Percussive Maintenance XL - Beetleweight Build Diary

I’ve been putting together a kiwi drive axebot for summer events, as a spiritual beetle successor to Percussive Maintenance, my long-time ant axe. Heavy inspiration from Ominous, which led me to experiment with kiwi drive on an ant first before designing this. Here’s a slightly older CAD before I started prototyping the weapon.

BBB drive motors, belt-driven axe with a OpBox Mk4 and running on 4S. The main chassis is a mixture of 4mm and 8mm HDPE and the skirt will be all TPU, as is the belt/upright guards.

I started prototyping the weapon a few weeks back, my first foray into belt systems - I wanted a belt so there was a built-in clutch to save the motor. I started with a simple post to screw in an idler. Pictured here with a temporary arm and axe head, which is literally just an old mild steel fork from the original Déjà Vu!

With this setup, I was getting good power out of the motor and not belt slipping, but when I upgraded to the full-length arm and Hardox head, the belt started to slip at around 40% power, which led me to redesign the tensioner.

Now there is a slot that the idler floats in, with screws driven through captive nuts to push the idler down, increasing the belt tension. Once appropriately tensioned, I got some full power swings with no belt slipping :slight_smile:

No events currently set in stone for this yet, but I’m hoping a BBB August event will be its debut. Aside from that, I plan to alternate Robodojo events with this and Déjà Two, as there are 6 events a year with your top 3 results contributing to the league scoring they use/


I’ve been thinking about armour configs recently. The existing design has a prow at the front, which while I think looks cool, protrudes out in front of the forks, which will cause problems against drums. I’ve redesigned the front of the bot so that it has a flat front, and fork mounts closer to the middle of the bot, intended for fighting against drums and beaters. The outer fork positions should still be fine against other verts. I’ve also designed the anti-horizontal config, which removes the forks and presents 12mm of TPU at the front. I’ll have to see when I get round to building the bot whether these configs will actually be in weight.

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Looking beautiful mate. MotherLoader MK2 had 10mm TPU pannels which stood up to a direct smacking from Baby Shrekt. Only problems I had was fastener pull-through which you can easily avoid.

I’m wondering if it might be worthwhile considering having the outer forks longer than the inner ones or some way to kind of buffer and contain opponents from the sides letting you whack away without them escaping the weapon path.

Ah, I see what you mean. Yeah that is probably worth mocking up. It’s worth mentioning I probably won’t be running more than 2 forks at a time as I don’t think I’ll have the weight for 4, so it would be a choice between running two inner forks (for drums and beaters) or two outer forks (for general use).

Done some more playing around with the design of the skirt. I’ve tidied up the front of the standard config, bringing all the fork mounts in line with each other and changing the front to be a two stepped wedge. I’ve had to tweak the division from the front/side and rear panels in order to keep each part fitting onto my print bed.

The anti-horizontal config then becomes the same, just without the forks as a solid wedge.

Finally, I also started designing a anti-top attack config, for hammer saws and Stratus.


I’ve gotten the lion’s share of Medway Gaming Festival prepwork done, so I’ve started turning my attention back to PMXL. I started printing off the cutting templates over the weekend, and mocked up some of the components using them. Surprisingly compact and snug, that weapon module takes up a lot of space!


Hype, I do hope you get to give this a spin in august, it’s a beautiful machine!

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Looks absolutely wonderful. Would be great to see it percussing!

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Rapture 2023 came and went, and with it, a delivery of CNC’d HDPE courtesy of Rob Weston! I immediately started work on getting back home and…

An initial assembly to get PoM sorted out! The drive still needs a bit of tuning to get all the omni-directional motion smooth, but it’s all working good, and quite nippy too.

Later on in the day, and I added all the chamfering and screw holes that couldn’t be done on the CNC, and made a proper wiring loom and link mount.

All that’s left now is to get the armour skirt printed, and sort out the hammer arm. I should have plenty of weight for those, as it weighs just under 1kg pictured!


This is so hype. Excited to see it in person soon!!

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Thanks Craig! After seeing Bop! and the other axebots at Rapture, it’s certainly galvanised me to get this done ASAP!

I fired up the printer today to start churning off the TPU parts. I’m still new to printing with TPU, so I kept a close watch on the print during the first layers…

And then kept checking in on the print every now and then…

Until this popped out at the other end of 8 hours!

The print did curl up slightly near the end, near the support at the back and on the corner, but it otherwise turned out great - 2mm walls, 0.6mm top/bottom and 20% infill. It actually came out a bit lighter than I expected.

And of course, I couldn’t resist a cheeky test mount! It likely won’t run with all the forks in battle, the two mounting positions are just to give me options.

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My printer has been working overtime to print the TPU for this bot. It finished the armour skirt earlier today!

The skirt adds quite a bit to the footprint of the bot! The TPU came out great, with only some very slight curling at one or two corners.

I also did a quick weight check - it’s US legal! Though if I add additional armour (e.g. for top attack bots) it will take it over. I’m hoping to get the last bits sorted this weekend for a full systems test.

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Got a little 3018 CNC last week and got it set up this weekend. Baby steps for now, but I’ve cut the first weapon arms (ignore the bit where I cut into one of the arms while cutting the second!)

I also got the last bits printed off, so the final assembly is here!

It’s ready to fight as it is, but the weapon is severely underpowered, torque-wise. I massively underestimated the torque requirements to get it to self-right, so after Sub Showdown I’ll need to do a fair bit of redesign to get a bigger weapon motor in there. For now, I’ve got some different pulleys on order to hopefully coax enough torque out of the system to at least self-right slowly.


Such a pretty looking bot! I always love a good omni drive.

Shame to hear about the self righting. The OpBoxes are listed as having a theoretical max torque of 7.94Nm, which, eyeballing your hammer at 20cm long, would give you enough force to lift 4 kilos. Obviously that’s theoretical, but it’s actual output must surely be pretty close to self righting!

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Yeah, that arm is just under 20cm. I thought I had done the number crunching to get something much higher than necessary, but I don’t fully understand torque! I assume the fact that pretty much all the weight is behind the pivot of the axe, it makes it more difficult. The forks will also not help by adding length on the other side of the pivot.

The pulley setup I tested was a simple 16T/16T, I’ve got a 30T pulley on order to make it 16T/30T. If that’s not enough, I think (after some internet lookup on pulley sizes) I can max out the pulley ratio at 12T/36T without changing any of the prints. We’ll see how that goes.

After a couple of attempts to correct the pulley ratio, I have now settled on a 16T/36T pulley combination, which gives the weapon enough beans to self-right with the forks attached. Unfortunately it can’t self-right if the weapon is extended when it is inverted, but hopefully that shouldn’t occur too often! The 36T pulley is the largest pulley I could fit into the weapon module without having to do a major redesign of the module.

The standard setup weighs around 1.4kg, which gives me a bit of room for additional armour for certain situations. I still need to finish printing the anti-horizontal skirt and the top armour package, and I need to prepare spare parts, but the bot itself is ready to go for Sub Showdown!

Here is a full systems check.

Sorted out the armour packages! First up is the anti-horizontal setup, which ditches the forks for a solid front. The skirt is slightly lower, and the top is raised by about 10mm to help protect the wheels from high overhead bars.

The top attack armour just adds 8mm TPU panels to the top of the chassis, and is compatible with both other armour configs.

The nice thing about the split skirt configuration is that I can set up so one side has forks and the other is set up for horizontals, in cases where I am fighting more than one bot at the same time.

With that, I’m pretty much all set for Sub Showdown, I just need to sort out some more spare parts and wait for some spring steel to arrive so I can cut out a weapon arm for use against horizontals.

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Phew! Sub Showdown has come and gone, so it’s time for a writeup! First of all, thanks to the BBB crew for putting on an amazing event, and seeing all the awesome new robots built by everyone else was great!

Joe Brown has been lightning fast with the upload, so you can watch all the fights here!

This year, Sub Showdown gave everyone 3 fights, a 3-way melee and 2 1v1s, with the top 8 going to eliminations. My first fight was Antithesis, a crusher, and Gra-b-ee, a horizontal grabber. I affixed the wide fork setup along with the TPU extra top armour accordingly. Knowing that my weapon wasn’t likely to do too much damage, I opted for an aggressive drive style I think I managed to do that quite well, either pushing one of them into the wall and repeatedly firing my hammer, or lining up hits while the other two were engaged with each other.

Also during that fight was this slick dodge:

Antithesis ended up in the pit as the timer ran out, leaving myself and Gr-b-ee in a split judges decision in my favour! Interestingly, BBB have shown the judges scorecards for this event, and it looks like aggression was what swung it for me!

Having never ran the weapon before in a fight, I decided to check it. Aside from some blunting of the axe head from hitting both opponents and the floor, the belt was also loose, so I tightened the tensioner (which may have resulted in something happening later…) and trued up the points on the axe head. I also changed the turning rates, as I felt I was oversteering a lot.

My second fight was against Blast Furnace, an axe/lifter multi weapon bot. I started by charging at them and got a couple of swings in before the bot lost all power. It briefly regained power when Blast Furnace lifted me up against a wall but it didn’t want to play.

I was concerned that there was some deep electrical issue, but eventually, I found that the fuse wire in the removable link was cut. I suspect that actually happened in the first fight, but the heat shrink was holding the two ends together just enough for a second fight to jostle it loose. The fuse wire was 15A, a holdover from the original Déjà Vu which ran a 3S battery with a max current of ~18A, so I couldn’t run any higher rated fuses. The 4S battery I use now goes up to 45.5A, so I made a new 30A removable link and all was good.

Fight 3 was against Strongerhold, a grabber that was a scaled-up ant that I’ve fought before. The slower low-end turn rate helped me here to keep the front pointed to Strongerhold for the most part. I lost the weapon belt halfway through the fight, I suspect because I had cranked down the tensioner earlier on in the day, but I managed to get a good push and got him into the pit.

I would have entered some whiteboards, but I started having transmitter problems - thankfully, with a factory reset and re-callibrating all the sticks/pots/switches, it appears to now be resolved.

Overall, I am very happy with the bot. It drives like a dream and the weapon didn’t immediately kill itself! I do want to upgrade the weapon to resolve the self-righting issues I had during testing, but I’m happy for it to continue fighting as is for now. Unfortunately, I didn’t get to test it against any spinners, but with Robodojo at the end of the month, there’ll be plenty of opportunities for that!


Hey, that’s cool that you got the scorecards and I appreciate you not going in for the kill with little one. She’s quite proud of little Grab-ee and was pleased he made it home in one piece.

Great sportsmanship - look forward to seeing you at the next event :slight_smile:

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