Like many precision measurement tools, Pin gauges are very simple, but also very effective. With proper use, these precision tools can measure holes to a high degree of accuracy.
The important words here are proper use and proper care. Because these pins are so basic and seemingly inexpensive, they are very often abused and misused. A bit like reliable people who make few demands, others tend to take advantage of them!
Proper care for pin gauge sets
Your gauge pin set should be calibrated at least 1 time each year sometimes 2 times per year (depending on usage). Keep you sets clean and free from rust. After use, wipe the fingerprints off and return them to the box. Keep a sheet of rust-proofing paper in the top cover to help control rust. With simple care, your pins will last for many years.
Fingerprints often have a PH level of oil that causes rust. Some people have particularly high levels of this in their body oil and must take extra precaution to remove the fingerprints.
Do not force the pin into the hole you are checking. This seems obvious, but in the rush to get things done, many pins become damaged for this reason. Gauling is when metal is transferred from one part to another. Gage pins can become damaged from gauling when forced into a hole that is simply too small.
Take care to clean the hole, remove any burrs and chips. Any of these can cause gauling and ruin the pin gage.
Pin Gauge tolerances
For general use in a tool and die, injection mold making or precision machining operation, a class ZZ pin is sufficient.
Class ZZ Gauges - These gauges have an inch tolerance of .0002” and a metric tolerance of .005mm. They are practical where good precision and speed are important.
Most shops use pins that are +.0002 and -.0000. This works out well because you generally need some clearance in the hole anyway to accept the mating part.
Class Z Gauges - These gauges 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. These pins are less common, but still very useful for precision machining and inspection.
Your gage pins should be traceable to NIST - National Institute of Standards and Technology and have a Certificate of Class Conformance - Assurance issued with each shipment of quality gages that conformance characteristics are traceable to NIST. Your gauge pin set should be calibrated at least 1 time each year (depending on usage)
Types of gage pins
The plug gage is the most commonly used. However, there are custom gages for any given job as well. There are thread gages, ring gages, taper gages, master setting discs, and many more. Any time you need to inspect a part, you need some kind of gage, and if you don't have one, you can either make one or buy it.
Quality suppliers would include Starrett, Meyer, Mitutoyo, and Vermont Gage. All of these companies offer high quality, traceable gages that you can count on.
Toolmakers, injection mold makers, and precision aerospace machinists use pin gages regularly. They are an indispensable precision measurement tool.