Metrology
Metrology
Feels Like A Go To Me
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Feels Like A Go To Me
George Schuetz, Mahr Federal Inc.

Once upon a time, an overly enthusiastic QC manager appealed to me, confused and dissatisfied.  Here he was, spending good money to purchase very high quality masters, but his inspection process was no better than before.  What was worse, his masters went out of calibration rapidly, pushing his costs even higher.  The problem was that he was buying more accuracy than he could use.

Choosing the right tool for the job applies to mastering, just as it applies to every other area of gaging.  While it may be possible to master a gage using a variety of standards, the best master for a job strikes a balance between accuracy, economy, durability, and ease of use.

Gage blocks are "primary standards," directly traceable to an "absolute" standard maintained by NIST, DIN, or ISO.  Masters are "secondary" standards, because their sizes are established by reference to primary standards.  While masters typically have a higher level of uncertainty than gage blocks, they are often the appropriate choice for production gaging.  Gage blocks, after all, are square, while masters are typically round.  If the parts being measured are round, and the gage is designed to measure round parts, the use of a round master will help avoid certain sources of geometry error.

A master ring or ring gage is basically a bore of a known dimension.  The same device can often be used as a setting master for variable inside-diameter gages (such as bore gages, air tooling, and mechanical plug gages), for go/no-go mastering of fixed ID gages (such as a fixed plug gage), and for go/no-go OD inspection of male cylindrical workpieces.

Ring gages are made from steel, chromed steel for durability and corrosion resistance, or tungsten carbide for extreme wear resistance.  They are classed by level of accuracy, with XXX indicating the tightest tolerances, XX, X, and Y being intermediate grades (in descending order), and Z being the lowest level of accuracy.  Class tolerances vary by size: larger sizes have higher levels of uncertainty.  Tolerances may be bilateral (i.e., evenly split between plus and minus around the nominal dimension), for use in setting variable gages, or unilateral for use as go/no-go gages.  For rings, "go" is minus (-); for plugs, "go" is plus (+).  Go/no-go gages may often be identified by a groove or ring on their knurled outside diameters.

Plug gages, for go/no-go measurements of part IDs, or for mastering ID gages, are also available in different materials and classes.  Plug gages may be reversible or double ended, with a "go" end signified by a green stripe, and a "no go" end signified by a red stripe.  Usually available only in sizes up to about 0.76", reversible plug gages can be disassembled to replace a worn end.

Plug gages are often identified by the names of their handle or mounting designs.  Taper-lock plug gages usually range from 0.059" to 1.510", and have a handle on only one end.  Tri-lock designs, also called discs, range from 1.510" to 8.010", and have handles on both ends of the mastering surface.  Annular designs, for sizes from 8.010" to 12.010", are like wagon wheels, with handles for axles.
 
Specialty masters are available for a range of applications and odd shapes, including slots, splines, and tapers.  Tool holder taper geometry is of increasing importance in precision machining, and manufacturers have begun to pay closer attention to taper quality.  Taper plug gages can provide an indication of whether an ID taper is too steep or too shallow, or if the bore entry diameter is within tolerances.  Inside and outside taper masters are also frequently used for setting taper air gaging.  Such special purpose masters make mastering and measuring quicker and easier, and usually cost more than standard gages.

In general, one should choose a master whose tolerance is 10 percent of the precision of the gage, while the gage's precision and repeatability should be 10% of the part tolerance.  For example, if part tolerance is 0.001", gage precision should be 0.0001", and the master's tolerance should be 0.000010".  It's usually not worthwhile to buy more accuracy than this "ten to one" rule: it costs more, it doesn't improve the accuracy of the gage, and the master will lose calibration faster.  On the other hand, when manufacturing to extremely tight tolerances, a ratio of 4:1 or even 3:1 between gage and standard might have to be accepted.

Finally, here are some general guidelines for the care and feeding of masters: store them in a secure place; use a wax- or oil-based sealant to protect against corrosion; handle carefully don't force or jam them onto the part; don't try to modify them; and when shipping for calibration, take steps to protect masters against damage and corrosion.