Calibrating Gages: With Master (And Engagement) Rings, Size Matters

A master ring, or ring gage, is basically a bore of known dimension. The precision hole is often used as a setting master for variable ID gages (such as bore gages, air tooling and mechanical plug gages), for go/no-go mastering of fixed ID gages 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 often classed by level of accuracy, with XXX indicating the tightest tolerances; XX, X and Y are intermediate grades; and Z is the lowest grade. Class tolerances vary by size. Larger sizes have more open tolerances because they are harder to manufacture. Tolerances may be bilateral for use in setting variable gages or unilateral for use as go/no-go gages. For rings, “go” is minus; for plugs, it’s plus. Go/no-go gages may often be identified by a groove or ring on their knurled outside diameters.

For example, for a 0.820-inch master ring, the following tolerances would apply:

Class XXX = 0.00001 inch
Class XX = 0.00002 inch
Class X = 0.00004 inch
Class Y = 0.00007 inch
Class Z = 0.00010 inch
Of course, the better the class, the higher the price. If you want to stay at a five-star hotel or get the highest grade for your engagement ring, you have to pay for it. It’s the same with master rings. The XXX ring is manufactured to a tighter tolerance, so there is cost involved. It may take longer to manufacture, take the skill of a higher paid technician or, if something goes wrong, it may have to be remanufactured and take longer to get.

Typically, the rule of thumb for selecting a master has been to choose one whose tolerance is 10 percent of the part tolerance. This, combined with the gage’s performance, should provide adequate assurance of a good measurement process. It’s usually not worthwhile to buy more accuracy than this 10:1 rule: It costs more, it doesn’t improve the accuracy and the master will lose calibration faster. On the other hand, when manufacturing to extremely tight tolerances, one might need a ratio of 4:1 or even 3:1 between gage and standard simply because the master can not be manufactured and inspected using a 10:1 rule.

Take a taper master, for example. Say the angle tolerance is 0.001 inch over a 12-inch long taper. Usually, a gage or master is not 12 inches long; rather, it might be 1 inch long. At this length, the same tolerance now becomes 83 microinches. Using the 10:1 rule, the master would have to be 8.3 microinches. Such a gage would be virtually impossible to manufacture or even measure.

But there are alternatives that can allow these tolerances to be measured and can reduce the cost of your masters. Masters can be certified to their class (XXX, XX, X and so on), or they can be certified to their size. This means you may have the tolerance that requires a class XXX master ring, but a suitable replacement might be a XX ring certified to size. What you will get with this ring is a certificate that documents its size at various locations as well as the calibration lab’s measurement uncertainty.

Now you know the ring met the XX class, and you know the exact size of the ring. You can use this information to your benefit. When setting the gage to its reference (usually zero), set it to the actual master size. In effect, you are getting XXX performance from your XX ring. You’ve saved some money and probably sped up the delivery of your gage.

Wouldn’t this be great if this worked for engagement diamonds, too? That zirconium looks awfully good.