round rails

[TrickyCNC]

I'd definitely recommend the aluminum extrusion. Or is it aluminium for you guys? It costs more, but is by far the easiest for a novice imo.

for ease yes, but remember it's not even just a case of buying an 80x20 extrusion 1700mm long ! a BIG chunk of ally is needed to be rigid at this length

[Jonathan]

for ease yes, but remember it's not even just a case of buying an 80x20 extrusion 1700mm long ! a BIG chunk of ally is needed to be rigid at this length

If you made a torsion box type construction out of aluminium or steel sheet that can be very strong compared to the volume of material it uses.
It's interesting to compare the young's modulus of aluminium and MDF:

1. We can loosely say aluminium is 14 to 28 (hard to find accurate figures for MDF) times stronger than aluminium based on the ratio of their young's moduli and similarly steel is 40.6 to 81 times 'stronger'. That implies that you can use 14 to 28 times less volume in aluminium than MDF for an optimally designed structure with each material.
2. Aluminium is currently roughly £5.50 per kg which equates to £14850 per cubic meter.
3. MDF, based on buying 18mm 8x4' sheets is £320 per cubic meter.
4. Jazz says £26 for a 7.5m length of steel, which is 4.4kg/m making steel £6627 per kg.

Therefore aluminium is 46.4 times more expensive than MDF per unit volume and steel is 20.7 times more expensive.

Taking into account the 14-28 ratio for the strength, we can conclude that the same strength structure should cost 1.7 to 3.3 times as much in aluminium (much more if you price up aluminium extrusion per kg), or 0.24 to 0.5 times the cost of MDF if you use steel.
So in conclusion... if just purely go by comparing the, for want of a better word, strength of the materials then steel costs less than MDF(!) and unsurprisingly aluminium is expensive. Clearly there's more to it than this simple analysis - as has already been alluded to there are many other factors in using each material that can override differences in cost.