The event, that is. Not the robot.

With term now well under way, the time I have to actually work on the robot has become more limited. Fortunately, there’s not that much grunge fabrication left to do on Arbor. It is, however, nowhere close to actually running. Major assemblies have been completed and most of the remaining tasks are just connecting the dots. Pictures of the dot-laying itself are below.

While waterjets are good, they’re not perfect, especially for sub-thousandths sensitive applications like ball bearing mounting. Ball bearings don’t like being compressed too much by interference fits – they get tight and a bit crunchy. Both bearing holes on the saw gearbox side plates were about 4 thousandths undersize, which is far too excessive for a press fit.

Normally this would be a simple boring operation to resize the hole, but it was greatly complicated by the fact that my brilliant design skills left no two continuous parallel edges to fixture to in a vise.

So the 85 pound milling vise had to come off. I elevated the piece on a scrap piece of 1/4″ aluminum and used that as a landing spot for the boring head. Otherwise, it was a smooth operation.

After cleaning up the bearing bore, I hopped on the 30 ton hydraulic press and slammed the bearings in. They are R14 double shielded ball bearings. Here’s the gearbox assembly just slipped together for visual effect.

I threaded the perimeter holes for a #8-32 screw. This was actually a hackaround for my neglect in laying out the 2D file for the threaded vs. nonthreaded side.

The threaded side, obviously, was designed from the outset with smaller pilot holes. Unfortunately, they all ended up the same size, which was a #6-32 clearance.

To remedy this without using nuts, I had to slightly enlarge the holes in the other gearcase parts and then use the existing #6-32 clearance hole to tap a #8-32 thread.

I fell into the trap again.

Remember a little while ago when I discovered that my 3/8″ (0.375″) aluminum was in fact 10mm (0.393″) aluminum?

I forgot that again, and designed everything around 3/8″ plate. Problem is, that just won’t do for the worm gearbox, which needs pretty precise spacing and centering of the worm gear to properly mesh. So the solution was to shave down the middle plates of the gearbox, which determined the bearings’ axial spacing.

A quick trip with a fly cutter reduced the thickness of the middle plates to 0.374″. I was concerned that the setup was going to be unsound, since I was effectly clamping a ring between two flat plates, but things turned out fine.

Uh oh.

What you see here is the center plates failing to fall in line properly. There are a total of four plates between the two large gearcase sides, two 3/8″ thick and two 1/8″ thick.

There are three reasons for this happening.

1. The 10mm aluminum vs. 3/8″ aluminum quandary. While  I had fixed the middle two plates, by the time I discovered the discrepancy, I had already installed the bearings in the side plates, and didn’t feel like going back and uninstalling them. Additionally, trying to face down the side plates would have been an incredible pain.
2. I reversed the bearing orientation relative to the worm gear. If you refer to the rendering of Arbor, notice that the ball bearings stick out of the gearcase side plates. This required substantial shimming of the worm gear between them, something that I did not account for when I placed my first McMaster order. To remedy this, I investigated reversing the side plates so the bearings pointed inwards. Each bearing then should have needed only 0.025″ of shimming.
3. …if the aluminum plate was actually 3/8″ thick. Nope. The difference between 20mm of aluminum and 3/4″ of aluminum is about 0.04 inches. So, the interior of the reversed side plates were already 0.04″ too close. Now, while the two 10mm plates in the middle would have offset that and made things work again, I already cut them down to 0.375″. That means there was about 0.08″ less width than there should have been: for a grand total “leftover gap” of roughly 0.16 inches.

This meant that the two 1/8″ side plates couldn’t be installed.

I sat there scratching my head for a little while before deciding to just keep shaving the 3/8″ plates down until they accounted for the discrepancy. They had to be symmetrical in the middle to fit the worm shaft between them, so I couldn’t just use one or the other, etc.

Each side plate ended up at about 0.330″

All said, this took about 0.08″ of width off the whole assembly. Fortunately, the design is flexible enough to accommodate that change.

Here’s the first bolt-through of the entire worm gearbox assembly. I couldn’t locate 2 inch long #8-32 screws, so some dinky philips head deals had to take their place for now.

While everything was bolted together, I decided to bore the worm shaft hole in situ. I just stuffed the whole gearcase into the vise using a step block between the “ears” to hold the pressure, then drove a 0.5″ cutter straight through in the appropriate location.

Observe, the worm gearbox. The way the worm shaft is installed is slightly nonconventional. Each center thick plate has half of the 1/2″ hole in it. These envelope the 1/2″ shaft bushing that is stock to the worm gearbox. The rectangular extension of the gear casing is exactly the width between the thrust bearings of the worm shaft, so it fits in snugly and is captured both axially and rotationally.

With the worm gearbox essentially done, I went back to work on the other subassemblies of the robot.

Here’s the actuator of yesterupdate, but linked with chain and filled with gears. It worked like a dream after some selective shaving of material from the C-shaped output casing.

Essentially, using an even (14) and odd(11) tooth chain in the same powertrain will result in circumferential distances that are roughly one pitch too long or short. While they sell “half link” chain connectors, they don’t do half pitch ones, because those are physically impossible.

So, I was left with a chain that was just loose enough to occasionally grind and lock against the side of the casing because the chain links exhibited a limited amount of cam action.

Well, that isn’t any good, so I stripped off some of the wall thickness where the chain tried to bunch up. Problem solved.

I knocked off more protoforms of the wheel hubs using Delrin. The front wheels won’t be taking direct torque from the motor, so I elected to make them from plastic to save weight.

Using the leftover 3/8″ precision-ground 12L14 rod from Überclocker (the one that machined like aluminum), I popped off most of the small standoffs and dead axles in the design. Most, because I haven’t designed the rest yet.

I see where this is going.

Here it is. The 100th build picture of Cold Arbor and…

… what IS that? That’s not even robot-shaped. That’s a Pretend-O-Pile. It doesn’t even qualify for Pretend-o-bot.

I need one more trip to the hardware store and some more machine love before the robot can “stand” on its own, so to speak. Then, I need to cut the cover plates and electronics mounting facilities.

No, I need to design them first.

This will end well….

Cold Arbor is reaching that point in a build where another day of work will suddenly make a pile of parts appear cohesive and robot-like. Most of the fine details of the two linear actuators have been addressed, and I’m almost ready to move onto making the saw assembly itself.

Additionally, I’ve had the chance to cut out more parts and put the entire frame together.

Pictured here are most of the parts for the swinging saw assembly and the two clamping fingers. These almost go together as-is.

Back frame rail temporarily slipped together. Drive motors mount to the two projections, and the clamp actuator is (mostly) integral to the frame itself.

Here’s the front frame assembly in the “glob on as much brazing alloy as you possibly can” stage of fabrication. I heat up all the requisite areas of the metal and liberally distribute and wet the surface of the aluminum at the tabs with the zinc-aluminum braze.

The interior fillets on this piece are some of the best joints I’ve done so far. I was able to properly fill these joints because of…

…my +1 Frayed-Aircraft-Cable-And-Chunk-Of-Copper-Tubing Brush of Oxide Breaking!

Instead of ordering $9 stainless steel pencil brushes from McMaster, I chopped one up out of some stainless steel aircraft cable and the nearest small tubing I could find. In retrospect, making the handle out of copper, a highly heat-conductive material, was probably a bad move.

The long and thin pseudobristles allowed me to get the brush into narrow corners where my stainless steel toothbrushes could never hope to go.

The stage after globbing is sanding the proverbial daylights out of the part. Using a large and wide belt sander helps establish a new flat surface that somewhat resembles the old. This actually makes the whole part look really nice, almost like it was meant to look like that!

Here’s the backside in the globbing stage.

… and everything put together. Well, kind of squished together for the shot, that is. It looks great, but how well will it perform…

T-nuts installed into the thicker frame components. The waterjet cuts accurate enough such that these things either press in with thumb pressure, or, in the worst case, require a tap from a rubber mallet.

Mocking up the rear actuator. The bottom plate hole pattern mounts a Banebots 20:1 28mm gearmotor.

I bored, drilled and tapped one of the Surplus Center sprockets to fit the 1/2″ ACME leadscrew. The screw itself has two flats to let the set screws grip properly, and an end-threaded hole to act as a physical stop for the clamp.

I ran into a foreseen-but-ignored problem when making the output shaft for the saw actuator. 11 tooth #25 sprockets have a hub that is barely over 0.5″ diameter. The output shaft of a drill gearbox is typically 12mm, or 0.472″. I already sized the output bearings for 12mm.

So that means there’s no way to actually attach the sprocket to the shaft. Too little thickness to set screw or cross-pin, at least that I was comfortable with. While I could have turned the drill shaft down, this required either changing bearings in the actuator body or making some kind of adapting sleeve. I thought the 11 tooth D-bore motor sprocket (from a scooter motor) that I found would save the day, but alas, it was 10mm.

Naturally, I take the solution that would allow me to abuse machine tools: Make the sprocket hub bigger.

Uh oh.

I turned a steel ring that was fitted over the existing sprocket hub. This increased its diameter to around 1″.

Then I welded the ring to the sprocket on the exposed end. While all this was fixtured on the lathe chuck, of course. The sensitive machine surfaces were covered with a welding blanket first.

I could have done this on a less expensive or important fixture, but the steel ring bore was a hair too big to align properly without wobble. I used the machine spindle to correct the wobble, and decided to just weld it right there while everything was still squeezed together.

Mmm… porosity. After depositing the weld, I turned the surfaces clean.

You can tell I didn’t focus very hard on cleaning the sprocket surfaces beforehand. Oh well – this isn’t going into space.

And all ends well.

Not really a pretend-o-bot, but more parts are assembled. I got some 1.5″ long cap screws to close up the gearboxes, so now they are actually complete.  I still need to find a replacement 400-size motor that can stand 18 to 20 volts, however – the stock BB motors are only safe to run up to 12 volts or so.

Things left to do:

• Finish the saw assembly, including all the random pins that attach things together.
• Give Deathrunner some windings!
• Front wheel hubs
• Waterjet the last of the components – electronics mounting provisions in particular, and the top & bottom plates.
  • Design this stuff first.
• Panic
• Panic
• Panic
• Panic
  • Panic
• Panic