Metrology
Metrology
When less is more: How a Class X Master Can Outperform a Class XXX
EN-US

When less is more:
How a Class X Master Can Outperform a Class XXX
George Schuetz, Mahr Federal Inc.

This may sound like the wrong title for a gaging column. You'd think we'd be saying something like, "When More is Better," and talking about how gage performance is improved when the gage is larger and has more mass. Take bench stands, for example. Gage stability is based on mass, so a large bench stand, with a heavy base and a substantial post and arm, is going to be much more stable than its lightweight cousin. Having a solid, more stable gage means less deflection, slower thermal reaction, and less measuring error. That makes perfect sense.

But in the case of using hard masters (rings and discs), less can be more. That is, you can actually improve your measuring process by purchasing a lower grade master. A lower class master, certified to size, costs less than its higher class cousin, and when used with its certified master deviation, will provide a more accurate reading.

Master ring and plug gages are made from hardened 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. The table below provides an example of how the tolerance tables work for the various classes of master rings or discs.

Traditionally, the rule of thumb for ordering a master ring for a gage requirement was pretty straightforward: buy the best you could afford. Until recently there was no consideration for what the actual tolerance was. It was assumed that the best class master would assure the best measurement result.

But things have changed in the world of gaging, calibration and traceability. To meet today's ISO and other standards, customers now require documentation about the measurement process on the parts that they are buying. Now, even if manufacturers buy the best class master, they still need a document that says the master is certified to a particular class, or a document that actually certifies the exact size of the master to show that the master was within the tolerances specified for the class of the master.

And here's where less can be more. Let's take an example with a 1" steel master ring. If you order a XXX master ring from a supplier and have it certified for either class or size, you will pay at least double the price of the same 1" steel master ring, class X. And not only will you pay less money for a lower class master, you will also spend less time waiting to get delivery from the vendor. And the XXX ring, if used regularly will be apt to go out of tolerance within a few years anyway.

"OK, George," you're thinking, "so far, this is not exactly rocket science. Where's the 'more?' Tell me how a lower class master can give me a better measurement?"

The better measurement comes from using the information you paid for in the certificate you received. Remember, that certificate gives you the exact size of the master.

The saying is that "information is power," and this is especially true with certified masters. Let's look at the same 1" master ring you had certified to class. It has a potential error of ±15µ". When you put this master on the gage and "zero it out," you are not taking into account this error. On the other hand the 1" class X ring has a tolerance range, or potential error, of ±60µ". But with it you also get a document that shows it is off by some specific amount, say +55µ". Now you can actually "dial in" this master deviation and when zeroed out the gage displays 55µ" instead of "0". What is really happening is that the gage setting with the X master is more precise than the gage set with the XXX master.

But now let's look at the fine print. Most gages don't have resolutions to 1µ". But many do resolve to 10µ", including gages such as air tooling, bench amplifiers and even some good digital and dial comparators. If you can squeeze an extra 40µ" of accuracy out of a gage that is measuring a ±0.0001" tolerance (the difference between the ±15µ" Class XXX tolerance and the +55µ" gage setting), you have just improved the process by 20%. Not a bad improvement while paying 50% less for the master.

The point here is that when you have the capability of displaying to a high resolution and you know the actual size of the master, you can get more accuracy out of your measurement process. By using a master that is less accurate—but knowing the size and using it in your mastering routine—you can get better measurement results: more for less.