Keeping accuracy within reach: part quality is at risk if a machining center cannot hold tolerances at the farthest reaches of its work envelope. This makes volumetric accuracy a key indicator of a machine's performance. One machine tool builder discusses the implications

Modern Machine Shop, March, 2005 by Russ Olexa

To solve these problems, Mr. Walker contends that machine tools need to be hand-scraped with fitted components, and hysteresis must also be controlled by tuning servos and manipulating the velocity and positional data for smooth motion and good control of the machine masses that are being moved around in space.

Although software is used by machine tool builders to offset machine inaccuracies resulting from spindle growth from heat, this software can't be used to offset inaccuracies of axis movement in the work envelope, because these inaccuracies can change depending on the part or parts being machined.

Measuring The Machine And The Part

Although measuring a machine tool's volumetric accuracy is very difficult, Mr. Walker describes two approaches to accomplishing this task.

Dr. Charles Wang, president of Optodyne, Inc. (Compton, California), has developed one method for measuring machine tool volumetric accuracy. According to Dr. Wang, procedures for volumetric calibration have been set forth in ASME B5.54 and ISO230-6 volumetric machine tool performance measurement standards. However, machine tool users have been reluctant to adopt volumetric calibration because following these procedures necessitated taking the machine out of production for 2 or 3 days. Optodyne's LaserVectorTechnique for volumetric calibration and compensation reduces the time factor to 2 or 3 hours, making it an attractive option for measuring this aspect of a machine tool's performance.

Renishaw Inc. (Hoffman Estates, Illinois) also has equipment to measure the volumetric accuracy of a machine tool. Clive Warren, business manager of the calibration products division, lists three devices that will give a user an accurate measurement of a machine tool. One is the ML10 Gold Standard laser measurement system used for comprehensive accuracy assessment of machine tools, CMMs and other position-critical systems. Another is the QC10 Ballbar System used for quickly checking CNC machine tool performance on a regular basis. The last is the machine checking gage. Mr. Warren says that all of these systems are portable, and the ballbar and machine checking gage are affordable, automated and readily used by small shops as well as large ones.

The Full Range

To machine parts accurately throughout a machining center's full range, it must exhibit the adequate volumetric accuracy. The easiest way that a buyer can make sure that a machine tool's volumetric accuracy is adequate is to cut sample parts where they are located in the work envelope and to check them to verify that tolerances are met. As Mr. Walker summarizes it, only machine tools designed and built with the precision to achieve high volumetric accuracy can hold tight tolerances regardless of where the cutting tool goes within the envelope.

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