Why is Full-form Calibration Necessary

In many facilities it is common to find laboratories measuring just pitch diameter and major diameter on thread plug gages. This practice is inadequate, and problems with the setting of manufacturing gages will occur due to errors with lead and angle that can go undetected. The majority of calibration laboratories are issuing thread gage calibration certificates specifying just pitch diameter and major diameter values. Without measuring lead and angle to their specified values (per ASME or other pertaining document), non-conforming gages will go undetected, and non-conforming threads will enter the manufacturing process.

If a threaded component or thread plug gage has lead and/or angle error, the functional size of the thread will increase, creating the following problems:

  • A technician using a set plug with lead or angle error will set ring gages incorrectly, indicating gages are oversized.
  • A technician using a work plug with lead or angle error will reject internally conforming holes and accept oversized threaded holes. When the mating part does not fit properly, many companies will compensate for this condition by re-working the product thread.

According to ASME B1.2-1983, ASME B1.16M-1984, and FED-STD H28-20, a proper calibration certificate contains measurements for the following thread characteristics:

  • Minor Diameter
  • Major Diameter (Front and Back)
  • Pitch Diameter (Front and Back)
  • Lead (Front and Back)
  • Lead over Engagement
  • Leading Angle
  • Trailing Angle
  • Included Angle

With the data provided by a full-form certification, the manufacturer is able to guarantee that their working gages are able to ensure conformance.

Without a full-form calibration certification, non-conforming threaded product will enter the manufacturing process which may lead to serious assembly or failure problems in the field.