Axes worthy of a Celtic legend
Updated: Nov 22, 2021
As mentioned previously, our featherweight Dullahan, takes its inspiration from the legendary figure of the same name. Originating from Celtic folklore, the Dullahan was often said to ride across the Scottish countryside, carrying a variety of possible weapons including a terrifying axe. This ghastly figure would spell death or doom to whoever entered its path.
To fully integrate this theme into our robot's design, we wanted to develop a powerful axe system that could really do some damage, while resembling the sort of weapon the mythical Dullahan would use. My team designed this wonderful bladed halberd style weapon, as well as an interchangeable scythe-esque pickaxe, which were waterjet cut from 6mm thick Hardox 450. The axe blades can be swapped out depending on our opponent - a pointed tip for sniping critical components, and a blunt weapon for fast repeated blows.
Packing a Punch
Meanwhile, I got to work on the system which would power this hammer. A popular choice for featherweight axes are large brushed scooter motors, although there are many different models with different power ratings and operating voltages. As most of these scooter motors are of the cheaper "made in China" variety, accessing accurate technical specifications online is often not possible, and this made choosing the correct motor more troublesome that it could have been. However, after much consideration I went with a 24V 350W motor - from researching builds from other roboteers, this seemed like a pretty high-end model. However, the caveat of using such a large motor is the hefty weight of 2.5kg which is almost a fifth of the entire robot! It dwarfs the size of any motor I've used before, including this 38mm beetleweight motor.
This beast runs on a 5s LiPo, and drives the hammer through a series of sprockets - a dual stage 7:1 reduction which was designed to keep Dullahan compact and protect the axe mechanism. Single stage reductions are usually more common in electric axe systems, however the very large driven sprocket must often be left unguarded and can be a weak spot in the design. The central HDPE guards in Dullahan prevent opposing robots from riding up the wedge and colliding with the sprockets, whilst also preventing the system from scraping against the ground when the robot is turned over.
Getting the chains down to the right number of links required some elbow grease and busted chain breakers however, as I chose a chain pitch of T8F, commonly used in Mini Motos. I had heard standard 8mm pitch chains were prone to snapping when used in a rigorous environment like combat robotics - T8F is heavier duty and fully compatible with 8mm pitch standard sprockets. It also provides more lateral tolerance when it comes to lining up corresponding stages, which is a great bonus. Slack in the chains was accounted for by some tensioners, which were m6 bolts with black nylon standoff washers.
To prevent the sprockets from moving along their shafts, it was decided that they would be welded in position. This included the sprocket on the motor itself, as the standard sprocket wasn't the same pitch as the rest of the system. Andy from ARDesign, the roboteer behind amazing machines like Telekinesis which I have fought in the past, generously agreed to help us out and did a cracking job. All that was left to do was press in some bronze bushings and clamp the shafts in place with shaft collars, and the axe was ready for testing!
You can see in the video how violent this weapon really is! I secured the system to my trusty workmate to prevent it from moving around too much, but the whole table jumped forward as the axe fired! That block of wood never stood a chance.
I feel like I can finally say I have a robot that really does the damage! The hammer is also an absolute hoot to operate, requiring near constant communication between the weapon operator and drive operator - certainly an intense test of co-operation!