Classification of Thread Gauges (Metrology)

14.2.
These can be mainly classified in two broad groups, i.e. one group comprising gauges used to check the product and the other used for reference. In first group are the working gauges which are used to check the product as it is being manufactured and the inspection gauges which are used to determine the acceptance or rejection of the product. In the second group are setting or checking gauges which are generally plug gauges with the help of which adjustable thread ring gauges, thrpad snap gauges and other thread comparators are set for checking size of master or basic gauges (threaded plug gauges representing the physical dimensions of the normal or basic size of the part).
In first group of gauges the working gauges are sometimes set to limits which are within the limits of inspection gauges. This practice ensures that any part which is passed as being within tolerance by the working gauges will also be passed by inspection gauges, thereby, reducing the possibility of a disagreement between the machine operator and the inspector in border line case ; a disagreement which often arises when working and inspection gauges are set to identical limits.
Working, inspection and master gauges differ in respect of the accuracy with which these are made. A working gauge is made to the widest tolerance and is, therefore, the least accurate, while master gauge has narrowest tolerance and is the most accurate. The accuracy of inspection gauges lies in between, depending upon the intended application.