A bit of help designing a spinning disc

[typhoon_driver]

I thought I would start this thread as a help for people thinking about designing a spinning disc as they are becoming more popular in the featherweight class.

It is very easy to spin a bit of metal up to a high speed but it is a very different matter transfering that energy to another machine. There are a few things you should keep in mind at the design stage.

What I refer to as the cutter bite is one of the most important parts of designing a disc (in my opinion) and the one that is most frequently overlooked. A good cutter bite can mean the difference between a disc grinding on an opponent or doing what it was meant to do.

To calculate the bite your disc do the following,

- divide the rpm of your disc by 60 to find out how many rotations it will do in a second
- multiply this number by the number of teeth on your disc. This is the number of teeth that will pass a particular point in space in a second.
- now to calculate how much of a bite you will get as you move a bot forward I use a forward velocity of 1m/s as it gives a relatively accurate yet conservative idea of how fast a bot could be moving in the arena during a collision.
- The number that you get from this calculation is the distance (in meters) that your disc will move forward between one tooth leaving a position and another taking its place.

Just to make it clear I will use hornet 2 as an example. The first calculation will be based on my 2 toothed disc and the second based on the recent single toothed disc for comparison.

double tooth

4000rpm disc / 60 = 66.67 rotations per second
66.67 * 2 = 133.34 teeth per second
1 m/s divided by 133.34 = 0.0075m = 7.5mm

single tooth

4000rpm disc / 60 = 66.67 rotations per second
66.67 * 1 = 66.67 teeth per second
1 m/s divided by 66.67 = 0.015m = 15mm

As you can see the single toothed disc gets double the bite and so is more likely to transfer the kinetic energy during a collision.

I just want to clarify at this point that I am not telling to cut teeth off discs etc but instead calculate your cutter bite and think about how effective it will be in the arena.

It is also clear from this calculation that the factors that affect your cutter bite are the rpm and number of teeth. It is not affected by the size of the disc. So to increase your cutter bite you can either decrease the rpm or number of teeth. But a decrease in rpm = less energy so it is about coming to a happy medium.

[grant_ploughbot]

Note - bars DONT have any better engagement as alot of people try to argue, they are exacly the same unless the end of the bar is just cut off flat. If thats the case, it effectivly has 4 teeth, as theres 4 points all at the same distance from the centre of the bar. That makes it much worse!
Keep as much weight around the outside as possible, that will make a HUGE difference. Also, and obvious one, to increase tip speed (add more energy) and keep good engagement you will want to increase the diamtre. 360mm, 2 teeth, 4.4kg disc, 3kg of that around the outside 3500ish RPM 4hp motors- sound good?

[typhoon_driver]

sound good?

Only if you get the speedos working

I will post more things I have found over the coming days. Mainly because i have exams and like to have something to take my mind off studying during my breaks but also to give you a hand if you are thinking about building a disc.

[Mouldy]

Never bothered doing those calculations, just did what was obviously the best.. most weight on the outside, big teeth on a nice angle and pretty fast with a punch behind it.. works out that 360 has a bite of;

8.6mm..good enough lol

[ady]

do what i do and hand it over to the experts lol,
if it dont work on the day blame them lol

[kane]

New thread title How to build more effective robot killing machines

[john_frizell]

How many arenas do we expect to be in operation in 2008 that can take spinners?

[geeza]

Just the one as I understand John. Robo Challenge

[cavecrusher]

and the DRG one

[dragonmaster_1]

it is the arc that intersects the flat plain, between teeth, that is important, not the time... time is nothing, arc angle of incidence is what counts, plus tooth profile, material interaction and engagement. By the information given so far, a heavy drill bit is as effective as a 2 meter bar of same weight at 1500rpm. By this thread, 20,000rpm antweight disks dont work