I want to purchase a straight edge which I can use to measure the flatness of heads & blocks and other such nifty things. My question is, what is considered a good tolerance for a straight edge? To make this a little easier, if I wanted to buy a 24" straight edge, how much should I expect it should be straight to across the entire length?
To confine answers, I'm not looking for what you think it should be, but rather what is standard for regular machining practices. References would be helpful.
what is standard
DIN 874 says less than +/- 3 microns over a 300mm "sharp edged rule"
If you purchase a "straight edge" from an engineering tool company, it will most likely meet this specification.
for a "steel rule" DIN 874 defines four grades
Flatness tolerances are
E.g. in microns
Grade
Length 00 0 1 2
--------------------------------------
0.5m 4 7 12 21
how much should I expect it should be straight to across the entire length?
This also depends on how you support the straight edge. There are specific procedures to be followed, some procedures involve using gauge blocks and/or other specialised test gauges in conjunction with the specially supported straight edge.
