Quote Originally Posted by CharlieRam View Post
I am just finding it hard to understand why flipping the rails will introduce vibration as dean stated and I can't understand how the plate can bend as it has the bearing blocks on each corner.
Charlie it's not rocket science mate. It's all about leverage and vibration. Quality of finish mostly comes from trying to get the lowest amount of vibration at the tool. So we use the shortest tool we can with the least amount of stick out from the spindle to shorten the lever.

Your design is a very long constant length lever that never changes no matter what depth we cut.? Your lever length is distance the spindle extends from the spindle mount plus the tool extension. Lets say total extension of 150mm.

Now if you swap the rails around your lever is variable length lever and only ever reachs 150mm when cutting really thin material and at full extension.
Every where in between and the leverage drops and the stiffness increases so there's less deflection at the tool. At full height our extension is pretty much just the spindle nose and the tool length so order of magnitude better than your design.

May seem like a small thing to you but it makes all the difference when things get hard.!! . . . . Your design is ONLY good for wood anything harder will show it's weakness quickly.
Also with your design then using thick steel for the back plate will add very little to the design other than making the Z axis motor work harder lifting more weight. Your gaining very little strength over aluminium because it's supported at the 4 corners and not extending, if it was extending like a lever then yes it would give more strength.

Regards the size and use of machine then I'll repeat what I've said many many times it's always best to build a machine designed to do the job intended and do it the best it can. JACK of ALL TRADES is always a compromise in some department and this compromise increases with size.
Small machines do it best because they are easier to OVER build.!