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Turnbuckles and bogies need to be added, then the 4 drive units and 2 passive wheels. Another couple of days to rough it out, then 2 more to get all the finicky details doubled checked before I start printing.
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Another good day. Design is finally roughed out. Lots of the dimensions and stuff are tolerances, but now I have something I can refine.
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Especially proud of learning to integrate components from other sources properly. SG-90 servo motors are now in the design (random open-source), as well as a ball joint from McMaster-Carr
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I'm still astounded by how I can do all this insanely powerful engineering design work for free, and my only cost is materials and time.
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The turnbuckle was the hardest thing I've had to model. 2 ball joints at either end of a variable length rod... I left out the screw constraint and the lock nuts. Made it cylindrical joints instead.
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And FINALLY, the full assembly is sorta done with all major linkages and joints represented. It was a bitch to get this right... there's like 6 ball joints, and 5 pin joints here.
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The thing is, attaching the 2 turnbuckles would immediately mangle the assembly into a weird train wreck. Since the geometry is coarse, all lengths are rough approximations, and OnShape doesn't allow ball joints to be constrained, the constraint solver creates pretzels.
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And initially I had the turnbuckles set too long, so the thing would really contort. So I suppressed one end of each turnbuckle, rearranged the whole thing to look right, then manually adjusted the turnbuckle lengths and positions to be approximately right...
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Then I unsuppressed end connections and the solver "found" the nearby solution and didn't pretzelize the design. Then I used the measuring tool to measure the turnbuckle length, and went back to the part subassembly and constrained cylindrical joint to be a few mm around that.
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Effectively, instead of solving a hard 3d geometry problem to compute the right nominal length, I found it "experimentally" in the 3d CAD design. Now I have everything close enough that I can do projections and back out final actual geometry. 😎 🥳
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Replying to
First attempt didn’t work because it turned out one of the 2 drive motors in ABTF build had terminals shorted 😭. Took a bunch of continuity testing to figure it out. Luckily I have 2 more plus half a dozen coming. They’re not cheap. $13 each, with integrated quadrature sensor.
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Feeling the urge to add a pair of RGB leds on my rover and have the leds go red for evil. In general I want to make my rover evil. You guys are lucky I’m pretty bad at this stuff.
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Next up: testing the quadrature sensor. 4 encoder wires hooked up to 4 encoder inputs with alligator clips ugh. Motor will run off dc supply for this test
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Back to rover after 6 weeks. I have 3 weeks to build a face-saving demo for demo day 😬 Dec 12. Printing parts for NIAM (Nature is Attempted Murder, the draft build for final build, Nature is Murder)
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Urgh. This part was sized big enough to hold the servo, but not big enough to allow it to be maneuvered in. 🤬
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Design for assembly is a bitch. I thought I had enough room to wiggle it in but no. Will try to saw in a compliance notch like I have on this mating connector. PLA is pretty stiff, but these notches work well.
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Alt solution is to mount with flange on the outside of the box, which wastes space and increases vehicle height by about 3mm but might be better than hacking away.
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Ah crap, this problem only affects the outer 4 wheels, and there may not be room to lower the middle wheels 3mm
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And FINALLY, the full assembly is sorta done with all major linkages and joints represented. It was a bitch to get this right... there's like 6 ball joints, and 5 pin joints here.
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But the assembly was already off anyway, since I am using 65mm wheels instead of 60, and adjusted the motor forks by 5mm, which means the outer wheels are now about 7mm higher than middle. Which is fine since the bogie is hinged, but it will be weirdly sunken middle look.
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Edit, 5mm, since only half of the extra diameter affects height... so the bogie crossbar will now have about a 5-7 degree off-horizontal gradient, making the thing look a bit like a grasshopper
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This has been an exercise in demonstrating to myself why premature optimization is the root of all evil. I was procrastinating on printing because I hadn’t worked out the exact target geometry of the design for it to be even keeled at nominal. Finally said fuckit just print.
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Good thing because this draft assembly is going to so wildly off in so many ways, I’m going to have to do a significant redesign anyway. But this will be good enough to test a bunch of things
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6 parts printed, 2 more printing now, 4 left. Already aware of a dozen design errors (all fixable in post with a hacksaw and sandpaper so not showstoppers for the prototype). But damn mech design is harder than they let on in school. Dunno how my professors let me pass courses
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The basic mechanical frame for NIM has 26 3d printed parts, 8 bits of aluminum tubing 4 off-the-shelf complex fasteners (ball joints and turnbuckles) and probably a couple dozen ordinary fasteners. It’s about 10x more complex than anything I designed or built in mechE school
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This is going to be a bitch to put together. But milestone coming up… metal-work. Need to cut that aluminum tubing into pieces with either a hacksaw or a dremel.
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Soft test assembly of the draft build. With construction paper tubes where the aluminum will go. Don’t want to cut metal and then discover a major error. This looks ok I think.
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First metal… cut 100mm strut.is there any use for aluminum filings? Saving it anyway. 9 more to go. Aluminum is much softer than mild steel, but hacksawing it is still a bit of a workout. Needs some filing
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Ok that was really quick. 5 minutes for 3 cuts. Takes some control though. The cutting disc wanders easily. And aluminum stock does get hot.
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Two rocker bogie sub assemblies almost done. Waiting for drill to charge so I can make the pin joint holes larger so these binding posts fit. Took a bunch of filing but I have a nice snug fit.
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Ugh this was a bitch to do. Tiny M2 mounting screws for SG90 in misdesigned casing, servo horn on bracket, N20 in fork. But good news, the conduit theory of wiring is probably going to work out though it’s gonna be a tight fit.
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