Snail Cam Spring Flipper - FW

In the oh so distant past of 2017, when I was just getting started in the world of combat robots, I found this video on Facebook of a cam driven, spring powered hammer. Combat Robotics | Facebook
I saved it as part of an inspiration folder for the future, but I didn’t really want to build a hammer. A flipper on the other hand seemed far more interesting. So I went to YouTube and found a video of what I wanted. garage door spring winch powered flipper - YouTube
That video has been stuck in my head for almost 6 years, so I figured… Hey, I can do that, right?

After a quick google, the general idea is far from a new one. In fact, the Ask Aaron forum had whole pages dedicated to answering questions about this, including one with the exact video I had seen on YouTube and an older, but better designed version of a snail cam flipper. Most usefully, it had a template for the ideal snail cam shape, which I shamelessly downloaded for my own use. Combat Robot Weapons - The Ask Aaron Archives

My actual designing begins, as I’m sure many of yours do, with quick random sketching. I threw this together in about 10 minutes to send to the rest of the team to show them the not-at-all-thought-out ideas I would drag them into building.

Once I had the team thoroughly convinced, or at least not completely opposed, it was time to get to work. I’m sure a lot of people would say the smart thing to do is open SolidWorks or Fusion 360 and make a CAD model. Not me! I don’t have any patience and want to see immediate physical results. So out to the garage I went, and in tradition Team Scrappers style, grabbed some scrap wood and set to work building a prototype.

First up, the snail cam itself traced from the image on the Ask Aaron forum. Initially I made this way too big, so drew a line 1cm in from the edge all around and slimmed it down a little to the size in the picture below.

This was followed quickly by the first of three test flipper arms and a frame to attach it to, to give me the physical representation of the whole system with which to mess around and find the right geometry.

Of course you never get things right first time, after the aforementioned slimming of the cam and taking a few measurements it turned out I had basically built a seesaw. Pivot in the middle of the arm, near equal distance of movement on both sides. Now sure, I knew before I started that this first arm wasn’t close to the right shape. But it was the first piece of progress I could present to the team, and it wasn’t exactly impressive.

No worries, I’ll just make another arm of course. Having already built one, this time was speedy, all done and made in under 15 minutes. Move the pivot and the cam runner back a bit, see what happens. Well, I wasn’t actually calculating how far back I could move it. I figured I needed to leave space for the cam to fit in the gap between the runner and spring attachment, so I just moved it a few centimetres. Test again, measure again. Improvement! Not much, but some. I was headed in the right direction.

One more go, move everything as far back on the arm as possible, and actually check measurements rather than guessing this time. Ah-ha! Much more successful. The actual flipping side now moves twice as much as the spring side.

So that’s where we are after day 1 of building. A weapon test that has a geometry that I’m at least satisfied with, enough spares in the garage to build a chassis and drive system to go alongside it.
The only things left to figure out before we go all out on the building is which spring do we need to power the flipper, how large a ratio gearbox is needed to turn the cam drive, and which events can we take it to when it’s built. As I write this I’ve just heard that Extreme Robots aren’t hosting featherweights this year, and most other events are hard to get to from the east coast of Norfolk.

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Will be cool to see this mech in action! Keep at it.

Yes unfortunate timing with XR ditching feathers last night, the remaining feather events are certainly not currently anywhere East, I think more Midlands stuff may pop up again soon though!

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Cool to see more spring flippers about! I ended up going down the snail cam rabbit hole for Léim Thart. I messed up a little with spring choice, but the overall idea is solid so long as you can get a strong enough motor do to the tensioning without stripping or stalling, which is the real hard part.

If you haven’t read it yet, I’d highly suggest giving the build log of Total Recoil a read. It’s an Australian robot that ended up going for a snail cam-driven FW flipper. There should be vids on his YT channel as well. It’s probably the closest build out there to what you’d be looking for.

And for reference here’s what the robot looks like in 2022: Combat Robotics | Facebook

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No longer having the goal of Brentwood to aim for, progress has slowed down quite a lot on the as yet unnamed flipper. On the other hand, the team is now entirely on board with the idea after I waved enough rough cuts of wood at them.

To start off, a few new parts have arrived. Most importantly, a selection of springs to play with to provide the flip. Hopefully one of them will be just perfect and I won’t have to spend any more time staring at data sheets that I can only pretend to understand.

Not to mention the ball bearing castor, a sign of lessons learned from running wide wedges on our previous bots. It turns out two wheeled bots want to ride on three points (who could’ve seen that coming) and dragging the whole front of the bot on the floor usually gives awful driving characteristics (again, who could’ve guessed).
Given that the only part we need on the floor is the wheels and the flipper arm, let’s hope actually thinking about how it’ll drive pays off.

It’s at this point that I must admit that I have managed to misplace the planned drive motors and gearboxes in my oh so definitely tidy garage. Somewhere in there is a pair of new in box Banebots P60s and RS775 motors, originally bought as spares for the (admittedly slightly rusty) drive of B.A.W 2.1.

But, that’s all good. I can just measure the set currently bolted into B.A.W. 2.1 and go from there. With both having the same drive system it turns out that, with a little adjustment of the arm test rig, the floorplan of this bot is almost identical to B.A.W 2.1, coming in at 360mm x 400mm.

This will be tightened up when we have the components finalised, and clearly has a fair bit of empty space in the front end, so given the chance I’ll almost certainly cut it down to a more traditionally UK wedge flipperbot shape. But before we go any smaller there’s one part to get that I haven’t been able to make a decision on, the gearbox to turn the snail cam.

There’s two steps to deciding on the weapon gearbox/motor. Number one, the ratio of the actual gearbox itself, and number two, how much we want to gear it down with a belt or chain.
We’ve only worked with chain reduction in our heavyweight Ill-Tempered Mutt, and I can’t say I’m much of a fan of it (the drive chains are still twisted from pre-pandemic (I swear I’ll get around to fixing it one day)). My preference is to do as much of the work as possible in gearbox, however, a bigger ratio obviously means a longer motor/gearbox. So the next step is to do some searching and measuring to see what we can fit without making the bot bigger.

On a totally separate note, I managed to dig the robot formally known as Cicatrix out of the pile its been under since I bought it years ago. I had bought it with the intention of learning how to run a 4 wheel drive unkillable brick, but a broken gearbox and world events put a delay on that. Maybe once this flipper is built, that’ll be next on our list.

Love it. I wouldn’t have associated plywood prototypes/trial and error designing with a snail cam flipper but you’re nailing it there I think!
Will you have the flipper running continuously or will you have a position switch to stop it when it’s “cocked”? We tried running Keith the Teeth (spring cam grabber) in full auto mode the first time out and it proved visually amusing but functionally unsuccessful.

Thanks. I tend to struggle with visualising things when they’re in CAD, so I like to build models out of scrap stuff I have laying around before CADing anything that actually needs it.
For the sake is cheap/easiness, I’ll probably set it up similar to a drive motor so I have to manually get it back to “cocked”. Once I’m satisfied all the hardware works and it won’t rip itself apart, I’ll invest in a position switch.

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