Ring gages used for gaging of externally threaded parts, aid in establishing physical limits for Maximum Material size and thread function. In addition, ring gages can help detect a lack of roundness or surface discontinuities on the threads.
A Go ring gage simultaneously checks the thread elements of form, lead and pitch diameter in a cumulative manner. It is manufactured to the high limit of the thread tolerance allowing for clearance of the minor diameter and major diameter of the screw. A Not Go gage is produced to the low limit of the thread tolerance, and ensures that the size is larger than the minimum pitch diameter of the screw.
When using a Go gage, a correctly sized screw thread will pass completely through the gage with little or no effort, whereas an oversized screw thread will not pass through the gage. Conversely, a part must stop in a Not Go gage within three turns. If it passes through the Not Go gage, it is considered undersized and is unacceptable. Because ring gages tend to have frequent contact with work pieces and are exposed to the everyday shop environment, it is important that they maintain a strict calibration schedule.
Thread setting plugs used to calibrate ring gages can be found in “Truncated” or “HiLo” designs. With the truncated setting plug version, the ring gage is adjusted to fit snugly on the truncated section of the plug, then carefully screwed onto the full form section. A properly fitting ring gage will have little or no difference in the feel while passing from truncated to full form. Reliability on the accuracy of human judgment or good feel can be a concern using truncated setting plugs.
The HiLo setting plug version removes most of the uncertainty by allowing the setting of ring gages while taking the judgement factor out of calibrations. The “Lo” end of the setting plug member represents the low tolerance limit while the “Hi” end of the plug member represents the high tolerance limit.
Adjusting the ring gage so it will accept the Lo or front of the plug with a drag without threading more than one and one half turns onto the Hi or back section of the setting plug, will achieve the correct setting. Areas the HiLo setting plug examines are worn flanks, angle checks and bellmouthing conditions.
Keeping ring gages clean and free of debris is very important. Oil, chips..ect. can aversely affect the performance of the ring gage not only during calibration, but also in use inspecting screw threads. Using bristle tube brushes frequently, which relate in size to the major diameter of the ring gage and compressed air, will help keep it from “loading” up with debris while in use.