Ready Steady Eddy

[JAZZCNC]

Gantry rails are not on a profile it is steel box section to be bolted onto a base so it can be shimmed level. I've also read all about the epoxy leveling idea.

Not talking about rails I'm talking about the profile you show the gantry sitting on, or should say sat on top of the X axis bearings.

Regards the rail position on the profile.? You are showing or presuming the profile is flat and smooth across it's surface but most i've used have some recess down the centre or between slots so you may find they interfere with plans.
Also if your using 15mm rail then you'll find the slots are wide with chamfered edges, this means the rail sits awkward and can cock when fastened down.

Q1: First are you sure you only need 4mm shims.? When I make Z axis even with 20mm profiled rails I need to machine approx 6-7mm channel down centre of both plates to give clearance for nut and bearings.?

Q2: Either.!!. . . . Steel is cheaper and strong but it resonates more so ideally needs filling . Profile is OK but not has strong and often not flat has folks realise and can make rail setup/alignment harder not easier like folks think.