Yeah they will probably chip if you try lathe them down.

Heres a shot of the one i tried to lathe down, all the ends of the teeth fractured off -



Then i grinded them down with a dremel in the toolpost and it came out much better -



Unless there's some explicit reason for doing it that sounds rather baffling to me lol. The weight saving over the course of a 26.2kg robot is probably insignificant. Would probably be better to just find a lower kv motor and keep two stages as they are.
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That sounds like a great excuse to get a tool post grinder!

OTOH, a Dewalt 36V motor would work perfectly with a 16:1 P80 and Would be less work. The voltage would be much more suitable for a brushless weapon motor as well.

Totally, i wouldn't mind a serious version The dremel is super handy though, specially for turning down colsons with the sanding drum.

Oh and i forgot to add it to my last post but i vaguely recall that riobots book have some segment on gyro effect.

ANND also forgot about the titanium center. It might work, but if anyone took off one of the bolts holding it all together which is probably likely then that's 5 grams of weight removed from the periphery and the whole shebang will be majorly off balance. Having it bend even a little is nothing major (i doubt it will get bent full stop at the thickness we are talking tbh) and for the cost of bisalloy just grab a handful of spares.
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I did an analysis of a spinning weapon's gyro effect for a school coursework way back when.

IIRC, if you're trying to maximise energy then a smaller, faster weapon will have a smaller gyro effect for a given KE.

However, if you're trying to maximise momentum (which I think is a better indicator of how fast your opponent will be travelling after impact with the weapon) then there is no way around it, more momentum = more gyro effect, whether the momentum comes from size/weight of weapon or weapon speed makes no difference, same momentum=same gyro effects.
The only way to reduce the gyro effect is to have a wider wheel base, giving the robot a more stable stance, making it harder for the weapon to initiate a gyro dance.

Momentum = I * w (inertia times speed)
KE = 0.5 * I * w^2
gyro dance moment = I * w * TS (inertia x disc speed x turn speed)
Robot stabilising moment = 0.5 x width x weight

You used a Dremel to turn down Colsons? How did it work?

With the disk construction, I am just throwing ideas out to see which ones stick; one of the advantages I saw with a two part disk is that there would be less machining time if we wanted a disk with a thick rim cross section and thinner spokes. With a one piece disk, there would be substantial CNC milling time to thin down the spokes.

On the other hand, CNC work on the spokes and hub allow for all sorts of tricky ideas; we could machine bearing pockets directly into the hub rather than welding them and the spokes could be designed for the greatest 3D stiffness.

Your earlier disk with 2 teeth had spokes like this:



Was that to keep weight out at the rim or to minimise stress after an FEA analysis?

With CNC, the spokes could be given a tapered cross section, which would add some weight at the hub but would possibly make the disk more resistant to bending from horizontal spinner hits.

going from hazy recollections.
but isn't KE ~ gyro?
I seem to recall last time i ran the numbers basically the KE of the disk was directly equal to the gyro forces? IE any increase in KE means a direct increase in gyro?
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Mechanical engineers build weapons, civil engineers build targets

[quote="Nick"]You used a Dremel to turn down Colsons? How did it work?[/qoute]

I've been lathing down rubber skate stoppers for my sisters roller derby team, and they're a fair bit softer than colsons but turn out looking basically the same as they started off with the toolpost dremel. I'll dig out a colson tommorow and give it a go! I wouldn't expect it to be any different tbh.

quote:

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Your earlier disk with 2 teeth had spokes like this:



Was that to keep weight out at the rim or to minimise stress after an FEA analysis?

---

That is my gut instinct too - after watching some videos of K2 vs Touro Light it works out in practice as well. K2 has much less energetic hits and almost no grro effect, while Touro delivers a huge punch and has a major gyro dance, even though it has a lower centre of gravity.

I'm thinking that a brushless motor with proportional control will be very handy...

It should be stronger being tapered, your maintaining the same cross sectional area of steel which should help things.
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Mechanical engineers build weapons, civil engineers build targets

@ Glen: I must be on too many flu tabs at the moment , I remember someone had a disk like that and the shape was supposedly derived from FEA analysis.

When I was designing the weapon bar for Scissorhands, Robert Woodhead analysed many different shapes in Solidworks and found that deflection was greatly reduced if the first 1/3 of the bar from the hub was thicker. That should broadly apply to spokes on a disk IMHO. A steel disk would be much stiffer than Scissorhand's Ti bar, but minimising damage from horizontal spinners has to be a good idea.

Apparently that's correct if i can read solidworks right (doubtful lol) These all weigh 9.59g with the same "rim" and "hub" dimension, so they should give a fairly accurate analysis.

Weakest:



Middle ground:



Strongest:



The last two don't deviate that much on deformation it seems, so its probably not that bad leaving it untapered for the sake of easy of making the part and keeping KE up.

Got me thinking now about that statement of 1/3rd the way up. Cause i guess it's acting like a lever so the force would be greater the closer you get to the hub. Might go have a look at one of the old bisalloy 80 disks that bender bent a little and see where its actually started to bend. BRB lol !

triple mega edit - Looks like that analysis is pretty spot on then, this ones bent right up near the hub. So investing a bit of weight to taper it out at the hub in the lower quarter is probably a good idea. Doing it in a curve would be pretty sweet (least i wont be machining that XD!)


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http://www.youtube.com/user/HyzerGlen - Videoooozzz

Can you switch your FEA to output max stress or safety factor? Displacement is rather misleading and doesn't show if any point is exceeding the yeild stress and if it will spring back or not. Also how long is it going to take to aquire and machine everything that is being talked about? I gave myself 2 months for design, 6 months for building and 3 months for testing, modifying and shipping, and ended 3 months behind plan with only enough time for one proper test before it went in the box. January 1st should be the goal to have drive and weapon working at the same time. You guys are a lot more ambitious with machining then I was, and must have deeper pockets

Read torque enduced precession http://en.wikipedia.org/wiki/Precession . The biggest difference between K2 and the Touro robots is the drivers. I saw Touro Light's driver as very aggressive and heavy on the sticks whereas K2's was very passive and more patient, causing him to be lighter on the sticks.

Very true about the driving styles, also K2 has a much slower top speed so it can't turn as fast with or without the weapon spinning.

I am already getting worried about the time and cost factors - I don't want the bot to turn into a talkfest! The disk is a good example; we have come up with some interesting options and good maths, but I realised today that getting the disk waterjetted and then CNCed as described would probably cost over $1K. There two ways to cut that down: a thinner disk with minimal machining or changing to a short bar with massive teeth along the lines of Scissorhands.

For testing, I want to try testing subsystems before the bot is assembled if possible. As the frame will be welded, its really hard to make alterations if a component doesn't fit or needs to be replaced with a different part.

You could waterjet cut a 10 - 12mm bisalloy disc and weld a ring of the same thickness to each side to increase the rim thickness up to 30 - 36mm, but keeping the spokes thin. If the tooth was just part of the centre disc the ring won't need much welding since they won't see much load. Probably would waterjet some sighting marks into the disc and ring so it'll be easy to jig. Would be a lot cheaper, require no machining and one afternoon to assemble.