It’s the end of January. Where’s my damned robot!?
After about a month of dormancy and on-and-off work, it’s time to get serious… lest I pigeonhole myself, again, in the position of working on the damn thing a day before setting out for Motorama 2013. I’m generally confident that Clocker was designed with the best and latest of my design-for-assembly methods in mind, and the progress (mostly in the past week or so for actual work) should demonstrate that here.
RageBridge and DeWut?
No matter how much I get done, though, the availability of the RageBridge assembled boards and the DeWut?!s will be critical in the robot’s completion. As of even right now, I don’t have a concrete bail plan for the DeWuts in particular if the parts do not arrive in time. Fortunately, I received notification that RB shipped this past Friday, and the DeWut order has been completed and may go out on Monday. Here’s hoping the magic of modern express logistics gets them in my hand by the end of this week. Both will probably need a little legwork on my end before I can offer them up to you all, but that’s all part of the battle plan. On a related note, the Hall sensor boards and mounts are ready for your experimentation!
We begin the build of Überclocker with my favorite production machine, the abrasive waterjet.
These parts were made from one of three plates of 7075 aluminum I caught a great deal for on eBay. 7075 may actually be my most favorite material because it’s one of the strongest aluminum alloys, yet you really can’t tell by machining the stuff. I probably could have made some of the material areas smaller to take advantage of the 50% increased yield strength of 7075 over 6061, but elected to not make design changes at the last minutes to save an ounce or two.
The plates were all machined without incident, save for two of them, where the insides are shifted relative to the border. This is a classic failure mode of constant-height waterjet cutters before motorized Z-axes were fashionable – if any part of the previous cut interferes with the head, the machine generally bumps the part into a new coordinate system.
While the damage was minor enough on one of them (the top X part) that I could have milled out the slots, it would have weakened the joint significantly due to the loss of material-on-material interference in the joint. The other one pretty much needed replacement, since the error occurred (seemingly) right before the final profile runaround. I elected to redo both parts at the earliest opportunity.
Also lined up for the first batch was the main lift gear. It’s the same pitch as Überclocker’s previous lifter gear, at 12DP, but the reduction ratio is higher (5:1) instead of about 3:1. This is to make up for the loss of the 216:1 integrated dual frankenboxen for speed reduction purposes. While the difference between a DeWalt gearbox in low gear (52:1) and another 5:1 is still outmatched by the reduction ratio of the IDFB, I think it’s less likely to destroy itself. The DeWalt motors are innately more powerful and torque-balanced than the 550 motors, so perhaps a 260:1 reduction is enough. In fact, it’s more than enough, but the maximum top speed of the lift would be an unnecessary ~15 in/s at the periphery. I’ll deal with the increased current draw, though, because hopefully RageBridge’s low speed exponential response and dynamic braking will make up for it. Maybe it’s time for a closed loop speed feedback…
The small gear is a steel pinion I purchased from McMaster whose hub will be removed and bore broached for a 1/8″ keyway.
Round two of parts. The top and bottom plates are made of my most recent favorite top and bottom material, 1/8″ G10/FR4 garolite in black. There’s some of the usual delamination from high pressure piercing. In the past, I’ve taken care of this by injecting copious amounts of CA glue into the bubble and then slamming it in a vise. A perhaps imperfect repair, but it at least brings some of the strength back in the bubble area.
The tensioner and drive sprockets were also cut at this time. These used the profile shifted sprockets I designed for Chibikart to account for waterjet taper. The tensioners are basically sprocket rings glued to a ball bearing as shown in the topmost example.
A little bit of stuffing with Loctite 609 retaining compound later, and I had the tensioner sprockets. The bore was designed such that they were a near perfect tapered press-fit as cut on one of the MIT waterjets I frequent the most. Different machines would necessitate familiarization before I am able to do such a thing.
Continuing the steps of small, easily pressables, I installed the fork shaft bushings and the outboard support bearing for the lifter motor. The bushings needed finish-reaming after installation since this bore wasn’t that perfect – luckily, I was able to borrow a 1″ adjustable reamer from one of the campus shops. A ring of 609 ensures their retention. After the finish-reaming, I decided to increase the diameter a little further to allow for some alignment slop when it came time to assemble the frame, since otherwise bushings will lock up with any small amount of misalignment.
Round 3 of cutting sees the front “reactive outrigger” parts finished and the replacement frame rails also finished. Now I can really get onto assembling the robot’s structure.
The legs, now that I actually hold them in my hand, are massive. If the bot ends up a little overweight, these are the first parts getting selectively lightened!
The order of assembly of Clocker this time mandates the fork mounting structure be assembled first. This then slides, with the frame’s back member, into the sides. In previous Clocker iterations, this would of course have guaranteed the need to disassemble the entire bot before any work can be done on it, but I hope I correctly allotted space this time around to swap motors and repair drive components without needing to do so.
A first look at the assembled frame of the bot. These pieces are just shoved together for now – there are more parts to make and assemble before I can install all of the t-nuts.
Another item of minor fabrication is attaching the clamp hub shaft collars to the components they will be driving. The two fork tine collars will be tightened securely, while the one on the gear will function as the slip clutch for the system. #10-32 screws were used for this effort since they fit flush into the counterbored holes in the shaft collar, and plenty of high-strength Loctite 262 were dumped into the threads to make sure I can never, ever take these things apart again. Ideally, I would never need to…
With most of the minor assembly complete, I turned my attention to solving an issue that had been on my mind since the first time Clocker was in a tournament. The clamp actuator has always been really slow, in part because I’ve been using highly geared motors on the Acme thread. Many matches in Clocker’s history have had missed grabs because the clamp just didn’t come down fast enough. Other clampbots in the past have used pnuematics and R/C servos for the clamp arm, so they’re quicker (but each has its own downsides).
One way to solve the problem would have been with a fast-travel leadscrew such as the one I used on Make-a-Bot with 8 threads per inch and 2 starts (so basically a 4 thread per inch). that would net me a 2.5x speed increase. Problem is, that would also entail remaking the Acme threaded sprocket – and I didn’t have either nut or sprocket one on hand. I decided that the force loss was acceptable enough to just take out one stage of the Harbor Freight drill innards which made up the gearbox for the clamp motor. This was a 36:1 gearbox, so taking one stage out is a 6:1 increase in speed. Because Clocker’s clamp is hypothetically not backdrivable (unless something truly terrible has happened), I don’t actually need that much clamping force to hang onto someone, especially with the big squishy rubber bumper on there.
So, onto the lathe the ring gear goes, and one pass with a parting tool was enough.
The actuator, reclosed with 1″ long #6 screws. I forgot about the fact that my little tension roller standoffs existed, though, and had to go back and trim down two of the 1.5″ long screws that were in this duty so they would fit those.
With that matter taken care of, I embarked on Epic Standoff Evening where I popped out many little round threaded things from tinylathe. With the exception of the tubular spacers (for the fork) at the top, These parts are actually all 7075 too – due to the magic of eBay, once again, I caught a great deal on 3/8″ and 1/2″ rods of 7075. Because some of these parts, namely the axle standoffs, were modeled as steel, I ought to be creeping slowly further down from the initial weight estimate, which is good.
Threading the ends of the standoffs led me to come up with quite possibly the worst tapping fixture known to mankind. No taps were harmed (I think…) in the production of this image. I only used the other drill to hold the piece steady – it was not counter-rotated. Really the way this came about was trying to figure out how to hold the round piece still to thread it without damaging the precision-ground surface, like what would happen if I threw it in a vise like I usually do.
One of the other simple operations was to trim off the hub from the spur gear. At this point, my 1/8″ keyway broach had not yet arrived, so I couldn’t broach the thing, but at the very least it can be prepared.
We conclude this address with pretend-o-bot #1. Still to go in this picture are making the drive wheels, machining the fork’s main axle from the giant aluminum shaft, machining the front leg parts, and finish machining the top and bottom plates. I’ve hopefully ordered the last round of random hardware needed to get the build done. Past that, it’s just waiting for the hired out parts, and possibly formulating a ditch plan…