Quality Magazine

Other Dimensions: Searching for Answers

February 22, 2006

In recent columns I presented answers to the most-asked questions regarding measurement and calibration, and I hope some of them have been helpful. As more companies start calibrating their gages and instruments, more questions arise, but many of them are answered in a 28-page document titled, "Searching For Zero."

About 12 years ago the American Measuring Tool Manufacturers Association (AMTMA) published "Searching for Zero" as a guide for anyone calibrating precision tools and gages. It created a stir at that time as many of the beliefs widely held in this field were shown to be just that: beliefs. Since then, it has become a valuable reference document on the realities of dimensional metrology. Many standards writing committees have used it as a source of information in their work.

The first edition sold 17,000 copies; the second edition is completely revised. The first edition included typical uncertainties for various types of calibration procedures so the reader would get a feel for the order of magnitude of it all. Unfortunately, while it did that, it provided fodder for some unscrupulous folks who adopted the values as their own when they had no uncertainty budgets to support their claims. The new edition lists the elements that have to be considered for an uncertainty budget instead of typical values that can be abused.

Two pages are devoted to explaining the concept of measurement uncertainty using simple examples to show how it is applied. This gives people better insight into the process, and coupled with hints from AMTMA members, a means of improving their calibration activities not to mention a better overall awareness of the realities involved.

The layout of the booklet is straightforward with general information in the front section followed by specifics for calibrating different instruments. The format for each is to deal primarily with the most-used method of calibrating a particular item, outline the parameters and the equipment used. A list of elements that should be part of an uncertainty budget is included for that method. This is followed by hints on how to improve the process.

One headache that affects those not directly involved in the manufacture of instruments and gages is finding which specifications, standards and practices apply to a particular item to be calibrated. Like the first edition, the current edition lists these documents and their sources so the technician can be up-to-date in this area.

The AMTMA has adopted a dispute resolution process to reduce the time and money wasted by people fighting over a few millionths of an inch or parts of a micron. Often the measurement uncertainty of either or both parties to a dispute is such that neither could legitimately declare a dimension to be out of limit, but no one wants to give in. The process outlined in this edition is designed to end such stalemates and is expected to be used by AMTMA members and others in this field, as it is universal in nature.

Technicians using precision tools and gages often rely heavily on their suppliers of these items for technical help and support. "Searching for Zero, 2nd Edition" will no doubt be a must read for people in dimensional metrology work. It is straight, to the point and has been reviewed by the AMTMA Technical Committee to ensure it reflects the realities of the industry.

Topics covered in this edition include, terminology of dimensional calibration, common sources of error, general hints for better measurements, an introduction to measurement uncertainty and its meaning, an explanation of an uncertainty budget and calibration guidelines for a variety of gages and instruments.

Copies of "Searching for Zero, 2nd Edition" are available from AMTMA headquarters or any AMTMA member company. Visit the AMTMA Web site at www.amtma.com for more information.

I must confess that I wrote "Searching For Zero." Before you start, no, I don't get paid royalties on each copy sold. I was paid a flat fee up front-$1. Of course, I made sure it was an American dollar rather than lower-valued currency with the same name.