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I have a set of pin gages which are labeled "minus .0002".
When placed into the muzzle of a new Miroku/Winchester M92 .357 Magnum/.38 Spl:
The .346 pin goes, the .347 does not go. I read this as.3462" bore.
What do you say is my rifles bore diameter?
People really -really- need the half thou pins as I find the thousandth increments far too coarse to be accurate for measuring firearms. If a .346" pin goes snugly with drag against the pin, and the pin is .0002" under, the measurement is close to the advertised diameter of the pin, .346" If the pin goes with very little drag and you can rattle it a bit, but the .347" will not go, your measurement is more like .3468" which is why the half thou pins are critical for these types of measurements.
If you had a .3465" pin, and it fit in the bore, a little more snugly than the .346" pin, then the bore is closer to .3466"~.3468" than .346"
Yes .3468" is the actual diameter of the Z minus .347" pin, but you won't fit it into a hole so close to actual diameter.
The guys who taught me about gage pins always told me that it takes .0002 to allow a pin to go into a hole.
I was also taught that a "minus pin" say a .375 could be from .3750 to .3748 in the case of ZZ pins. So if you have whole and half and both plus and minus you can get a really good idea what size a hole is in a geometric way.
.375 ZZ minus would be .3750 to .3748 (lets say this goes in)
.375 ZZ plus would be .3750 to .3752 (and this does too)
the size is somewhere in here.
.3755 ZZ minus would be .3755 to .3753 (this one does not go in)
.3755 ZZ plus would be .3755 to .3757
What I mean by geometric is that the hole may be square, triangular, or oval, and the gage pin cannot detect that condition. We used GDT (geometric design and tolerencing) in Industry and it tries to pin down ways to coordinate hole size and true position tolerances to create things that will fit together.
The tightest tolerance reamed holes we did in everyday work were +.0005 minus 0, much closer than that and one would be ID grinding or something similar IMHO. Also when you start to get to really close tolerances the temperatures of everything, the part and the gauges starts to be more important.
Quote:
Class ZZ Gages - These gages have an inch tolerance of .0002” and a metric tolerance of .005mm.
They are practical where good precision and speed are important.
Class Z Gages - These gages have an inch tolerance of .0001” and a metric tolerance of .0025mm.
They are one half the deviation of the ZZ for a better fit.