Shiny things and milestones

A project log for "Tenacity": Yet-Another-Sawppy-Project

Inspired by Roger and others, I'm making a SAWPPY-Alike rover, adding my own tweaks and mods.

SteveSteve 08/05/2020 at 01:550 Comments

I've decided to plant a flag on some of the 3D-printed aspects of Tenacity, makes notes to myself(and here) and move on. I've got lots left to do and can't really do it when axles snap every 10-30 minutes. I've been working on Tenacity for a bit over a year now and have been through a bunch of iterations of the drive, steering and suspension components and am ready to move on. 

So, the question is:
Can you, in 2020, 3D-print yourself a robot that weighs almost 20 lbs and have it support its own weight and move around ? 

I'm going to answer that with a qualified "Yes". First, you're not going to do it  just with PLA, or even just with PETG. You will need Nylon or some sort of Nylon-like blend for load-bearing parts and even then you're going to be replacing long, thin parts pretty often. But Tenacity did take its first rolls on all 3D-printed parts, except for the 1515 Aluminum body frame, center suspension rod and various nuts and bolts. I was able to set it down on the ground and the rocker-bogie shafts and the axles on the wheels and the corner steering all bore the weight of the rover.

So, where I'm going from here is moving back over to metal for the long, thin parts like the axles and shafts(about which in the next post) because as much as I've enjoyed being a total materials geek, I really need to get to where I can have Tenacity reliably rolling around and start testing some software, get the ROS stack firmed up, etc.   I'm excited that I got as far as I did, and certainly just having the printed parts get me this far has really helped bring my version of SAWPPY to life.

ABS Steering & Drive shafts
Snapped off clean.       
A few hours
These snapped  when going over a hard bump.
Sometimes along the retainer ring slot, but more often where the part was held by a set-screw.
Carbon Fiber-Nylon blend(NylonX)
Rocker & Bogie shafts, steering shafts,
Deflected, but held up the weight of the whole rover. Broke under shocks/stress.
Rolling/Operational: 10-30 minutes
These snapped at the retainer-ring slots as the rover entered a turn at speed and the weight/stress shifted to one side.
Nylon, even with carbon-fiber stiffening, really wants to be part of an object with some "bulk" and not a long, thin part.
Carbon Fiber-Nylon blend(NylonX)                                                                                  
Drive & Steering Shafts, motor couplers, wheel hubs.
Steering shafts: Last longer than the rocker/bogie ones, but still eventually snapped.

Motor shafts/axles: Rarely broke, deflected a bit.
Steering : Hours to days.
Motor/Drive axles:
The motor/drive axles are the real success story here. They are short enough to cross some sort of mechanical stress and volume threshold, even when the rover was going along at a good clip and swerving into turns, with  the steering servos twitching left and right.
I think I replaced maybe one or two of them and on analysis, found them to not have been printed at 100% infill. The solid ones didn't break.

The couplers and wheel hubs were also pretty indestructable and are still in place.
Again, chunky, substantial parts vs. long, thin ones.
Polycarbonate (PC)                                            
Drive & Steering shafts, motor couplers, Wheel hubs.
Split or shattered within minutes, sometimes even when a set-screw was being tightened. Even at 100% infill. MinutesThe PC is the one material that was an unqualified failure.
I thought it might provide the stiffness that the NylonX was lacking, but for this application it was just brittle. I gave up when a coupler cracked while tightening the set-screw against the motor shaft.

PC might be good for drone frames or something, but it's not good at this scale or application.