Team for Crab! - Long Overdue Antweight Thread

I’ve been meaning to write up my various antweight builds so it’s time to stop procrastinating. This post will be a bit of a retroactive build log since it covers about 18 months of tweaks and improvements.

Time for Crab!

The first of my current stable of antweights is Time for Crab! This guy started off as a silly idea based around an N20 motor with a leadscrew. This worked well enough, but the fastest leadscrew motor I could find was still very slow and the 3D printed fitting at the other end was rather fragile.

Running this at our first mini robot event at Nottingham Hackspace it also became apparent that the clamshell body was a problem. It took a full 5 minutes to get the batteries in and out, and pinching the wiring was extremely when closing it up (especially in a hurry) was quite likely.

The slowness of the claw mechanism was the main weakness however so I ended up designing a new mechanism using some custom turned parts. The claws became symmetrical with a turnbuckle mechanism in between. Some hex barstock was turned down at both ends leaving a small hex section in the middle that serves as a hub for a gear. One side has a left hand thread and the other has a right hand thread so turning the middle section pulls in or pushes out the claws. There’s a floating gearbox that’s prevented from turning by the chassis which holds a low geared N20 that drives the claws.

Time for Crab! has evolved quite a bit over 18 months or so of events. It now has squishy TPU claws, with various geometry tweaks to improve the behaviour when grappling opponents, and the much needed hatch to make battery swaps and basic maintenance much less painful.

It occasionally gets zombified when the googly eyes take a hit.

Big Loader

Big Loader started off as an idea about using a dynamic centre of gravity to make a good lifter robot that would fit into a 4 inch cube, but eventually pivoted into a slightly silly robot that can flummox grabby control bots by twisting out of their grasp, but equally has had issues with unconventionally exiting the arena by climbing over the walls due to driver error.

The first version had the motors mounted in a tubular section that joined two drive pods, with a large gear built into it. The rest of the robot was an L shape that the tube ran through, with a scoop at the front and the electronics and battery in a box directly above the drive motors.

A motor under this electronics pod drove a pinion that rotated the drive section relative to the rest of the robot. Long wires to the motors would wrap around the drive tube and eventually prevent it rotating any further.

The idea was that as an opponent was lifter the centre of gravity would move backwards providing counterbalance to help lift an opponent of the same weight while fitting into the 4" cube in the resting position.

This never fully worked, partly because of the scoop design, but mostly because in the default position the robot would flip tilt backwards as it started moving. It was possible to work around this by rotating the drive pods 180 degrees after the start of a fight to move tracks behind the main body of the robot at which point it could still be a very effective pusher.

It took a couple of attempts at redesigning Big Loader before I settled on an alternate configuration worth building.

Big loader v2 kept the 360 degree rotation while losing the tall electronics box. It also got rid of the issue with twisting the motor wires by keeping the main chassis and motor pods in the same part of the robot with the passive scoop being the only part that rotates.

The motor pods and scoop arms are made from FR4 PCB material of various thicknesses to keep them as low profile as possible. 3D printed gears interlock with a 0.8mm FR4 scoop arm, and rotate around a 3D printed hub. The motor sits inside this hub and the drive pods plus a small spacer keep the scoop arms captive. It’s probably best illustrated with this all 3D printed prototype. The aluminium shims turned out to be overkill so the current version has PLA running on PLA which isn’t ideal but seems to work perfectly well.

The scoop is rotated by a pair of gears that were initially driven by a N20 with a T shaped gearbox, but every one of these I tried proved to be both very slow and very low torque so the chassis was redesigned to hold an N20 mounted transversely that drives a D shaped 3mm countershaft between the gears on either side. So far this has proved very effective, though there have been some issues keeping the motor in place in the new design (shown here with some battle damage courtesy of Dissector - which fortunately missed anything vital).

Big Loader has proved remarkably effective at twisting loose from grabby opponents, but has a weakness against spinners due to the thin and brittle FR4 parts and 3D printed scoop. I’ve recently been testing a TPU version of the scoop to try and avoid this.

The main remaining issue with Big Loader has been getting the scoop to stop rotating when using a standard radio, since it’s driven by an N20 with no endstops. This has led to a few embarassing defeats including rotating its way out of the arena before I could stop it at AWS 67!

Recently I’ve moved from using a Hitec Aurora 9 radio with a radio module to one converted from a cheap drone transmitter. One of the surprising benefits of the no-name radio is that it has a pair of shoulder buttons. Programming these so the throttle stick does nothing until a shoulder button is pressed, with the left shoulder button also reversing the throttle output, provides a way to drive the rotation with full control of the speed but the ability to instantly and reliably centre the stick hugely improved Big Loader’s performance at SCAR 6 (though it was hampered with various other technical issues) so I’m looking forwards to seeing how this works at the next event.


Mlemp is my newest antweight, and it evolved from a 4 wheel drive chassis I threw together and printed in a hurry as a loaner bot for Nottingham Hackspace’s Hacktvate tournament last year (under the name Cardinal, because of its redness). I printed it in TPU on a whim which has proven useful and it’s possible that in addition to being robust the squishyness helps traction by keeping all 4 wheels on the ground. Unfortunately since this was thrown together in a hurry it doesn’t use my usual antweight link PCB and instead I ran it with a pin header sticking out of the back which proved to be its downfall.

After Hacktivate I realised that the screws that attach the top and bottom parts of the robot would make a great mounting point for attachments, so with SCAR 6 coming up and the 4" cube restrictuon going out of the window I printed a variety of tongues out of some Mylar sheet I had lying around. The tongue attachments hopefully explain the name of the robot. :stuck_out_tongue:

The first attachment was inspired by a certain robot named after the 4" cube rule, and is named the Hoop of Shame, though its effectiveness is probably very situational.

The second attachment I’ve tried out so far is just a giant scoop made from an A5 sheet of Mylar. With this attachment I was able to bully a vertical bar spinner around the arena for all but 3 seconds of the fight (when I fed my own wheel into his spinner and had my link sniped shortly after), though it didn’t fare so well against face spinner Danger Cone which was able to grab the sheet of Mylar just so, yeeting Mlemp from the arena.

Since then I’ve built up an embarassingly large collection of experimental tongue attachments, which I’m hoping to test out at upcoming events (at least ones where they’re legal to use) and am currently printing a revised version of the chassis which puts the link somewhere safely out of the way and should close up better than the current version.

That brings me roughly up to date with antweight shenanigans. :slight_smile:


You’re the king of interesting weapon mechs at ants! Time for crab has a soft spot in my heart

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That hoop attachment reminds me of an idea I have for a beetle that would trap opponents using this weird foam rectangle I have lying around.

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A new robot has joined the squad.

This is Grump Truck and it’s an antweight version of ForkHead (which is a beetleweight version of Big Loader). The circle of life continues.


Another new robot, very different vibe.

This is Tri-Tongue. The idea entered my brain fully formed, name and all on Monday evening. Getting it built is the only way I can see to get that bit of my mind back.

I guess it’s a vertical spinner, optimised for maximum lick.

The tongues are cast from the same soft polyurethane rubber I bought for Grump Truck’s wheels, minus the black die, and overmolded on a TPU core. At least that’s plan A.

They’re cast in a 3 part printed mold and the first pour was semi-succesful. I keep underestimating how thick the stuff is when pouring, especially with the pot life being only a few minutes.


Is this a sportsman antweight, or is it a normal one? I imagine that spinner to be very… floppy, but I don’t have any experience with what it’s made of, it just looks like rubber. I guess it’s part of the them though.

It’s not really for a sportsman event, but I have the materials for casting wheels (and they have limited shelf life so they’re wasted if I don’t use them) and because if it works it should be fun to drive and entertaining to watch.

It also fits the informal theme of my robots so far, which is trying to make non-destructive robots that can stand their ground or be really annoying for spinners to fight. So far that’s been at least partially successful. :slight_smile:

The tongue is really soft bendy rubber, that also means it has a lot of grip so I’m hoping it will be capable of launching opponents.

The risk might be that it stretches too much when spun up and hits the chassis. That will definitely happen at some RPM, whether that gives me enough speed for it to be effective is the big question.

I’m trying to build a similar launching vert for Rock and Robots (the UK’s only regular sportsman ant event) with a standard gearmotor and some Lego Technic tyres I have lying around. If I’m lucky I won’t be turned around for bringing a spinner to an open top arena (despite it being a 3rd wheel strapped to the weapons joystick).

Still, a very interesting drum spinner you have there. Another one for your (possibly too) vast collection of antweights.

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Grump Truck had a great outing at June Ants.

Teamed up with Jack Franklin’s Luchador Jnr it went undefeated in the doubles tournanent, winning us first place and a free meal.

There were a few hiccups in the first fight - a tyre came loose and jammed up one side of the drive - but Jack was able to cover for me while I crab walked as far from the pit as I could get.

I fortunately left an extra position for the drive standoffs to I moved them away from the rear wheels to avoid that happening again.

Later fights went smoothly and we managed to pull off some good teamwork to control most of the opponents.

It took a tiny bit of damage to the drive pods against a horizontal spinner, but that was easily replaced and didn’t hinder its functionality any.

Meanwhile TriTongue was expectedly cursed.

I had an absolutely nightmare getting it ready, with all sorts of 3D printer issues I don’t usually experience doing their best to get in the way.

I printed a new chassis for it on Friday morning due to the original design not having space for all of the components. Getting everything installed in the new chassis I managed to over-countersink the screw holes in the pulley that drives the spinner, and the screws wore away the motor windings.

After swapping the motor out I had everything up and running on a FS-14X transmitter (I thought I’d use a non-DIY transmitter since it’s a spinner) and was introduced to the lovely world of FlySky. It turned out I had a newer receiver that apparently can’t be failsafed from the button on the receiver, so I had to borrow an I6S to set the failsafe.

Somewhere during that process the elevator channel went haywire, intermittently either outputting nothing or outputting the same value as the aileron channel.

At this point we had to get to the event, so I added some larger cast wheels to Time for Crab! as a backup full combat entry.

At the event I took that receiver out and I (well, mostly Sam) swapped it for a new one which fortunately worked.

I’m guessing some loose wire strands bridged between the two channels and damaged the elevator channel, but I couldn’t see any evidence of this when examining the old receiver.

Having the robot finally up ready (somewhat after the event had started) I was able just about cram the electronics in (with a tiny 120mAh battery) and enter it.

The first two fights were characterised by the drive belt for the spinner breaking and me eventually driving into a pit (in one case seconds after a very jammy escape from the pit area as it went down).

The third fight was similar, except I managed to not drive in the pit, and was able to get the tongue in contact with Twisted Lifter a few times flipping it over. Eventually a screw backed out from the tongue spinnern and the belt broke, but I was able to keep it away from the pit and eventually managed to prop Simon’s robot against the wall for its first victory.

All in all a great event. It was fun to bring some new robots instead of the old faithfuls and nice to see one of them achieving some success. :smiley:

Grump Truck will get a few tweaks ahead of AWS 71 next weekend. The mechanism that rotates the drive pods is binding up at some positions, which is annoying but clearly didn’t hinder its performance too much. I’ve printed a new set of gears for it with a slightly higher module that should allow me to leave a bit more clearance without skipping, so the tolerances can become less crucial.

If I get to it, TriTongue will get a new slightly wider and more spacious chassis to make fitting the electronics in much easier, and a bearing support to keep the brushless spinner motor in plane with the spinner - it’s currently mounted to a TPU bulkhead down the middle of the robot which is obviously suboptimal. I also need to look at the geometry of the front end to see if I can do a better job of feeding opponents into the tongue.

That said, when it did make contact with a suitable surface on the opponent it seemed to work as expected, gripping them and licking them into the air.


Great work at the weekend Steve! Thanks for being a great 2v2 partner :smiley:

Grump Truck is a fantastic little bot - I’m a big fan of it (and forkhead).
TriTongue howevr, is absolutely cursed.I really cant work out if I love it or hate it :rofl:.

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