Internal G-Series Screw Thread
The internal G-series screw thread is not assigned a class-of-fit,
and thus has no class-of-fit letter designated.
External G-Series Screw Thread
The class-of-fit designation only applies to the external G-series
screw thread and by association the ring gages. The ISO-228 screw
thread specification offers a choice of two pre-designed classes-of-fit
for the male G-series screw thread: Class-A and Class-B.
What are the similarities?
In the ISO-228 G-series screw thread standard both classes-of-fit
have the same basic size. They are identical. This means that the
maximum material size is identical. Since the GO gauge checks the
maximum material size, the GO gage is identical for both the Class-A
and the Class-B. The GO gauge then can accurately be marked: A;
B; A/B; or it can be left undesignated.
What are the dissimilarities?
In the ISO-228 G-series screw thread standard the minimum material
condition is affected by the selection of the class-of-fit. The
Class-A produces a tighter tolerance product, the Precision Class.
The Class-B has twice the product tolerance of the Class-A and would
be considered the commercial class.
Selection of Class-A may increase screw thread product production
costs. It is common that the tighter the tolerance the higher the
cost of manufacturing. Conversely, there may be a cost saving on
the purchase of the gages. The Class-A NOGO gauge is usable to measure
product that will pass both Class-A and Class-B requirements. For
this reason the gage makers frequently stock only Class-A NOGO screw
thread ring gauges.
Selection of Class-B may decrease screw thread product production
costs. Again it is common that the wider the tolerance the lower
the cost of manufacturing. Conversely, because the Class-B NOGO
gage is only applicable to measure screw thread product designated
as Class-B. For this reason gauge makers sell less of the Class-B
NOGO screw thread ring gage and thus designate the Class-B screw
thread ring gauges as a non-stocked special gage, with the expected
higher price and longer made-to-order lead-time.
In conclusion, here is a suggested G-series screw thread ring gauge
purchasing strategy. Purchase the GO G-series screw thread ring
gage and use it for either Class-A or Class-B regardless of how
it is marked. If your requirement calls for Class-A; purchase the
Class-A NOGO screw thread ring gauge. If your requirement calls
for Class-B; and you are only making a few parts; purchase Class-A
NOGO screw thread ring gage. If your requirement calls for Class-B;
and you have larger production runs; purchase Class-B NOGO screw
thread ring gauge. If you have trouble keeping track of this; buy
both classes-of-fit screw thread ring gages as your screw thread
product requirements dictate.