LandBearShark 2 is finished, but it isn’t snowing anymore.

chorl3 Comments

Well, I can’t say that I didn’t already finish LBS2 like 2 weeks ago. I did, in fact, and it’s been around the block a few times already. But sadly, since then, there’s been no snow in the weather forecast. Meaningful amounts, anyway – a few flurries fell here and there last week, but what good is that? Do I really need to take this thing to New Hampshire!?

I was counting on some snowfall to let me combine a little testing video with the rest of the build, but given that seems unlikely to happen, here’s the build in its entirety.

The first step in rebuilding is to retire the old design. Here is LBS pulled out of under-table storage. A little dusty, but it still looks kind of glorious.

The big ATV style crashbar is not returning in the new design, because really, it was just too over the top. It was funny and contributed to the very unique look of the thing, but it also weighed about 10 pounds (made of surplus 1/8″ wall steel tubing…) and the mounting design was very much optimized for the old frame and I couldn’t quite make the design work with the new aluminum one.

Otherwise, electronics aside, many component parts of LBS are being reused. I don’t think I’m quite at the level of the Ship of Theseus yet…

LBS has been reduced to components. I took the opportunity to perform a mass cleaning of all the track parts because they had been subject to substantial dirt and grime buildup. The chains, especially, took a distressing amount of soaking in brake and carb cleaner and like an entire roll of shop towels. Chains are terrible things.

Its former frame was used as a collection bin for most of my other retired aluminum-bodied projects. I was intending on pitching it in a shop’s aluminum recycling dumpster, but ended up putting it up on Reuse (the free stuff mailing list) for people’s amusement. I figured that had I been a happy froshy bunny during this time, I would be so extremely excited, jumpy, and otherwise bunny-like if somebody posted a pile of waterjet-cut aluminum parts to Reuse that I might start building project of my own spurred by the simple fact of possessing these items. That was the plan, anyway. Paying it forward for the next happy bunny that bounces into MITERS.

(Some of it was used immediately in a productive fashion).

One of the first build exercises was spacer and standoff-making. Most of these short ones shown are parts that will go into the motor mount, suspension, and new bogies.

I’ve been gradually liberalizing from my usual hardline t-nutting policies in favor of using material more effectively when the design calls for it. In fact, LBS was one of my last major giant t-nutted plate assemblies (the other being Make-a-Bot). I actually can’t think of anything I built from scratch in 2011 and 2012 which made gratuitous use of plates where they weren’t effective.

I’m thinking it’s about time to update the How to Build your Robot Really Really Fast document I wrote some time in 2010 to be a more thorough treatment of various design for assembly manufacturing methods, rather than a cursory overview of standoffs and t-nuts.

With a few of the necessary spacers done, I turned my attention to assembling my Fake Andymark Gearboxen. I’m a fan of their very inexpensive (compared to industrial suppliers, anyway) hubless spur gears because for most robotics-related purposes, hubbed spur gears add unnecessary bulk and weight if you are just making cluster gears anyway, or using another method of power transmission like splines or direct coupling to the driven member. In fact I’m such a fan that I’ll plug them some more: SPUR GEARS!

(Some mixing and matching may be involved.)

The way my intermediate gear shaft goes together is simple. Take a hex shaft chunk, stuff a bushing into it (drilled and bored on tinylathe), then start piling hex-bore spacers and hex-bore gears onto it. A hex bore custom sprocket is in the middle somewhere. On each end is a bronze thrust washer to keep everything in check axially.

The whole assembly was purposefully made a few thousandths of an inch shorter than the length of the center standoff – involving shaving a bit of material from one of the hex bore spacers – that it could spin freely once lubed up a little, and kept itself in place. Bam, fast-build gearbox without machining complex shafts and retaining features.

I’ll also admit I am a slow convert to the hazardous, addictive, and self-destructive world of hexagonal shafts. They’re just so easy.

The two Fake Andymark Gearboxen completed. These have no mounting bolt pattern – they are hung from the center Big Shaft of the vehicle, and kept from moving by chain tension on one side and the chain tensioner on the other, upon which they brace against.

Onto frame assembly. Did I say I wasn’t going to make T-nutted things any more?

Nah, no way. I’ve not become that unprincipled. There are still a few holding the frame together, but they are no longer the majority contributor to frame rigidity. In this case, it’s pretty much just for making the right angle joint that will be backed up by long threaded rod-and-spacer preloaded columns (see Carly Rae Jepsen’s build style), which can add much more rigidity than an equivalent floppy plate span.

The bogie frames this time are much lighter in section, and maybe a little too flowy looking. The reason is that these never took subtantial structural loads anyway – recall that even in LBS version 1, they were hinged in the center at the Big Shaft. The load path goes from the rider weight into the Big Shaft, where it is met by reaction force coming from the ground, going through the track sprockets and into the track axles. But that load is expressed primarily in torsion (out of plane twisting) of the bogie frame sides, because the suspension is so damn stiff that it’s basically a truss member. LBS has always ‘sagged’ a little as a result of this torsion (think overloaded car), so there was no reason to keep using the huge heavy side plates.

The much lighter cross section of the new bogies saves about 3 pounds and cuts the design down to basically the bare amount needed to connect the dots in terms of mounting points.

Now it’s starting to look like some kind of cracked out Mars rover, or a kinetic sculpture. Everything gets slid onto the Big Shaft at once, secured by shaft collars. I switched to a 3/4″ diameter axle this time – LBS1 had a 20mm one, which is a dimension I have no clue why I picked initially, but 20mm shaft collars are espensive and I needed a few more of them anyway. 3/4″ was the closest size to 20mm that still allowed me some space to clear the CIM motors, and wouldn’t be bendy under rider weight.

After a brief game of Which One-off Spacer Is This?! I began to put the track wheels back on, and also slipped the motors onto the Big Shaft.

Without the outside bogie frames keeping the axles on center, slipping on the track itself was easy – the whole assembly just kind of bent in enough. I did have to spreader-clamp the axles to make all the screws go in, though. Overall, the track tension has increased over version 1, since it was known to be somewhat loose.

After the track pods were slipped back on, the project reached criticality. At this point, with the chains not hooked up yet, I was supermanning it and coasting down the hall – not very far, of course.

Moving onto electronics, I’ve punched together the electronics box and mounted the switch and cooling fan. Little rubber grommets have been installed in the bottom where the motor wires will enter. I also drilled, tapped the eventual RageBridge mounting holes and installed standoffs.

The box mounts to the frame using these little hanger hooks which interface with bolt heads sticking out of the electronics box. The arrangement is self-securing (the force vector of the box being loaded downwards tends to pull the hooks tighter together), or at least theoretically so – if not, zip ties will rescue it. The rear hooks were swung out, the entire box slid into position and rested on the front hooks, then the rear ones tightened down again.

Much of the wiring was recovered from the old LBS, and I made the RageBridge wiring harness to match it. Here’s one of them mounted to test for fit and wire clearance.

I took some time to finally repair LBS’s somewhat decrepit batteries. These were made 2 years ago and suffered a balancer cable short & fire some time afterwards. Since then, they have been wrapped in bubble wrap and duct tape, charged and discharged without regard to inter-cell balance.

To my surprise the cell banks were at most 50 or so millivolts out from eachother. While still alot, it’s quite a testment to the durability of A123 cells. Too bad the company itself… isn’t very.

I remade the balance harness, this time carefully routing the cables out the side of the pack, then wrapped the entire thing in some foam rubber with Kapton and fiber-reinforced strapping tape. After a night of balance charging, they were all leveled out and ready again.

The two RageBridges were stacked together with some 3/4″ tall standoffs, and linked via a small custom Y-cable going to the receiver. The bottom one powers the cooling fan through the 15V rail (which, incidentally, is going to be missing from RB version 3 to be replaced by a 5v fan output).

Each RB controls only 1 motor per side – the system is set up in mechanical parallel. Hypothetically, if one controller fails, the other can still move the vehicle at 50% power, but it of course depends on the mechanism of failure. If the failed controller becomes a short, then it would be very hard to power against the shorted motor, for instance. There was no intent of providing redundancy, just a convenient means of controlling more current than one Ragebridge can effectively put out.

With no custom software needed, it was drop the batteries in and go. LBS is basically a dumb ROV at this point, no different from one of the battlebots. The RageBridges were put into “mix” mode because the simple 2 channel Hobbyking radio does’t handle any of that fancy stuff.

After putting the other battery in, things started getting…. crowded. Batteries are retained on the bottom with a healthy dose of Velcro, and the virtue of being confined keeps them from jiggling around otherwise.

The wiring harness is admittedly a rat’s nest, but hey, salvaged wiring. It’s also be a good chance to test RB’s robustness under non-ideal wiring conditions.

Closing the top up… I designed this version to be way easier to service in case something goes wrong inside because the electrical box lid is removable through the top, after the board itself is removed.

And here’s the 98% complete shot. At this point, I didn’t yet receive my threaded rod to finish the two standoffs in the front and rear. Without those, the frame bowed a little when I stood on it. But it was functional enough for some superman-style hallway blasting.

There was one problem I discovered during this testing. The motors could exert so much tension on the chain that they were physically bending the rear bogie frame inwards, collapsing the hollow cutout and making the chain jump off the sprocket.

Well, that was dumb. The placement of the chain tensioner was pretty much in the middle of a totally unsupported span. I could, in fact, unbend it with some big channel-lock pliers. I definitely hit copy and paste a few too many times…

To address this issue, the rear bogie frame side was recut to be solid and the tensioner mount itself was made a little fatter and angled.

Here’s a picture of the bottom of the beast, showing the drive chain setup and the batteries. Still, missing standoffs (which have since been added).

I don’t have any testing video yet, since there hasn’t been exciting enough weather to do so. The new arrangement, however, has demonstrably increased torque and better steering response too. The Ragebridges are synchronous rectification drivers, meaning the motors exert a torque against any external changes in speed unless commanded to that speed, so the tracks have increased dragging ability on either side. It can alsofinally turn in place, even with rider weight on it. The top speed is right around 8 or 9 miles per hour.

Once the weather gets more interesting, expect some updates with videos!

The DeWut Motor: Next Steps, and Design Files for Making Your Own DeWalt 3-Speed Motor Mount

chorl4 Comments

Alright, with the Long Weekend of Nobody Being Open coming to a close, I’ve pretty much finished the ‘version 1’ of this project and am moving onto another fork of it. As discussed previously, the DeWut is an effort to make the new generation DeWalt 3-speed hammerdrill gearbox actually useful. The ‘old generation’, according to my sources, has totally stopped production, and are getting harder to purchase spares for. DeWalt has been the foundation of so many robots I’m surprised they don’t straight up sponsor events.

The short story is that I am skipping over a waterjet-plate stacking version (… what? Are you out of your mind?!) directly to investigating an all-billet, CNC machined version. That seems a very not-me thing to do, and actually I’m going to build a few of the aforementioned waterjet-compatible mounts for my own amusement, but as a potential product, I am in the process of gathering quotes and potential suppliers.

The long story…

After completing the body of the first version gearbox, I went ahead and carved the future shaft out of some 1566 precision-ground steel stock. Luckily for me, the new generation DeWalt gearbox has a rather normal double-D shaped output adapter, so it was a short mill pass to turn the round shaft into the proper mating shape. Unlike the old style gearbox, which had a…. what the hell is that? Apparently a massive stress riser. This would be only a manufacturability test, because untreated steel is going to be far too weak to handle the torque output of the motor. Based on a few FEA torque simulations, to hold the torque of the motor at stall in low gear at 100 amps, the double-D area is going to see about 180ksi of shear stress.

Ouch. That’s some serious steel – comparatively few alloys can heat treat to the 200ksi+ range and not be brittle as glass, and we all know how my last adventure with “heat treating” went. This is a subject I’m not so familiar with and so will probably have to leave to the professionals when I make these into production parts. For my own amusement, though, I’m probably going to buy a sampler plate of steel rods, make the axles, and try heat treating the ends to various degrees.

I also came up with a new arrangement for the motor’s rear retaining plate. Previously, it only held onto that little nub that the motor’s tailshaft sticks into, and was fairly unstable. A more stable method is enveloping the ‘nub’ and pressing entirely against the motor’s endcap. Unfortunately, the brush holders have conductive crimps that end flush with the endcap surface, but can still short on metal. Hence, a ‘gasket’ of sorts is necessary, shaped to the endcap’s outline, and I’ve modeled that as the little yellow-beige-brown thing to be made out of a thin insulating material.

Additionally, the structure of the Nifty Barrel Shifter retainer has changed. It’s now removable from the side and the idea is to hold onto it with a nut from either side. The bolt slotting narrow a little before opening up to the proper center, so it ‘snaps’ into place and is less likely to fall off. This arrangement makes it easier to change gearing (if needed) but also allows the whole thing to be assembled before picking the gear ratio, since it does not have to be stuffed onto the gearbox while it is being lowered into position.

But the most important addition conducive towards kitting this assembly up is the alignment marks on the side. Starting with 1 little indent slot on the side, just line the plates up in incrementing slot count order. I figure the X shaped motor back end is obvious enough on its own. There’s 9 in the structure, then the gasket, then the motor retainer, for 11 total.

This is what the retaining plate looks like now. Of course, one option to bypass the need of a sketchy nonconductive shim which COULD conceivably become conductive by accident, is to make the thing out of a nonconductive material. Garolite (G10/FR-4 grade) was my prime candidate, but electrical grade fiberglass ought to work too. Anything in 1/4″ short of nonfiberous squishy plastics ought to be stiff enough to hold the motor in place.

Moving the retainer plate onto the motor endcap means the whole assembly becomes basically 6″ long minus the shaft length. A nice, even number to work with!

For the purposes of easy cutting, I arranged these plates into separate 1/4″ and 1/8″ assembly files with little “sprues” in between the plates. This ensures that they stay together and cut as one part, and can be extracted whole.  It’s a little more dangerous than individual parts, but machine time is billable by the second and precious seconds are spent cycling the nozzle, so continuous cutting is marginally cheaper too. For small parts, the sprues prevent them from falling into the tank.

However, I think that’s as far as I’m going to go with this design. Here’s why:

Huh. Well then.

That’s just for the 1/4″ parts! Note that I tiled 4 of the assemblies together, so the real cost is about $38 per set. Add to that the 1/8″ plates (another $20 per set) and the heat treated, custom machined steel shaft (estimating $30+ each), plus all the hardware and the laser-cut gasket. I’m basically looking at a hair under $100 just in cost. And only in quantities 10 and up, assuming I have like $8000 to drop on this right now.

As funny as modern digital manufacturing is, and as great as it is if I really just needed 1 of something, I think I can get a much better price by falling back to good ol’ subtractive machining. Hey, as it turns out, waterjetting in real life is expensive.

So, as previously mentioned, even though I could pop out a few for my own use, I won’t be able to introduce these as a viable product (in my opinion – feel free to differ). So what’s next?

DeWut? Design Files

I’m going to make everyone else do it. Contained in the above ZIP file is the two tiled parts to be cut (0.25″ and .125″ aluminum) and a 2D drawing of the gasket to be made from something nonconductive, in DXF version 2000. Also included are the original Autodesk Inventor 2012 3d models of the assembly and an exported version in X_T (Parasolid) which can be imported back into individual solids.

The assembly should only fit together one way, basically described in the first post.

There are a few #4-40 holes to tap in order to make blocks from the plates at first.

The mounting holes up front are properly sized to be threaded 1/4″-20, no more than 3/8″ deep.

The shaft solution, however, is left as an exercise to the reader (the shaft model is included).

The model as-cut should fit the new-style DeWalt transmissions (397892 series), possibly with a little stuffing.

There’s no warranty expressed or implied.

Next Stage

Based on my experiences sourcing the hub motor parts for Chibikart, I believe I can get billet mounts for the gearboxes contracted out through a Sketchy Chinese Dude With a CNC Machine for substantially less price. Leaving the thickness quantum domain also means I can make the gearbox mounts better fitting and have multiple threaded mounting holes, etc.

I basically began translating features found in the stacked plates into a single, solid model. There were optimizations for weight reduction made, as well as thicknesses and details changed to fit the manufacturing technology.

This is the end result. Now, it’s not totally free of potential laserjetted parts, because I’m keeping the Nifty Barrel Shifter holder. It will still rest on the tie rods that hold the thing together.

Check out that new motor mount. It’s a 2-piece clamp affair out of necessity because the DeWalt motors don’t face mount to the transmissions, they stick into them a good 1/4″ or so. Hence, the idea is to tighten together the gearbox and shaft portion, then stuff the motor in, then tighten the mounting collar. With a 1/2″ of support on the collar and additionally 3/8″ of support in the aluminum block, I think the motor’s not going to go anywhere short of having the output shaft shoved through it (by which time, terrible things have happened). This obviates the need for a rear retainer.

Overall, I’m looking at 3 aluminum machined parts, 1 heat treated steel part, and a derpy laser-cut thing. Because the NBS holder is not structural, I’m content with making it from Delrin or ABS plastic.

Incidentally, the mounting holes match a Banebots P80, at least on the front. This whole thing is basically a lighter P80 type motor with 3 speeds!

Of course, I’m not leaving the world of 3DRP forever. I wasn’t going to send anything out without a sanity check for the critical mounting dimensions. I turned, appropriately, to a Dimension 1200ES to produce these mostly hollow plastic shell representations of the gearbox. The Dimension printer is much better at making parts ….. on dimension…. than the Makerbot Flock.

Since it cost 3 times more than the aforementioned Makerbot Flock combined, I would expect nothing less…

And hey, everything fits! I verified that the little slots and cutouts to cater to the DeWalt housing were within reason – if they fit in slightly shrinky plastic, then they will be a little looser in aluminum unless Sketchy Chinese Dude with a CNC Machine is really that sketchy. The overall length has remained essentially the same – this was basically a direct plate translation, after all. The Nifty Barrel Shifter holder is not seen in this picture, nor are the #10 cap screws that will keep the two halves firmly locked together.

The next step from here is to pitch all of these parts to my favorite SCDwCNCM as well as mfg.com (my new favorite snack) to compare prices vs. features. At this point in the year, I’m going to consider getting prototypes in before the holiday breaks a total not-happening, so expect some more news on these DeWuts in January.

By the way, this is what is inside an allegedly “empty” Stratasys Dimension ABS build cartridge.

…seriously? That’s like 50 feet. I only feel slightly ripped off.

Luckily, this filament is 1.75mm diameter just like the latest generation Makerbot Flock feed, so I’ve been making heart gears using the Replicator. Productivity!