Quality Magazine

Other Dimensions: And the Other Answer is...

December 1, 2005
More common metrology myths are debunked.

In a recent column I gave answers to some of the more frequently asked questions received by folks who make and calibrate gages and measuring instruments. Some readers may have thought I was being negative since every answer in that column was "NO!"

I wouldn't want you to think I was a negative person since I can be quite positive at times, not to mention neutral as well. To prove the point, I offer the following YES! answers to more common metrology questions that come up from time to time.

• YES! Standards call for Class W setting plugs to be used to set adjustable thread ring gages. Unless you specifically ask for Class W tolerance, however, you will get Class X from the industry. Why? I don't know. Perhaps it is because folks have enough trouble measuring a thread gage to verify that it meets Class X limits and that makes Class W out of the question in practical terms. On the other hand, it could be that most users opt for Class X because it is cheaper.

• YES! Measuring instruments claiming protection to IP65 standards are handy if you are measuring workpieces half-submerged in coolant as shown in many of the advertisements promoting them. Don't forget that the work has to be clean, which is unlikely if it is drenched as shown in the advertisements.

• YES! There is a difference between an expensive instrument and a cheap instrument from the same manufacturer. If the cheap model was as good as the more expensive model (options excepted) the manufacturer would not have the expensive model in his line up. When the price is lower, you can bet other features are lower, such as accuracy, quality, speed and repeatability.

• YES! If a laboratory is properly accredited to ISO 17025 it does not need to be registered to ISO 9000 because that standard is already part of the 17025 standard.

• YES! The operator of an instrument or gage should determine what the calibration frequency should be. This would be based on the circumstances and data from ongoing calibration of the item.

• YES! Gages can be calibrated using a coordinate measuring machine. You can also calibrate them using a dial caliper, but not very accurately-in either case-for most practical purposes.

• YES! Variable gaging can be more accurate than fixed limit gaging for the single element that is being checked. But most variable gages on their own do not check all the characteristics that a go/no-go gage does and are often poor indicators (no pun intended) of functional size or whether the mating part will fit.

• YES! Linear pitch of a thread plug gage can be checked using an optical comparator. Unfortunately, the measurement uncertainty of such a process is too high in relation to the tolerance for that feature, making the results of little value.

• YES! Operators can "measure" thread plug gages using a pitch micrometer or a thread comparator, but not accurately enough for the readings to be taken seriously.

• YES! Cheap sets of thread measuring wires are out there: usually centerless ground. Their tolerance for diameter alone is often greater than the tolerance on pitch diameter for a thread plug gage, which means they are not satisfactory for calibration work.

• YES! The metric system is used worldwide. How it is applied can be radically different from one country to another, especially when it comes to fixed limit gages. And in the case of thread gages, the method of measurement is significantly different as well. The same applies to inch-based thread gages. Many importers of such gages and laboratories calibrating them are not aware of this.

• YES! There are rules for measurement and calibration. No one can break them without compromising the results obtained. But that's life on the physical plane. You could try to escape them by working in a parallel world of the kind that physicists are now considering. But to span two realities requires abilities attributed to a medium and that takes a lot of training for most folks.

Following the rules on this plane is easier, because this is where the lawyers and the courts will be waiting for you when your measurements cost other people money-or lives.