hi

i wonder how that really will be with the specifications /size/ you stated in the beginning? Are you sure you did not forget something? What type of gantry will that be?
I can afford to increase weight if needed, I have gone for 20mm supported rails and can beefed up if needed. I want the machine to be able to surface and etch stone, maybe even v-carve it. Accuracy is more important that speed,

The bed is going to be deep, but have fixing on it to bring parts up to Z, and troughs to allow water for laser or plasma cutting. When using stone, the surface if the stone will be close to the Z top ( least angle ), but enough drop in the bed to allow me to get a wide bit of foam on the 4th axis.

My initial gantry design,2* 100mm or 150mm by 40mm steel with drops, Y rails on the top, bit of engineering to get close to an A frame for the z plates. So pivot point is very close to the z bottom ( basically, z bottom minus the bit length )