[mactec54]

I didn't really have a problem with the first beam design being sufficient to cut aluminum. My main issue was with fabricating that beam. It would take a lot of work machining, drilling and tapping the parts to assemble this beam. If it does warp when laser cut, then there really isn't much he can do to straighten it out during assembly of that beam. We all know that mounting profile rails directly to a warped beam is unwise. You could instead purchase an I beam or C channel. Bolt additional pads to mount the rails to and have them machined flat.
Don't get me wrong. It all can certainly be done as designed and it can be done well. It's just not a design that I would recommend given the difficulty of fabricating when off the shelf steel profiles could be used with as good or better results.

And as for aluminum extrusions being sufficient, they most certainly can be. They are also not nearly as challenging to fabricate a machine with.

As you say both can be used, but no matter what you do with aluminum extrusion, you still can't get away from the fact that it's Modulus of Elasticity does not compare to steel, Aluminum being 9.6 and steel A36 ( plate ) is 29.0

To make any kind of beam that will do a good job you have to fabricate it, Aluminum or Steel, these small beams are not hard to fabricate, C-channels have been used before, all parts of a fabrication like this have to be welded, to be effective, after welding you Stress relieving, and then machine where needed

Most extrusions are not suitable by it's self, the 2 15 series under the same test as the 3/8 steel plate started to deform at less than 150 lbs. the steel went over 2,000 lbs. once it had the top and bottom plates, adding the linear rails also took it higher, as it did for the Aluminum Extrusion as well

The density and Frequency of aluminum is also low compared to steel, the shear Modulus of Elasticity for Aluminum 3.9 Steel 11.9

So no matter what way anyone looks at it you can make a light weight steel Beam that will out do any Aluminum Beam comparison

How something like this is fabricated is key to a successful Beam, Gantry support, is also a major part to get right, most have a Beam that is ok and the support is weak

I do use Aluminum also and for Beams sometimes, always fabricated, but almost never use Aluminum Extrusion

Even if you buy off the shelf steel profiles, there is nothing that is going to be suitable straight from a supplier, you are limited to wall thickness and size, so by the time you modified the piece you brought from the store, you could of fabricated, both fabricated or brought from the store, you still have to stress relieve and machine

Aluminum extrusion you have to machine also, lots don't, it has no surface that is suitable to mount linear rails too, this company machines ( 2 ) faces https://us.misumi-ec.com/vona2/mech/...num+Extrusions

[LeeWay]

I did machine faces on my 80/20 mill that I built, but it really was not needed. When you tighten the profile or plate to the extrusion, that face of it then becomes flat. They are flared up at the slot to lock something down flat with minimal torque on the fasteners. 80/20 is anyway. Now I have not built a machine that used only aluminum extrusions. My router has three steel flat bars to help support the gantry beam. My plasma cutter has steel flat bar skinned to one side as bearing tracks. My 80/20 mill and lathe don't really count here, but they were also beefed up with extra plates and gussets.

At least some that are building aluminum extrusion machines do reinforce the beam and gantry risers in some manner. The Z axis movement across the gantry alone is probably enough to skew simple aluminum extrusions.