Hi Thunza - Make the flanges thick and the Z plate thick - as thick as you can. You have other hurdles to clear. Are you welding this? Bolting? brazing etc. There maybe a commercial channel in alum or steel? Mine are laser cut and bent. You also have to consider if you are going to machine the rail foundations. Then there is access to bolts on the saddle cars, you have many more issues to resolve in this area. You have the drive nut under the spindle clamp. This will be a pain to maintain as you will have to remove the spindle to check or undo the drive bolts etc. Don't bury any bolt heads!! Is the spindle 2.2kW if so consider downsizing it to 1.5kW or even 800W. A 2.2kW is a big beefy spindle and not needed for wood or aluminium. The size of thr 2.2 will drive many areas to be oversize. If your trying to keep things compact go with aq 800W water cooled spindle... The only reason to go with a 2.2kW spindle is to get a ER20 collet so you can run 1/2" tools. Thats serious cutting territory and your machine won't match its capabilities... Peter

edit - ductility and material stiffness are not related. Some of the stiffest materials we have are ceramics and they fail at <1% strain. Metals are ductile because they have grains that are held together by friction just like lego blocks so they can slip along each other when stretched. Crystalline structures have no slip are very strong and stiff.

speed of machine cutting metals is nearly irrelevant. The machine still has to be stiff enough cutting slow or fast. maximise the geometry of every part, chase stiffness with every part. What is the work envelope you are aiming at?