GAGING: PLAIN CYLINDRICAL “PIN” GAGES

GO-NOT GO plain cylindrical gages are used to check plain holes to ensure the holes are within tolerance. If the GO member is long enough to check the entire depth of the hole, it is also checking for straightness from top to bottom.

Special design “locating” gages can be used to check the position of holes. One of the most common uses for plain gages is to check the minor diameter of tapped holes produced by a drill prior to tapping.

Plain cylindrical gages are available in four classes, “XX”, “X”, “Y”, and “Z”. “XX” gages are made to the tightest tolerance and used for master or setting gages, or for part tolerances that are very precise. “X” tolerance are sometimes used for master gages or gaging close work. “Y” and “Z” gages are generally used for inspecting work pieces only.

How do you know which class to select?

Selecting the right class of cylindrical gage is not only important to ensure that it rejects “bad” parts, it is equally important to minimize rejecting valuable “good” parts.

If a lower class gage is used on a tight tolerance part, the gage tolerance will consume most of the part tolerance, rejecting good parts as being bad.

Gage manufactures call this “thievery”, stealing part tolerance for the gage tolerance.

To ensure accepting the maximum number of good parts, select a class using the “5% rule”.

That is, the gage tolerance should be no more than 5% of the part tolerance. For example, if the part tolerance is 0.001”, the gage tolerance should be no more than 0.00005”. Select a class that has a 0.00005” tolerance or less.

Gage catalogs or gage standards publish charts showing the classes and tolerances.