[mactec54]

What do you guys target for horizontal and vertical parallelism accuracy when mounting (square) linear rails?

I've had mixed success trying to mount my Y and Z rails using digital calipers and a dial gauge to test how parallel they are but I think the quality is a limiting factor. In theory, my digital calipers are accurate to 0.01mm but I have trouble achieving the same measurement twice for a fixed distance. In reality, I only trust their accuracy to 0.1mm. I place even less faith in my dial gauge / magnetic base stand set (which seems to just produce random readings as I slide the bearing block back and forth).

What would you guys consider to be acceptable mounting parallel accuracy for rails over a 30" or 48" slide?

The rails seem to function ok if they are parallel to 0.1mm in that the stage slides up and down without that horrible grinding noise that seems to occur when I get it too far out of tolerance. That doesn't mean it will perform well on a finished CNC machine though.

Do you add any adjustment screws for fine tuning? I find it very difficult to make 0.05mm adjustments to rails by hand. I am wondering if there are some other tools to help me....

I achieved the best results (so far) by just using machinist parallels between the rails as I drill the screw holes in the mounting surface. Still not much better than 0.1mm though.

That is why Linear Rails are used on a machined surface, with machined pockets, or a machined edge with tapered wedges or screws the clamp them to the machined surface, they want to be better that .1mm , .025mm would be close to what you should have for a hobby build, this also will relate to how much clearance the Bearing Blocks have, if you have Bearing Blocks that have a preload, then the rails have to be perfect, if they are just general Bearing Blocks with no preload than you can get away with up to .05mm over the complete travel, they wear quite rapidly if Bearing to Rail alignment is not correct

You can not use a digital caliper to set these Rails up, you need to have a minimum of a .0005" indicator or a test indicator

Check the Linear Rail manufactures web site, they have examples of how accurate your Rails have to be mounted, for them to work correctly