My Achilles Wheel…

As I’ve previously mentioned my last Pi Wars bot was rudimentary to say the least. This time I wanted something that would push my skills further and also result in a better designed / built bot. There’s nothing like jumping in at the deep end so when I got my hands on the 3D printer last year, I had an idea. A mad, stupid, almost impossibly, ridiculously complex idea but hey – this is Pi Wars, right? I’ve kept this largely under my hat during the development of these as I honestly wasn’t sure I could get them to work…

MECANUM WHEELS!!

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So, I saw PiBorg and their mecanum wheeled robot at a couple of events and pretty much fell in love. This combined with witnessing the performance of the omni-directional bots last time was enough to convince me to attempt to do something similar. I looked around and found prices for mecanum wheels to be more than a little eye-watering and therefore decided to see if I could 3D print some. It became pretty clear, pretty quickly, to be honest that the full design is waaaaay beyond my capabilities so I adapted an original set I found on Thingiverse which were just so freakin’ cool.

The first frame appears on the printer…..

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They were designed for Lego motors originally so I adapted the hubs to work with Pololu 4mm hubs and gear motors. I also reconfigured the roller attachment to work with M3 bolts, rather than Lego axles.

Roller up…. Roller up.

Each frame takes around 7hrs to print at 0.2mm resolution – fortunately all four printed first time with no errors.

And now the fun really begins…. 4 Wheels. 8 rollers per wheel, 2 bolts per roller.

I’d originally intended to print in NinjaFlex – a really cool rubbery 3D printing filament but sadly it turned out my printer couldn’t use this so I had to get creative.

Printing the basic rollers….

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(Each roller takes 19 minutes to print)

I can print in PLA or ABS, both of which are basically solid plastic and therefore completely unsuitable to be used as rollers as they have no grip at all. So – a covering was required. To quote myself and many other Pi Wars contestants – ‘How hard can that be?’

As it turns out, very. Annoyingly, swearingly difficult actually. I have now tried and tested seven types of covering. That’s all 32 rollers removed, coated, replaced, tested. Often twice – just to be certain.

Hanging ’em out to dry….

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The list of what didn’t work includes…

  1. Balloons
  2. Heatshrink – thought this was guaranteed but failed completely!
  3. Rubber Cement – cracking idea from Graeme C. – sadly it peeled right off.
  4. Latex – dipping each roller many, many times to build up the layers. Very weird stuff – and ripped straight off.
  5. Grip Tape – stuff skateboarders use. Useless, wouldn’t adhere to rollers at all.
  6. PlastiDip – rubber coating stuff. Took ages, cost a fortune. And ripped off, again.

It can be seen from the above that coating and grippiness wasn’t an issue, durability was. So, as I was about to throw in the towel, Mark from Astro Designs came up with idea number seven…

7. Self Amalgamating Tape – aka plumbers tape.

And….. we have a winner!

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Here’re are the rollers wrapped with tape and ready for testing. It had become a proper love hate relationship with these damn wheels by this point in the process.

Thankfully, I think we’re looking good. Whilst the bot is ridiculously ‘slidey’ the wheels do actually work.

I conservatively estimate I have spent 100+ hours on the wheels alone so I am thrilled that I have a bot with ‘proper’ mecanum wheels. Whether this offers any advantage in the competition remains to be seen!

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The tape offers enough grip to get the bot to move and it’s durable enough not to tear straight off. It’s a complete riot to try and drive the thing however. It may yet be christened ‘Uncontrollabot’ !

It has of course made autonomous driving and the associated challenges much, much harder than they may well have been. But hey, we’ll cross that bridge when we get to it.

In about two weeks time :-O

Until next time….

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