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September 2009

Automation Founders Circle

CNC machines can make parts right ‘the first time,’ thanks to Beckman Award winner John Ziegert

By Jim Strothman

When programming a computer numerical controlled (CNC) machine to make a metal part that will be mass-produced, for example a golf club head or bicycle pedal, the manufacturer can usually afford to make some mistakes and toss a few parts in the dumpster until the CNC machine is programmed exactly right to cut the desired shape.

However, in applications such as aerospace, where planes and spaceships are not mass-produced, a single engine part can cost $50,000 to $100,000, or more. Manufacturers such as Boeing cannot afford to waste even a few parts.

“They need to get it right the first time,” said Dr. John Ziegert, professor and Timken Chair in Design and Development at Clemson University. In 1993, Ziegert and a college graduate student he was mentoring at the University of Florida invented a device that enables parts to be machined with sub-micron accuracy “the first time.”

Called the laser ball bar (LBB), the time-saving and cost-saving invention resulted in Ziegert winning the ISA’s 2009 Arnold O. Beckman Founder Award, to be presented during the Honors and Awards Gala on 5 October, in Houston, Tex., the day before the 2009 ISA EXPO, 6-8 October.

“We knew both time and money could be saved if there were a direct way to measure the machine tool’s dynamic contouring accuracy over an arbitrary three-dimensional path without the need to machine an expensive test part,” Ziegert said. “In effect, the spatial coordinates of the dynamic tool path measured by the LBB can function as a ‘virtual’ test part.”

Time-consuming method

Prior to the LBB’s invention, instruments for machine tool metrology generally were designed to measure small errors in the motions of individual machine axes. A three-axis machine tool has 21 error motions that have to be measured.

This resulted in very long measurement times, especially considering that, once the measurements were completed, additional time was needed to combine the calculations into a kinematic error model to predict the spatial positioning error of the machine at arbitrary points in its workspace.

The LBB uses trilateration to evaluate spatial positioning accuracy of computer controlled machines. Trilateration is a method for determining the intersections of three spherical surfaces, given the centers and radii of the three spheres.

“The LBB’s trilateration methodology measures the spatial coordinates of points along a CNC part (cutting) path,” Ziegert said. “The six sides of the tetrahedron formed by three base sockets (attached to the machine table) and a tool socket (mounted in the machine’s rotating spindle) are measured. The coordinates of the tool position can be calculated geometrically.”

Speeds measuring process

The LBB uses a novel parallel kinematic metrology approach to allow direct measurement of the spatial movements of the tool with sub-micron accuracy over large work volumes. A complete set of measurements for a machine can be accomplished with a single instrument and setup, in a small fraction of the time required for traditional methods.

“ANSI/ASME (American National Standards Institute/American Society of Mechanical Engineers) had just put out the first national standard” for evaluating the accuracy of CNC milling machines using another measurement method, Ziegert recalled. “Using the method they described would take $50,000 worth of instrumentation and 12-to-24 hours to complete the measurements. Using our instrument, we could do the same test in 20-25 minutes.”

Instrument for evaluation of spacial positioning accuracy of computer controlled machines using trilateration.

As described by Ziegert and his graduate assistant, Dr. Christopher D. Mize, in a 1993 presentation to the American Society for Precision Engineering (ASPE), the LBB contains a heterodyne laser displacement interferometer. The interferometer’s measurement axis is aligned between the centers of two precision spheres joined by a telescoping tube assembly.

Interferometry is the technique of diagnosing the properties of two or more lasers or waves by studying the pattern of interference created by their superposition. The displacement measuring interferometer essentially uses the wavelength of the laser light as the “tick marks on the ruler” enabling extremely precise measurements.

Interferometry is an important investigative technique in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, quantum mechanics, nuclear and particle physics, plasma physics, and remote sensing.

Formed LBB company

Ziegert and Mize use the LBB’s interferometer laser beam technology to measures changes in the length of the LBB itself, in combination with a novel method to allow the instrument to be initialized to provide the absolute distance between the ends. Once initialized, the LBB can measure the absolute distance between several fixed and moving points on the machine structure.

The University of Florida turned the laser ball bar patent over to Ziegert and Mize. They formed a company, Tetra Precision Inc. in Gainesville, Fla., to manufacture and market the instrument. Ziegert has been president of the firm since it was founded in 1994.

Given in honor of Dr. Arnold O. Beckman, founder of Beckman Instruments (now known as Beckman Coulter, Inc., Fullerton, Calif.), the ISA award “recognizes a significant technological contribution to the conception and implementation of a new principle of instrument design, development, or application.”

Ziegert’s citation credits him “for conception and implementation of the laser ball bar, an instrument for measuring the spatial positioning accuracy of multi-axis machine tools.”

Beckman, who served as ISA President in 1952, was an internationally recognized scientist, educator, executive, humanitarian, and civic leader. He died in 2004 at the age of 104.

Continued LBB development

Ziegert continued to work on improving measurement technology and using the LBB to solve other measurement challenges following its invention.

For example, for the 1995 ASPE proceedings, Ziegert and Narayan Srinivasa, coincidentally of the University of Illinois at Urbana-Champaign’s Beckman Institute, described a method to predict thermally-induced error maps in machine tools by combining a LBB with a Fuzzy ARTMAP neural network.

After selling LBBs for several years to national laboratories such as Lawrence Livermore and Sandia, as well as to aerospace firms, Tetra Precision recently sold the technology to CAD CAM Solutions GMBH, Kaufbeuren, Germany.

This has enabled Ziegert, a past president of the ASPE (2001-2002) and currently editor-in-chief of its Precision Engineering international journal to continue to pursue his research interests at the Clemson University International Center for Automotive Research (CU-ICAR).

CU-ICAR’s web site (www.cuicar.com) describes the research center as “an advanced-technology research campus where academia, industry, and government organizations engage in synergistic collaboration. With more than $200 million in commitments, CU-ICAR represents the ultimate public/private partnership, directly fueling a knowledge base critical to the automotive industry.”

Ziegert has specialized in high-precision measurement since his college engineering days. He earned a BS in mechanical engineering at Purdue, an MS at Northwestern, and his Ph.D. in mechanical engineering at the University of Rhode Island, where he wrote his thesis on Kinematic Calibration of Industrial Robots.

After working as a project engineer for General Motors’ Pontiac Division, Ziegert left GM to launch a career in academia. He has taught and done research at Northwestern, the University of Hawaii, Brown University, Roger Williams University, R.I., the University of Rhode Island, California Institute of Technology, the University of Florida, and now Clemson University.

He is a co-holder of 12 patents related to that field of interest, was principal investigator or co-principal investigator on 16 funded research projects, has co-written more than 40 refereed journal papers, and has more than 55 published conference proceedings.