Why can one not use their thread gauges to cut thread?

[RJSakowski]

I absolutely HATE using 3 wires but I use them because I don't have a thread mike and I believe the 3 wire method is the only accurate alternative to a mike. I know some guys use the one wire method but as it uses the thread crests on one side it will only be accurate if the flats on your new thread are exactly the same as the thread you'r trying to replicate. The one wire method removes 50% of the error that would be introduced by the no wire method. (Just measuring the OD)

Not so. Two measurements are made, one with the wire (M2) and on without(M1). The measurement without the wire determines the location of the surface opposite the wire and effectively cancels out any effect caused by crest geometry.

The formula used in the three wire method is E = M-3W +.86603/n while the one used in the single wire method is E =2M2-M1 -3W +.86603/n. There is an increase in potential error in the calculated pitch diameter from about .0004" to .0006", assuming all measurements are +/-.0001". The biggest factor in determining error comes from the measurement of the wire diameter.