Come Here And Say That Again - build dairy

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Come Here And Say That was my first featherweight that actually got into an arena - it was built in an afternoon as a bit of a joke for the Inter Hackspace Robot Wars but went on to win it. Its since been to a few live events and survived getting thrown around by the floor flipper but never really did much to worry anyone competitively. It had two ride on scooter motors and gearboxes, a Sabertooth 2x25 ESC (which didn't die!), old school 3Ah Technobots nicads and salvaged 10mm polycarbonate. Eventually one of the motors died so now I'm building a slightly more serious replacement. Come Here and Say That Again will probably only be a little more competitive, as I'm trying to spend as little as possible again by repurposing some spares from Deadlock. As they're from a heavyweight they're a bit out of place in a feather, but it should be fun if it works.

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The new design is similar: a big wheeled wedge with a polycarb chassis but this time with a shock mounted hardox wedge, lifter hidden underneath, massive motors and go kart wheels. Hopefully this will also be a quick build since its relatively simple and I have almost everything on hand.

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These are the motors and batteries from Deadlock and the wheels from my ride on power tool drag racer - I'll be using two of each. Just the motors, batteries and wheels come to ~8kg, so weight is going to be tight.

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Some of the bits bought just for CHASTA

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I've just started the machining, these sprockets may ultimately end up in Deadlock, but for now they're being stolen as they also happen to give the right gear ratio for an overpowered feather.

This weekend I'm hoping to make the majority of the plastic parts.

I had a reasonably productive today today, making 8 of the chassis parts whilst snapping two cutting bits. More time was spent being frustrated with my poor Fusion360 CAM skills than actually making parts, so hopefully tomorrow will go better as I figure out the process.
One bit of weirdness I couldn't figure out that caused one of the snapped bits was the feed rate randomly increasing mid cut. They were regular 2d contour toolpaths and occasionally the feedrate would increase ~10x just for a few millimeters, then drop back to normal. The code looks fine in the mach3 toolpath viewer, but the I don't know really what I'm looking for.

I also need to get better at photographing transparent parts:
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So a heavyweight inside a featherweight? this is going to be awesome. If you're interested, Rory from team Nuts recently did something similar with scooter motors, he posted a video of it. I hope it enters next Insomnia!

Thanks!
Rory's is even dafter than mine, I think we're both going to Insomnia so it'll be interesting to see how they compare in the arena.

I got most of the plastic parts finished today, with no more snapped bits. I had been worried about the hubs fitting the wheels as they're such a 3d shape, they took a little dremel tweaking but now they fit fine:
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Traditional pretend-o-bot:
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So, it looks like I've made a daft mistake. I was checking the weight of each machined part and comparing it to the CAD model. All the machined parts are within 1% of what I expected, but the motors were way off - 450g in CAD when they're 1.8kg in reality! I don't know how it happened as they're correct weight in the Deadlock model. Regardless, I'm going to need some drastic weight saving measures to get this back on track. Current favourite is to ditch the kart wheels as they're the next heaviest part after the motors but I really want to keep the aesthetic of big fat wheels.
:-(

Does it bring you over the limit though? If it does, that would be an issue and in that instance, I'd probably agree with the wheel change or perhaps some weight saving holes in the armour (though wherever the latter will help substantially I don't know).

The CAD was projecting a weight of ~13.3kg without wiring, so yeah, its a problem. If I change nothing the CAD now predicts I'll be 2.5kg overweight. the whole polycarbonate chassis only weighs 2.6kg...

Ohh err, not good. I guess the wheels are pneumatic, not solid?

Have you thought about putting some small weight reduction holes in the chassis? Just don't overdo it to the point where it compromises the structural integrity. Should bring it down some, but other than that, I'm not sure what else to say. Thinner tires? Foam instead of thick rubber? Helium instead of air? Okay maybe not the last one, not sure if that's against the rules or not, haha.

If the chassis weighs 2.6kg and he needs to lose 2.5kg, drilling holes would leave him with virtually no chassis left :crazy:

What about making wheels from thin HDPE with tyre wrapped round them, akin to what Tough As Nails has? I imagine the kart tyres would weigh a fair bit, so doing that would save weight while keeping the aesthetic similar.

Well I did say some, not all. :B

Though that second part is a pretty neat idea.

perhaps you could get 3D printed hubs instead of metal ones? might save a bit of weight and im sure you could knock them up in CAD and get something pretty solid printed

Will you be using those dual board VESCs?
And those motors are 80kV on 5S?

They're nominally 50kv motors (though we measured them as closer to 70kv), running on 10s. Originally it was going to be two of Deadlock's 5s 5Ah packs, but with the weigh issue I'm going to drop down to a more sensible 2.2Ah but still at 10s. I'm using two of these vescs: http://www.ebay.co.uk/itm/VESC-New-4.../182714259982?, which is basically what we have in Deadlock at the moment.
At 50A it should be able to wheel spin with an opponent on top and have a top speed of ~25mph, but I think the arena's probably a little too small to reach that.
Does anyone know what size the Robochallenge arena is?

If those are 50kv motors you are going to hit the erpm limit on the vescs

Edit.. derp I am confusing myself on kv lol it will be fine

Robo Challenge arena is 3.6m x 4.8m if I remember rightly.

The wedge has arrived, the bends look good but its also heavier than expected. Sigh. The '3mm' Hardox (That I was expecting to be around 3.2mm) is actually 3.4mm. I've looked at the different wheel options and I think I can loose enough weight there for this and the motor mistake, fingers crossed...

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mixed progress this week, looking back lots has got done but its not gone that well and its going to be very tight for weight and time. I had a terribly unproductive weekend, making a bunch of mistakes, but the last couple of evenings have gone fairly well.

The closed cell polyethylene foam for the wheels arrived. The density feels about right to me. fairly stiff but with some give and should save 2kg:
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The lighter batteries also came, they save ~500g:
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I find it amazing that such small batteries can put out so many amps now. Hopefully these can perform to spec. With this change and the foam tyres, I think I'm about on track to be in weight. fingers crossed.

I've now countersunk the hardox wedge:
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machined the foam:
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made a lightweight plastic hub:
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trimmed the tread from the kart tyre:
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then stretched the tyre over the foam:
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The wheel's overly complicated but is lightweight and I think will do the job. I expect large chunks will get taken out by any spinners it meets but I think that's probably inevitable with big exposed wheels.

I've also machined the thick wall aluminium tube axle to take the wheel bearings and eclips:
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It doesn't feel substantial enough for such exposed wheels, but I'm hoping the fact the wheels are foam will limit how much force the axle actually sees.

Another little thing I'm trying that I thinks fairly new (which I originally wanted to use on Deadlock): a dual removable link with push button quick release pin:
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The sprung balls are locked out unless the button is pushed, so the link is locked in.

I've also machined the mounts for the wedge rubber
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and machined more dense foam to mount the batteries:
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I rushed this machining, taking much more aggressive cuts than the wheel foam (and forgot to use conventional milling, as that seems to work better on foam), so the finish is really rough but it should do the job.

The mounting brackets have now arrived, so I tried making a little 3d printed jig to help bend them. it worked perfectly for the first bend but then started cracking and yielding and subsequent bends weren't as good. I need to come up with a better way as I've got another 50 to bend...
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Here's how CHASTA is right now:
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So, getting there. Progress needs to speed up a little if I'm going to be done in time though.

Very nice work Mark, look forward to seeing it :)