Adding 3D printer ability to my CNC / advice needed please

[Trispectiv]

There are few (or none) printers that use a 470mm long bed supported at the corners only. The reason is the aspect you mentioned yourself, you have a 470mm long sheet of aluminium supported at the ends only. There will definitely be some deflection at the middle of that length. If your deflection is 0.1mm and you are printing 0.1mm layers, you have a deflection of one full layer thickness at that spot. It can be suppressed by using a raft or setting a thicker first layer in your slicer, but you will soon find these methods inconvenient.

I would assume a 5mm aluminium thickness would be required, but it also depends on the aluminium alloy in terms of stiffness. You will probably have an extra sheet of glass/mirror/borosilicate on top of it, which will be your actual printing surface. The extra glass layer will tend to remove the deflection. However, even so, if in theory the glass manages to keep its straightness while the aluminium is slightly bent at the middle, the missing contact surface will mean less heat conductivity.

All printing surfaces over 300mm I have built were not supported by the 4 corners method. 3 parallel beams were laid under the bed - one on each side and one at the middle. No springs were used and during calibration the beams were shimmed under after checking the surface with a dial indicator.

Silicone heaters are effective but more dangerous then the usual 12V/24V stuff. I would add a thermal fuse or two under that bed.