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June 25, 2014
Begin Sand Drive prototype after planning since spring 2013

I had put my build on hold to help deal with a family matter and am just now getting back to thinking about my build. In fact, it's good to have my mind obsessed with something as constructive as this:

I think I confided in a few builders that I was thinking about a foot drive no one has built yet. All R2 units can operate on hard wood floors, and most on carpet, cement and some on asphalt. One I heard can actually drive over GRASS! The only thing someone hasn't gotten their droid to move over is SAND... except with a sheet of plywood.

I'm doing R&D on a SAND DRIVE. A tank tread using as many currently available parts as possible. I'm trying to fit the tank tread, motor and housing all in a standard foot shell.

My goal is to come up with a sand drive that will be easy to swap out with anyone's existing foot drive, or a quick-release foot to swap out a "carpet foot" for the "sand drive foot" and back whenever the terrain changes. There's a lot to consider here! Some droids are light like a styrene build, some aluminum droids are like 200 lbs.

Like I said, this is R&D, and I'll document what I'm doing for everyone to see, and will undoubtedly be asking for technical help from the R2 Builders Club.

Here goes:

I went out and bought some parts and stuff from Home Depot the other day. I've been thinking through this for weeks and it's time to do a mock-up.
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Next step was to take 4 old CDs and trace a rear bicycle gear. I made 4 sprockets and my "plan A" was to find a motor to go inside the tread. The only motor I could find suitable was the NPC 2212 Motor since it has a gearbox with both axels - left and right - and it's familiar to many R2 builders. I went for it because of known torque output, electronic output, etc. I made a cardboard mock-up of the motor from prints I found online. It's freaking HUGE! Usually you find the NPC 2212 housed in the battery box, but with a tank tread, the motor must be inside the tread.

Plan B: I now make a 5" sprocket from cardboard:
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Now I assemble the drive belt and motor mock-up. I just barely can get the motor in there with a 5" front sprocket. Note the 4" half-section of copper pipe I cut for tank tread pieces resting on top of the front 5" sprocket.
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Well, the fit calls for maybe 2" to show below the foot shell. I'll need to lengthen the chain by 4 to 6 links to utilize much needed tread contact to the ground. The rear sprocket can be moved back to the pencil mark, but because of the larger diameter, the 5" sprocket won't quite make it that far forward and still have adequate clearance.
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With a 2" sprocket I was planning on having the tread be 4" wide to get as much surface area as possible on the sand, but with a 5" sprocket the clearance narrows to 3" at the top of the foot shell, so now I'm re-thinking the tread size. It fits tightly in my foot shell at 3".
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Here's a shot through the side panel. I need to calculate where to put the attach points and adjusting points for access when the complete foot is assembled.
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This upside down shot shows how the motor will hang low on the inside of the tread.
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And here is how the pipe will be spaced on the tread. Plan A for attaching the now 3" sections is to solder them to the chain. We'll see how THAT works. (Sarcasm.)
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I've mentally constructed this "sand drive" foot drive system hundreds of times in my head and I keep coming back to needing the motor to start with to get the final measurements right. Everything depends on building from the motor out; the sprocket diameter and spacing, washer width, outside housing bolt clearance, how far the unit fits up into the foot shell, how much hangs below the foot shell, how much room there will be for the drive structure and its fit to the foot shell, the adjustment placement within the mechanism and it's accessibility when assembled, the shape and construction of the sand drive tank tread structure, and finally the electrical wire placement and quick disconnect to power the motor.

I'm now researching the material to cut the sprockets out of. I will hand cut and file the 6 sprockets for this one sand drive foot by hand from either aluminum or zinc. It must be less than 1/8" thick to fit in the chain, yet beefy enough not to have the teeth on the sprocket bend or break with use. When the time comes I'll scribe lines on my stock plate using the right size sprockets and buy a new blade for my band saw.

I need to make a mock up with the full sized motor and the size sprockets I'll use. I'll use a 1/2" wooden dowel to simulate the motor shaft and cardboard for the sprockets. I need to know where the drive sprockets will need key slots and where the key slot on the drive shaft will be. I need to know where to cut a groove in the shaft for the retaining washer, cotter pin, or e-clip. I'll need 2 on each side - one for holding the sprockets on the shaft, and one to hold the motor and tank tread on the sand drive frame.

I need to somehow fasten the copper pipe halves to the chains temporarily to calculate how much space to allow without soldering the whole thing together only to discover I have to remove chain links to make it smaller or hopefully add links and make it BIGGER! Then I'll know better the structure I'll have room inside the foot shell for and will fold aluminum foil to simulate the aluminum stock I'll bend for the structure, and how and where I'll be fastening to the foot shell.

There are also a few hundred other variables I need to see during construction which depend on the previous step and problems to solve at most every step before I can complete this prototype.

I'm waiting to find an appropriate motor. Best bet still looks like the NPC 2212. Later I'll need another one EXACTLY the same for the other foot, and the electronics, RPM, and specs must be familiar to the club.

While waiting I made a test box containing a switch, a pilot light and a potentiometer for testing the completed sand drive installed in the foot shell without having to hook it up to a droid.

I also have 2 bicycle wheels from one of the bicycles I cannibalized for the sprockets and chains that I set aside to fabricate a platform to hold the wheel upright and a place to put weight above the foot to see if the sand drive can actually move over sand with the weight of a droid on it. Since an aluminum droid is about 200lbs, the sand drive must be able to move well with 100lbs on the test platform.

Here's a couple of shots of the box:
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I found a place that sells aluminum sheets, and they had scrap I bought for under $20 that was .100 thick. A tenth of an inch will be a perfect fit to make the sprockets for the chain I have. So now I superglue the bike sprockets on the aluminum and blacken with marker and scribe around it. When I traced the outline of the sprockets to the aluminum I was amazed at the inaccuracy of the sprockets I got from the bicycle! I guess they don't have to be good to work. But I want these to look nice, and these sprockets look like junk!
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Once I had the outlines all scribed on all 8 pieces I started cutting with my piece of crap bandsaw. (It wants to walk about 30 degrees to the left - I have to push material slowly through sideways to cut a straight line. And one of the guide wheels came off with the ball bearings falling all over the place.) Two bandsaw blades later and I have half of them cut. I bought a new blade yesterday and hopefully I can wear through the last 4 sprockets.
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Tonight I finished the first sprocket. I carefully measured the depth of each cut and filed to fit the chain. It appears to be a little more accurate than the original sprocket which is pretty sad when you consider I'm shaping these by hand.
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I also carefully filed a bevel on each of the teeth to prevent the chain from jumping out of the sprocket.
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Washers are common 1/2" flat washers sku 802521 from Home Depot - which measure 1.075 in dia with a .525 hole and are .080 thick. I need 2 on the shaft next to the motor on one side and none on the other for sprocket clearance to the motor. Two are needed outside of each sprocket. I'll see how much I can file off each washer later. I still think I can cram the 10 washers, 4 sprockets, 4 e-clips and 2 frame attach points on the shaft in 4" but it's going to be real tight! Here is a washer with burrs and one I filed with the small drive sprocket.
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After careful consideration I decided to get a new NPC 2212 drive motor from the manufacturer. The reasoning is this: First, it's repeatable - motors from other sources might be different RPM, voltages , etc. 2) It's a familiar motor to the builders group. 3) It's got a shaft on both sides. I received ONE motor and tested it out and see I wired the test box wrong. Apparently I have the indicator light wired in series with the motor. I'll fix it later. Now I fit the gears I've made thus far. Now I have both sides done except for the groove for the e-clip and key piece on one side. It's got a little slop which could be a problem. Here's a shot trying to get a piece of the key bar cut to .080 and stuffed in the groove. I've got the drive sprocket notched out here.
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The key piece in, a shot of the size difference between the small drive sprocket and the sprocket that will carry the tank tread. It's huge and still will barely clear the motor. The tolerances on this project are insane for a handcrafted drive!
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Here is the first side completely assembled.
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The Hillman 5/8" ACP E-Clips (Home Depot) just fit over the shaft after making a score into it with a small file. The clip measures .040 and the file width is .050. Ran the motor and held the file to make a deeper groove.
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Cut the back 4 sprockets today. I need to finish-file them all and then head to Home Depot with them and my calipers to figure out the rear axel assembly.
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Finished 2 more sprockets tonight. Six done two to go! Making these with nothing but a bandsaw and files is hard work but very satisfying. As far as I can figure I'm within aircraft blueprint tolerance. Notice the outer sprocket has shorter teeth. They are about .200 so they don't protrude through the chain. Also I chamfered 2 bevels, one about 15 degrees and one about 30 degrees, on each tooth to prevent the chain from jumping (I hope).
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Here's the front sprocket set and the rear sprocket set in the relative position they will have in the completed drive.
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Going to try and make some off the shelf parts from Home Depot work for the rear axel assembly. I got so excited with the parts I found I clouged it halfway together out in the car. I got a brass pipe nipple LFA-746 for the outside rotating portion of the rear axel, the 4 sprockets and washers will attach to that. That whole unit will rotate with the drive sprockets and tread sprockets. I need to drill through the sprockets and their adjoining washers and fasten them together with small short nuts and bolts to make 2 rear sprocket assemblies out of them. I may also solder the inner washers to the brass for stability. Still haven't decided. A 4 1/2" long 5/8 bolt through the center of the brass has a little slop, but I can always shim it if it's a problem. I'll attach the frame to the outside of that. R2
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Next step is try and test fit it inside of the foot shell and hope that the chain links line up. If they don't - and since plan A and plan B had flaws - then it's on to plan C. (I got 22 more letters before I run out of plans.) After a test fit I'll make foil mock-ups of the aluminum I'll bend for the frame and see what the simplest way wold be to attach it to the foot shell. R2
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Got home later that night and stuck it all together. Though the rear axel assembly is just stuck together to see what it will look like, I wanted to check the fit of everything to the foot shell. First I had to cut the motor shaft and I swear it was like I was cutting off my fingers. Figuratively. I hope I left enough on each side. Painful decision! What's MORE painful is the cutoff wheels and cheap drill I was trying to use! I ended up using a hacksaw blade wrapped in tape. I really really really miss working in a shop with real tools! By this shot I appear to have about 1/4" clearance once the tread is attached. Had to put the drive assembly in upside down to get it to balance, the motor will be at the top. The piece of folded aluminum foil is simulating the support to the rear axel to check length and bend. R2
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This shot shows the motor orientation inside the tread. Couldn't have the front sprocket much smaller and have it clear the motor. I'll be dealing with as much as 3" of the drive assembly showing below the foot shell. Next it's getting the rear axel assembly, sprocket assemblies, washers, spacers etc. all attached together and everything spaced the same as it is on the front motor shaft. R2
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I assembled the front axel, put in the key which locked drive sprocket to the front shaft.axel, and replaced the e-clips with smaller ones. The 5/8" e-clips are too big. I've decided to use 1/2" e-clips and make a deeper groove in the shaft. The file doesn't work as well as the hacksaw blade. I turn on the motor and hold the blade still. This makes a perfect sized groove for the clip. R2
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Next, I needed to see if the chains would match up. I've been dreading encountering a miss-match in length between the drive chains and the chains that are the tread! Well... The chains were one half link miss-matched! I solved the fit problem with a technique I learned from an aerospace engineer at NASA on Edwards Air Force Base: I got a bigger hammer. Here are a couple of shots of both chains clouged together before I designed and constructed the center bracket that holds the sprockets the correct distance apart and has a slip slot to tension the chains.
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I drilled the rear sprocket assembly's washer and sprockets and bolted together with very small nuts and countersunk screws. Had to saw the ends off the screws and file flat. Since everything is being done by hand I've had to resort to things like drilling the holes at obvious patterns for easily recognizable pieces for re-assembly.

I took some 1x1 aluminum square tubing and cut the rear half of the center slip bracket. Fortunately I found the right washers to space everything right.
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I'll replace the wing nut later with a nylon self locking nut so it's loose but won't fall off. I'm also thinking about putting the tension rods (that will push each side of the axels away from each other) INSIDE the sprockets because I've just about run out of room outside.
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I'm as inversely proportionally adept to mechanics as I am inept at electronics. While trying to get my control box wired and ready to test my prototype I let a generous portion of the magic smoke out of the potentiometer. Twice. I haven't a freakng clue what I did! I apparently didn't do permanent damage though. Here is a short clip of the first test of the drive assembly.

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Now comes fitting the drive into the foot shell. I made a couple of rear brackets and had to straighten and re-bend them a bit, but have a general idea of what will work best. I have to disassemble and make a couple more cuts and a bend for it to fit, and will probably make 2 new ones because I've about trashed these first 2 with all the bending. Right now the point where I bend the outside rear bracket interferes with the access door.
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Now for the tank tread! I'm soldering the copper pipe pieces to the chain with a torch. I was sure to close the door to my room so I don't set off the smoke alarms in the apartment seeing as how I built the jig to hold the chain and pipe pieces out of wood!
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I've re-soldered some of the pieces a bunch of times because each time I try and get the chain to bend I pop the pipe off the chain. I've got the flux gumming up the chain and the solder isn't quite strong enough! I was originally thinking brazing would be the way to go, but on my budget all I can afford right now is a torch and solder. I better get out the JB Weld and reinforce each of the pieced if I'm lucky enough to have a movable tread and all pieces stay attached. The tread is sitting in a bucket of acetone right now and I'm hoping the rosin flux is soluble enough for the a-tone to un-gunk it!
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Total pain in the butt to get all the pieces soldered on AND have the chain not get gummed up by flux. I soaked it in acetone over night and finally got all the pieces soldered and re-soldered when they snapped off when I freed up the chains. I reinforced the joins with JB weld and cut some wood pieces to help with applying the canvas.
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I glued the canvas to the copper with Elmers glue. Poor choice. Next time I'll use the same caulking I glued the end pieces on with.
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I spaced the canvas between the copper pieces with a square piece of wood so there would be enough material to go around the smallest sprocket without binding.
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Cut a bunch of polypropylene tubing into 3" lengths and slit them down the middle. I put one on each of the edges of the copper sections after filling the center with marine caulking. Cured the tread over night.
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Put a drop of machine oil on each of the chain links and moved them around to work it in and then fit the tread to the drive assembly. Here you can see the turn buckle under the pipe piece that's protecting the motor wires. I had to install an angle bracket between the motor and the front piece of the bottom support and bolt the all-thread to it. The rear - the left hand thread piece - is bolted inside the square pieces that the rear axel goes through.
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Now to fit the foot shell and the drive together. I'm designing the front brackets with the idea that I'll have to be able to disassemble it through the foot shell door panel. You can see the markings on the rear bracket indicating where I need to make more bends to clear the door.
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After bending the supports to fit inside the foot shell I gradually devised a way to fasten the whole thing inside. I'm using a 2" square tubing section and will countersink screws to save space. This is the simplest configuration I can come up with at the moment, so I'm gonna run with it.
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This shot shows how it will line up better.
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This shot's a little blurry, but I wanted to document how I need to JB weld some nuts in place where there would be no way to get tools or fingers up there during final assembly.
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I've got the front bracket done here. You can see the interior of the foot shell with the front bracket assembly clearly shown. Everything is so incredibly tight that I've got to use countersunk screws in 6 places. I have about 1/8" clearance when the foot shell door is on.
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I had to re-bend the outside rear piece again because it wasn't clearing the door and it broke from metal fatigue and such a tight bend. I fabbed a new one and drilled the holes. It appears to be OK by eye, so I JB welded a nut on the rear of the rear inside bracket. For a production run I'll use self-locking nylon nuts on everything.

I haven't posted my final ideas yet, but I'm working on a way to encase the motor and tread assembly completely to keep sand out of the chains, sprockets, and motor. I have yet to invent the sprocket skirt and the chain guard. With that done, and a support stand with wheels to keep the foot upright and platform to add weight for testing, it will be off to Tatooine! I mean the beach!
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OK, I took about a year off from my build due to a family matter. It's now July 2015. In the above pictures you can tell the rear brackets are rather rough. so much so that the last thing I did last year is tighten the pictured screw and it popped the nut and JB weld off because the surface was curved and tightening would bend the aluminum.

Durring the last few days I was able to fab new rear brackets. Here they are mounted on the Sand Drive:
(Left Foot Outside)
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(Left Foot Inside - note protection for the motor wiring and quick disconnect)
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(Viewed From Front)
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Here you see the new brackets I fabbed installed inside the footshell.
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To prevent the foot from tipping over when testing it on sand I would need a support structure to keep it upright and able to put weight on it to simulate the weight of a droid. I threw together a test cart from the rest of that old bicycle and some scrap wood. I cut off the ankle of one of my mock legs to attach the footshell.
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OK, here comes the chain guard:
This is a chain guard so that the Sand Skirt which is going to attach to the tread to keep sand out of the chain won't get chewed up in the chain. My first few tries are a FAIL, which I'm including so others won't go down the same road.

Here's the raw material on the inside of the sand drive where the motor wires come out.
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But you can see by these 3 pics trying to form the plastic is not working with a heat gun while mounted on the sand drive. It pulls away from the chain instead of falling down...
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It doesn't leave enough clearance for the sand skirt...
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And it doesn't lie flat on the large gears.
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Here are the fails: I've made 2 attempts and have material cut for a 3rd try. But there IS no try - just do or do not. Thanks to input from club members I'm going to make a mold/jig to form this material before trying another material.
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So to see if this is going to work at all I made a jig to hold the plastic and rigged an oven from scrap wood and a heat gun. FAIL. Heat too hot and not evenly distributed...
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With that done, I decided to give it one last shot, and after Googling if high-density polyethylene (HDPE) out-gasses (negligible) I brought it up into my apartment and set my oven on about 300 degrees F to see what would happen. It took nearly 2 hours to ramp up to temp, but it worked. Next step is to build a good jig to hold the plastic and make the chain guards. BTW, plywood out-gasses. My apartment smelled like a wood shop in the winter. Delightful!
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