary.

  • Trying to fix the overextrusion.

    When I printed a set of parts using Colorfabb PLA/PHA, I found that the parts were not fitting together very well. This was true of all the parts I printed. I finally printed a flow calibration thing, and determined that my flow needed to be down at 88% instead of 100%.

    The part on the left in the picture was when the flow was at 100%. The part on the right was after adjusting the flow to 88%. The part on the left would not easily take the captured M3 nut, the part on the right fit the nut perfectly.

    The part on the left would not take all three bearings without having to bend out the uprights. The part on the right took them easily, with the slightest of gap left over.

    From another angle, the part on the left shows poor surface finish, and the M4 screw doesn't seat properly in the recess for its head. The part on the right sits deeper into the recess, and the bearings run smoothly.

    The part on the right, printed at 88% flow, looks and operates better than the overflowed parts. The flow could be adjusted upwards slightly though. 95% would probably make it even more perfect as it shows slight signs of underflow (thing lines and gaps). I also printed with a single shell, and 95% infill, with very short infill lines (down to 0.2mm). This ensures that all the tiny little gaps, particularly between the perimeter and the holes for the M3 captured nut, get filled in.

    I tried printing in various orientations, lying on the side, as pictured, turned out to be the best.

    Once done, they installed nicely.

    NOTE: The M4 - 20 is just barely short. It barely catches the first nut thread. An M4 - 22 would be a more perfect fit, and would not really cause much trouble in terms of rubbing against anything.

  • NOTES for Shoulder

    I was short 1 M4 hex nut. Probably dropped on the floor somewhere. Providing a couple extra of these tiny parts would probably be good.

    Good design on the motor shaft mounting. Without that slot, there would be no way to tighten the grub screw.

    For the thermal inserts that go behind the tensioners, I imagine there might be force exerted on that insert from the tensioner. An insert, that has a flange on the internal side, would probably be a better thing. Then, as the pressure from the tensioner pushes on it, it will just press against the plastic, rather than pushing the threaded insert out the other side. Right now, it's relying on the strength of the plastic to hold it in place, which may not be strong enough after enough usage and warming.

    Also, probably good advice to print all parts with at least 3 or more shells so these holes have some strength to them, particularly because they get degraded by the process of inserting these heated parts.

    It might be better to have a screw in insert rather than a heated insert. That way the bite will be better, and you won't compromise the integrity of the plastic as much.

    The M4 - 20 should be an M4 - 22

    Give some advice on which direction the cables should be oriented coming out of the motors. In my case, I faced them towards the "back". I'll find out when I'm done whether that was the best choice or not.

    The order of assembly was good. I haven't put the toothed belt onto the pulleys yet, but it's good that there's space open at the bottom to help facilitate that. I might have been tempted to screw the shoulder to the big gear and stick it on the base, just to show progress, but then I would not be able to get the belts on.

    Overall, once the parts are printed with proper tolerances, the thing goes together fairly easily. Having the metric drill bits for reaming would be a bonus.

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