This is a classic management paper (also a key reference for the neat little book, Obliquity, by John Kay) that I often recommend to clients, but for people/org management. But it applies to this solo engineering project too.
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Finishing up the mechanical design of ABTF with a pair of designs for a strut cap for motor strut, to hold motor in place. One is a hatch style that will screw on, and should work no problem, the other is a shoe style that I'm hoping will work without screws, interference fit.
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Hmm I suppose I could have printed this as one piece, with just a push-fit hole for the motor 🤔
There would have been an overhang, but I like the idea of a 2-piece assembly, not sure why. But will try single-piece design for NIM.
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Beginning test assembly of Accessory Before the Fact (ABTF). The Bill of Materiels (BoM) even for this simple test rover is 65 distinct parts (+ 4-8 more depending on what interconnect design I settle on to finish the wire harness 😫
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67 [+4-8] actually, miscounted
16 mechanical parts
36 fasteners (12 nut-bolt pairs, 12 screws)
6 motors
1 computer
1 battery
7 interconnect cables (not counting 4 integrated ones)
4-8 connectors, not shown.
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The steel rod at bottom right needs to be hacksawed into 2 axles the size of that toothpick, so that will add 1 more.
At this rate I’m guessing the main Nature is Murder rover will weigh in at ~200 parts. Damn, engineering is complex.
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Discovered an idiotic oversight. There’s no way to attach the servo arm to motor bracket without obscuring the center hole through which you attach servo motor. And if I try to attach motor first, attaching arm is hard but then then motor won’t fit through hole in chassis 😥
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But drill to the rescue! Just drill an access hole in bracket! And of course I mess up a bit. Next bracket, hole will be designed into the print.
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Next, attach arm using 2 tiny self tapping screws. But holding it in place is a challenge in itself. No worries, we use an M2 nut and bolt to hold it in place (the arm/horn center hole is tiny but I have M2s lying around, yay!)
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Now another issue. The self-tapping screw is strong enough to penetrate horn pinhole but apparently not the bracket. It simply pulls up the horn 😡
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Gotta break out the tiny hand drill and create a starter hole. Beginning to appreciate the value of having the right tools at hand. I bought this tiny jewelry hand-drill set a while back precisely for this sort of bs. But again, gonna design the hole into the print next time.
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Managed to strip the head of one screw in self-tapping false-start, despite using small screwdriver. But can tighten it with needle-nose pliers. Lesson: never force tiny parts, they strip easily.
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But… horn finally attached and temp center nut-bolt removed. Whew! Now for the motor.
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Motor attached to chassis and bracket without incident.
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One down, 3 more to go.
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Well, all 4 steering serves and brackets attached. This proto rover will have some major alignment issues.
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Next up, make axles for the 2 passive wheels. Hacksaw time. But first a short recess to go take a consulting call and make some money to pay for all these shady R&D shenanigans.
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The lessons never end. The shaft needs to be 26mm according to the design dimension, but the toothpick piece I used to test the assembly is 27.3mm. Which to use?
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I go with the toothpick. Figure the compliance of strut will create a nice compressive fit. But I forgot steel is much stiffer than wood, so it’s a tighter fit! Still it fits. Btw I’m very impressed with my hacksaw precision. I’ve matched the toothpick exactly 😎
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Second axle I decided to go a little shorter. Note the scratches on the shafts and leftover stock btw… hard to wield a hacksaw precisely without it jumping around a bit. Next time I’ll make a cutting guide or something.
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Passive wheels in!
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Aaaannnd drive motors in as well 😎
The assembly is done! Note holes on drive motor strut fork caps… can add screws there if necessary but interference fit holds motor in place well enough. That was a bitch to get right btw.
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Here’s some posing with steering angles… stylish lil guy eg?
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Let’s add the computer, battery and soft assemble the partial wire harness (battery and servos connected; drive motors not yet… will add 2x6= 12 more wires/4 more cables to this picture).
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Let’s power it up 😎
This is just led blinky running but it’s still neat. I got much further today than I expected. Now for toughest part… rest of wire harness. Unfortunately crimping and/or soldering may be needed and I suck at both.
Then the hellish challenge: programming
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Back to the mechanical design of the main rover... just spent 2 hours figuring out how to get a mcmaster-carr ball joint and some standard nuts and bolts into my assembly, and detected some geometry issues that would have been annoying to discover after printing...
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Finally actually earning the mechanical engineering degree they gave me in 1997. I think the mechanical design is now about 60% done. I'm now aiming to build a model accurate enough to simulate.
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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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Back to rover after 2 months. Finally got beaglebone blue to drive a motor. Result of a half day of work that should have been 20 minutes. 🤬
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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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Or at least I think that’s what’s wrong. Will need to do some more testing to be sure.
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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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