To get to this design you gotta remember that as much as possible stock on hand is being used. Decided on a box beam as analyzed in post #19. Turned sides inside out to get the offset for the Y ballscrew to fit between the rails. Side plates don't cover the complete length of extrusions since 48 inch wide thin plate (5mm) stock is available but the rails (and extrusions) are 60 inch. Double plate gantry side supports have been recommended here so 16mm plate was used for those and the rest of the end plates.

As you can guess there is aluminum plate, sheet, and extrusions in abundance to use. All sunk costs. Below is what the structure inside the gantry looks like before adding the end plates.

Click image for larger version.  Name: Gantry Guts.jpg  Views: 4009  Size: 208.3 KB  ID: 24020

Ends are complicated but the CNC knee mill in the garage will sort that out if necessary. Could use some advice on how to simplfy them.

I am worried about resonance. If people here think that resonance will be a problem with a manufactured box beam like this I will probably abandon it and try an "L" configuration. Looks like a lot of people here are having success with less overkill than this. How do the chinese big box beam gantries avoid resonance?

Thanks,

John C