ok, first attempt using sketchup 8

[Jonathan]

It's only a suggestion, take it or leave it. I post these things to benefit other people who may want more from the machine than Dean.
Adding the plate to make a box is very simple and should make a big difference but if Dean never wants to cut aluminium regularly there's no point. 0.01mm tool deflection might not sound like much, but when you start cutting metals it is - surface finish, material removal rate and tool life are affected. Running the calculation with cutting force for aluminium gives 0.058mm.

I used 8mm web thickness for the RSJ, not 6mm, in all the calculations as that's a standard size - with 6mm tool deflection is about twice as much.

[JAZZCNC]

It's only a suggestion, take it or leave it. I post these things to benefit other people who may want more from the machine than Dean.

Great Jonathan but in a thread like this surely the target is to help the person who's building come to a design that suits his needs not others.? Thats how approach every thread.
First and foremost I try to advise with clear uncluttered information and suggestions to help them arrive at a design that will suit the intended purpose and do it the best it can. . . No more no less.!! . . . . If I can help them achieve this in a cheap and easy way then I most certainly do and get a warm fuzzy felling doing so:naughty:

Over complicating and over building past the usuall built in safety margin is just wasteful in both time and money.!

The simple FACT to DIYCNC is that it's NOT POSSIBLE to easily and cheaply build a machine that can cut every material like a purpose built machine optimised to cut the same material . . . . Belive me because I've tried very hard.!! You either build the machine to do the very best it can at that intended job at a sensible price and accept it's short falls in other area's. . . . OR . . . . Throw lots of serious money and time massively over build and have it perform in all areas average.!

Now back to the job in hand.

Dean for your MAIN need of cutting wood the RSJ will perform perfectly well without any extra bracing, welding or what ever. It will cut Ali but obviosly with restriction and greatly reduced performance compared to if it was built for that use.
Accept it's limitations and you'll love the machine which will become your faithfull servant but push it past them and you'll hate the bloody thing because it will try to kick you in the bollocks at every inopertune chance it gets.!!

Personally I'd use the RSJ but want to fill in the front and rear with thin covers, these would be serving 2 purposes with a frindge bennifit of adding strength.
Firstly functional, Secondly cosmetic, on the front they would be protecting the screw from debris with the slot idea on the previous design and on the rear it would be mainly cosmetic possibly houseing wires etc.!!. . . . but heres the fringe bennifit.?
To do this I would cut plywood braces that get bolted between the flanges and thru the web of the RSJ the covers would be just thin plywood screwed to the blocks. The bolted blocks would increase rigidity and combined with the plywood increase strength.

I've attached another pic to help explain.

Edit: Added coverd in pics for completeness.!