GO-NO GO GAGES

The concept of Go-No Go gaging  is so simple in its logic that it has been known to delight philosophers.  For them, a Go-No Go situation is the very rare case where there are only two alternatives - black and white - with no gray in betwee- n.  In practice it is not quite so simple as this, since there are a few borderline cases which require a little human judgment.  But the ground rules for using the cylindrical plug gage are certainly not complicated.  If the Go end of the gage slips through the hole and the No Go will not enter, this is necessary and sufficient proof that the hole is within the tolerance limits assigned to it.  Pick up the taperlock plug gage and examine it.  This gage was made to inspect the hole which the design engineer specified must be .500" +- .002".  The GO member of the gage is therefore no smaller than .498-inch, the low limit of the hole tolerance.  The NO GO member is no larger than .502-inch, the high limit. 

Now the question naturally arises, "How much larger than. .498-inch can the GO end of the gage be?  And how much smaller than .502-inch can the NO GO be?"  In other words, how much tolerance can the gage maker be allowed?  Though he may spend many tedious hours finishing off a piece of work like a jewel, he still works under the same natural laws as the man with a boring tool.  He must have some tolerance too.  The generally-observed rule-of-thumb, which has some statistical reasoning behind it, is that the gages can have a total tolerance no greater than ten percent of the part tolerance.  In the case of the .498"/.502" hole, a total tolerance of .004-inch has been allowed.  Then the overall tolerance permitted for the gage cannot be more than .0004-inch, which must be distributed between the GO and NO GO members.  Splitting it evenly between them would be permissible.  The GO diameter could have the limits .4980"/.4982" and the NO GO .5018"/.5020".

There are ways of dealing with the permitted gage tolerance which will seem quite logical when the life cycle of a fixed limit gage is better understood.  As an example, the idea of a wear allowance on the GO member should be studied by those who plan to go into quality control work.  For the present, let us settle for the fact that the gage in hand was made to one of the several classes  of tolerance which have been adopted and standardized by American gagemakers.  Class Z, the coarsest of the standard tolerances, permits the gagemaker to deviate .0001-inch in one direction only on each of the gage members.  So in this case, the GO member can be .0001-inch larger than .498" and the NO GO can be .0001-inch smaller than .502".  When the tolerance is thus restricted to one side of the "name" size, with none permitted on the other, it is called unilateral tolerance.  When tolerance is permitted on either side, it is called bilateral.

In the future, metric dimensions will probably appear with increasing frequency on American drawings.  This is not a serious problem in dealing with simple linear dimensions like these, where conversion factor: one inch = 2.54 centimeters = 25.4 millimeters can be used.  Metric threads do present a sticky conversion problem as will be seen in discussion of threads.

Since the very small decimal numbers which must be used in discussing gage tolerances will be new to some readers, it may be helpful to know what they are called in the language of the machine shop.

.001"    one thousandth (of an inch)

.0001" one tenth (of a thousandth)

.00001" ten millionths

.000001" one millionth

Using this vocabulary, then, the figure .00012" is read "one tenth and twenty millionths."  Standard gagemakers' tolerances for cylindrical gages up to .825-inch in diameter are as follows:

Class XX    .00002" (twenty millionths)

Class X    .00004" (forty millionths)

Class Y    .00007" (seventy millionths)

Class Z    .0001" (one tenth)