Quality Magazine

Other Dimensions: And the Answer is...

October 1, 2005
Common metrology myths are debunked.

Gage manufacturers and calibration laboratories are asked many technical questions during the course of the day. Often the person asking the question has a pet theory about the answer and disputes what the gage maker or lab offers as the "right" answer.

This can be quite frustrating for the person trying to give reliable, technically correct answers. It is common for a customer to receive an answer supporting his theory or practice coming from someone who may know even less than he does. But because it supports his view on how things should be, it is more believable than answers that shoot his theories or practices down in flames.

Most folks ask questions because they really want to know. When they get conflicting answers, they often don't know where to turn for accurate ones. Others, however, feel that gravity, physics and all that other stuff don't apply to them. The latter group is responsible for giving energy, and thus life, to some of these metrology myths.

At the risk of being hung, drawn and quartered the next time I venture forth in public, I offer the following answers so others will not have to continuously do so:

• NO! Setting your adjustable thread ring gages to a working thread plug gage is not an acceptable practice by any standard I am aware of. I know setting plugs are expensive compared to working plugs, but they are exclusively designed with this purpose in mind.

• NO! Your handheld digital micrometer is not good enough to calibrate plug gages to any known standard tolerance with any degree of certainty. It doesn't matter how many digits its display has.

• NO! The fact you can repeat a measurement many times over does not mean that it is an accurate measurement. It means your measurement process repeats well which is essential to an accurate measurement.

• NO! Gages don't automatically come with a correct calibration report. This is because it costs more to provide this benefit and most customers won't pay for it.

• NO! Certificates of Compliance are no substitute for a calibration report and should never be accepted in lieu of one, no matter who issues it.

• NO! Gage makers' tolerances are called by that name because that's whom they are for. They are not meant to be your recalibration acceptance criteria.

• NO! Steel gage blocks are not as stable as the Egyptian pyramids. They move around over their life, meaning that even if they're never used, their calibrated "length" may change.

• NO! Directly measuring pitch diameter of an adjustable thread ring gage will not give you as good a setting as using the correct setting plug. The reason for this is that most folks who follow this practice do not-or cannot-measure linear pitch, lead variations or roundness variations in the ring, which must be done to arrive at its functional size.

• NO! A calibration report from a non-accredited OEM is not considered acceptable if there is a properly accredited laboratory available to do the work.

• NO! A two-day seminar or workshop cannot create a dimensional metrologist. It doesn't matter whether you believe in intelligent design or the theory of evolution, it takes a lot longer. Having said that, it is possible that reincarnation could explain some fast learners.

• NO! Calibrating gage block lengths using optical flats is not a practical proposition. It may be a great way to demonstrate certain optical principles, but that is all.

• NO! A calibration laboratory registered to ISO 9001 standards does not have to meet the same criteria as one that is accredited to ISO 17025. In the latter case, the laboratory will meet the ISO 9001 requirements and much more. ISO 17025: 2005 notes that a laboratory complying with 17025 meets all the requirements of ISO 9001.

• NO! Class ZZ tolerance gage pin sets are not made to be used as go/no-go plug gages. The standard they are made to allows them to be out of tolerance a 1⁄4 inch, or 6 millimeters, from each end and still meet the standard. This makes them next to useless for checking blind bores.