If all goes to plan, there'll be another crusher come the end of summer, although I think I said the same thing a couple of years back

I'd recommend HDPE Calum; cheap, thick, and really nice and easy to work with. Although Tom has a good point about Ti and sparks (always looks cool ) if you don't have the facilities to work with Ti, then HDPE is a good substitute. A 500mmx500mmx12mm sheet from http://www.directplastics.co.uk is only, like, £12ish I think, including postage. Plus its easy to cut and fit a replacement piece if it gets chewed up by a spinner. Gary also makes a very valid point about air being your friend.

Good luck with the crusher. I'm finding in the planning stages of mine that its going to be quite a bit of work desiging the claw, getting the material and getting it made, plus there are a lot of little obstacles springing up that you don't initially expect. I'm banking on the chassis construction being relatively straightforward as long as I build it strong enough to cope with exerted forces. I reckon it'll all be worth it in the end though, and it will be good to follow your build and hopefully draw some helpful parallels with it.