Precision Straightegdes -that don't cost a lot

[John Arnold]

That is exactly what I intended to do with the steel rulers I have, but when I tested them on a surface plate, I discovered that it wasn't necessary.
I found mine in a salvage store for 25 cents apiece.

[tadol]

The machinery making steel rules is pretty darn accurate - you can get 2, and check them against each other ( hold them to each other, then edge to edge, rotate and check again ). In the old days, you could make a set of winding sticks, pin 2 hardwood sticks together, joint them together, then take them apart and check them to each other, much like joining a top or back. A reject fingerboard ripped in 2 maybe? Plus its alot harder to ding a top with a wood stick than a heavy steel bar -

Maybe I don't understand how precise things need to be -

[simply rod]

First post! Long time listener, first time caller.

Not sure if it would help the OP, or anyone for that matter, but I use a $1 50mm plastic ruler as a notched straight edge to check for neck relief before leveling frets. It doesn't need to be perfectly straight for my purposes... just in the ballpark. I lay the ruler on top of the fretboard, use a marker to record fret position, then use a cheap soldering iron to melt notches in the side of the ruler.

I then lay the notched side along the fretboard, and holding it in place with one hand, bend it side to side at each end of the fretboard as well as in the middle. If there's no resistance, it means there's a gap. I adjust the truss rod until I get even resistance across the length of the board.

Doing the leveling is a different story. It requires much higher tolerance, but I have to keep in mind that although my straight edge may have very high tolerance, the sandpaper and adhesive on top of it have much less tolerance. Then there's the issue of crowning. I'm sure my precision leaves a lot to be desired!

[Rudy4]

Web-based supplier such as:

https://www.flatground.com/catalog/catalog.jsp

I purchased a 24" cast aluminum level that had two milled sides about 30 years ago for use where I needed a good edge referance or sanding surface. It is fine for jointing top and back sets and the like, and I use it fairly often as a guide for my router to machine the matching top and back edges. I'm betting there must still be a manufacturer of a good milled edge cast level now.