[GME]

I have mounted linear rails to numerous surfaces. I have seen few cases where linear rails mounted to aluminum extrusions without machining them first works well. Same goes for most any "mill product". Aluminum or steel, they need to be machined. And it will say so in the linear rail mounting guide. If you read the specs for mounting linear rails and then look carefully at the extrusion mfgr's production specs, you can see that aluminum extrusions are not made to accept linear rails.

It's easy to tell.... Loosen the bearing block bolts, if it rolls easier, then you need to machine the surface, or shim under the rail.

I guess I got lucky, although I took an extraordinary amount of care in prep and assembly. I know it's highly unlikely that my Hiwin assemblies meet the published specs, but I can say that everything moves the same, whether individually, loose or tight. I know, because I checked it as part of my assembly setup checklist. Movement was the same in all circumstances. Stiff, but consistently smooth. Perfect? Probably not. Close enough that I cannot detect any difference? Yes. I attribute some of my success to my design and the care I took milling the extrusions for square ends and for length. I would have milled rail mounting surfaces, but my mill isn't even close to large enough for that. Also, there is no practical way to take a bolt together machine to an industrial mill. Too many opportunities going and coming to knock the frame out of alignment. Too much to go wrong. Welded steel is a completely different matter.

Morale of the story: If you can afford it and are willing to part with the money, buy a welded, stress relieved and milled CNC. I can afford it, but am not willing to part with the money at this point in my retirement. So, I either accept the compromises inherent in my decisions, or I forego a CNC altogether. I chose the former, which is apparently a choice you would never make. Good for you for being able to stick to your principles. I wish we were all so lucky.

Gary