Help cutting threads on a lathe

[berol]

When looking in the Machinists handbook, I see for a 1/2" - 13 class 2a external thread the major dia has a spread from max 0.4985" to min 0.4876" and the pitch dia max 0.4485" to min 0.4435" 1st, What is does the 0.0015" in the Allowance column mean 2nd, What is the 0.4822" in the min (small d) column. 3nd, How do I decide where within these spreads to shoot for when cutting the thread. Will external threads work properly with a corresponing same class intenal thread if both have finished dimensions that are anywhere within their respective tolerance spreads. Thanks, berol

[agrip]

Berol - -

It pays to read footnotes.
The allowance is for a coating to be added, and still fit.
Class 3 has none so the mfr has to reduce the screw size to assure a fit.

The 0.4822 is for hot rolled material "cement contractor anchor bolt threads" kinda stuff.

Hth Ag

[John Foster]

Berol, I don't have a handbook in front of me but basically the tolerances are set up so that all nuts will fit all bolts, that is, the nut limits will be larger than the bolt limits.
Assuming that you are home shop, unless you have screw pitch mics for the threads on the bolt and plug gauges for the nuts, what you are aiming for is that one fit the other. It is nice to know about the limits but not necessary for home shop. Example; a spindle nose on a lathe. Your goal is to have a good fit for your backing plate on the spindle with no slop nor wiggle. If this is done yo should be within limits. John