What is a fixed limit gage The word gage

							                                           What is a fixed limit gage?

The word gage, when used by itself, is about as general and unspecific in its meaning as the companion word
tool . In discussing gages, we must first narrow the field down to a manageable size. First, then, we are
talking about production gages, which came into being as a necessary step in the development of assembly-
line production. When a manufacturing company undertakes to produce a complex assembly, like a new
Wankel engine, it is generally economical to assign production of the numerous parts to subsidiary plants and
subcontractors where the specialized machines and skills are available. It follows that large numbers of shafts
and bearings, screws and nuts and other assorted parts may be stockpiled at the assembly plant without
having been test-fitted. If it is now possible to select parts at random and fit them into a functioning assembly,
it is only because gages existed and this manufacturer knew how to use them.

Machine operators, working on parts with critical dimensions, were provided with gages, so that they could
keep a check on wearing tools and other machine problems which prevent absolute uniformity in their work.
The parts were then carefully spot-checked with gages in the subcontractor's inspection department before
being shipped, and again at the assembly plant when received. This defines the area occupied by production
gages. They are not often used in a home workshop. But in a complex industrial organization, they are all-
important.

In order to limit this discussion to a manageable level, we are going to talk exclusively about fixed-limit gages,
which are basic and foolproof in their use. Fixed limit, or Go / NoGo gages are 'Attribute gages'. Attribute
gages check the extreme limits of the product tolerance letting you know the product is within its manufacturing
limits. This will eliminate the extremely sensitive measuring equipment, which must be used under controlled
laboratory conditions. It will also eliminate many devices, which have been designed to check concentricity,
squareness, parallelism, exact position and other relationships between features, which are required in
assembly. We are now restricted to checking a blueprint dimension given for a single feature - the diameter of
a hole, the effective size of a screw thread, thickness, length, and the like.

PLUGS AND HOLES

The nominal, or "name" size of this hole is 1/2 - inch. If the draftsman wanted to give the impression of great
precision, he could name it .500000-inch. But that, too, is only a name. The nature of matter is such that no
hole made with human tools can be precisely .500000-inch across all diameters measured at various angles
and at various depths. When measurements are made in millionths of an inch, there is no such thing as
perfectly round, perfectly straight or perfectly parallel. Further, no two holes will be precisely the same. A little
matter has worn off the drill during the first pass and now is slightly smaller for the second pass.

Given this state of affairs, the design engineer must decide how close to .500-inch this hole needs to be. This
will depend, of course, on the performance expected of it when the parts are assembled. If it is simply a
clearance hole, it can be .500" more or less. If it must freely accept a half-inch shaft ( running fit ), it can be
no smaller than .500". If it is to make a rigid joint with a half-inch shaft ( force fit), it must be no larger than
.500". Unnecessary precision is expensive. The more leeway the design engineer can allow the machinist
who makes this hole, the cheaper the part will be. So he draws upon his experience and decides, let us say,
that a variation of .004-inch would be tolerable. This is the tolerance on the dimension. His instructions on
each of the three holes mentioned above would be revised as follows:
                                          Type of hole Drawing call-out

                        No smaller than .500"                 .500"/.504" or .500" + .004"

                         No larger than .500"                 .496"/.500" or .500" - .004"

                         .500", more or less                  .498"/.502" or .500" +/- .002"

Instructions of this type have a clear meaning to machinists and inspectors. The hole may not be perfectly
round and it may be slightly tapered along its length. But the diameter may not be less than the low limit or
greater than the high limit. A machine operator would obviously have to make several instrument readings on
each piece to be sure these conditions existed. This would require more of his time than he had spent making
the hole. So he uses a cylindrical plug gage, which has been made for the sole purpose of checking this
particular hole. It's quick, it's easy and it does not require any special skill.

Go-No Go

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 between. 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 taper lock 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 gage makers. Class Z, the coarsest of the standard tolerances, permits the gage
maker 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 gage
makers' 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)