Centrifugal Enforcer Build Report - Part 2

"We're going to need bigger guns!"
Dick Durkin, Split Second (1992)

Once I got closer to having a platform that would move under its own power, I began to get really excited to see what my chosen lawn-mower blade and motor would do. I also began to worry that Centrifugal Enforcer would either A)Fail to come in underweight, or B)Tear itself apart on the first good impact with an opponent. Only time and a trial by fire would tell. Centrifugal Enforcer was built on an extremely rapid pace, basically 3 full weekend days and 6 intervening weekday evenings. This accounts for ALL of the machining, wiring, etc. so you can imagine that testing and "refinement" were almost nil. This is by far the fastest I've ever built a bot, and I'm pleased to say that the speed of construction had less to do with weaknesses than the relative quality of the design and materials at hand.


A dimly-lit shot of the mounting flange that came on the surplus pancake motor. Nylock nuts threaded down onto the screws, but even with only a 2" chassis height, the pancake motor is so flat that I had to shim it up onto 3/8" nylon standoffs to get the output shaft to stick up high enough out of the chassis for the blade to clear the top plate.
A shot of the underside. I used 10-32 "Low Head" cap screws whose heads were a bit shorter than other socket-head or button head capscrews in order to retain every possible inch of ground clearance.
Here was my simple yet effective solution for coupling the lawn mower blade to the stubby output shaft of the pancake motor. The two halves of this hub are threaded so that they function like a giant two-piece shaft collar. A ball end mill added a relief for the roll pin in the shaft, but its mainly for clearance since the shaft is so short. The real mechanical coupling and torque transmission is from the clamping.
Massive 3/8"-24 cap screws thread down into the hub to affix the blade. The blade itself is a 100% unmodified 22" replacement lawn mower blade from Home Depot. I wanted to avoid having to modify or construct a blade so that easy replacements could be had quickly if the originals got bent. I bought two blades to start with, but so far the original has survived nicely.
The underside of the hub. (Normally this would be attached to the motor first and then the blade would be affixed.) Keeping the clamping seam off of the blade centerline simplified the hub design and allowed it to be smaller (and thus lighter) but necessitated the angled cuts fit underneath the blade. The other two holes were merely to save weight.
Another weight-saving measure I experimented with this time around was to forgo using power distribution blocks (which I had neither the space nor weight to indulge) in favor of simply bolting a set of ring terminals together and then heat-shrinking tubing over the junction. Several power distributions were built in this manner since main battery power had to be split to two ESCs, and then the output of each ESC (positive and negative) had to be split to two motors in parallel.
A simple SPST Automotive relay triggered by a Team Delta RCE200A D-switch provided weapon control, although no braking was available since I couldn't find a SPDT relay of sufficient capacity in the week before BotBash. The weapon only took about 20 seconds to stop when run on 7.2v (as it was for a good chunk of BotBash), but when run at the original 14.4v it took a good bit longer. Next time around I'll be able to get the proper relay and add some terminal braking.
As with Tangled Marionette, I used interlocking Anderson Powerpoles to build physically polarlized plugs that served as both charging ports and removable master links. This made it easy to plug in the chargers with the bot's top still on, and eliminated trying to find and mount a pair of high-current switches, but once or twice the Power poles did seperate at the most inopportune times.
After the fact I've discovered that Anderson makes a mounting kit for physically locking down a pair like this to prevent seperation and make their mounting more rigid. Perhaps I'll try those next time. At any rate, this pair of master links served to activate drive and weapon power separately.
A shot with all of the various bits and pieces in place. The wiring turned into a rats next in no time, but at least it all fit. Notice the plastic top. At the last minute I realized that the original carbon fiber top was putting me about four-tenths of a pound overweight, so I switched to the only lighter material I had on hand: Acrylic. Yes, I know. Acrylic bad, polycarbonate good. If I'd had Lexan I'd have used that, and probably would have fared much better.
On the bench at BotBash, before our first fight. As usual, cardboard and duct-tape create last minute covers for the sharp edges. You can see the charging jacks in place keeping the batteries topped up. You can also see that I've swapped to a new battery pack (near side) for the drive motors. The hobby battery I was using was failing and Team Cosmos alum Ted Zeiger stepped up and loaned me one of his spare packs. Tight fit, but thanks Ted!
Image courtesy of Team Cosmos
During our first fight the Acrylic top shattered (surprise, surprise), so we switched back to the original Carbon Fiber top. You can also see where the chassis got bent when the blade self-impacted the chassis, which is what shattered the top. Running with this top meant we had to lose one of the spinner batteries, which took a lot of punch out of the blade, but at least we could keep fighting.

On to Event Report

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