Establisihing reference for rail alignment

Hi - I'm new to this forum and it's an incredible resource for CNC machine builders.

I am looking for specific advice on how to go about establishing a level surface and straight reference line for mounting profile rails on a steel or aluminum box tube when you don't have the option of getting an accurate reference surface established via mill or surface grinder. I have a vertical mill with 24" bed travel, so I'm good for smaller parts or the entire z-axis of a gantry CNC machine.

I've read through the many posts on epoxy leveling. I have no issue going that route, but am thinking would prefer to use a dense filled epoxy (aluminum and/or stainless powder) for best compressive strength. I read some of Jazz's recommendation to use a mix of hand grinding and spot leveling, and I know how to hand scrape ways, but I don't have a large machinist's straight edge - just a 12 x 16" granite surface plate.


I would like to mount my rails on the side of the steel box tubes, so a self-leveling epoxy pour is out. But if I can get a level and straight alignment on the 2 side box tubes, I can certainly shim and epoxy bed the 2 box tubes with attached profile rails onto the machine frame once they are aligned parallel to each other.

Regards,

Chris Krumm