Immediate verification

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END USER: Flint Tool & Die, General Motors Co., (313) 556-5000, www.gm.com. CHALLENGE: Verify machine tool accuracy when producing large automotive dies. SOLUTION: A PC-based verification system. SOLUTION PROVIDER: Inora Technologies Inc., (734) 302-7488, www.inora.com

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The dies that stamp automotive body components combine massive size with high precision. General Motors Co.’s Flint (Mich.) Tool & Die facility manufactures stamping dies for major car body parts, including body sides, hoods and roofs. The largest dies are 180 " long and weigh up to 60,000 lbs., and tolerances are as tight as 0.001 " for a body-side contour. “The body-side dies we build stamp the whole side of the car,” said Robin Miller, the facility’s daylight machine shop group leader.

GM’s pattern shop casts the ductile iron dies to near-net shape with about 10mm of excess stock, and Miller’s shop mills, drills, taps and thread mills them to finished dimensions. The shop has 10 SNK bridge-type vertical machining centers, seven of which have 200 "×100 " tables to handle the large workpieces.

The stamping dies don’t operate individually; each is part of a series used on a progressive stamping line. “We normally build a four-die line,” Miller said. “We will have a draw die, a first-trim die, a trim-pierce die and a flange die.” 

As a result, the clearances need to be correct when the dies come together. “If we are going to flange something on a flange line, say to marry a hood inner into a hood outer, the flange tolerances are about 0.003 " or 0.004 ",” Miller said. 

“A lot of times we’ll machine one half of a die on one machine and another half on a different machine, and trying to get both halves to match so we have proper metal clearance is extremely challenging,” Miller continued. “If one machine is out of spec just a little, it can give us a lot of trouble.”

Verifying machine alignment and accuracy is crucial, but previously the verification process was slow and inconsistent. To check machine alignment and accuracy, machine repair staff set up a steel square on the machine and ran indicator checks on it. It was a process that took 3 to 5 hours, depending on the group members doing the check. 

To speed the process and maximize machine accuracy and consistency, the shop looked for a new way to verify alignment. The solution was the Inora SRS (spatial reference system) from Inora Technologies Inc., Ann Arbor, Mich. The system consists of a geometrically and thermally stable reference artifact, which is placed in the machine work space, and a software package that analyzes dimensions gathered from the artifact by the machine’s measurement system. 

The artifact is assembled from six NIST-traceable carbon-fiber bars to form a rigid pyramid with a steel sphere at each corner. The bars are essentially temperature-insensitive, varying only 0.1µm per meter per degree Celsius of temperature change. At the end of each bar is a compensator that accounts for any thermal expansion or shrinkage of the spheres. The system verifies horizontal or vertical machine tools and measuring equipment, such as coordinate measuring machines and laser trackers. Standard bars range in length from 180mm to 2,000mm to fit different size machines. 

Courtesy of Inora Technologies

In the Inora spatial reference system, the machine tool’s measurement system gathers dimensions from a geometrically and thermally stable reference artifact, which is placed in machine’s work space, and software analyzes the data to verify machine alignment and accuracy.

System operation involves assembling the artifact and placing it in the machine, probing the four spheres and importing the data to a PC. The process consumes less than 30 minutes. “The SRS is designed to be simple enough that the user of the machine tool or the measurement equipment can quickly and easily monitor his own equipment,” said Nate Enstrom, Inora business development manager. 

SRS users are not required to orient the artifact to the machine’s coordinate system, eliminating that error source in the verification process. Moving the artifact within the machine and repeating the process checks the machine’s accuracy across its volume.

Enstrom noted that the SRS complements tools such as lasers, squares and granite plates that physically adjust the machine tool. After those tools calibrate the machine in each individual axis, the SRS verifies the relationships between the machine’s angularity, positioning and scales, and pitch, yaw and roll. This confirms if the adjustments have been correctly performed. Because the SRS hardware and software are stable and independent of the machine control, the data collected “ultimately shows how the machine will perform as a correlated 3-D device,” Enstrom said.

“The SRS gives us positioning checks for our X, Y and Z axes,” Miller said, “and it also gives us a squareness check of X to Y, X to Z and Y to Z. We can also calibrate and check our probe to make sure it is correct.”

Regular use of the system allows the shop to compare assessments and track trends, and schedule preventative maintenance or repair so it reduces impact on production schedules. “It gives machine operators and owners immediate capability to make informed decisions about their machines,” Enstrom said. 

Prior to using the SRS, an undetected machine alignment problem might produce machining error, causing scrap or rework. Now, scrap is minimal and rework is no longer a major drawback, according to Miller. 

Other stamping facilities occasionally send a problem die to the Flint facility for rework. “They give us 3 or 4 days to remachine it and get it back to them,” Miller said. Because the accuracy of its machine tools is verified, the Flint shop can meet those demands.