First Ever Robot Build: Invertible Featherweight Vertical Disc Spinner

[TheNeonMatt]

This is my first ever robot build and after reading a bit on this forum and watching some featherweight videos on youtube I have decided to try and build (as the title says) a featherweight disc spinner that is also invertible. So far I have decided to use two cheap 12V 550W drill motors for drive and probably two Turnigy 85A ESCs that I plan to try and hack myself, not yet sure on the weapon stats as I'm planning to attach everything drive wise to a chopping board as a test run and work from there.. I don't know the dimensions of all the internals but once I have them I will adjust my google sketchup plans to include them. Any help on materials and thicknesses would be extremely useful as I haven't sorted any of that out yet, (I only have basic tools at the moment so was looking at a HDPE structure with metal plating in places, I'm also planning to get a drill mount/pillar drill to make accurate holes)


Here are my basic plans that let me get the shape and rough size and I'm also planning to add a hinged wedge to the back:

Any help on the design, components, and weapon would be appreciated.

[Niels - Team K.a.L.M.]

First up, welcome.

Try placing the dics and the wheels closer together. This will reduce the gyro effects a little.

[TheNeonMatt]

Thanks for the advice

I saw them talking about that with Archangel in the Hardwire 2 Build diary and have tried to bring them closer but it will either mean I have to make a smaller disc or a steeper angle on the wedge. Does a particular angle work better on a wedge? Also how does the disc size relate to the hit? I understand how weight affects rpm as in more weight is harder to spin up to speed but I've heard stuff like bite (I think is theoretical amount of the tooth on the opponent), KE (I'm guessing is kinetic energy of the disc) and KV but I don't fully understand how they relate to one another.

Finally would a larger base help stop gyro or am I looking at it wrong?

[maddox10]

Hi NeonMatt.

On the pillar drill. Even when you're not building a robot, it's a dandy tool to have.
But, don't buy too small. The £75 versions from Ebay or the DIY are capable enough for basic feathers or smaller machines. But the small motor will struggle with the better materials or bigger holes in metal. Especialy when using cheap and/or blunt drills. Better to safe up for a bigger one with a 500W or better motor.

On the TZ85's. For a basic drill drive, the BotBitz 30 A speedo is more than good enough.
Also, I myself buy TZ85's from BotBitz. Not worth my time to reprogram those myself.


KV stands for rpm per volt input. A 1000 KV motor will run 5000 rpm on 5V.

Kinetic energy aka KE. The faster a weight is moving, the more force will be in that object. A slow light disk will contain less force than a fast heavy disk.
Keep the same speed and double the weight, and the force contained in the disk will be doubled.
Kep the weight and double the speed, and the force contained in the disk will quadruple.

Wheel placement and such. In a vertical spinner the wider the wheels from each other, the less the gyro effect. The closer to the axle of the disk, the more weight on the wheels, so more control/torque availabe.

[Max]

Sorry for the thread hijack but I've got a 650W pillar drill and it struggles even drilling a 6mm hole in mild steel, the motor itself does not stall but the belts slip even in it's lowest gear ratio. Any ideas why this is happening? Should I replace the belts with slightly smaller ones?

[harry hills]

Either that or add a tensioner.

[RogueTwoRobots]

Has your pillar drill motor got a spring/tightening bolt on it? On my 500W pillar drill, the motor is on a hinge. When I want to change the belt ratio, I slacken the bolt and it releases the tension from the system and the motor hinges towards the front of the drill. Once I've moved the belt onto the other pulleys, I push the motor back against the spring, re-tensioning the transmission, then tighten up the bolt which holds the motor in place.

My drill was slipping at first until I got the hang of that, but now it gets through steel without issue.

[Flag Captured]

Welcome!

Personally, I'd have a good look at Gary (typhoon_driver)'s machine Hornet 2 because it reminds me of that sort of design, may be able to pick up a few ideas there!

I do feel like the wheels are a little too far back, perhaps you could shorten the bulkheads and beef up the body a little? The disc can easily cut into the body shape as long as you design it right. Archangel's will be doing that for the most part, to try and get everything as compact as I can to save on weight, I guess. Also means that all the weight is on top of the wheels and so you'll get more grip, Will (Team_Shock) wrote loads of helpful tips from his experience with the various versions of Inertia XL on my build thread, definitely read that if you can find it (amongst the posts of me pretending progress is happening when it really isn't)

S3 from robot wars could be a nice design start-point too, now that I think about it, may be a little too big if the disc you want is larger, but it's certainly something to look at. What size/speed weapon would you want on this? That definitely is something you'll want to decide on before going too far with your design, given you'll need to contain all the energy in it too - it's all well and good having a monster of a weapon, but you need to make sure your bulkheads are up to the task too, so that it doesn't chew itself up after a few hits.

What sort of dimensions and all that were you thinking for the disc/wheels too? That might help out a starting point on exactly what parts you'll need for it all. For reference, Inertia XL's disc is about 300mm diameter and I believe that Hornet 2's was pretty similar. I think Maddox's Valkiri is one of the bigger verticals about at around 400mm on the disc, and most of those are about 2-3kg in weight from what I remember hearing. May be of some use to you when designing, possibly!

[typhoon_driver]

Hornet 2 had a 250mm from tooth to tooth disc and then I made it a single tooth. First one to do so Only had a 2kg disc but then it was designed for the days of 12kg featherweights.

I would make the angle with the bulkhead less accute and bring the axis of rotation of the disc closer to the axis of rotation of the wheels. With hornet 2 I mounted the drill motors on the outside. Everyone said it was a bad idea, no one ever took one out. The issue with such a design is that the vertical bulkheads have to be bloody strong to take side impacts as I discovered to my detriment. Hornet 2 was 750mm wide which is bloody wide for a featherweight. It was good on one on ones but terrible in melee matches with more than 2 robots in the arena because of such a huge turning circle. I wouldn't bother with the hinge etc at the back. More to go wrong and hang you up on.

[TheNeonMatt]

Thanks for the advice guys, I'm currently trying to re-cad the robot with as many of the internals as possible and resizing it to what you have all suggested. I have found some drill mounts to put inside but the way the screws/bolts go in is confusing me because the ones coming from below/above will get in the way of those coming from the side. On the outside I have increased the angle on the robot from 30 degrees to 40 degrees so if the disc is already around the 300mm mark it will be able to come towards the wheels more otherwise I'm going to have to rework it again. The idea of the wedge was to act as a backup in case the disc stops working and it also puts some more weight over the wheels. The bulkheads where going to be 20mm HDPE is that strong enough to take attack/hold a disc. Does Inertia XL have a build diary because I'm sure there would be some really useful information in there?

Forgot to mention that I'm going for Banebots wheels I think because they are easy to screw onto drill motors and they are about 98mm in diameter

[typhoon_driver]

20mm HDPE - no
20mm nylon - yes BUT I would do what I did with hornet 2 and have hardox or steel braces around the front to strengthen it.

I could have fitted the drills inside but putting them outside gave the machine more ground clearance and more space inside.

[TheNeonMatt]

Just got a couple of quick questions:

Does this switch count as a removable link or does it need to be a whole section of circuitry?
http://www.fingertechrobotics.com/proddetail.php?prod=ft-mini-switch

Does a smaller disc work as well as a bigger one if the rpm is higher? When does the disc become pointlessly small/should be replace with a drum for a better attack?

Quick CAD update:
I found that the original disc was ~250mm, so the disc will have to be smaller (hence above question). I would like to keep the robot within ~45 degrees because I think that otherwise it will start to look too tall.

[Niels - Team K.a.L.M.]

That switch is against FRA rules as used in europe.
The Australians and the americans do use them.

The FRA rules specify that a link has to be removed from the machine as a clear way of disarming the machine.
An action that is aided by the use of the indication LED that will turn off once the switch is removed.

[Flag Captured]

Well, the way I see it is that if you get a larger disc with a slower rpm, and a smaller one with a faster rpm, they may well have the same tip speed but at lower rpm ranges the disc will get more bite and engage more frequently, if that makes sense?

It all depends on your design though, you want to maximise bit as much as you can - even though 720's drum spins at about 15k and is only about 100mm diameter, it has a single tooth design and well over 10mm of bite if I recall. That plus the speed it runs into other machines at means it can get monstrously large hits. (Again, all what I've heard, apologies if any of that is wrong!)

If your disc ends up pushing 10-12,000 rpm then I'd consider a larger disc to be honest. There's lots of variables though like weight and how much of the mass is towards the outer ring... That's why Archangels bar has those heavy tool steel teeth, it contains more energy at lower RPMs, so may get similar/better performance than one with more equal weight across the bar hit-wise but will also have the perk of getting regular engagements.

Theoretically, I think anyway. The extent of my robot building really goes as far as the computer game, so I'll probably need someone to verify haha.

[typhoon_driver]

ha 720 would get nowhere near 10mm of bite unless it was moving forward at a rediculous speed *waits for smart arsed comment from mouldy*

Tooth engagement is a balance between forward velocity and rotational velocity. I wrote a big post on it a while ago.

Energy in a disc is all about the moment of inertia of the disc. A useful tool to work it out is here, granted it's all in US units but you can do a quick conversion

http://www.teamcosmos.com/ke/ke.shtml

For reference, the original hypnodisc had around 6kJ of kinetic energy, typhoon 2 could store 40kJ, boner's drum when it was doing a few thousand rpm was 0.7kJ, supernova in robot wars was around 30kJ, middleweight typhoon could store around 12kJ.