NIST Takes On New Role In Machine Tool Standards

By playing a key role in updating and developing international standards for evaluating machine tools, researchers at the Commerce Department's National Institute of Standards and Technology are advocating U.S. perspectives in a technical arena that is increasingly defining the terms of manufacturing performance and equipment sales in the global economy.

In acknowledgment of its long-standing research program on machine-tool characterization and accuracy-enhancement methods, NIST's Manufacturing Engineering Laboratory recently was named secretariat, or administrative headquarters, for the International Organization for Standardization (abbreviated ISO) subcommittee on test conditions for metal-cutting machine tools. Part of the Geneva-based organization's technical committee on machine tools, known as TC 39, the subcommittee is responsible for more than 50 standards pertaining to tests of various types of machine tools.

"It's a perfect match for the NIST group," says subcommittee chairman Tony Bratkovich, who also is engineering director at the Association for Manufacturing Technology, McLean, Va. He notes that representatives of the participating nations, including Japan and Germany, world leaders in machine tool sales, indicated their regard for NIST's technical qualifications by accelerating the process for transferring the secretariat to NIST from a European standards organization.

The subcommittee's standards establish common technical ground between the buyers and sellers of machine tools by defining terms and prescribing methods for tests of accuracy and other key performance variables. A small but growing number of U.S. firms also are using these guidelines as the basis for their own quality assurance programs. In addition, several large manufacturers are beginning to require suppliers to incorporate standard testing procedures into their quality programs.

Since NIST assumed the secretariat role in late 1995, the subcommittee has been operating at a hectic pace. It has secured international approval of 11 new or revised standards, while 10 draft standards are being circulated for industry comment. In many of the new and draft international standards, U.S. perspectives have been accommodated.

"Inserting U.S. technical views into the standard-setting agenda" was the main motivation for NIST's involvement in the TC 39 subcommittee, says secretariat executive secretary Alkan Donmez, who is the head of NIST's Sensor Systems Group.

For example, four pending ISO standards for evaluating machining centers incorporate elements of a U.S. standard, which was developed under the auspices of an American Society of Mechanical Engineers committee chaired by NIST mechanical engineer Denver Lovett. Soon-to-be-finalized revisions of guidelines for evaluating temperature effects on machine tool performance and for assessing positioning accuracy and repeatability also will make the ISO and U.S. standards more consistent.

In addition, Donmez points out, testing methods and measurement tools, such as laser interferometry and recently developed telescoping ball bars, are being incorporated into the various standards under the subcommittee's purview.

As a general rule, according to Donmez and Bratkovich, U.S. and ISO machine-tool evaluation standards have reflected differing, although not necessarily conflicting, philosophies. U.S. standards tend to employ a "deterministic" approach, using average values to establish, say, the practical range of a machine tool's cutting performance. The approach simplifies identification of machine-based errors, such as those due to misalignment, that are amenable to improvement, Donmez explains.

In contrast, he says, ISO standards have relied on an approach that complements the statistical process control methods used in the quality programs of many manufacturers. However, the statistical approach allows for uncorrectable, random sources of errors. Therefore, the range of potential cutting errors is assumed to be infinite. In fact, most of the apparent random errors are due to uncontrolled conditions, such as external temperature changes.

Both approaches have their merits and appropriate uses, Donmez says. Pending revisions of ISO guidelines for evaluating temperature effects on machine tool performance and for assessing positioning accuracy and repeatability accommodate both, he says, adding that comments on the drafts have been favorable.

"Finally, machine tool builders and customers all over the world will be talking the same language," Donmez adds. "Common definitions mean that U.S. builders of machine tools will have an easier time communicating with foreign customers. They also will have a direct method for comparing the performance of their machines with that of their competitors."

Among U.S. manufacturers, interest in international standards is growing from traditionally low levels. Bratkovich characterizes U.S. involvement in the machine tool arena as progressing from a "sporadic approach to a more disciplined one." One driving force is the recognition that, increasingly, demonstrable compliance with standards is a necessary ante for entry to foreign markets or a precondition set by prospective foreign and domestic customers. Major manufacturers, in particular, also recognize the value of being able to realistically characterize the capabilities of their equipment so that they can use their resources most efficiently and productively, Bratkovich says.

On the other hand, Donmez notes, standards must keep pace with changes and needs. He notes, for example, that concepts dating back to the 1800s are embodied in some of ISO's general standards and definitions, which underlie more specific guidelines for tests of machine-tool performance. The test and measurement techniques remain valid, he says, but they are much more useful for the builders of machine tools than they are for the users. One of his major long-term goals for the new NIST-based secretariat is to revise the standards from the users' perspective and to emphasize "measurements of functionality."

As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. economic growth by working with industry to develop and apply technology, measurements and standards.