Joe's 1000x600x200 steel and aluminium router started

[devmonkey]

So what I plan to use the machine for is mostly wood and aluminium milling and occasionally very very light stel milling. Based on what you and everyone are saying I am leaning towards top/bottom mounted rails since stiffness is critical.

Have you built any machines with face mounted rails and 160x80 extrusion gantry that was not stiff enough to mill aluminium efficiently?

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No every machine i've built, including one that was substantially made of MDF with an underslung gantry rail with only a single igus carriage on each X rail on which you could rock the gantry visibly by leaning on the spindle, were capable of machining aluminium. You just need to back off the speed and depth of cut if you lack stiffness, all the accuracy is delivered on the finishing cuts anyway, just as with a normal mill, tool deflection particularly with the thin tools we use in routers is usually as much or more of a problem. That said it is easier to produce accurate work on a stiffer machine, so depends what you are making. If it is machining a few plates out with accurate vertical edges and holes that is one thing, if you are running 3D paths to build molds etc then you need a stiffer machine, and a machine that is significantly stiff over the Z travel you need.

Note though that if you want the rails to sit flat on extrusion it does have to be milled irrespective of which face you bolt them to. I don't think many people do this on this forum but most still get completely acceptable results.