jes's build log

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jes
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jes's build log

Post by jes »

Thought I'd try and document the build of my first antweight.

I'm going for a 4wd wedge pusher, and part of the plan is that it should work upside down just as well as it does rightway up, with a switch on the controller to invert the drive direction.

I spent some time in FreeCAD last night and came up with this:

Image

The chassis is made of 2 identical halves that bolt together, and the wedge at the front is free to pivot so that it always touches the ground. The motors are just held in place by the chassis being bolted together, there's no actual "motor mount" as such.

I also plan to add some horns that stick up on "top" (whichever side that is) to stop other robots from just driving over the top. Basically a double-ended horn that rides in a slot inside, so that the weight of the robot pushes the top horn up, and the bottom horn rides against the ground.

So far I'm estimating 160g without any horns, and that's at some pretty dubious 3d print settings (5% infill, only 1 perimeter) so I probably have to design the chassis a bit more sparsely.

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MarkR
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Re: jes's build log

Post by MarkR »

That looks like a pretty slick design, 4wd pushers / wedges always seem to be quite competitive despite being one of the most "Boring" setups :)

I like the creative idea of clamping two halves of the shell together. I also use a 3d printed shell to hold drive and weapon motors and for the most part it works ok.

The biggest problem I've had with that setup is that it's tricky to repair after damage, you may want to take some spare chassis halves to the event in the case of a really big hit which smashes the whole thing up.

Also, wheel guards are a bit of a double-edged sword, because a 4wd pusher can lose more drive from one jammed wheel than from one missing wheel, many times wheel guards contribute to a loss. Also stuff like debris can wedge itself in the wheel guard and lock the drive.

Mark
Robots: Betsie - RaspberryPi controlled flipper bot with gyro stablisation - too clever for her own good?
Stacie - tidy flipper; 4wd driven by hair bands

MySolderIsOlder
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Re: jes's build log

Post by MySolderIsOlder »

Also... watch out for flat/square sides. In a fight it's surprisingly easy to get stuck on one side or the other with all four wheels spinning to no avail as the room counts you out. For my 'bog-standard 4wd wedge' bots I like to stick a couple of machine screws in on either side, protruding just enough to prevent a stranding. Weighs next to nothing, works a treat and easy to replace if one gets pinged off by a spinner.
Stuart (Anthony's dad)

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jes
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Re: jes's build log

Post by jes »

MarkR wrote:
Mon Jan 13, 2020 2:21 pm
wheel guards are a bit of a double-edged sword, because a 4wd pusher can lose more drive from one jammed wheel than from one missing wheel
This is an interesting point! I had assumed that wheelguards were a no-brainer, might have to rethink that.
MySolderIsOlder wrote:
Mon Jan 13, 2020 4:51 pm
Also... watch out for flat/square sides.
It's quite hard to see in the picture, but the rear and sides are rounded for precisely this reason :). They're perfectly straight horizontally, but curved from top to bottom. If it gets flipped onto the back or the side, it should roll one way or the other back on to its wheels.

I've found that it's quite hard to keep the motors and wiring in the right place while fitting the top half of the chassis on. I saw some cable-tied motors in another thread (peterwaller's, perhaps?) so it might be worth making some cut-outs in the chassis for cable ties to hold the motors in place while the chassis is split in half.

It's almost ready to bolt up for the first test drive (albeit without a wedge yet), just printing some temporary wheels at the moment.

Image

As you can see it's a pretty godawful print, partly because there's hardly any infill and only 1 perimeter, and partly because the PETG doesn't seem to extrude very predictably in my printer. I only started using PETG last week after a couple of years of printing PLA, and I'm not sure I like it so far. I have some Taulman 910 coming soon so hopefully I can just abandon PETG and start getting the hang of Taulman 910.

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Kyro
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Re: jes's build log

Post by Kyro »

i use cable ties for my motor mounts...

but seeing as you just need your motors to be held in place while you attach the second half, i suggest using some double sided tape...

i personally use carpet tape (for attaching acetate) as it holds far better than normal double sided tape does although this is probably overkill for you to just hold some motors in place while you attach your second half...

regarding wheel guards, i use them on my bots, but i have enough clearance so that if a tyre gets knocked off, it won't jam up the whole wheel...


a last piece of advice, i would save the weight at the back of your bot by removing the part that covers the wheels... basically have the treads fully exposed... this allows for you to always have wheels touching the ground even if you end up far up someones wedge, enabling you to be able to push back/drive off...

hope this helps

Luke
Team Rocket
Esper(4wd flipper)
Mantis (4wd grab 'n' lift)
Dominus (4wd SPINNAAH)
Fennec(4wd flea and 1/2 cluster)
Gizmo (4wd flea and 1/2 cluster)

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LimaHotel
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Re: jes's build log

Post by LimaHotel »

Oh, I absolutely second making sure you have full rear-wheel ground contact. This will allow you to reverse off a Wedge that may get under you instead of being high centered and helpless.

Regarding printing, I know shakey had success with abs and I know Peter has success with nylon filaments. I personally prefer just cutting and bolting polycarb, as then it feels like I've actually made something instead of just producing it, so I've never actually printed anything and so take my "knowledge" with a pinch of salt.

I also use cable ties for... Probably too much really
Jiggy Sawdust
Jammin' Good with Weird and Flippy
The Pushbot from Mars

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peterwaller
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Re: jes's build log

Post by peterwaller »

Just to help stress the point about ensuring there is still contact between the drive and the arena when tipped back here are a couple of mine where the armour gives good protection but the tyre still sticks out the back.
Image

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jes
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Re: jes's build log

Post by jes »

Ah, if cable ties are good enough to use for the real motor mount then I might be able to save a bunch of weight. I'm not married to the idea of using identical chassis halves, so I could have a lower half that the motor cable-ties to, and an upper half that is just hollow inside. Worth considering.

And exposing the tyres at the back is a splendid idea! I hadn't thought of that, thanks all. I certainly will do this and have already done a first iteration of modelling it.

Image

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jes
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Re: jes's build log

Post by jes »

Haven't printed any more parts yet, but this is the latest CAD:

Image

I don't think I'll have space/weight for the retractable horns idea, so I've put a step in the wedge so that robots can slide part of the way up the front and then get caught on the step so that they can be pushed into the pit. It's curved so that they get nudged towards the centre of the wedge instead of sliding off the side.

Currently the wheels are plastic and not very grippy, but I intend to make hubs out of PETG or Nylon, with silicone rubber tyres moulded over them.

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peterwaller
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Re: jes's build log

Post by peterwaller »

That is looking good.
You are obviously putting a lot of thought into the design, hopefully will be seeing at an event soon.

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