a few questions need answering please?

[RogueTwoRobots]

3mm steel would do well against some spinners, with the exception of the insanely high-powered ones. Hardox would be the next step up from that; 3.2mm hardox is close in thickness but more expensive than the generic 4mm hardox used by many robots. 6mm thick hardox is OTT for featherweights, the 4mm stuff works fine. It formed the back end of Drumroll and never sustained any damage except for paint scratches.

If that costs too much, plastic is the next best alternative. Polycarb would work but you'd probably need 8mm-10mm minimum to defend against spinners and it starts to get expensive at those thicknesses.
HDPE is much cheaper and something in the 8mm-12mm range would be fine. It absorbs the energy from spinner impacts more effectively than polycarb and is easier to work with and repair/reform if damaged.

EDIT: In response to your most recent post, I reckon 20mm thick HDPE armour is overkill. You don't really need that much. For top and bottom panels, it is your choice between steel and HDPE but 5mm HDPE might be a bit flimsy; I use 8mm stuff when using HDPE for a base which is more rigid.

[mattsdragons]

so if i went for 15mm for the sides just to be safe then 8mm for the and bottob i would have something which could stand up to most spinners?

[RogueTwoRobots]

I would say so. Although it's worth noting that you could have the thickest, strongest armour around but it wouldn't do you much good if the way it's mounted is weak (i.e. using duct tape instead of bolts, just as an extreme example) so make sure the construction is solid and it should have a good chance against spinners.

[psychostorm]

so if i went for 15mm for the sides just to be safe then 8mm for the and bottob i would have something which could stand up to most spinners?

It should do.

Mind you, that's like saying will this chicken give me food poisoning - depends on how it is made.

The eye panels on Kaizer @ Leicester were 10mm HDPE. Alright, they fell off due a lack of screws but they survived it.

I'd beg to differ Jamie - Kaizer lost less panels when I taped them than when I had the screws in!

[mattsdragons]

mabey if i rivit and screw them in?

[psychostorm]

Pop rivets tend to snap when hit with a large sudden force

Screws will work provided you aren't as bad a joiner as I am

[jonny]

why don't you tap and then weld most of it? leaving the top panel unwelded so the insides can be accessed.

[mattsdragons]

i dont have the equipment to weld hdpe

[PJ-27]

This is a good question actually, I was wondering about armour. I've done the opposite and made our top and bottom panels 15mm HDPE to protect against axes. Hoping to avoid spinners......

[ady]

matt, 10mm sides is plenty, 8mm base, 6mm or 8mm lid. make sure it all lines up nice on the corners/joines, no sharp/sticking out edges. i always bolted on hdpe with good old nuts and bolts which were countersunk.