Portable Arms Inspect Body-In-White

April 1, 2003
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A worker uses portable CMM to check tooling on the Saturn production line. Photo: Joe McClure/Saturn.
Saturn Corp., known for its no-haggle pricing, takes a no-haggle approach to the quality of its sheet metal. It’s done right. Period. At its Spring Hill, TN, plant, the company spares no effort to make sure the tooling, and the subsequent parts, are correct. The quality-assurance technicians on the maintenance teams in Saturn’s body systems fabrication unit have adopted state-of-the-art technology to inspect product and ensure this quality. This technology includes portable coordinate measuring machines (CMMs) and inspection software from Romer/CimCore (Farmington Hills, MI) that compares the gathered data with the original CAD file.

Since 1999, Saturn has bought four 12-foot arms from ROMER/ CimCore. Three are 1000i Series machines and one is a counterbalanced 3000i Series; Saturn also has ROMER linear rails in three sizes, 4, 6 and 10 feet, that make these 7-axis CMMs. Inspection software for the arms includes three licenses for Power-Inspect, which is included with all ROMER 3000i arms. PowerInspect is developed for ROMER by Delcam Inc. (Ontario, Canada). Quality assurance technicians can use the PowerInspect software to generate visual reports of the inspection results of a part or tool. These reports can be invaluable for explaining needed changes.

The portable CMMs check tooling and the way the tools fit into the components. “Statistically speaking, we are doing significantly better since we began using the portable CMMs,” says Frank McMahon, body systems maintenance team leader in fabrication maintenance. “The current car presently has a Six Sigma variation of 1.8 to 1.9 millimeters. World-class is 1.5 millimeters and we are well on our way to that.” This variation level was achieved while Spring Hill tooled up two new vehicles in less than a year. This required a commitment from the top Saturn executives.

“Saturn leadership,” McMahon adds, “supports the portable CMM approach to troubleshooting and quality assurance for body-in-white production.”

The portable CMMs “are used to check relationships among parts when clamped into a tool for welding,” says Dave Schutte, inspection technician for the underbody resource team. “We rely on these systems for checks of part surfaces against their CAD models. As needed, we check weld-assembly tools after they are installed,” he says. “We troubleshoot any part-location problems with holding fixtures. We also check parts from stamping suppliers and weld-assemblies in search of root causes for build problems.” In the body sides and framing unit, “the portables (CMMs) are used to verify the true locations, in reference to the car’s coordinates, of specific parts,” says Bill Gilliam, framing and side quality technician. “We use them to check whether the part was made to its design specifications and to fine-tune the tools. This helps us hold parts more stable dimensionally and stay closer to the centers of our tolerance bands.”

Portable tool inspection

Prior to the portable CMMs, tooling could only be checked by a conventional CMM by taking the tooling out of the production line and carrying it into the CMM room—the same way sheet-metal assemblies are inspected. From a production standpoint, this ranged from impractical to impossible, so automakers were forced to rely on manual methods. Small tools are checked with hand gages that were difficult to use correctly and not always verifiable. For big jobs, Saturn used theodolites, which are optical gauging systems that are similar to surveyors’ instruments.

In theory, measuring the sheet-metal components when they come out of a welding tool will reveal tooling flaws. But such measurements are indirect—they must be interpolated—and many other factors can intrude. For example, the stresses of spot-welding and differences in metal temper and chemistry can cause distortion of steel components coming out of a perfect tool. The only way to recertify the tools is by checking them directly.

Portable CMMs can do this on site, as needed. At Saturn, this is the essence of ensuring that the sheet metal frame, the so-called body-in-white, is dimensionally correct before it goes to final assembly. At final assembly, additional inspections, using vision systems and other technologies in addition to the portable CMMs, will verify that hoods, roofs, doors and trunk lids are correctly attached.

“For me, the most important thing is the flexibility of being able to use the systems anywhere and measure anything,” Schutte says. “The portable CMMs are free of the reach and position limitations imposed by the machines in the CMM room. Probes can only reach the tops and sides of what’s being measured. Reaching inside, behind and underneath can usually be done only by a portable CMM.”

To expand the measuring range, Saturn also uses linear rails that are magnetically clamped inside tools or fastened to the floor with a tripod riser stand. “The linear rails are a big plus,” Schutte adds. “They fit inside the big body welding tools. The greater reach of the rails lets us move the arm closer to whatever we are checking. This keeps probed points close to the arm’s base where its accuracy is highest. Gravity never stops pulling on those arms.”

Schutte and Gilliam also check tools to make sure they conform to the car body’s orientation as Saturn specifies.

In addition, Schutte and Gilliam use the portables to ensure that the clamps, holding fixtures and locator pins hold the sheet metal’s joints and seams precisely where the welding robots expect them to be. They check locations of mounting brackets and studs—and the welding tools that put them in place.

Portables also check the way individual sheet metal stampings fit, or nest, in spot-welding tools. This is a three-step reverse-engineering effort. “First, the tool itself is checked,” Gilliam explains. “Then the sheet metal is checked as it nests in the tool. It’s checked a third time, after we weld it, to ensure proper placement.” The variations reveal whether problems are traceable to stamping tools, to the spot-welding gun clamps, to the robot’s program, or to the fixture itself such as locations of mounting pins or the dimensions and orientation of the tool.

ROMER/CimCore
(800) 218-7125
www.romer.com

In-House “Doctoring” Saves Hours of Setup

Reductions in machine tool set-up time typically come in small increments. However, this is not the case on the four vertical machining centers used by the instrument department of Howmedica Osteonics (Allendale, New Jersey), a maker of replacement hips and knees. The instrument department reduced set-up time of standard jobs from more than two hours to five minutes and eliminated $1 million of inventory by implementing a quick-change pallet fixture system, tooling carts and Renishaw’s (Hoffman Estates, IL) NC1 noncontact laser tool setter.

Howmedica Osteonics uses Renishaw’s noncontact laser technology for quicker tool setting. The NC1 is commonly used for in-cycle broken tool detection, but Bob Mykytka, Howmedica Osteonics’ manufacturing engineer, found it just as helpful for hands-off setting of tool length and diameter.

“Machine shop floors are a buzz of activity, and it’s often difficult to stop and examine how current procedures can be improved,” says Mykytka. “But it was evident that long set-up times were killing throughput of our short-run jobs and rapidly increasing back orders. Our set-up procedures needed significant upgrading,” he says.

Howmedica Osteonics’ instrument department produces more than 4,000 parts for 1,000 tools and devices that are used for shaping bones and installing orthopedic devices. Most of the parts are machined from 17-4 or 300-series stainless steel. The instrument department produces them using four verticals, three horizontals, four lathes, three screw machines, two grinders and a few small manual mills. Most of the parts are processed on the verticals, so Mykytka focused on those machines for the new set-up program.

Set-up time hurts productivity

According to Mykytka, the previous setup procedure had an operator with a part order sheet collect tools from the tool crib; assemble tools in a tool holder; install, indicate and tram fixture in the machine; and then manually perform tool offset and diameter compensation and dry-run the program—a two-hour process.

Mykytka assembled a team to brainstorm ideas for reducing set-up times. “We wanted something similar to an automotive industry flexible manufacturing cell, with all machines in-line, all fixtures already mounted and rail delivery to machine, but this was obviously beyond our budget,” he says. “We set out to design the poor-man’s flexible manufacturing cell.”

Mykytka decided to use standardized pallet fixture tooling on all the verticals and found one covered in dust in the tool crib. The system uses a pallet receiver that mounts permanently in the machine bed and receives an aluminum plate onto which the fixture is mounted. The plates slide on and off the receiver in seconds, providing positioning repeatability to 0.002 inch without the need to tram or indicate. Mykytka began testing the quick-change fixture system on one of the pallets of a dual-pallet machine. That way, when the machine was needed for production, he changed-out the test pallet, allowing the other to be used for a job, ensuring no production time was lost.

With the first piece of the set-up puzzle in hand, Mykytka investigated ways to automate the tool setting procedure to allow single-button cycle start with a “hands-off” tool compensation procedure. For that, Mykytka attended a trade show and bought Renishaw’s NC1 noncontact laser tool setter.

The NC1 allows setting of tool length and diameter to be carried out at normal spindle cutting speeds, enabling identification and compensation of errors caused by radial runout of the tool and tool holder, notes Mykytka. The NC1 can measure tools anywhere along its laser beam (up to 2 meters in length), eliminating a positioning move specifically for measurement and minimizing cycle times. The laser tool setter can measure tools as small as 0.2 millimeter with resolution of 1 micron.

NC1 noncontact laser technology has a compact and robust laser-based transmitter that sends a visible beam to a receiver unit. System electronics detect when a tool breaks the beam, and output signals are sent to the machine’s control, allowing the position of tips, teeth or cutting edges to be instantly established. No additional M-codes are necessary for basic system functionality, simplifying installation.

A continuous stream of air flows through special apertures, Renishaw’s MicroHole, to protect the NC1’s optics. There are no moving parts in the protection mechanism. The NC1 runs off a single air supply and requires no adjustment or cleaning once installed. “The NC1 is right at home in the belly of a machine tool surrounded with coolant and metal chips,” says Mykytka.

After he proved-out the fixture and NC1, the real work began, installing the receiver plates and NC1 on each of the verticals and converting the more than 4,000 jobs going to the verticals. The tool room worked to mount all the fixtures to the plates, while the programming department updated the part programs with an NC1 macro so the operator could just push cycle start and the machine would touch-off tools. This stage of the project took one year to complete. All but three jobs that have height restrictions now use the plate system. Some even use indexers, vices, double vises and other such fixture devices mounted on the plates.

The final step was streamlining tool retrieval and installation. Tool carts were purchased that allowed operators to have all the tools selected and waiting for them to install in the machine, instead of wasting time at the tool crib picking out tools while the machine may be sitting idle.

The department also started keeping the most commonly used tools permanently installed in the tool magazine. Most machines have 20-tool capacity, so tools 11 through 20 were strictly end mills of different sizes that are used on almost every job. This way, the operator may have to load only half as many tools. The tools are only removed when they are broken.

Under the new setup procedure system, the operator goes to the cart parking area to choose a cart, loads tools into the machine, quickly slides in the fixture plate without having to tram or indicate and hits cycle start. The NC1 macro in the program automatically touches-off all the tools, then the machine starts making chips.

The experience has led the company to use the system in other departments. The femoral knee department currently has one NC1 mounted on a robo-drill. It is used for tool setup and broken tool detection. “If a drill breaks during a drill and tap routine and the machine continues, all the tools after the drill would also break,” says Mykytka. “A quick check with the NC1 between cutting routines ensures this does not occur.”

Renishaw Inc.
(847) 286-9953
www.renishaw.com

BENEFITS

The portable CMMs perform crucial roles as they check the tooling and the way components fit into those tools.

Currently, the body systems fabrication unit has a Six Sigma variation of 1.8 to 1.9 millimeters. The goal is to reach 1.5 millimeters.

  • The portable CMMs are used to check relationships among parts when clamped into a tool for welding. Part surfaces are checked against their CAD models.
  • The software’s graphic capabilities allow users to develop visually compelling reports.


BENEFITS

  • Setup time was reduced from more than two hours to five minutes. More than $1 million in inventory was saved.
  • The NC1 sets tool length and diameter at normal spindle cutting speeds, enabling identification and compensation of errors caused by radial runout of the tool and tool holder.
  • The system is also being used to detect tool breakage.

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