Take a master ring used to set a reference on an air plug as an example. The standards define the physical dimensions, the class of the size (that is, how close to the specified size the ring is), the surface finish and form of the ring, squareness of the hole, roundness and cylindricity. Because of these defined conditions, the master ring can be used at any orientation or height along the axis of the ring. While the certification for the size of the ring will typically be measured at three heights at the 12 and 3 o’clock positions, there are form definitions that ensure that the diameter is the same within the roundness specifications that are applied to the ring. This makes it possible to use the ring as a reference anywhere.

When the operator places this master ring on the air plug or on the mechanical bore gage, he or she can zero the gage at any location within the master. It is not uncommon for the user to move the master up and down along its axis, or turn the master to look for diameter variation. This is possible because there is ample clearance between the gage and the master. Additionally, there are no specific reference marks for aligning the master to the gage.

However, there are special masters made for applications which are not defined by industry standards. In these cases, the manufacturer defines the characteristics of the masters and where those characteristics are certified for the size. Special masters that might fall into this category include taper masters (both rings and discs); masters used for special air gages with multiple diameters; and custom masters for fixture gages.

Many of these special masters use witness lines or other marks to indicate the locations for setting the gage. Others are of a specific shape, which will fit the gage only in one direction so there is no opportunity to move or rotate the master in the gage. In these cases, the master is most likely measured at the same specific locations where the gage will measure parts. A certificate will usually indicate these locations and their deviations.

Other special masters, while not necessarily designed for it, can be moved or rotated in the gage, or even be reversed 180 degrees. Taper masters can fall into this classification. When placed in a clearance style taper gage, there is plenty of room for the master to rotate in the gage.

The size on some masters is only certified at a specific location, as indicated by the marking on the gage. Because there is not necessarily any standard defined for the master, other locations may not be within acceptable limits. Often these special masters are so complex that they cannot be held to the same specification throughout their form as with a simple cylindrical master. Thus, the only real sweet area of the master is where it is marked.

Despite the design limitation, rotating the master may seem like the natural thing to do. This is a common procedure when using an air ring with a master disc where the part is going to be rotated in the gage to pinpoint out-of-round conditions. Since the zeroing process involves simulating the gaging process, it would seem appropriate to rotate the master. Nonetheless, doing so may cause the master to wear.

With some types of gages or tapered air rings, there is a reference anvil mounted at the base of the ring. Rotating parts on this anvil may not cause problems, but continually rotating the master on it may eventually cause wear to a depth that adds a small variation to the measurement. This may not even be noticed when the master is re-certified. When the calibration technician is measuring the master, it is placed either on a flat surface or master pins, both of which span the worn area. Therefore, the heights for certification will vary from the heights at which the gage is actually being used. This will cause a gage bias.

The easiest way to avoid this is to instruct operators not to rotate the master in the gage. They should also pay attention to the witness marks noted on both the air ring and the master. On the air ring, the witness marks indicate where the air jets are located. On the master plug, the witness lines note where the master has been measured and certified. When the gage is zeroed, the master witness lines should be aligned with the air ring witness lines. When this is done—and repeated for verification—the gage is being zeroed at the area on the master plug that has been certified.