Jeff,
The comments of the other contributors are all valid. We are a calibration house that does the calbration of tons of plug gage sets. We have several recommendations to our customers regarding plain plug gages. First, when we do a calibration we offer the service of "dip sealing" the pins after the calibration. The customer would then modify their procedures and label the boxes that the rule is to remove the dipseal before use. We find that most companies use roughly 30 % of the pins in a set, therefore making the calibration of all of them yearly, redundant and costly. With reference to using a micrometer to calibrate a .0002 tolerance plug, this does not meet the 4 times rule of calibration. The tool being measured must be calibrated by a tool at least 4 times more accurate. Manufacturers specifications for micrometers are typically +/- .0001, giving a range of .0002 to the mic's accuracy defeating the validity of the calibration. There are several ways to address this. If the work you are producing does not truly require .0002 accuracy pins, if your tolerance is more wide open, you may want to open the acceptable tollerance of the pins to say .0004 or .0008 thus making calibration by a micrometer valid. Or, obviously you can procure a more accurate measuring tool such as a calibration center. The quality of .0002 pins in general is often in question. They are manufactured from 52100 steel that does through harden, but the hardening process does not cause the complete transition of austenite to martensite. Without complete transformation, steel will grow with time as the transition occurs naturally. Gage manufacturers solve this process by cold stablizing the gages multiple times to complete the transition. The question of if Import gages are treated in this manner is often in doubt. Further .0002 gages are typically not round. The are manufactured by Cylindrical Grinding which leaves lobbing in the gages. Higher quality gages are typically Cylindrically Lapped, which produces a round pin. Several good american manufacturers produce pins in this manner including Glastonbury Southern Gage. Finally, many of the import sets are shipped with paper wrapped around each pin with a cosmoline type substance that is time consuming to remove. Good American sets like those manufactured by Van Keuren are not packaged that way. I hope this information helps. If you would like more info please feel free to call. Art Stauff, Angrave Metrology & Supply 203-574-1316
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Art Stauff
Angrave Metrology & Supply
