[How do I?] measure linear rail flatness

[Wreck™Wreck]

A large granite surface plate might work, but you would have to 4 corner level the base first before trying to measure flatness. I assure you that cast base is not likely to be perfectly perpendicular between its precision top surfaces and its feet.

Indeed, we have a 4' X 8' X 24" granite plate, excellent for such work. Probably not accurate enough for hobby work however.

 

[cs900]

wow, that thing is thick!

 

[KBeitz]

Shoot or skim a laser across the top...