Hi Gecko et al - My journey with UHPC has sort of ended, It does not offer the stiffness it had promised so has sort of faded a bit for a bespoke machine. It makes sense to commit to moulds for a production machine and I would consider it for that application. Since my interest is in production machines this is still being pursued. Just need to find the outlet for many machines vs single builds.

My next choice is aluminium because I can deal with that in my workshop. The ply machines I have built have exceeded my expectations and I can see I could make ply machines, plasticise the surfaces with epoxy and they would then be suitable for flood/mist cooling. But timber does move (due to environmental conditions) so it would be another stepping stone to a metal machine. So therefore billet AL or laminated aluminium is the material of choice at the moment. Gecko originally want a machine of 300x400x350mm envelope to machine aluminium (with a wish for steel and ti) I think Al is totally doable so stay in that space. Steel and Ti require a different spindle and we cover that in Milli as well. So Gecko make a high rail design or a moving column design. That's my recommendation. A bolted together design can be adequately stiff and its portable. Two configs examples are attached. I prefer that the part stays still this allows optimisation of the mechanics and gets the mechanics up out of the muck. A bolted machine does take patience as there are lots of holes and threads or holes and nuts to deal with (if you do it yourself) but its a step by step approach that gets you to a known place. Welding is hit and miss unless stress relief is available and its a step forward, step back approach. If you have access to a mill and can make the parts to the design spec then everything bolts together as expected (just like a plane does, unless someone forgets to tighten the door plug bolts). The assembles can be epoxied together if permanent connections are OK. You will place a stake deeper in the ground at some point... when you do, carry that philosophy right through in CAD and resolve your design. Even if you do not use it its done. Partial designs litter the hard drives & sketch books of us all.... At 300x400x350mm I'd be looking at the moving column. Less drives to get square and keep square and the spindle is better aligned to the bearing footprints. So my vote is for the M1 arrangement in your case. Peter

edit - I also prefer the rails to be on top of the gantry and the saddle to be an angled bracket. This separates the X&Z bearings allowing a narrower saddle plus allows access to all bearings. A narrower saddle does affect the footprint of the machine considerably (if you go with a gantry design). I also think it helps with reducing the spindle overhang.