Metrology
Metrology
Picking the Right Gage and Master
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COMMONLY ASKED QUESTIONS:
Picking the Right Gage and Master
George Schuetz, Mahr Federal Inc.

 In this job I get asked a lot of questions.  In fact, I did some figuring the other day, and estimate, conservatively, that we have probably answered hundreds of thousands of gaging questions over the past 10 years.  Some of these questions have been absolutely brilliant.  They have pushed me to learn more about my business and our industry, and to grow professionally.  Some have even helped me develop new products.  Others have been, well... less brilliant.  Those asked most often concern picking gages and masters.  We have talked about various aspects of this process in previous columns, but I thought it would be well to list the questions and answer them directly.  Then, next time someone calls, I can just read the answers!

 Without a doubt, the most common question I am asked has to do with selecting a gage: “I’ve got a bushing with a .750” bore that has to hold  ± 0.001 in.  What kind of gage should I use?”  There are a number of choices: a dial bore gage, an inside micrometer, an air plug, a self-centralizing electronic plug like a Dimentron®, or any one of several other gages.  But picking the right gage for your application depends basically on three things: the tolerance you are working with; the volume of components you are producing; and the degree of flexibility you require in the gaging system.

 For tolerance, or accuracy, we go back to the historic and often dubious ten-to-one rule: if your tolerance is ±0.001 in., you need a gage with an accuracy rating of at least ten times that, or within one tenth (±0.0001 in.).  But that’s not all there is to it.  The gage you pick may also have to pass your own in-house GR&R (Gage Repeatability and Reproducibility) requirements.  Just because we, as gage manufacturers, say a gage is accurate to a tenth, doesn’t necessarily mean you, as a component manufacturer, will actually get that level of performance from it in the field.  GR&R studies are designed to show how repeatable that specified accuracy is when the gage is used by a number of operators, measuring a number of parts in the manufacturing environment.  Since this incorporates the whole idea of ‘usability,’ it makes the process of selecting a gage more complicated.  There is no single standard for GR&R studies, but generally, it is a statistical approach to quantifying gage performance under real life conditions.  Often this is expressed as the ability to measure within a certain range a certain percent of the time.  As “10%” is a commonly quoted GR&R number, it should be noted that this is quite different from the traditional ten-to-one rule of thumb.  But that’s a topic for at least a couple of future columns.  For our purposes here, suffice it to say that if passing GR&R is one of your requirements, you should discuss the details with your gage supplier.

 Component volume is also of prime importance in picking a gage.  How big is the job? How long will it last?  How important is it to the shop?  This will dictate how much you can spend on a gage or gaging system.  Generally speaking, the trade-off here is speed and efficiency for cost and flexibility.  You can get a system that will measure several hundred parts an hour, twenty-four hours a day, if that’s what you need.  But that system is not going to be good at measuring a number of different parts, and it’s not going to be inexpensive.

 The flip side here is flexibility.  It may well be that the decision to buy a gage is based not so much on a specific part, but on overall shop requirements.  That may be a different gage from one which measures a single-sized hole with optimum efficiency.  Finally, consider what you intend to do with the reading once you get it.  In short, do you need digital output?

 After gages, the next most common question concerns masters: what grade and kind to buy.  “Do I need XX or XXX, and what’s the difference?”  The answer here is a bit more direct.  There are several classes or grades of masters, depending on accuracy.  These are Z, Y, X, XX, and XXX, with Z being the least accurate and the least expensive.  Class XX is the most common, with an accuracy rating of ±0.00001 (up to ±0.00005, depending on size).  What class you buy is determined based on the gage, not your part.  Should we use the 10:1 rule of thumb? If your 0.750 in. diameter part has a tolerance of ±0.001 in., pick a gage that is accurate to a tenth (± 0.0001 in.) and a master that is accurate to one-tenth of that, or ten millionths (±0.00001).  In this case, that would be a grade XX master.

 But now, here’s a rub: let’s say you have a tolerance of five tenths (± 0.0005 in.) and you are using an air gage with an accuracy of twenty millionths (±0.00002 in.).  That is certainly better than ten-to-one for the gage, but what class of master do you use?  One that is accurate to two  millionths?  If so, you’ve got a problem, because no one makes them.  What you do in cases like this is buy a master that is Certified for size.  This means it will be accurate to within five millionths (± 0.000005 in.) of the certified size, and will indicate the variation from nominal.

 Finally, people continually ask me about chrome plating and carbide.  “Why should I pay extra for chrome plating, and when do I need carbide gage blocks or masters?”  The answer here is simple, and has to do with the hostility of your gaging environment.  Chrome plating protects against corrosion.  It is also much more wear resistant than plain steel.  So if you have a corrosive or abrasive environment, chrome-plated gages and masters are worth the cost simply because they will last longer.

 As for carbide, I generally recommend using blocks and masters of a material similar to the part you are machining, because of thermal expansion.  Carbide has a coefficient of thermal expansion about one-third that of steel.  If the temperature in the shop changes -- a not uncommon occurrence -- your carbide master will not grow at the same rate as your gage or parts.  However, carbide is extremely corrosion resistant.  Also, it has the highest wear resistance of any master material now in use.  If your environment is so corrosive and violent that steel and even chrome plate do not hold up, carbide may be the answer.