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Slingshot - Demon Designs - NSW

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Slingshot - Demon Designs - NSW

After many years of saying next year, Here's the build report for my full combat co2 flipper bot, Slingshot.


Drive: Banebots p60, 70mm wheels and 930kv brushless motors
Armour: 6mm hardox front and sides, everywhere else is 3mm with a 3mm ti base and carbon fibre tops.
Electronics: Redbrick 70A
Weapon: 50mm bore full pressure co2 flipper with integrated 1/2" valve.

First port of call was to make the 3mm titanium base plate. This was half done on my CNC router as a semi successful experiment and finalised on the milling machine.

Base with drives attached and a 3dp mockup of the battery bay and cylinder to get an idea of how large it would be - turns out it's quite small. Wiring it will be fun. sigh Razz

Each of my flippers (citizen flips, polaris etc) has had a major reduction in weight each time, and slingshot is no different. A lot of these components are extremely overkill so quite a bit of weight can be removed in non critical areas. This solenoid coil for the valve is one. There's a very heavy resin outer case with a 5mm steel inner piece. I can accept the increase in current from removing the steel.

Can't recall how much lighter it was but at least 100g and much smaller.

More pretendo-bot. The main tank is a 12oz, the buffer will be a cut up and rewelded soda stream cylinder.

Good thing about having the robot fully designed before you start is building becomes a matter of just mindlessly making the bits. The part on the left is the end cap for the buffer tank, i threadmilled the holes in for the dump valve and there's an m8x1 thread in the other side for a banjo fitting that comes off the main tank feed.

On the right is the 3mm titanium battery bay i was planning to weld to the base plate.

Until i made a dumb and snapped it. I tried to use a radius die in my metal bender and it drew into the thinnest section. Bit of a bummer but live and learn.

The replacement is CNC routered 3mm aluminium that bolts on. I quite like removable elements as it leaves you with some wiggle room later on if you want to change components. Especially handy for batteries where you can rarely buy the same model year to year..

Put some work in to get the settings right for carbon fibre. A tricky material but was extremely happy with the results. This is just hobbyking chinesium carbon fibre.

Slowly accruing parts. A couple of frame assembly jigs, mounting tabs etc in there.

Removing more weight from the flipper system. This is the mid section of the valve, the original is a giant brass piece. Those grooves for the o-rings are 1mm too. tricky as to make!

Heavy brass part.

Much weight saved

Pretendo bot 3. Things are coming together and there is progressively less space for the wiring - eek. My plan to run the main tank gas line over the wheel is starting to look like a bit of a dud here as well.

The buffer tank end in place, and you can see why the coil had to be chopped down, its almost flush with the base.

Starting on the cylinder base with the valve integrated within. Decided to use 2011 grade alloy which is a bit more brittle than 6061 but it machines far better. This is the first operation done.

Next its removed and chucked up with the inside jaws in the lathe and the rear is faced to the correct thickness.

The new design allowed me to do a nice 3d milled smooth gas entry port. Polaris just has a sideways hole. That extra hole with the boss to the top left is for a planned vent solenoid that i never ended up making.

Another operation, this time to bore the valve body internals. Gotta get that 5th axis made some day.

Final weight saving operation, being machined on a fixture.

The entire ram base. 50g or so more from the base itself could be removed and maybe 10g by redoing the piston from Aluminium. But this is already about 300g lighter than polaris' setup. Very happy with it.

Little Cylinder being lathed from 2011 aluminium. The inside was bored with a massive boring bar and a special finishing insert. Far less taper than was inside citizen flips and polaris' cylinder.

Polaris on the left and slingshot on the right. The new setup is much much smaller and not a significant amount less powerful really.

Vent holes and a UHMWPE bushing made. I've tried tons of plastics but they all inevitably swell or go brittle with the freezing temperatures and moisture present inside the system. The UHMW does very well here.

Can also see the shoulder bolt at the bottom. This serves a double duty of bolting the buffer tank on and being an anchor point for the bungee cords.

With the ram itself done, on to the buffer tank. This part is the inlet stub that gets welded to the side of the buffer. The other end slots into a hole in the cylinder base and two o rings seal it up. Nice and radiused for good flow of course Smile

Don't know if having it flush or protruding makes a difference to flow. Maybe one day I'll do some CFD analysis on it for laughs.

The buffer in place. The stub and mounting tabs get welded to the bottle, and bolt sideways to the cylinder. This takes the load off the actual stub and allows it to be far lighter.

Polaris uses a giant 3/4 inch steel threaded bung to attach the bottle to the ram.

Hardox from the laser cutter arrives. Gotta get it all ground down for TIG welding.

Titanium base fits just lovely. First job is to weld on the mounting tabs.

The basic frame tacked together. These are the critical elements, the rest of the frame and armour doesn't need to be accurately welded in, as long as these parts are!

Can see how the cylinder base bolts to the side walls and the ti base to strengthen the chassis.

Welding the tabs on for the lid. I didn't use rivnuts here as i wanted the tig practice. its been a long time Razz Standard m6 nuts fuse welded on, works pretty well.

All the armour panels tacked up. Hooray for CAD.

Last bit of fab for the frame was to roll some main bottle mounts from 1.6mm steel.

Most of the bot assembled, along with the flipper arm. I like to weld in sections and let it all cool down to minimise warping, so the frame took most of the day to weld while i went and did some other jobs.

First fitup of the gas works. Everything pneumatic stays bolted inside the chassis while the baseplate with the electronics and drives comes out. I was hoping this would aid in repairs. Its a nice system.

My 8mm rod didn't show up so before welding weekend, so these $2 screwdrivers will do fine. Genuine chrome vanadium!

One thing that was missed in the cad was the big knob (hehe) on the bottle was hitting part of the frame as it was unscrewed. Turns out that's a big waste of weight anyway. So i chopped the end off leaving a square stub, and printed some square drive screwdrivers to turn the valve. Worked very well.

So things turned fail here. My mill was playing up and the tools were slipping out of the tool holders on cuts that weren't even that heavy. Here the cutter yanked out on the finishing pass and busted through into a bolt hole causing it to leak aggressively. Fortunately araldite patched it up good enough.

The entire Cylinder and buffer unit assembled and ready. Dumhed on here welded the tank up for me Smile

Even had time to 3d print some little take aways for the kids this year. Gotta make a stack more next time, they disappeared real quick :O

Wiring time. The brushless drives are so efficient now days i was able to wire the robot with 16 gauge wire and use a tiny tiny fingertech power switch to save more weight and space.

This aluminium plate holds the reciever and the flipper micro-controller. I was hoping to eliminate this with a taranis radio but that never showed up before the event. Next time!

My 8mm rod finally turned up so that became the safety lock bar, with scanned fist topper!

Testing time was a.. partial success and fail. the bot drove great and the flipper has a lot of punch. But it would jam occasionally. Dang. Reason is the angle is so great at the top of its stroke that the piston is jackknifed in its bore. That angle needs to be reduced.

And to do that, the flipper arm needs some surgery. Using a dremel the top plate is trimmed open and bent

This adjustment makes the angle equal but in opposite directions when the flipper is all the way down, and up, keeping the maximum angle between the piston end and flipper arm to a minimum.

Fixed. Looks like a little rally hood scoop now.

I also made a second piston with a wider piston ring at the bottom this time to reduce the possibility for the piston to jackknife.

Wedges ground hellaflush

Gooooold! That dulux gold paint is so good. 10/10

And the insides wired. Still a bit of a hack really. wasn't too happy with it but it did the job in the end.

Lucky i noticed this one while doing the final assembly, My diode was blown up! leaving it this way would have killed the flipper ESC for sure.

Done and ready to be packed.

Spare arm with its loosely bolted on tongue. I was going to use this one against any robot that had some super low wedgey bits. Probably could have tried it against abomination but don't think it would have made much difference.

These little faces go in between the chassis rails in the middle to try stop the wiring falling fowards into the path of the arm. I was gonna have a couple of different expressions but time ran out.

And last is his safety stand, Routered that out while the uber to the airport was coming haha. Gotta use those scraps somewhere.
www.demon50s.com - Minimoto parts
http://www.youtube.com/user/HyzerGlen - Videoooozzz

Post Wed Nov 13, 2019 8:56 pm 
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Joined: 07 Jul 2004
Posts: 4435
Location: Sydney

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Awesome write up,
that stand though, too professional
Mechanical engineers build weapons, civil engineers build targets

Post Thu Nov 14, 2019 1:29 pm 
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Joined: 15 Jun 2004
Posts: 8511
Location: NSW

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Yay for quality build reports!

Whats on the list of improvements for the next version?
Team Knightrous

Post Sun Nov 17, 2019 12:20 pm 
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