Metrology
Metrology
How to Become a Master of Deviation
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How to Become a Master of Deviation
George Schuetz, Mahr Federal Inc.

A few issues ago we wrote about how "less" can be "more," by purchasing lower grade master rings and discs and using their certified size their Master Deviation to actually improve the accuracy of your gaging process. Well the topic has sparked a number of questions and not a little confusion; primarily because we did not have space to talk about actually applying this technique on the shop floor.

First, let's quickly summarize: master rings and plug gages 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. Of course, the higher the class, the more difficult the master is to manufacturer, the longer it takes to get, and the more you pay. Tighter tolerance masters also wear more quickly. Traditionally, the rule of thumb was to buy the highest accuracy you could afford as it was assumed the best class master would assure the best measurement result.

However, because of ISO and other standards, masters now come with documents that certify their actual size. The difference between the nominal class size what the master is supposed to be and what the size of the master is actually measured to be is referred to as the Master Deviation. The uncertainty of this number is most likely smaller than the deviations you will find with a master made to a specific class size, and you can use this number to actually improve your measurement performance using a lower class master.

For example, a 1" class XXX master ring has a potential error of ±15µ". A 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". Many gages, such as air tooling, bench amplifiers, and even some good digital and dial comparators can resolve to 10µ", and by "dialing in" this master deviation when zeroing the gage, you may be able to 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). Thus, you improve the process by 20% while paying 50% less for the master.

Using a dial indicator gage with a master deviation is pretty easy. If the master is off by +0.0001", the setup person would adjust the gage to master at 0.0001" rather than zero. In effect the operator is making the gage read what the master actually is. If a part is measured at the true nominal size, it would read 0.0 on the gage. This is even easier on many of the electronic gages in use today. Most of them have a procedure for entering the master deviation into the setup routine of the gage. Once set, whenever the gage is "zeroed" it will show that actual deviation as seen on the certification.

However, there is a trade off in that this process will require a little more work on your part. In order for this technique to work effectively on the shop floor, you and your operators will have to start keeping track of the master deviations by gage, and ensure that everyone using the gages is aware of the master deviation. Processes will have to be set up to regularly measure and certify the master to its particular size. This number must be tracked by your quality department, and users must be aware of the offset and account for it. The number is apt to change annually so this is a long term process that has to become an integral part of your gaging routine.

But these efforts will also pay another long term dividend. Once you have set up the shop wide process of using master deviations, you can start monitoring changes in the masters. Calibrations from year to year would normally show some wear in size and probably a small amount of change in the form of your masters. If over the course of a year, you find that the size has changed more than 10% of your part tolerance or more than 20% for tighter tolerance parts something significant has happened to your process and you might want to question some of the parts you have measured since the last calibration.

Thus, changing to a Master Deviation based measuring process requires a different process on the floor, but it can save big dollars in master costs and rework in the short run and provide an excellent long term check on your manufacturing process.

By using the Master Deviation from the Calibration Certificate to set up your gage, overall performance of the measurement can be improved, often using lower class masters.