Archive for December, 2012

 

Now Presenting: How to Build Your EVERYTHING Really Really Fast!

Dec 30, 2012 in MIT, Bostoncaster, Cambridgeshire, Stuff

It’s done!

I’ve been practically a recluse for the past week and a half while I turned over every stone in my brain and mined the depths of my archived data drives for build report pictures. Compiling all the information that I and my peer cloud have thrown into our collective cauldron of engineering-under-duress tactics and weapons of engineering design warfare, I hereby present

How to Build Your Everything Really Really Fast

HTBYERRF will officially supercede my old How to Build Your Robot Really Really Fast. I hope to keep adding to it and also take user contributions for example images. Unfortunately, I have some gripes with the way Instructables now lays out images – there’s no clear order for them any more, and they’ve also gotten rid of the ‘bottom row’ of images, which means some of my old references to “above” and “below” in Chibikart and Scooter Power Systems now make no sense. So perhaps more example images will not really help the situation…

Either way, merry new year. Go build some robots!

Additionally, this is the first thing I’ve …built? manifested? which I actually would like to see sprayed all over the Internets. Ultimately, I see more discussion and knowledge-sharing as beneficial, and I think this new document can help spur it. Feel free to sprinkle the link or page where you feel appropriate.

oneTesla on Kickstarter!

Dec 28, 2012 in Stuff

I have a reputation for trashing on Kickstarter products, but this one is near and dear to my EMI-induced heart palpitations.

The backstory: A group of students at MITERS have been working on a singing Tesla Coil kit. Having witnessed and supported small aspects of its development through this whole year, I have to sing (via plasma) the praises of the level of design and engineering that’s gone into it.

It’s now live on Kickstarter and has already made its way onto Mashable.

oneTesla also has an official website. Everybody go coil!

Introducing Überclocker Advance!

Dec 21, 2012 in Bots, In Progress, Project Build Reports, Überclocker ADVANCE

I’ve been in hiding for the past few days out of fear for my life EVERYBODY IS IN FINAL PROJECT DOOMSDAY MODE!!!! PANIC!!! since I’m a TA/lab instructor for the Media Lab’s famed MAS.863 How to Make (a Huge Mess) out of (Almost) Anything class as well as 16.842, a systems design class based on the DARPA Model Based Amphibious Racing competition (MBARC). I have rants prepared summarizing my experiences in both of those, but that’s for another day.

What’s on deck right now is a brand new version of Überclocker, my ‘flagship’ combat robot (flagbot?), which has been out for retirement since at least 2010 or something. It’s what I have been slowly hammering away at for said past few days, stretching sporadically into the last two weeks or so. I’ve realized that it’s rare to see a explain-the-CAD-image on this site any more, which makes me a little disappointed since so much of the idea synthesis that ultimately determines the fate of a build is contained in the design period, and perhaps my thought processes can help in the design work of others. So in an attempt to go Back to My Roots, I’m going to have an explain-the-CAD picture post summarizing the status of this Überclocker build, now entitled Überclocker ADVANCE! for reasons that are difficult to explain using words.

Let’s face it – Clocker has been a little slow-moving compared to most of my past robots and current vehicle-like things. In 2008, it was completely terrible because I basically square-wave stepped into infinite machining and fabrication resources from almost none, without the attendant knowledge and theoretical foundations to use them. In 2009, I rebuilt it using “every trick in the engineering textbook”, and t-nuts, to wholly unsurprising consequences. 2010 and 2011 were marked by dismal failures brought on mostly from a lack of true concern, since at the time I was well-distracted by the aforementioned vehicle projects, and didn’t really take upgrading and repairing the bot seriously. Finally, in 2012, I stepped back and really upgraded the robot, and it did well consider how terribly I ended up operating the damn thing.

The design of this bot actually has a year and a half of history, at least. My first inklings of a desire to totally start from scratch basically started after Dragon*Con 2011 with this concept solid model, made in a few hours.

It got the basic point across – I was tired of having low ground clearance and tiny wheels with their traction limitations (hence the big wheels). Clocker’s broadside attack weakness (since it’s so damned long, at 27″) was fresh in my mind, so I wanted to make the sides rounded in order to give me some complementary leverage in a pushing match. The legs, while they worked reasonably well, were bulky and some times caught on the stage edges and elements, so I wanted to replace the rollers with a ball shape. All of these things were on track for  addressing in version three. But, that model lasted about as long as I spent designing it.

The next burst of inspiration built off this sketch-model in later 2011. It was on the flight back from my most recent Singapore trip that I went through like all 3 of my spare laptop batteries on the plane and followed up on the sketch-model’s selling points. The end result was this.

My god, it’s round. It’s so round. And it was even mostly circular! Did I mention round? Curvilinear?

This was a very strong candidate for the design at the time. It would still involve a fair bit of waterjetting magic, but the frame was out to become much simpler than Überclocker Remix’s original 2009 frame. I actually spent some serious time making the circular edges as … well, circular as I could.

Sadly, I just straight up forgot about this for a long time after landing. Some part of me still said that the shape was impractical, that I wouldn’t have far to tilt before I lost traction, or that the waterjet puzzle was too complicated to be robust. But damn does it look good.

Fast forward another year, and I’ve basically had enough of this thing. Yeah, it was round. But my reservations were correct in every way I cared about. Überclocker original (2008) and Überclocker Remix (2009) both had very distinct and unique shapes – very flat and sleek, with angular elements everywhere that were neither positively contributory nor very easy to make. My recent Great Awakening with Null Hypothesis, a desire to return to the ugly square drivetrain-dominant bot that just worked more than anything else, pushed me strongly towards ditching the desire to be circular. In the modern Battlebot match, the giant brushless spinning weapon staying alive and driving is, in my opinion, something like 75% of the game. So long as you keep rushing at the other guy, strategically or haphazardly, you’re more likely to curry favor from the judges if you last. In the recent past, Clocker has had… well known issues… with that.

The new idea floating in my head was to make the thing bone simple, at least a simple as a 4-actuator 4-wheel drive bot can get. The frame needed to be just big straight rails. The dual-motor setup for the lifter had to go (because ultimately, one motor jamming or stopping will cause the whole thing to quit functioning), the whole thing needed to get smaller or at least get no bigger, and I really needed more ground clearance.

Images in my mind began forming of basically shoving the fork and clamp assembly onto Null Hypothesis. That bot was fast, had infinite traction from it’s 40A durometer 2″ wide “McMasterBots” wheels, and almost unlimited traction-positive angles because of the big overhung wheels. What it translated into in real life was a bunch of bad whiteboard sketches, including…

Yeah, what?

That looks like a combination of Omegaforce, the Wubba-wubba-Bot that never was, and some agricultural implement. It lasted as long as it took for me to sketch it out.

With the final project blitz happening to every lab class on campus including the two I was involved with, I hunkered down in my stuffnest and began creating geometry in Autodesk Inventor, for realsies (but interrupted roughly every 5 minutes with a different variant of “Do you think using a ____ on my _____ is a good idea? Is there one in the shop?”).

Yup, that it’s. That’s the whole robot.

I’ll admit that my reasons for creating DeWut!? were mostly self-serving. I needed to get away from two things:

  1. Using sketchy-ass 18 volt Chinese cordless drills in anything. Null Hypothesis basically ditches a motor every match. While these drills may have been better in the past, modern product design committees have cut so many corners out of them that they’re pretty well rounded off … literally. The cases have gotten curvier and prettier, but the material quality has really been shat out and redigested. I think they’re still fine for 12lbers, and 30s if they are not overdriven at all, but NH clearly pushed them too far.
  2. Repacking the guts of non-sketchy 18 volt American cordless drills (the DeWalts, which are made in China anyway) into my very sketchily-made aluminum gearcases. You can buy this for $200 already, it’s better than my version, but they’re never in stock.

Hence, if I could get the DeWuts made as a stock solution, then I can design everything around them. And soon, everybody can!

With that model in mind, I quickly started jotting down the geometric outline:

It’s still round.

I just can’t let that go. Cold Arbor was kind of round, and it looked great (but that saw is something I will be ashamed of forever), so I threw it in. The base form is that of Clocker as it exists now – two “pods” on the sides and a simple box frame in the middle, and the fork out front. In this picture, I was trying out Null Hypothesis’ 2″ wide giant caster wheels for looks. While I liked it, it would have caused the bot to be almost 2 feet wide, so they were not the final choice.

The hardest part about starting a new build is usually anchoring the design. Where the hell do you start, and what part do you start with? I found that making the outline made the choice much easier since it was then easy to see right in front of you what is holding the bot together.

After Clocker’s 2012 adventures, I tacked on some more items onto my list of grand design intents. The full rundown was now:

  1. More practical and easy to build, as previously described
  2. Higher ground clearance and increased maneuverability, as previously described
  3. The ability to resist or defend against broadside attacks, because round
  4. Making the legs actually useful.
  5. Making the clamping action much quicker

Here’s what I mean by all that.

Making the legs actually useful

Clocker’s “reactive outriggers’ are probably its best feature. The idea is when a 30lb opponent is lifted, the weight shifts forward and the robot gets up on two wheels and the rollers at the end of the outriggers. Hence, the robot still maintains traction and can move while carrying an opponent without hoisting it up all the way. Many historical clamp type weapons like Darkangel and Complete Control (both Clocker inspirations!) have static outriggers that function only to prevent tipping.

Here’s a picture all the way back from clocker 1 in 2008 that shows the principle.

That’s where the fun begins. The idea is basically to spin the opponent in a circle and then let go – not causing damage per se, but it looks intense. Some times it backfires. Other times it’s a big hit at the event.

The problem is that it really only works for a limited range of opponents because of how short the legs on the original Clocker and Remix (2009-current version) were. A little too chunky and Clocker would just faceplant. If they were too small or compact, then I don’t really get enough displacement to break rear traction. If the ratio of leg length to wheelbase were higher, then the range of spinnable opponents would also increase because it would both let me tilt forward with less weight and be more stable in that configuration.

Next, the “doubly supported” legs of the original bot, and Remix as a consequence, were a severe pain to remove if the drivetrain needed servicing (and my goodness did it need servicing…). It used a different hex wrench diameter than the rest of the stuff on the side, and there were 2 screws and 2 washers to line up correctly to remount them.

Some times, a single big chunk of metal is warranted over a creatively sculpted series of smaller chunks. I wanted this build to use a single, thicker ‘leg’ per side that could easily be removed and swapped if needed, from one side of the bot.

Making the clamping action quicker

Unlike many clampbots of smaller weight classes that use R/C servos or larger weight classes that use pneumatics, Clocker has an electric linear actuator to reduce complexity while still offering good grip strength – an R/C servo of this size class exists, but all I know about it is that it’sreally expensive. So, the clamping action is admittedly a little slow. This has resulted in a quite a few missed grip chances in the past.

This particular grievance isn’t a major design element, since Clocker’s clamp actuator was pulled from Cold Arbor and can be customized in several ways. I’m thinking of either switching up the leadscrew from a 10 TPI to an 8tpi fast-travel screw (with 2 starts, so effectively 4 TPI), which would make for a 2.5x increase in tip speed. Else, I can remove a whole stage of gearing from the chopped 36:1 drill gearbox that runs the actuator in order to effect a 6x increase in speed.

It’s a little hard to decide, since I’d have to weight the costs and benefits – namely, how much clamping pressure do I really need? If I could get 6x more speed and not really sacrifice how hard I can hang onto the opponent, then it’s worthwhile. Alternatively, I’ve used the clamp as an emergency lifting arm in the past, so maybe I don’t want to sacrifice so much torque.

Oh, wait, I forgot one thing…

Not needing 3 different sizes of hex wrench, 2 of which must be ball ended, and 20 minutes in order to fix anything.

Probably the worst thing about Clocker is how hard it was to pull anything. At D*C2012, I had to take out the lifter gearbox in order to remove one gear stage from it that had stripped out and binded up the whole thing. It took pretty much exactly 20 minutes to take the robot apart and put it back together, just as I had experienced in the shop. This design goal kind of goes with the simpler and more practical frame design.

Now, where was I? Onto the actual evolution of the design.

I very rarely use an outline of the bot as a design guide, but this time I founded it immensely helpful to visualize how all the parts will interact, roughly, before committing a part file to it. Above is shown an arrangement of the parts as conceived fairly early on, including the Third DeWut that will run the big fork. That’s right – no more weird dual-motor gearbox.

I was fighting back and forth about whether to do direct-drive to one wheel and chain/belt to the other (per side) or an indirect drive to both wheels with the motor in the middle somewhere. It was primarily finding a balance between 3 variables – whether or not the bot needed more ground clearance, the kind of speeds I could get with either method, and where I had to stuff everything else.

If the rear drive wheel were mounted in-line with the motors, then I would be limited to a maximum theoretical ground clearance (i.e. without any type of bottom armor) of 0.75″ with 4″ wheels. If I designed Clocker solely for smooth-arena combat, the ground clearance would be only 0.25″ at most using 3″ wheels, but this is not the case, so 4″ is pretty much required. 0.75″ clearance is what Null Hypothesis and the latest Überclocker all run with, and it seems to be fine for the stage combat scenario of Robot Battles. It would limit me to three speed ranges dictated by which gear I put the DeWalt geaboxes in – they have a 450, 1450, and 2000 RPM ranges (at stock voltage, rated by the company).

Now, I’m dead set on overvolting 18v motors to 24v at least (or rather, 25.6v for 8S A123 cells), because it’s a glaring sign of n00b to run motors at their rated voltage. At the very least, 7S must be used to be comparable in drive power to the current version of Clocker. This depended on how creative I could get with placing the battery itself. At 8S, I would see a (theoretical) top speed of 24mph in the middle gear. Yikes… that’s pretty high. But the alternative, 8mph, in low gear, is really really slow. At 18 volts or 6S A123 cells, the top speed would be a more tame 17mph, more to my liking but a little on the high side. So, wheel-on-motor would be a good choice if I was satisfied with 18 volt electrical systems and 0.75″ ground clearance.

However, if the wheels were not directly in line, I have more options. I could run 3″ wheels at below motor center line to retain the same level of ground clearance , but more manageable speeds. The motor location would be significantly more flexible. It’s wholly possible to run a 1:1 using chain or belts and with the motor not directly connected to either wheel – in this arrangement, my speed at 24v would be still 18mph, which is excellent.

The next variable to consider is how much tractive authority I wanted. By this, I mean how far can the robot be tilted or rolled without losing tractive authority? This would dictate my ability to escape from bad situations – the speed might be enough to avoid them, but if I ever got in one with a low clearance bot with little stubby wheels, it could be worse. Bigger wheels will always help with this problem.

I decided it was worth trying a 1″ ground clearance experiment using 4″ wheels. It would be a new design direction for me, since I have classically favored flat robots. Ideally this would make Clocker virtually impossible to wedge under because it would take incredible effort to break its traction fully. I wanted to leave space for the option of 8S packs, even though it meant a mid-20s top speed, because I could always back down from there and save some weight if that was warranted. A greater tractive authority combined with high speeds makes a bot much harder to catch.

The culmination of all this reasoning and pulling tradeoffs back and forth is many hours of positioning components and thinking of what parts go with the configuration, and roughly how fast it would go. Some times, a good arrangement existed for battery and motor placement, but there was not really space left for the Ragebridges. I made configurations with one Ragebridge per side (instead of 2 stacked on top), the battery in the front (not optimal for center of gravity), and even offset motors.

Ultimately, here’s what it came down to:

I had to release one constraint to settle upon this, and that’s the bot’s width. Clocker is already huge for a 30lber, covering a 18 x 27″ footprint. That’s bigger than some former 60lb Battlebots lightweights. Part of it’s unavoidable with this kind of design, where I have to contain a majority of another opponent.

Only by letting myself build a 19″ wide bot could I fit an up-to-8S pack in the rear along with the ragebridges. The motors and battery pack were now all rear-biased, which was favorable for CG reasons.

The observant would notice that I went back to the 1″ wide wheels after the previous shot. There were 2 primary reasons for that move. First, I really wanted doubly-supported wheels with static (standoff-like) axles. This increases the rigidity of the frame over a single supported wheel, and also lets the outer frame rails act as wheel armor. And second, those 2″ wide wheels would have pushed the bot width dangerously close to 2 feet.

After this part of the design was roughed out, everything else began falling in place. There’s really only one place to put the clamp and fork, really.

The next big challenge was how to mount the fork assembly. Clocker’s current configuration is a little “torsionally unsound” in that the force of a 30lb opponent capture in the fork is reacted entirely by the front frame cross-members twisting. Said front cross-members are also just flat plates, which are known to be very poor in torsional loads. Without the top and bottom plating to support them, the whole thing just lurches back and forth if any load is applied to the fork. While the latter configuration is acceptable (loaded top and bottom armor), I don’t like it as much because it depends on a material much less stiff than the aluminum (i.e. sketchy McMaster FR4 garolite plates) to handle the loads – I’d rather have a more “atomic” structure.

In the above image I’ve whipped up a pretty simple first-pass attempt at the clamp motor and pivot mounting structure. At this point, I was still relatively unsure about how to attach the whole thing to the frame.  The two big top-level choices were a CRJW style standoff tower (preloaded like mad) or just two crossing trussed-out 2.5″ tall aluminum flat plate members, separated a few inches. CRJW’s build style worked out very well with respect to overall stiffness, so I initially favored it.

I was also split between chains or gears for the main lifting drive. The first Clocker used #25 chain, the second used giant custom spur gears. At first, I figured chain would be easier to make an assembly with  because it was narrower yet more flexible (in terms of positioning the components). Hence, at this point, I still had a narrow assembly set up for a #35 chain (for more durability over #25) assuming I’d drop a sprocket in there.

However, what I eventually realized is that chains need space to exist, and I’d need to cut huge gaps out of the frame to pass the chain through. A sprocket combination that got me the needed external reduction in other to not make the fork a fucking hammer meant the large sprocket was almost approaching 6″ across!

I could more easily get 5 or 6:1 in a set of spur gears, whereas the same ratio in a chain necessitated a 9 or 10 tooth sprocket, known to be extremely weak and highly stressing on the chain. So with my brief excursion into the dreamland of chain drive complete, I returned to modeling the assembly to favor a set of big custom 12 pitch spur gears. The assembly would have to get much wider, of course, but this was a minor adjustment.

Here’s a random picture of a gear.

Needing a bit of mental break, I decided to get really creative with a spur gear and embedded the “overclocked” Doomsday Clock motif that appears in every Überclocker. About 3 people will ever get it, and it doesn’t actually make sense to put on the bot. But hey, it’ll look pretty in the model!

I went through an entire round of parts arrangement with the standoffs-style structure that involved lots of shifting the motor and gear around. I wanted the ability to use all 4 corner holes for fastening, else the continuous structural loop would be sacrificed, reducing stiffness. But this generally involved crossing a spur gear (or a chain sprocket), so the gear had to be moved or the fastening hole had to be moved. Keeping the standoffs spaced as far apart as possible maximizes the stiffness of the assembly, but that was of course in direct conflict with whether or not I could stuff a motor and gearing into the same projected space.

After a while, I began realizing that the conflicting goals were pretty much irreconcilable given my choice of constraints and the desired size and aesthetics of the bot. Taking apart a bunch of standoffs would also be a serious maintenance problem (I’d need a clear, straight-shot space across the bot to pull a threaded rod out of). Maybe some crossing spans weren’t so bad after all?

They weren’t. Making the pivot axis of the fork directly over the motor (as opposed to offset in front of it) meant that I didn’t have to make as large of a cutout in the frame rails as I had expected. This allowed the condensation of the assembly to only 3.5″ wide – basically, just enough to contain the motor itself.

In this arrangement if I torqued the pivot axis hard (like hanging a 30lb opponent a foot away) the twisting load is taken up by shearing 4 2.5″ wide bars across their width, basically. Much better than twisting the same bars about their own center axes. This is incredibly difficult to explain in more detail without a thousand more words dedicated to it, or a cute drawing/diagram.

I’ve closed off the structural loop around the fork motor now, and am pretty satisfied with how this turned out.

I try to not optimize anything too hard until the whole system has materialized to some degree, so I moved on immediately towards filling out the less critical parts of the bot, like the fork tines. These were laid out using the outline as a guide, but not for dimensional accuracy. Check out the fish hooks on the end – I’m gonna keep them in “production” just because they look pretty cool (uh oh…), and I also foresee them aiding in sliding under someone’s side armor and catching them. Worst case, I’ll sand them off, so who cares?!

I added the top clamp from Clocker’s 2012 incarnation, which was a newly built assembly, to see how it looks. I’m going to keep this clamp because it’s already built to work with the geometry of taller robots. The pretend-o-bot is starting to form. At this point, I’ve gone back and diddled with the geometry of the motor mount some more in order to get a more favorable “angle of decent” of the fork. As it turned out, a totally centerline pivot point forced the descending part of the fork to be very shallow, which made the ‘active’ part shorter but also let the fork swing down lower (before it hit the motor mount). Here, there was a tradeoff of “do I really want Clocker to lift it self off the ground?” – while it seemed advantageous for making sure I win the wedge war, it would be a disaster for expedient driving and maneuvering since the bot would be effectively high centering itself.

So, in the end, a steeper angle won out since I could hard-stop the fork just barely above the ground. I’ll deal with wedges as they come.

I’ll be reusing the clamp actuator too. Some time was spent playing the Geometry Game (midway down in this post) trying to maximize the range of travel without having the motor impact anything. This time, the components played out in my favor and made a little corner that the motor could stick into without running into the pivot shaft, as well as being protected on most sides by the bot structure!

A geometric example of a new leg has been added, too. The new design calls for this to be machined from solid 3/4″ aluminum. Chunky? Yeah, definitely. But, I need the stiffness if it’s going to be single-supported and stick out that far.

Here’s a bit of SCIENCE!! which I used to sanity check myself when designing the leg. I basically took a reasonable guess at how much instantaneous force the leg will see if Clocker just ran into a wall for no reason – say 1500 lb-force, applied directly to the big roller screw. Then I assumed the bot was infinitely stiff and could hold the leg still at the rear where it is attached. Then I told Inventor to pull some magic and tell me how much it deforms. Result: Probably about .25″ in compression and bending, and I’m more likely than not going to bend the roller screw.

That’s okay, I’ll make spares. A 0.5″ wide leg made Inventor yell at me for large displacements – that indicated some degree of hopelessness.

Realistically, a static FEA calculation isn’t going to capture the whole picture. The bot is not infinitely stiff – if it dives into a wall, the frame will most likely bend significantly at the mounting point, too, absorbing some of the hit energy. The suspension spring could also take up some of that force. The only question, really, is if the whole thing will just stay bent after it, which could be found out with More Analysis I’m not currently in the mood for. Just ship it.

At this point, I was starting to look at just making small refinements. I’ve taken the liberty of shortening the bot a little closer to original dimensions. This was accomplished by swapping spaces with one of the chain tensioners – before, I was limited in how far back I could move the fork pivot axis by how close I could move the tensioner to the main drive sprocket.

Well why not just swap them then?

I threw the current version of clocker in just for a size comparison. As can be observed, the ratio of robot to fork has decreased somewhat, and the ratio of leg length to wheelbase has increased. The frame itself is a tad shorter, but wider. And much taller. Overall,  Clocker ADVANCE occupies a bigger bounding box, but most of it is pretty spindly and empty.

When I was happy with the placement of parts, I began the t-nutting.

Now, I promised to not t-nut so prolifically any more, but this situation warrants it, I swear! The little gussets and brackets will double both as frame binding elements as well as top and bottom plate mounting points. The difference in this case being the top and bottom plates are made no longer structural – just to hold the guts in, not to take loading (short of direct impacts, which will be guarded from with piles of ablative material). These are far less egregious than Clocker Remix’s frame.

Additionally, the presence of the U-shaped gussets in the motor mount strengths that region from twisting even more.

The best part? The lifter motor pops out after undoing 4 screws accessible from the front. It drops out the bottom and can be immediately replaced. The drive motors will take a little more thought.

One issue I ran into was how to retain the gussets from moving in the Z-axis. The last picture showed pretty well an underconstrained joint – i.e. in the absence of friction, it could still slide out the top or bottom. Only friction retains it in real life.

By insetting the fingers fully into slots, I capture them in the Z direction, too. The downside is making the left and right chassis rails 0.125″ taller per side. I found this inconsequential because INFINITE GROUND CLEARANCE. Now, with these captured slots, there is also a clear assembly order for the bot – everything in the middle first, side plates go on last.

I turned my attention to the legs now, and devising a real mounting solution for them. They pivot directly on the front drive wheel’s axis, on a shoulder screw (which also anchors down the front drive axle standoff itself. I devised entirely new shock absorber things for this build, because I need to go up in spring stiffness to counteract the longer lever arm. The basic principle is still the same, however. Waterjetted from the same chunk of metal I will presumably make the legs from, then secondary machined.

I’m considering making a little extension to the frame to put these parts in “double shear” mode which will once again increase their stiffness. I decided to leave this until after the rest of the bot was modeled, since by this point I was getting close on weight.

Notice the blue string running around the model sprockets? I decided to try out Inventor’s chain drive designer for realsies this time. Prior to this, I’d only used it to generate sprocket profiles for machining. But as it turns out, it will tell you exactly how many links you need and whether or not you have enough tensioner travel to last the life of the chain, because chains stretch a few % with age (The answer for me was no, not for 10,000 hours anyway). You select existing cylindrical axes and tell it how big each sprocket is. You can even say a certain axis has an allowable amount of wobble (to make cam style tensioners) or can move in the XY plane a certain amount (for linear sliding tensioners). Then it will update whenever the sprockets are moved, and yell at you if you move them to an impossible position or you need to adjust your tensioners.

Wow. Computers are pretty damn cool.

In seeking more structure for the outer side plate, I decided to extend said tensioners to become standoffs in their own right. These have off-center holes so I can rotate them and then tighten down the long screw that binds the two plates together.

I added simulated top and bottom plates for the final almost-finished look.

At this point, the bot “weighed” 31.0 pounds. Uh oh… All that solid metal has to go. I want it to weigh 28 pounds or so in order to include overhead from wiring and screws I did not yet model (most of the big bolts were put in already).

Clocker Remix is very much “gothic cathedral’d out”, my term for making structures sparse and spindly to reduce weight, like… gothic cathedrals. I’m sure those guys did it less for weight and more because they were badasses, but whatever. However, it was done rather haphazardly – I have truss elements that really don’t do much and could have been totally absent (Did you know that trusses triangles are ideally all equilateral?)

And that’s it.

After selectively trussing out most of the plates and adjusting the height of others, the bot is now at 28.6 pounds as-modeled (with more big screws added, too). The side plates have gotten much lower (and name-emblazoned), which saved a ton of weight. I added one more standoff to raise the stiffness of that outer rail some more. The lowered sides also makes the motor mounting screws that much more easier to access. The only plate not hollowed out right now is the very back, which I’ve decided to keep solid because the bot otherwise has no rear armoring.

And the back shot.

in conclusion,

this is the longest post ever on my website at about 5500 words! I keep upping this number for some reason. It’s been a long time since a pure CAD-based brain-dump post, and I must say it was rather refreshing. Airing out decisions that you have made, makes you think about them more and critique them a little more impartially.

Construction on Clocker will commence as soon as everything opens up again after the Christmas-New-Years-What-Have-You holiday season. The target is Motorama 2013 (more strictly Robot Conflict @ Motorama 2013), an event I haven’t been to since Clocker and Arbor’s collective dismal losses in 2010!

 

The latest on the DeWut? project

Dec 15, 2012 in dewut?, Project Build Reports

It’s been a little while since I went “Make it your damn self!” on the DeWuts and left everyone hanging with the waterjettable pieces. Since then, the billet style design has evolved some. I’m proud to announce that it has been sent off for manufacturing by Sketchy-Ass Chinese CNC Co. Ltd., to return to me hopefully by mid January. This is a product which is in immediate need by robotland  ever since the old style 18v DeWalt “Team Delta” systems stopped manufacture, so, oddly enough, it might be my “launch product” instead of Ragebridge! Here’s what’s been going down.

This is the fully modeled design as of two weeks ago or so. As can be seen, I’ve actually bothered to model the DeWalt 3-speed gearbox! I’ve made the gearbox and motor available as a downloadable widget, if you want come up with your own design. The files are in Autodesk Inventor 2012 format, as well as a STEP and Parasolid.

While I tried to make a workable output shaft for the motor, I began to realize that it was perhaps more fruitful to replace the final output stage altogether. The 3 speed DeWalts have an advantage here because their antibackdrive (“that thing which makes it so you can’t crank on a drill’s chuck and have the motor turn”) system is very simple and planar. The idea would be to replace that ABD stage with a custom-machined ‘socket’ of sorts that wraps around the output carrier and has an integrated 1/2″ keyed bore, so in principle any 1/2″ keyed shaft can be used with the motor. If this is not clear from the above picture, then it will surely be elucidated by…

So, basically, the output stage planetary carrier has 5 little claw things. It’s easy enough to make a doohickey that wraps around those 5 claws. Normally, roll pins fit between those claws which are just barely smaller than the distance between one face of the weird decagon output coupler and the outer ring with 4 nubs on it (seen in the previous image). If you attempt to backdrive the drill, the decagon hub turn just enough to wedge the roll pins against the outer ring, locking the whole thing up solid.

This whole arrangement of course contributes much backlash to the system. While I could just say “take these 5 little derpy pins out”, that’s one more step in the instructions which, if not followed, would surely result in undesired behavior as the ABD rapidly alternates between locked and unlocked. A custom output coupler would also alleviate those concerns.

This is what the output coupler looks like, a 5-sided flower thing. In real life, this would be waterjet-cut from a high alloy steel like 4140 and moderately hardened. The shafting is a piece of stock McMaster 1045 steel shaft I bought to test fits.

The new output carrier pushes right against the inside of the inner ball bearing due to a chance alignment of English and Metric units. So, it truly is bring-your-own-shaft – the motor doesn’t provide any retainment force.

With this problem taken care of, I began addressing some fine details. With larger, heavier motors like this, face mounting screw holes are often not enough to keep the whole assembly planted under shock loads. A second set of mounting holes is provided at the rear to keep the heavy motor end anchored. These holes are designed to be 3″ apart and 1.375″ between centers. Why the weird dimensions? Because it’s compatible with a Banebots P80, just like the front mounting hole pattern.

This revision of the design also saw these little gearbox-retaining nubs on the inside, which help with setting the torque clutch tightness without having the motor installed yet. It allows more modularity in the assembly since previously the motor was the only thing pushing back against the torque clutch plunger (pressing on the spring steel wear washer immediately next to the gears, anyway).

The next logical step in the design was to combine the 5-sided flower thing with a shaft. This would fully constrain the output shaft, allowing direct coupling to a wheel.

Here’s what the whole thing looks like in mushy 3d printed plastic form:

This is the version I’m sending out to be manufactured. The integrated shaft is specified to be made from 1566 steel as-rolled 1″ (/25mm…) round, which should offer a yield strength in the mid 50s to 60ksi (400-ish MPa in Unamerican Units). So, the total setup if I were to kit this up would be:

  • Integrated output shaft
  • Output mount with 2 FR8 type ball bearings
  • Motor mount
  • Motor clamp
  • The nifty barrel shifter holder
  • 4 hex nuts to constrain the NBS holder
  • 4 long cap screws to hold the output and motor mounts together
  • 2 short cap screws to screw down the motor clamp
  • 1 set screw to adjust the torque clutch
  • A retaining ring, because retaining ring.

I’m wondering if I should make a version that has the “socket” output carrier such that the motor can hitch into any existing 1/2″ keyed shaft. The 5-sided flower thing will likely be available separately. I’m also going to pre-emptively make it available in downloadable form for your own waterjetting amusement (Inventor, ready-to-cut-DXF, STEP, and Parasolid). I strongly advise making it out of a high carbon or alloy steel for strength reasons.

For now, enough product development. I need to turn my attention to more pressing matters…

Cool thing of the day

Dec 12, 2012 in Stuff

I don’t know why this hasn’t been all over the internet, but it’s huge and in my opinion shows great promise and is a boon for the future of personal fabrication.

The MEAM waterjet was a Mechanical Engineering senior design class (not unlike our 2.009 but 2 semesters) which produced a functional abrasive waterjet cutter for under $10,000. No matter how proof-of-concept it is, waterjetting for under $10,000.

Maybe I’m just so into waterjetting that only I find this amazing. Maybe I’m just super excited that there exists a potential for DIY waterjets as much as there is for DIY 3d printers and DIY laser cutters. Metal is the last frontier of these hobby class fabrication devices. Nothing can really do metal yet.

Or maybe I’m just miffed that my own bad waterjetting ideas haven’t made it beyond chicken scratch status on sheets of crusty paper.

Yet.

Here’s the team’s whitepaper on the machine’s design process. There’s no videos of the thing, nor really any good pictures. As far as I’m concerned, that’s a documentation fail!