If you will be using glass on top of the bed to print on, then you could get away with setting a few small (e.g. M3) countersunk screws into the top surface. Alternatively, as the ribs are for supporting the aluminium, perhaps you can get away with attaching the ribs at the edges of the plate only.

It's also possible to bond aluminium with epoxy resin, if you prepare the surface first.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Good news, the aluminium plate will only be 800 x 800mm now. Also, the ribs will be machined out from the bottom of the plate. It's not too expensive, not too heavy, and definitely more rigid. It might be decided to just print onto the aluminium plate but perhaps with PET sheets on top or a metallic skin.

Thank you

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The (getting better) Noob

Zapadee zoopidee.