Metrology
Metrology
Making a Budget and Sticking to It
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Making a Budget and Sticking to It
George Schuetz, Mahr Federal Inc.

Over the past few months we have talked a lot about the gaging process and evaluating it through various testing methods such as GR&R studies. We have also seen that there are other factors that influence gage performance, such as gage linearity, long term stability, and bias from the gage design. Combined, these measuring system-based factors that influence measurement results are called the 'Measuring Uncertainty.'

For this reason, it's not too surprising that measurement uncertainties are disclosed by facilities doing measurement standards, such as gage blocks, master rings and discs. But it is also not uncommon for measuring uncertainties to be taken into consideration on instruments used for the inspection of products. Only by determining the uncertainty of an inspection system for production parts can you determine what part of the tolerance band is "left over" for actual production.

Thus, every measuring instrument or system has an uncertainty budget that you need to know in order to determine if it meets your needs. Drawing tolerances—which are often extremely close—are narrowed even more if the measuring uncertainty is too high. The upshot is that imprecise measuring devices increase production effort and, therefore, cost.

The present internationally approved standard for the determination of measuring uncertainty is the GUM method (Guide to the expression of Uncertainty in Measurement). The first procedural step in determining uncertainty is the enumeration of all the influence quantities. Here's a partial list of influencing factors that may cause measurement errors:

i. Environmental influences: Temperature (fluctuations), radiant heat (e.g., from the operator or the lighting), air refraction/gradients (influencing optical based systems, including lasers), humidity, vibrations and shocks.

ii. Influences related to the part being measured: Fixturing method, alignment, distortion through measuring force and dead weight, size and type of the gaging and datum surface(s), roughness of the gaging surface, undetected form errors of the gaging surface, form errors of the datum surface(s).

iii. Influences caused by the reference standards: Whether an internal reference or an external standard such as a gage block, master ring/disc, etc., standards can influence the measurement by being out of calibration, dirty, nicked, or by having some form of surface finish issue.

iv. Influences caused by the operator: Misinterpretation of the drawing specifications; excessive clamping force when fixing the part under measurement; selection of the wrong probes; selection of the wrong parameters (e.g., wrong profile filter, excessive measuring speed); programming errors and computation errors; heat radiation; physical shocks. For this reason, it is important that operators have comprehensive training in metrology, and in the correct adjustment and operation of their measuring instruments.

v. Influences caused by the measuring instrument: Gage design, robustness for the measurement, alignment issues, sensor linearity, deviations of the measuring axes, irregular movement during measurement, errors in the electronic indicating and control system (e.g., rounding errors), software errors.

The choice of measuring instrument has considerable influence on the number of factors that determine measuring uncertainty. In effect, just as you (hopefully) set a budget for your monthly living expenses, choosing a gage or measuring system sets a budget for the measurement uncertainty of your part tolerance.

For example in your personal budget you might plan on spending 25% of your total budget for rent. Anything more than that might negatively affect the way you live. But once set, this becomes a fixed cost, something you can't really change unless you take a drastic step like moving. The same is true with your gaging process. The gaging uncertainty you determined for your measuring system might be up to 25% of the total tolerance range for some tight tolerance parts. This may be a lot, but just like the drastic step of moving to change the rent, you may have to change the system to change the uncertainty. But there are trade-offs.
 
The cost of acquiring a gaging process with a lower uncertainty and thus using less of the tolerance span may be well outside of your dollar budget for gaging. With such tight tolerances today, it is not uncommon that even the best gaging process will use up a significant part of the overall tolerance. On the other hand, lowering the gaging cost with a process having a high uncertainty to tolerance ratio may be too risky and cost more by letting bad parts pass through.

All planning decisions should take into account the total tolerance band and the budget that you can afford to live with when choosing the gage for the part measurement.

             

Just as you set a budget for your monthly living expenses, choosing a gage or measuring system sets a budget for the measurement uncertainty of your part tolerance.