El Beast - Initial design phase, comments and critique welcomed!

[Zeeflyboy]

Sorry I am not native English speaker, therefore I can be easily misunderstood.

X rails: pre-machining rails do not count due:
- rails must be parallel - your design is frame from 4 profiles. Forget on straightness (you are assuming this will help with using expensive profiles.) think about other two elements which are connecting both profiles.
You have 2 plates - potentially nonparallel, two profiles Mitsumi + two profiles catted and machined by... as I said check the stiffens of the bearing, lets assume the total error of non-parallelism is 100 microns, and that half of this error must be compensated by bearings, the other half by brackets. This means 25 microns per bearing - now check the stiffness and you will see the load which is in the "system". If you wish to remove error, you need to do machining in the assembled condition...

All profiles are going to be from the same place (mitsumi now, given I want the milled stuff) actually, given that they almost certainly should be cutting the bracing profiles one after the other without re-positioning the cutting gate they should be very, very close.

They can always be massaged with my current machine which will happily repeat to substantially better than your stated 100 micron example (as confirmed by some fairly expensive glass scales mounted to both X and Y axis). It should be fairly easy to get them to within 10's of microns rather than 100's...

Check the I (Area moment of Inertia) of profile and plate, most likely 25 mm thick plate is stiffer than 50 mm profile or it is close to it. And whatever will you do: If the profile and plate are not planar, when they will be assembled the result will be non-straightness of the rail "bed" surface.

Y gantry: problem is torsional load not the bending - check the I and calculate displacement applying 100 N force at the end of the tool

This is a bit beyond me, I have never used any simulation software and not sure where to start.

I'm simply going on the basis that common sense Garden Shed engineering tells me a combined total of 32mm of solid aluminium plate and 40mm of heavy profile that makes up the gantry cross section is overkill for a 550mm span gantry on a relatively light duty table top machine. Certainly it is at least as beefy as most DIY machines of that scale that I have seen.

The second problem is: Y gantry length must meet X rail width - within micron range otherwise you are facing with additional load to the X bearings... Not mentioned that both side surfaces must be parallel, - if not additional load on X bearings again

I don't really see this being an issue as long as it's longer rather than shorter. All it requires is to try inserting a shim brass sheet of various thicknesses between the gantry arm and mounting plate until you find a thickness that slots in snugly.

Regarding the idea of setting the true angles with bolt - by definition this means you are incorporating elasticity in relative stiff system. Means also you have nice oscillation.

How is it incorporating elasticity? Once the angles are set the bolts on the respective parts are torqued down clamping everything rigidly in place... you aren't relying on the part just resting on an eccentric bushing. Where does the elasticity come from?

You are right system is relatively rigid. I am escalating problems caused G&T and consequences of those.

You've lost me again lol - what's G&T when it's not in a glass with some ice?