Chewing spinning weapon RPM limits??

[daveimi]

As above really, whats the limits on using something like pussycat had? After reading around I think 500rpm and a non-hardened blade?

[Bodge Job]

If your in the robo challenge arena there are no speed limits, you just can't use hardens blades. The 500rpm bit is for it be be classed as a spinner in the first place.

[Eventorizon]

Edit: Nevermind

[harry hills]

I feel this slightly relevant
http://www.teamvelocityrobotics.com/...eat/index.html

[typhoon_driver]

Ignore the above advice. If you want to chew/cut into something then you want a blade that spins as fast as possible. Go upwards of 5000rpm with a 4 tooth disc and you will chew and cut rather than smack.

If you want to know the theory why then let me know and I will explain.

[daveimi]

Yeah, ERM I asked this with regards to a Pussycat-esque weapon. Just wondered if anyone knew how fast their stuff span and if you needed hardened blades to be effective (which I know aren't allowed). I know the basics of heat/friction and being fast enough to cut / chew as opposed to kinetic energy transfer but if you can shed any light at all I'd be grateful for the help.

[daliad100]

More teeth, high rpm means less bite i.e. you get less of the opposing robot in between the teeth so you will grind rather than give a big wallop.

Back of the envelope says (60*closing speed)/(no. of teeth * rpm) = distance other robot gets into your disc. You don't really take more than a couple of mm when machining stuff if you want a reasonable finish but in this case you don't care about that (or at least I hope not) so you can probably take bigger bites of about 5 -10 mm. Too much and you start throwing the other robot which isn't clamped down (like in a milling machine) but you can try to keep on top of them.

Also you will want to use a smaller diameter weapon so your teeth will impact more obliquely sending them up and forwards and hopefully back onto your disc if you follow them.

Make of that what you will

[typhoon_driver]

Ok I€™ll go over the mathematics behind an effective disc as it may be helpful for others.
It all comes back to something I like to call cutter bite. A large cutter bite means that your disc, bar, drum etc will transfer more energy as a smack whereas a small cutter bite will result in cutting type action (think axe vs chainsaw. Axe takes big chunks whereas chainsaw takes many small bites to perform the same function)
To work out the cutter bite for any design, divide your rpm by 60 to get the number of rotations per second. For example we will use a disc with 2 teeth spinning at 4000rpm (fairly typical). So we would have a rotational speed of 4000/60 = 66.67rps. We then must divide 1 second by this number so 1/66.67 = 0.0150s. Then divide this number by the number of teeth in your disc, drum, bar etc. So in our example 0.0150/2 = 0.0075 (units are in seconds per tooth). What this essentially tells us is that if we have a disc with two teeth spinning at 4000rpm, it takes 0.0075 of a second for one tooth to leave a position in space before the tooth behind takes up this space again. If you also play with the numbers you will realize that the more teeth you have, the less this time is. So for 4 teeth it is 0.00375seconds/tooth and 8 teeth 0.001875seconds/tooth.
Now what we are interested in is a distance. The distance that the robot will travel forward in the time between teeth. Or in other words, how far into an opponent we will get to transfer energy.
To know this we have to know the forwards velocity of our robot. So lets take a conservative number of 1m/s (if you watch videos online not many spinner collisions are at high speed) or roughly 2mph. So we have a velocity and a time. To get the distance multiply the two. So for our example with two teeth moving at 1m/s we get 0.0075*1 = 0.0075m or 7.5mm. So what this means is that your tooth will only get a maximum of 7.5mm into an opponent before it hits at 1m/s forward velocity of your robot.
This also highlights why it is beneficial to have as few teeth as possible on a disc or drum with 1 being the optimum.
The other thing to remember though is that when the disc rpm drops, the cutter bite increases however with a lower rpm you have lower kinetic energy in the disc so it€™s all a balancing act.

[grant_ploughbot]

Don't worry too much about the hardening rule - that's something that was agreed to be ruled out a number of years back due to the progression of arena's. Many people run hardened discs of various techniques.

[daveimi]

That's great thank you, it explains a lot. I'm working on The Roasting Dish of Doom as a starter project but behind messing around with that I'm looking at building something serious for 2014 and FW Champs. I'm half way through sorting out a Pussycat type weapon but more 2WD and invertible with a fast chewing weapon to snag/pull/rip stuff off. But I was looking at these types of robots and the successful people on here seem to be the people that understand the physics of spinners, flippers, and designing for performance from the outset.

I knew that converting a roasting dish into a robot was never going to be a massive winner, but I have an extremely good idea about the basics needed to survive the FW Champs. I've spent hours on Youtube watching the fights and I understand now why people put drives where they do, how all the flippers have a similar geometry, how the spinners and drums use similar principals etc.