Quote Originally Posted by BriceO View Post
Thanks for these explanations, I used the Item site to calculate deflection of the profile (http://product.item24.co.uk/en/home/...ral-41665.html) for 1200mm and 400N. If I do it right I have a deflection of ~0.01 mm.
I was thinking that Aluminium was a good compromise between weight/strength and offer a good flatness. I don't have enough experience to determine how bad is it.

Sadly, I already have these "light" profiles. Should I go for steel box section instead for X and Y? Do you have any advice plz?
The deflection calculation will probably be just simple bending, so either simulates a plunge in Z or simulates the sag caused by the weight of the spindle assembly. This is usually easy to get a low value.
What really tests the machine design is cutting in the X direction (confusingly many US builders call this the Y direction, so be aware if you are reading different forums) because this puts a moment on the gantry and tries to twist it. This is usually the limiting factor and will result in much more deflection at the tool tip than the Z direction loading.

However, based on the fact you have the extrusions already, and you mainly want to cut wood, then stay with them and build it as per the top picture in post #26.

Also be aware that the aluminium profile is not especially flat unless you have it specially machined.