We’re beginning to reach the “minimum entropy point” of the design now, which is the point where I know exactly what needs to get designed and do those in sequence. For me, this usually occurs at the 2/3rds or 3/4ths mark (roughly), when I see the light at the end of the CAD tunnel. Basically, after this, the design sequence will probably get more chaotic as I visit and revisit portions of it.

This post will start where the previous left off, with me beginning to sketch the basic form of Overhaul’s characteristic rounded upper arm.

During the design of OH1, we went through a few arm shapes and construction methods. In large part, the rounded shape was dictated by the desire to avoid bends or miter-welded tube sections to maximize strength, since OH1 at this point was still focused on crushing. We’d already designed the lift arms to be the “tubes and AR400 side plates” method seen again in OH2’s lifting forks, so it was a matter of maximizing strength while minimizing weight.

The arm plates had large circular cutouts to reduce weight without adding stress rising sharp corners, and also looked cool because it was a Preda Raptor knockoff looked like historic machine tool cast iron girder frames. With a bit of reinforcement, that clamp arm turned out to be the most rigid thing on the bot, even without the reinforcing cross-tubes we designed to be welded into the holes.

I wanted to keep the design and look (adding to the visual continuity of the bot) so I started modeling the same way we did for OH1 – with a really big circle. The inner circle is not concentric, rather driven by the radius of the circle forming the hub/attachment portion and the desired tip width. The third constraint in the size of the inner circle was…

…the welded crosstubes. Through some geometry-mancing (geomancing is taken), I came upon a solution for the outer and inner circles which incorporated commonly available sizes of steel tubing that had the holes spaced all roughly the same distance apart. The spacing is not exact, but visually hard to discern the differences. That’s it, I’m keeping this design. It cannot possibly get better from here.

Modeling in the cross tubes. They keep the front portions of the jaw, where the highest compressive forces are seen in an attack, from spreading or buckling. OH1 relied on the tooth joint itself and a sheet of steel that was rolled to match the curvature of the outside of the jaw, welded to the top edges of the side plates.

I quickly modeled a tooth that could be machined from some tool steel in principle, but left it highly unoptimized. I just wanted something visually there to guide the rest of the design, and intended to come back to this later. While I could have this manufacturered, it’s shaped in a way that would require a larger, more expensive block of steel to start with, and also more setups and operations.

The tooth could be used in the ‘hooking in’ direction shown, or it could be turned around. I used the triangular hole pattern specifically for this, since it would keep the design versatile and allow different tooth placements and possible other attachments depending on who I’m facing.

With the top end of the bot visually anchored, it’s time to start thinking about actuation. For starters, I dropped in the “sketch model” actuator I made for the application, which contains a SK3 motor, a P80 gearbox, and some arbitrary solids that may or may not make sense. This allowed me to see how much it didn’t make sense and how porked I was for space, which was very. I spent a while figuring out the best angle to mount the actuator such that it remained serviceable, didn’t stick out too much, and most importantly, could get me the jaw travel I wanted without running into anything else. I’d say the latter two were the most difficult, since the design was anchored substantially in general by the time I got here.

One way to get better actuator placement was to back down on the size of the motor, but that’s for manlets I wanted to retain the high powered grab-or-smash capabilities, especially in conjunction with a very bad idea I began prototyping in November.

After some more design evolution, I’d like to point out the three key features of the new upper clamp actuator. It’s still custom, for a higher force to weight ratio than what I could buy commercially.

First, I decided on a compromise position between the upper “I want to use my motor as landing gear” position where it’s highly exposed unless I gave OH2 a massive steel mohawk, which would be KINDA COOL… or the lower “I want to use my motor to absorb a Tombstone hit” position. The position I chose is in the middle.

This keeps the actuator securely within the two walls of the upper clamp arm. However, it causes a significant bending force to be applied to the leadscrew because it’s pushing off center. The proportion of the bending the leadscrew sees is a ratio between the distance offset from mounting axis (the big hole) to the leadscrew’s axis, and the spacing of the two bearings of the leadscrew (the purple object – there’s another one hidden from view to its left). This meant I had to be careful with OH2’s clamp force calculations, as they could now easily exceed the bending strength of the leadscrew.

Notice also the new motor choice – now an A23-150 Ampflow motor, the “nanoAmp” motor. It’s relatively new to the market, and is roughly the size of a common FIRST Robotics CIM motor, except higher performance. This design change was driven mostly by my desire to exert a constant force on the grabber – something which the “hobby level” brushless systems can’t do yet. I wanted to be able to do this since ball screws are very low friction and will backdrive easily, so if I don’t want to lose grip on someone, I need to keep a bit of power going into the system for holding. It was then not difficult to design around a Ragebridge 2 to use the current-limiting feature, so I couldn’t accidentally bend the actuator.

According to my calculations (… I just used that for real… oh god … oh god) the maximum safe current was about 70 amps using the A23. At this point, the tip of the jaw can push with around 2500 pounds-force. That’s in low gear – in high gear, the leadscrew isn’t a limitation, since the motor can only push around 1000lb-force at stall current and I should not be stalling the A23 at full input voltage unless I was interested in suddenly making a surprise flamethrower.

Obviously not the 6000+ pounds that OH1 was capable of, but we never were able to use it in a match anyway. This new limit would necessitate redefining the mission of the bot; I’d only ‘go for the crush’ if the opponent had armor worth doing that to, such as under 3/16″ mild steel – OH1 managed that at a current of around 80 amps on the F30-150 clamp motor, yielding a force of roughly 5000lb. It managed 1/8″ steel at around 40 amps, which is still within reason for this new actuator.

There are, of course, a slew of unknown factors I have not accounted for – for example, immense friction between the trunnion surfaces could raise the maximum compressive force the clamp can exert before the motor “sees” an excessive bending moment. Or my leadscrews are made from Chinesium instead of induction-hardened E52100 bearing steel. Like I said – every fuckup I might make, right here for you to see, ON TV.

The sound you hear is everyone rushing to make weight for some titanium top armor.

Second (yes, now we get to Second) it uses what would have been one of the large cross-tubes in the upper clamp arm for one of the trunnions. This was a “might as well” type decision – I was going to have to intrude upon the volume of that tube to place the actuator anyway, and so would have had to remove that cross tube from the design. Using the cross tube allows me to still use it for some degree of anti-buckling of the side plates.

Third, wait, what do you mean “high gear and low gear”? (Go back and read the section about My Calculations™) Aren’t P80 gearboxes single speeds?

Not if you are depraved like me. Introducing the “P90X” gearboxconcept:

That is a design for a two-speed, shifting Banebots P80 (with significant modifications). I studied the design of the DeWalt gearboxes and took note of how they accomplished multiple speeds, and duplicated it using the parts of the P80 with some custom machining. In short, the ring gear of a 2-stager is split in half, with one half either forced to be locked to the housing with pins, or spin freely relative to the housing but locked into the first stage gear carrier effectively skipping that stage. Using the 16:1 as a starting point, I can get either 16:1 or 4:1.

This is a project which is basically its own story to tell, so I’ll leave the details of its construction to the later build posts. I’m comfortable with using this idea for now, since if something goes wrong or if it’s unreliable, I can always drop back to a 1-speed normal P80 like a pussy depending on who I might be facing.

Taking the ‘solid block sketch’ of the actuator and making actual features in it now. This part will be carved out of a big aluminum block – a one piece billet trunnion & gearcase. Inside, I’m making features to mount bearings. Using the maximum bending force calculations, it was pretty easy to size some big angular-contact bearings for the job. They require preload, which the ball screws I’m going to get have an adjustment nut for (They’re from the same vendor as the ball screws for OH1).

With some details in, I started placement of the assembly to fine tune external dimensions. This is the “motor high” position now.

And the new “motor low” position. Still not a fan of potentially exposing the motor to a wiggling 250lb opponent, even though it gets me more range of swing on the forks:

As seen here, the “motor high” position current causes the motor to be the hard stop for the fork actuation. To remedy this, I was going to give the clamp arm sides a small extension behind the motor. The “motor high” position causes me to lose around 7.5-10 degrees of possible fork swing. I think I can deal with this.

Moving on to more details! First, I had to reconcile the size of the actuator body with the necessity of mounting things into it. This meant making it ever so slightly wider, to accommodate both case screws and discrete gearbox mounting screws.

I briefly entertained the thought of integrated P80, meaning swapping the gearbox front output plate for an integrated bearing pocket & ring gear holding socket, but decided against it.

In retrospect, I probably should have done this to save some motor mounting volume – it’s not like it’s hard to remove the front plate from a P80 if I had to swap stock gearboxes, and my P90X design could also be accommodated since it simply replaces the center ring gear portion. As long as I was having something super custom machined…

I’ve hollowed the other block to have motor mounting features and the output bearing now. From there, it was cutting away stuff that didn’t have to be there. The openings in the case are to accommodate a larger output sprocket than what otherwise could fit fully inside – I could have made the walls ridiculously thin, but why even bother at that point? Just open the thing up and gain some more sprocket diameter.

Doing it this way let me use a 1.8:1 external reduction to the ball screw, instead of the 1.5:1 I could have gotten at most otherwise.

The last part of the actuator to be designed was the….

ball screw nut shaft

Alright, I said it.

Yeah… that. The discrete rod end bearing (“ball joint”) took up an extra inch of travel for its own mounting threads, so I designed an integrated one. The extra travel gained is worth about 2.75″ of travel at the tip of the tooth due to the leverage ratio.

Test fit of the actuator in the design. Seems legit!

The retaining method for the trunnion tube, a 2.75″ OD steel DOM tube, is a Giant Snap Ring. Also note the small mini-mohawk that I gave to the upper clamp side plates. This extends about 1/4″ past the motor and will hit the top plate first. The tip radius there is fit to weld a section of 3/4″ OD steel tubing over for more robust contact area – I’ll determine the need for this “dynamically” later on.

Time for a brief aside from actuator and clampy-jiggy modeling. I’ve been thinking of ways to retain the rotating shell of the SK3 motors since starting the project, but always put it off. It’s essential that this gets supported, because otherwise, the whole spinning mass of the big outer rotor is being supported by a little aluminum stick in the middle. A China-luminum stick. That’s just asking for an impact to break the whole motor off.

#nope

So, two of the P80 mounting screws get extended another 10mm (using 50mm instead of 40mm cap screws, for instance), and hex standoffs get screws onto the extended studs. A little plate with a flanged bearing sits at the end of the standoffs, and the flanged bearing rides on the 10mm shaft nub on the back of the SK3. Self-contained and rearrangeable.

Back to clampy-thing!

The next step is modeling Overhaul’s characteristic self-righting ears. Everyone seems to think they were decorative, but they were vital to OH1 being able to self-right.

Überclocker, for instance, is short enough with the fork and clamp down to fall onto its back, from where I can swing the forks up for quick righting. The curved upper arm of OH1 (and OH2) means the tilted upside-down position, where the bot is roughly at 45 degrees rolled over, is stable. This is bad. We discovered 3 days before ship that OH1 could not get out of this position in any way.

The ears were added to artificially shift where the stability point was, such that with some arm movement downwards, we could guarantee the bot falling onto its back, and then being able to push upwards and over. OH2 should be able to “dynamically self-right” easily due to the sheer speed of the fork, but better to have insurance.

I decided to make the ears one-piece from a bent steel plate for easier fabrication. The weldment would use two points on the upper curve and then a tangent to one of the cross tubes for alignment. All-around, better than the freehanded triangles of the OH1 ears.

Cutting out a few ounces where it wasn’t needed to support the weight.

And here we have them attached to the upper clamp arm.

The shape and extents of the ears are not settled. In fact, they will NOT be settled, made, and attached until we get OH2 together for a self-righting test. By Inventor’s center of gravity calculations, this shape should work in most arm-down positions. But real life testing will be needed to validate this part.

Up next: Electronics, electronics, and more electronics. As a mechanical engineer, I am obligated by oath to leave “the electrical stuff” for last!

In this part, I’m moving on from the aluminum parts of the bot and putting in some REAL STEEL. First order of business is designing the front armor, the angled pontoons for which Overhaul 1 was somewhat famous for. Next, I’ll put in the lifting forks, and finally the Razer upper clamp arm which is supposed to Razer come down on opponents and mercilessly crush them like Raz… hey, wait a minute.

Welcome to the Overhaul 2 Design & Build series, episode 4: A New Work Plane. Because that’s what’s going on here. So many work planes

The work planes are where sketches are drawn to make the solid pieces. While Inventor will let you sketch directly on the surface of a part in an assembly, I wanted to keep this part a little more separate from the assembly. Therefore, these work planes are defined from the assembly parts, but will still be around if I delete the part (e.g. to replace it with a new version), or edit/delete a frame part.

I honestly didn’t know what I was aiming for when I started. I knew the general shape of things from the sketch model, so I just started making a basic wireframe. The best term I could probably use to describe these first few versions of the pontoons was “Gundam armor”. Impractically faceted and pointy, and likely very hard to make in real life.

Here’s one completed example. I wanted too many things out of one design – the flat sides for easy attachment to the shock mounts, the sloped outsides to deflect KE weapon blows, and the sloped upper panel to protect the liftgear and better act as a w*dge passive-traction-leverage-implement. (It’s no longer polite to say w*dge in BattleBots!)

That’s how many work planes I got up before I decided to just start over. This was the accumulation of 3 revisions, so it included a lot of angles and facets I ended up ditching.

Here we are, after tossing THAT part in the dumpster of bytes. This shape more approximates the original sketch, with its sloped sides and double-angle front. I initially did not pursue this because I was thinking that modeling the non-straight sides would be a pain. To generate the surfaces, I use the wireframe (made of many sketches on different planes) to generate boundary surfaces in Inventor.

The double-angle front armor was borne of some musing about how to overcome horizontal spinning weapons. You can usually do only two things with big KE weapons – absorb them, or deflect them. The former usually ends quite badly.

We saw in the Bite Force vs. Tombstone final how effective usage of material geometry can bounce KE weapons around, since no matter how  much energy you pack in a weapon, your bot still only weighs 250 pounds. The idea of the double-angle pontoons is similar to that of a Jersey barrier, which is designed to bounce wayward cars back towards the correct side of the road. The lower angle slope guides the weapon upwards, and then the sudden higher angle slope destabilizes the now elevated weapon. The height of the angle change is set just barely above the average blade height of all the big hitters at BB season 1*

Adding to this deflective geometry is the soft rubber mounting points, which on a good connect will squash and cause the pontoon to come into contact with the ground. The hope is that this will basically mean the opponent suddenly reacts against the arena floor, adding to the bounce effect.

That’s the theory – we’ll see how it works in practice. But the effectiveness of even a single angle, solidly mounted deflective plow can be seen in the snippets of Captain Shrederator vs. Stinger (trust me, it was even more awesome in person) as well as the Bite Force final.

*The sound you hear is everyone rushing to build variable height weapons

Adding more of “the details” to the pontoons – now they have vertical panels and bottom plates. These are all still surfaces. The plan is to use this part made of surfaces as a reference to “grow” the final plate models from.

For now, I left this surface as-is since at the least I’ve defined the shape of the pontoons and therefore the shape of the front of the robot. I moved onto the next Most Critical Module, which was the lifting arm assembly.

I burped this out after a few hours of thinking about how best to attach the forks. Fundamentally, it’s a big center hub with an ear attached to it where the lift actuator will eventually mount – dimensions and spacings for this were guessed, since I hadn’t designed that yet and wanted to keep it flexible. A similar structure appears on Uberclocker Remix on its main lift shaft.

For OH2, I wanted the forks to be easily removable in the event of oh god I got owned but somehow won. To do this, the fork arms can’t be made a permanent part of something, and the attachment method also needed to be rigid in bending as well as be able to handle what might end up being thousands of ft-lb of torque if I actually use the full crush force on something.

I decided on a dead shaft system where the fork arms also have bearings (rather, just bushings) in them with the hub a separate piece. Eight 3/8″ threaded studs are anchored into the hub, and the forks are retained by 8 nuts once they are slid onto the hub. The way to quickly remove the forks would be to remove the lift shaft (two bolts), then undo 8 nuts per side, and then slide them out.

The lift hub is therefore a totally indepedent structure, so I could conceivably run OH2 even without arms or without the clamp for some reason.

 

Because this thing will be reacting all of the clamp actuator force through itself, I decided to check in on how the material was going to handle it. Having a 5″ round wad of solid aluminum in the center is great, but not for weight. I wanted to see how far I could hollow it out while retaining rigidity, so it was back to the good ol’ FEA tool.

If you haven’t ever seen FEA before, it basically does all of the stupid bending-twisting-beam calcualtions you had to do in engineering school by hand with a stupid lookup table (IF YOU’RE LUCKY AND HAVE A VERY GENEROUS PROFESSOR), on very finely subdivided elements of the part such that the result is a model of how a very complex geometry reacts to forces and deforms, broken down into simple numerical problems for the computer to crunch through quickly. Quicker than you. No, I’m not bitter about my Mechanical Engineering degree or my classes at all. Why would you even THINK that????

In a typical FEA simulation, you set the constraints (how the part can’t move), set the loads, the materials, and any contact conditions (e.g. between 2 parts, between 2 materials), and let the computer have at it. There’s many ways to do FEA WRONG that I won’t get into detail here.

Here, I’m doing a bending force analysis on the point where the clamp actuator mounts. I wanted to see if this would cause problems with the whole thing deflecting when OH2 mashes down with a few thousand pounds of force. The conclusion is that no, this hub is fine. Double shear for the win!

I usually design to Yield Strength for materials (the point which a material bends). It’s more common to design to Ultimate Tensile Strength – the point when the material breaks. I prefer Yield because if something bends in battle, you’re hosed, but this won’t put any planes in the air…

Having run through the hub once, I started generating the lifting forks. They’re going to be made from some steel tube and some steel plate, cut out into scientific shapes and joined in less than scientific ways by yours truly.

At this point, I was still interested in curved forks. I wasn’t quite sure how I was going to make these yet, but hey, why not try CADing something different? The tube profile was generated with a Sweep of the square tube profile (modeled without fillets).

Now in solid form…

I did something slightly nontraditional here, and made each of those elements you see – the tube, the tube caps, the fork curved square tube, etc. a separate solid body in one single Part file. This let me easily reference the parts off each other. To break this down into things I can cut, I highlight and save each solid body individually.

Modeling the “fish hooks” at the end from what will be waterjet-cut steel plates.

Time to see if this fork can stand up to the maximum force exerted by the upper clamp arm. This is a view of the “finite elements” in “finite element analysis” – the computer solves iteratively how each of those little triangular sides deform based on some relatively simple continuum mechanics rules. I modeled for 2500 pounds-force per side, at the tips. This is basically the full, err, bite force of the upper clamp arm (ideally each fork seeing 1/2 of the load).

First attempt: Dorky mild steel tubing.

Nope. Utter failure. Everywhere.

Second attempt: 4130 steel, normalized (not heat treated). This was assuming I could get 4130 in those needed sizes, which later proved to be a problem. This is “edgy” – the yellow on substantial parts of the tube means that on overload conditions like mashing someone into the wall, it could bend and fail there.

Okay, how about using geometry instead of more hardcore alloys? I modeled a single rib that runs the length of the fork arm on the inside. This is not preferable at all, since I don’t want to have something sticking up right where weapons can reach it, but I was interested in how it changes things.

The rib option is workable. This result is a little deceiving, since I had the force concentrated on a small region of the material (the hook), so it will deflect more than everything else. Really what I’m paying attention to is the green-yelow on the rib itself and the tube being largely blue – the rib has transferred most of the stress out to itself, which is what is expected.

The bolt holes are still a little concerning. They are, however in regions which will be tightly bound next to other parts, which simple FEA modeling doesn’t capture.

Instead of the rib, what if I just went to a thicker steel section? Here’s 1/4″ wall tubing (still using 4130 – not even sure if it existed at this point!). The performance is better than the 1/8″ tube version without the rib, but actually not too much. Since the rib is further away from the imaginary axis which this arm would bend along, it can resist the forces far better than more steel closer to the axis. Geometry!

I stuck with this for now, for the purposes of moving on. All of this analysis was really completed in one evening, by the way, so this wasn’t days and days of running simulations. I was willing to trade the inability to use all of the upper clamp force for some “easy” – roll a tube, weld some plates on, whatever.

I’m going to skip ahead a few weeks here, since I like the continuity of this thread about the forks. You’ll get to see the “final” fork design, even if the photos of OH2 from here until a few posts more will be showing this “old” design. Basically, after finishing the bot in whole, I began revisiting parts which I felt I had left “sketchy”, including the forks. In the intervening weeks, I had called around getting quotes and prices for sourcing the arm tube. I had thought I had a handle on sourcing 2″ square 4130 tubing, but I was either imagining it or they changed the Matrix between the design and my revisit.

Bottom line was that I thought using 4130 was no longer realistic in the (Time * Frustration) factor department. Therefore, I decided to capitalize on known available materials, and good geometry.

This time, I’m using a “tube and side plates” approach where the side plates define the shape of the arm and also carry the majority of the weight. The tube really is there for closing off the top and bottom surfaces (sideways rigidity) at this point, and I could even replace it with a rolled curved plate and have an H-profile beam.

This design kind of looks like the original “rib” design, except the ribs are sort of discretized into the new side plates.

The plan was to cut them from the same material as I was making the upper clamp – AR400 plate.

AR is my new favorite  material after building OH1. It’s just so good and versatile, and is my mental go-to if I need a ~140ksi yield strength material that can remain mostly flat. Usually, stuff with that high tensile strength isn’t easily welded, but they designed this alloy specifically for tacking Iron Man suits together in the oilfields with tin cans as filler material or something, so it’s also easily weldable. You do lose some strength in the weld area, but again – geometry can help overcome, or at least offset that.

Throwing it into the FEA module. I modified the location of the force so it wasn’t causing signficant deflection of a single point (the hook) – instead, it’s centered on the large planar face.

Some geometric changes and lightening holes later, and this is the curved arm design. The holes in the side lost about 2 pounds without substantially raising the stress levels.

The geometry here is modeled as “bonded”, meaning no movement at material boundaries is permitted. Clearly this isn’t true in real life – to approximate it, I’d have to weld substantial portions of the holes to the arm tube, or the movement will cause one or the other part to exerience more stress than designed. FEA is one of those things which is tobe melded into everything other aspect of a design, not the end-all answer to a problem.

Skipping forward again another week or two, I was discovering that the tube rolling process was going to be problematic. First, the sides won’t be flat any more – the tube will bow out. Second, there was no tube roller nearby that could handle the material – not in my network, anyhoo. To get the tubes rolled by a known bot-friendly company would have been 2 weeks of leadtime since they were on the west coast.

So I decided to do the final refactor of the design into something which can be built right now with materials and methods on-hand. This means angles.

The “look” of OH2 had been defined by the curved forks, so I wanted to keep an element of them in the final design while hiding the square tubes. Conveniently, the curved side slats are also excellent gussets. The idea was to approximate the curve as best as I could with regular square 2″ tubing.

The above shape with the “elbow” of square tube sticking out underneath…

…which I hid with an “inverse fish hook” coming from the side slats. Hey, they look intentional now!

Looking at the analysis of this part, the problem areas are where I’d expect them – the areas of the side slats near the elbow primarily. (After this picture, I increased the fillet radii substantially and the narrow neck near the elbow was no longer an issue)

As I had modeled these new AR400 plates with anticipated cutting tolerances, there’s gaps in them which I could not force to be “bonded” contact. As a result, you can see the corners of the front angled plate being “highly stressed” – causing the safety factor minimum to be artificially low in that region. In real life, these are going to be welded securely.

Alright, time to undo the timeskip (this was “roughly week of 3/7”). Now we return to the pontoons and upper clamp arm.

 

I took the surface reference model for the pontoons and literally grew those surfaces into 3/16″ plates…

…and then did my usual “edge stiching” finger joint method on the individual plates. This will make the part very easy to self-fixture for welding.

More details, such as internal gussets, have been added to the pontoons, with more to come.

With the front pontoons done, I brought in some more components as a bit of “work-ahead” – thinking about the next step while I’m working on the current one. I knew what I had to do for the upper clamp arm, so I just had to crank it out while thinking about electronics mounting, the bane of mechanically-minded robot builders everywhere. Can I just cast the whole thing in hot glue and be done with it?

Coming up next: The upper clamp arm geometry and actuator!