"When you only have a hammer, all your problems start to look like nails."
Old Saying

After Scarab's spectacular drivetrain explosion at Bash last year (at least I made the highlight reel), I vowed to build something vastly more robust this time around. So, I started thinking about what I wanted to do differently: Definitely enclosed wheels. Definitely stay in my intended weight class. Definitely be a force to recon with. Definitely put the hurt on all those RC-car based 'bots that were at last year's BotBash. I got lucky finding some cool surplus stuff, and an idea began to form. The result was Tangled Marionette.

The biggest change for this year is that I changed jobs, and along with that gained access to a machine shop. After convincing the nice guys in the Mech. Eng. group that I wasn't going to sever any limbs, I got access to this lab. It made a WORLD of difference in my build capabilities. On the left is the drill press, center is the band saw, and to the right-rear the mill. Right foreground is the set of arbor presses. Off camera to the right is a sheetmetal brake, and a shear. Most importantly, they had all the little tools I needed only once, for about 10 minutes each, that would have cost me thousands to rent or buy.

Here's where I've spent a lot of time in the last couple of months. You can read about doing milling operations all day, and believe me, the theory is good. But there's no substitute for experience. My last few milling operations were 1000% cleaner and easier than my first few. Last year my only power tools were a 30 year old Jigsaw, a $50 drill press, and a set of files. My problems didn't all look like 'nails' any more, but already I'm wishing they had a rotary table and a few other goodies...

The basic drivetrain for Marionette was to be two mirror image "pods" made from extruded Aluminum tubing. The wheels were suspended in the middle, and a chain drive ran the length of the tube. This is similar in concept to how Carlo Bertochini's Biohazard was built. This picture is also an example of lack of milling experience. With this setup I was getting a lot of material flex that was making for rough cuts. Later I learned how to clamp the material properly.

Just as with Biohazard's setup, the wheels and drive mechanism rides on fixed axles, therefore I needed a smooth running system onboard the wheels. I found these steel Boston sprockets at a local place, they were perfect. I pressed out the bronze bushings they came with, and loctite'd a small 5/16" bore bearing into each one. These things would spin for more than two minutes if left alone once they were pressed into the wheels. 2" x 5/16" shaft shoulder bolts made convenient axles, albeit a little heavy. Nylock nuts kept them captive in the tubes.

I needed a carrier for the bearing on the opposite side of the wheel from the hub of the sprocket. My one and only sponsor this year was Tom Deadrick from Kartboy Kustoms. He lathed up a set of brass bushes that I put another set of bearings in. These were pressed into the other side of the wheel using the handy arbor presses in my newfound workspace. Big thanks to Linco in San Diego for letting me pick through their bins of Colsons to find the right wheel for the job.

The reason I found these Boston sprockets so convenient was that the outside diameter of the hub on them matched exactly the inside diameter of my 2.5" Colson caster wheels. Friction fit alone would probably have been enough to transmit the torque into these wheels, but just in case I slotted the material in the wheel and pressed roll pins part way into the hub of the sprockets. You can just see the end of one in this picture, just before I pressed it the rest of the way in on the arbor press. As far as I can tell, these never came into play, but you can't be too careful in this game.

Right about this time is when BotBash registration opened up, and they required a picture be submitted to verify you actually had a robot. This is what I sent, with one side of the drivetrain actually built. Yes, originally I was going to try to fit an EV Warrior in for a spinner motor, but I wasn't sure I could make weight. For the record, the operating name at that point was "Centrifugal Enforcer", which my wife came up with. That'll most likely get used next year some time...

So here's plan 'B', a lifter motor. I got this little gearhead from C & H Sales for less than $20. It's 2 3/4" case height seemed perfect for the 3" tall tubing I was forming the drive pods from. The front and rear bulkheads/armor are structural and give the box some shape. They're simple 6061 plates I got from IMS and cut to length. I love the fact that I can buy 12" x 3" x 1/8" Al plates off the shelf from Industrial Metal supply. Those guys rock! Oh and by the way, the brass-colored rings that mount the motors are actually modified 2" capacitor clamps for mounting filter caps to chassis...

The output shaft of the gearbox is centered on the box, but I wanted the lifting arm to be flush with the top armor when it was put on, so I made up this block to join the extruded channel stock of the arm to the output shaft. The slit is to allow the unit to act like a clamp (its cross drilled and threaded) so that I can really clamp the unit down to prevent it sliding along the shaft. In essence, its a combination hub and shaft collar. A standard keyway helps with torque transmission, I added that after the picture was taken with an 1/8" mill and a file.

The large bored holes in the unit above match holes in the channel and they let me use 1/4" shoulder bolts to transmit the torque of the arm in shear, rather than relying on the threaded portion of a normal bolt. This was MUCH stronger. The bevel on the right side is to allow the assembly to clear the can of the motor as the unit reaches its max lift.

Pretty tight fit. Here I'm mocking up the arm with some of the extruded channel, trying to get approximate lengths and to verify that the arm will fit right down between the endcaps of the Pittman gearhead motors.

Another size of aluminum tubing gets chopped up to make battery clamps to hold my hobby sticks. Still owe a big thanks to Dan Danknick for cutting me that deal on batteries. The original plan was to run at 14.4v nominal, but later in the build process I bumped up to 21.6v due to other circumstances. This made the Pittman motors much happier since they're wound for 24v and were a little slow at 14.4v

So, batteries mounted to bulkhead, motors and drivetrain in place, and lifter motor mounted. Observe what lengths I tried in order to squeeze my Vantec RDFR23 into that space. Looking back, I'm REALLY glad I didn't stay with this method, as it could have really allowed my precious Vantec to get damaged. In order to prevent this arrangement I actually switched temporarily to a pair of Team Delta RCE220 H-brides as their smaller package let me get everything between the tubes. However, as it turns out, I was about to find that I had PLENTY of space and weight to play with, since the lifter system was putting me WAY overweight.

On to Part 2

Copyright © 2001 Puppetmaster Robotics. All Rights Reserved. Original Images and Text are property of Puppetmaster Robotics.
Please contact webmaster@puppetmaster-robotics.com to request permission before reusing.