Making a Budget and Sticking to It

We have talked about the gaging process and evaluated 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 linearity, long-term stability and bias from the gage design. Combined, measuring-system-based factors that influence results are called the “measuring uncertainty.”

For this reason, it’s not too surprising that measuring uncertainties are disclosed by facilities making measurement standards, such as gage blocks, master rings and discs. However, it is also common for measuring uncertainties to be taken into consideration on instruments used for inspecting products. Only by determining the uncertainty of a production system’s inspection system can you determine what part of the tolerance band is “left over” for actual production.

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 present internationally approved standard for determining measuring uncertainty is the Guide to the expression of Uncertainty in Measurement (GUM) method. 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:

• Environmental influences. These include temperature (fluctuations), radiant heat (from the operator or the lighting, for example), air refraction/gradients (which influence optical-based systems such as lasers), humidity, vibration and shocks.

• Influences related to the part being measured. Fixturing methods, alignment, distortion through measuring force, and dead weight all influence the part being measured, so do the size and type of the gaging and datum surface(s), the roughness of the gaging surface, undetected form errors of the gaging surface and form errors of the datum surface(s).

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

• Influences caused by the operator. These can include misinterpretation of the drawing specifications; excessive clamping force when securing the part under measurement; selection of the wrong probes; selection of the wrong parameters (such as wrong profile filter and excessive measuring speed); programming and computation errors; heat radiation; and physical shocks. It is important that operators have comprehensive training in metrology as well as in the correct adjustment and operation of their measuring instruments.

•Influences caused by the measuring instrument. The measuring instrument can influence gage design, measurement robustness, alignment, sensor linearity and measuring axis deviation. It can also cause irregular movement during measurement; errors in the electronic indicating and control system (such as rounding errors); and software errors.

The measuring instrument has considerable influence on the number of factors that determine measuring uncertainty. In effect, 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.

For example, in your personal budget you might plan on spending 25 percent of your total budget for rent. Anything more than that might negatively affect the way you live. Once set, this becomes a fixed cost, something you can’t really change unless you take a drastic step, such as moving. The same is true with your gaging process. The gaging uncertainty you determine for your measuring system might be as much as 25 percent 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. 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 tight tolerances, it is not uncommon that even the best gaging process will use 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 passing bad parts.

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