#hmrevents

[heavymetalrobotics]

Welcome to part 2 of our event recaps from ARC Scrapbots 2! In part 1 we went over the first half of the event which was building a robot out of junk in just 3 hours. Now we get to the fighting! My first fight was against a robot built by Leon.

One of the cooler looking robots from the day, Leon's bot was built from a VHS tape box, and used the "flipper" electronics kit to turn it into a biting/grappling weapon. Foam board teeth and eyes on top gave it a monstrous appearance, like some kind of robot sea beast. From a strategy perspective this was a difficult opponent for me. Full-body shell spinners like the robot I made need space to get the weapon up to speed, and control-based robots like Leon's are excellent at shutting that strategy down.

Immediately they closed on my bot and used their jaw weapon to stall my robot's spinner. Despite trying my best to back away to make some space, they kept chasing my bot down and grabbing onto it, even hooking one of my weapon teeth in their jaws. Eventually I got my robot to work itself free and got the weapon up to speed, just as Leon's bot seemed to stall out (this will be explained in more detail in later posts). I used to opportunity to attack and ran the spinning shell into their chassis at full speed...which did nothing. I backed away to get the weapon spinning again, but it merely twitched instead.

It seems that the shell was still rubbing on my bot's chassis slightly in just one place in the rotation and would stall out if it stopped at that point. I ran it in reverse for a second to free it, then full speed forwards to get it functioning normally again. By this point Leon's bot had recovered and was moving in to attack again, so my spinning shell ended up hitting their front teeth again. This time though one of the decorative eyes was knocked off their robot! Technically that is damage I caused!

I managed to spin the robot free from my opponent pretty easily this time, but in doing so had difficulty telling which direction my own robot was facing. That is one of the biggest downsides to shell spinners. They are almost by definition round and symmetrical, making it challenging to understand which direction the chassis underneath is facing. By the time I got that sorted out they had cornered my bot against the wall of the arena. I backed away once again to get the space needed to spin up, but that put my bot dangerously close to the arena pit.

When trying to drive a robot with a horizontal spinning weapon like a shell, the torque from the weapon motor actually steers the robot in the opposite direction while driving. For this reason shell spinners tend to drive in swinging arcs rather than straight lines. Since I was so near the pit, I opted to keep the weapon turned off to maximize the control I had over my bot while escaping from the corner. Unfortunately the weapon stalled out again and before I could free it, Leon's bot grabbed hold of it.

Despite having superior traction with the foam rubber treads, my bot was simply too light weight to push my opponent. I tried to steer to the side to maneuver around them but fell victim to another of the shell spinner's driving quirks. Since the shell was pinned by my opponent, it remained stationary and the chassis turned underneath. So rather than getting free and driving around to their side, my robot spun a 180 degree turn and drove itself down into the pit, losing me the fight.

All in all, it was a good fight even if it was short. Driving shell spinners is really difficult, and control bots are their biggest weakness, so I wasn't surprised at the result. The main positive thing I took from this fight was that my robot worked wonderfully! It was responsive, the weapon spun up to a reasonable speed (most of the time), and I even caused a little damage to some cosmetic items. I went back to the pit table afterwards with no repairs needed and confident that with a little bit better driving I could take some victories in the remaining fights.

Below is the link for the fight video from ARC's youtube page so you can watch the fight for yourself. It is timestamped to this fight in particular. Up next for me was a spinner robot built by my pit table-mate Ben. I will go over that fight in the next post. See you then!

[swdjb]

via Dark Brotherhood

[heavymetalrobotics]

ARC Scrapbots Event Report Part 1

It's time to get started on our event report from ARC Scrapbots Challenge! This was a very unique event unlike anything else in robot combat so this report is going to be a little different than usual. In this part I'll explain the event format a little bit and go through the first stage, and then in subsequent parts I will go through each of the fights in detail.

In most robot combat events, competitors arrive with their custom made fighting machines that they've spent hundreds or thousands of hours designing, building, testing, and engineering to be at peak performance in the fighting arena. Scrapbots Challenge is quite literally the opposite. Every single robot that competes must be built at the event, in 3 hours (roughly). Competitors are provided with a common, basic electronics kit consisting of motors, electronic speed controllers, and a battery. They are then tasked with constructing their robot using nothing more than what can be found in a large pile of household junk. The format is based on the old TV show Scrapheap Challenge (called Junkyard Wars in the US). That was once one of my absolute favorite shows, and I have long talked about wanting to try a similar thing in combat robotics. When the opportunity came up for me to participate in ARC's version I jumped at it immediately. Silly combat robots are my favorite kind and here was an event consisting entirely of silly robots!

The event took place at a makerspace, so there were plenty of tools freely available for use to build our robots with. Upon arriving, each competitor was instructed to add their own household junk to the pile on the table, and then given a small numbered token. The pile was quite small when I arrived but had nearly doubled in size by the time the event began. All manner of random objects could be found in the pile: disposable cutlery, plastic food containers, VHS boxes, old cell phones, computer speakers, broken children's toys, bits of rubber, old 3D prints, lego pieces, aluminium drink cans, and much, much more. Anyone could pick what they wanted from the pile, so there was no guarantee that something I could see early on would still be there when it came time for me to pick the scrap for my robot.

The numbered tokens were an important part of balancing the difficulty curve of the event. People who were new to combat robotics were drawn into groups 1 & 2, while those with experience were placed in groups 3 & 4. When it was finally time to begin, the groups were each called one at a time in numerical order and given 3 minutes to pick 5 items from the pile to use for their robot. This ensured that the less experienced competitors had the best selection of pieces to choose from, and more time to work on their robots while the later groups were still picking through the pile. I ended up in group 3.

Rather than waste precious time sitting around while the first two groups were picking their parts, I had a look over the electronics kit and began to lay them out so I could begin planning my robot. After all, there was no sense in picking out pieces that would be too small to hold everything. In addition to the electronics, there were also 3 different sizes of 3D printed hubs to make it simpler to attach things to the motors. I had one of the "spinner" kits (the other kind was a "lifter" kit) which meant I would need to also plan for a spinning weapon for my robot.

When my group was called to the scrap pile, I had a difficult time finding suitable parts. Many of the good pieces had already been taken by groups one and two, and I quickly ran out of time before finding what I wanted. Once every group had their chance to pick through the pile for their initial parts, it was opened up for anyone to go in and get more pieces whenever they wanted. I ended up spending a LOT of time going back and forth looking for more parts, something I definitely need to improve on the next time I try an event like this again.

What I ended up picking the first time around were two bottle caps with holes in their centers, a clear plastic bowl, a giant googly eye, and a sheet of thin green plastic that was very flexible, similar to the material that file folders are made from. I was hoping to use the bottle caps as wheels, the plastic sheet as a base plate, and the bowl as a shell for the robot. But everything else the robot needed I had no idea for and had to figure out as I went.

I didn't have much of a chance to take photos during the building process because of the limited amount of time available. Pictured above is what I constructed for the base of my robot. I cut a square out of the green plastic as a base plate, then cut small rectangular recesses in the sides to mount the drive motors in. I spent probably far too long cutting up an aluminium drink can (from an energy drink I think) to fold into motor mounts. While I knew I could simply hot glue the motors into place, I was anxious about the risk of the glue getting into the exposed gears and locking them up for good. Further, glue doesn't help much to resist torque, so I wanted a direct mechanical connection to take that load. I ended up folding the aluminium into a roughly omega shape (Ω) so I could fold the "wings" under the base plate and glue them in place. Some creative slots cut in the aluminium also meant I could slide them into the recesses in the base plate, further adding to the mechanical strength of the connection. This was honestly my favorite part of the whole build. It's been a VERY long time since I have tried to work with joinery techniques, and also quite a while since my brain has gotten a good creative workout like this.

Knowing that traction would be a problem with bare metal wheels, I found a small strip of foam rubber, cut it up, and glued it to the outside of the bottle caps as a wheel tread. I then used the smallest of the 3d printed hubs provided (which I cut down further) to mount the bottle caps to the drive motors. Several times the glue holding the hubs to the caps came loose, probably because of how smooth the metal was, so I ended up scratching up the inside of the caps to add texture for the glue to adhere to better.

By this point I had decided to build a robot type called a Shell Spinner. So to mount the weapon motor vertically, and keep it stable so it wouldn't come loose, I hot glued a Lego Technic piece down the center of the base plate and press fit two wooden dowels into it. Then I duct taped and hot glued the weapon motor between them. Keeping this as close to vertical as possible at all times was super important because the clearances between the chassis and spinning shell would be quite small.

(Forgive the post event photo, I didn't get a good picture of the finished chassis at the event.)

For the rest of the chassis, I found a metal bracket that just happened to be the exact shape of one side of my robot, so that got duct taped as the front wall. Not able to find any other scrap that was particularly suitable I dug out some foam board from the pile, cut it up, and used it for the back and sides of the chassis. At this point I was running out of time. I think I only had 45 minutes left when I started working on the chassis walls, and I didn't even get around to the shell yet! To cover over everything I picked out a plastic lattice that had once been part of a model kit and had all the pieces removed from it. I thought that it would be pretty cool and unique to have a sort of "cage" over the top of the robot rather than a solid opaque lid so that the electronics could be seen through it.

With the chassis mostly finished it was time to work on the spinning shell weapon. At first I thought about securing the "teeth" to the sides of the bowl by running them through it, but the first attempt at puncturing it resulted in the bowl cracking. Not having enough time to sort out alternate solutions, I simply hot glued the pieces directly to the outside surface. This was very tricky as I needed to hold everything at an angle to a curved surface while the glue set and cured. Somehow I managed to get them attached the way I wanted, with one piece supporting the other so they wouldn't just break off the first time they hit anything. I ended up using the handles of disposable razors because they were orange. At the time I was thinking about theming the robot based on one of my old designs from video games (which was orange) but ultimately I decided against it, and by this point it was too late to change things.

You may be wondering why I used the handles of razors instead of the sharp blades themselves for the weapon teeth. The answer is simple. If I included anything as sharp as an actual razor blade I would also need to make a safety cover for them so that nobody would get injured while handling the robot outside the arena. Time was rapidly running out (I think I had maybe 10 minutes left at this point) so that wasn't an option.

With the remaining few minutes I glued the cut-off parts of the wheel hubs from earlier to the bottom of the robot to keep it from tilting forward and back too much. Then it was decoration time! Shell spinners are not a particularly appealing shape (they are round almost by definition) so I felt I needed to do something to make it look nicer. A few small googly eyes around the top of the shell and a plastic egg in the center did the trick. I had initially thought about the big googly eye going on top but it was larger around than the shell and blocked the view of the chassis inside. I'm pretty sure one of the other builders at my table ended up using it in the end. Lastly, I happened to find blue, pink, and white straws in the pile, so those got glued to both sides of the shell to add some more color while it spins around.

I don't have any photos of this next part so have one of the makerspace as it was set up for Scrapbots.

Not long after I finished the decorations, time was called. At this point it was mid-day, and the event had a 1 hour break for lunch. However, we were all permitted to spend our lunch break however we saw fit. Those who had finished their robots went out to get food, while many kept working through the extra hour to get their robots finished. I was one of those who kept working.

First up was testing to be sure the controls worked properly and the shell would actually spin. To my surprise, I had mistakenly calibrated the fail-safe in the radio system incorrectly. I had assumed that a single direction spinner would use the throttle stick on the transmitter from zero percent at the bottom to 100 percent at the top. However, the electronic speed controllers provided were bi-directional which meant that they treated the middle point as zero, and the bottom as -100 percent throttle (full speed reverse). So when I tested the fail-safe by shutting off the transmitter, the weapon turned on at full speed in reverse. Oops. I eventually solved this by moving the weapon control from channel 3 to channel 4 on the radio, which meant that it was controlled by moving the throttle stick from side to side. This not only was already set to fail-safe correctly, but it made it easy for me to control the weapon. I could simply let go of the stick and the spring would push it back to the center zero position to shut the weapon off.

The bigger issue though, was that the weapon didn't actually spin! The clearances between the chassis corners and the inside of the shell were too small, and at some points in the rotation they would rub against each other. Essentially the chassis was acting as a brake. I spent most of my lunch hour cutting and grinding away at the corners of the chassis to try to free up the weapon. In the end I made it work, but there was still some slight rubbing which would occasionally stall the weapon out during fights.

Finally, before fights were to begin, we had to weigh our robots. Normally ARC runs robots in the 150 gram weight class, but for Scrapbots the weight limit is increased to 250 grams. Part of the reason I chose to make a shell spinner is because of how challenging it can be to keep those designs within the allowed weight. The shells are usually really heavy, and so the chassis needs to be small and compact to account for that. Well, it turns out I did too good of a job on that front. My finished robot only weighed 157 grams! Without the decorations it would be completely legal to run in the normal 150g antweight class. Next time I try a competition like this I plan to make a larger robot that uses the full weight limit.

That brings us to the end of the building portion of Scrapbots! Up next will be the fight reports from each time my little robot was put into the arena. Everyone got 3 fights to qualify for a knockout bracket, so there was plenty of robot fighting action to be had. Tune in then!

[heavymetalrobotics]

Our social media presence wouldn't suggest it, but we've been busy and competing! In April 2025 we took part in ARC's Scrapbots 2 event! It was a tremendous amount of fun and we can't wait to post about our time there.