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Shop Solutions: Portable CMMs Help Maintain Fit and Finish at GM


When engineers at General Motors Corp. (GM) want to make sure a vehicle is fitting together properly--or find out why it's not--they often turn to the Design Check and Vehicle Assessment team at the company's sprawling Warren Vehicle Engineering Center (WVEC; Warren, MI).   

The group of five men use tools including a portable coordinate measurement machine (PCMM) and laser scanning to quickly and accurately validate tolerance and variation stack-ups and perform other tasks.  

Buyer surveys have shown that the way the sheet-metal skins of cars fit together--"fits and finishes" and overall appearance--is a big factor in perceived quality and the decision to buy a vehicle. The Design Check team constantly checks vehicle fits and finishes and helps strengthen and refine virtual builds based on the GM Math Model. When there's a problem, the team quickly provides precise dimensional data to engineers and designers assigned to virtual design validation. They also perform other dimensional measurement and analysis tasks, including what-if studies; design-check measurements from WVEC body, chassis, and powertrain engineers; and benchmarking competitors' vehicles.

"For us, this is all about the physical integration of as-designed parts--how they fit together--rather than the dimensional measurements of individual parts," says Team Liaison Engineer and Leader John C. Sturgis.

The primary tools for Sturgis' team are a pair of portable CMMs with 8 and 12' (2.4 and 3.7-m) working envelopes. Supplied by Romer CimCore (Farmington Hills, MI) and equipped with PowerInspect software from DelCAM Inc. (Windsor, ON), the arms replace measurement tools such as rulers, tape measures, and height gages. They also do away with the need to move parts and assemblies back and forth to a stationary CMM.

For measuring large objects, such as a vehicle frame or underbody, the team also uses a Romer GridLok Extended Reference System. The floor-based navigation system simplifies orientation of the CMM to the car body and eliminates the need for "leapfrogging," which results in additive positioning errors. Accuracy of the portable CMMs as the WVEC team uses them is ±0.2 mm.

One of the Design Check team's biggest challenges is inspecting the underbody alignment of an entire car to check the body framing weld-assembly system on which the vehicle is built. Sturgis and his team leave nothing to chance.

"We align to the vehicle with the GridLok system to create a full body global alignment, the same as would be done with a fixed-axis CMM," he explains. "We use the body-build datums on the vehicle, and if we need more we can glue on additional datum targets. Once we get all this set up with GridLok and its data plotted into the car-body coordinate system with PowerInspect, we take the car out into the shop where we can lift it up and get underneath to measure exhaust system locations, placements of bumpers and bumper fascia, and other parts of the underbody."

The software preserves the body coordinates and the arm's reference to the vehicle, allowing team members to use the same vehicle reference and coordinate systems even for measuring jobs that require two separate layouts.

To examine hinge alignments and the quality of fit-ups between doors, door frames, and body pillars, the team uses the PCMMs and math model data downloaded to the software to check a rectangular array of lines a few inches apart on all the exterior parts. Often referred to as a "gap and flush" check, the measurements are part of GM's Class A surface inspections for fits and finishes. The scans can be color mapped to show deviations from nominal as the surface is described in the math model or in a supplier's CAD system.

"Those color maps show whether any section of the surface is out of tolerance and, if so, how far out," Sturgis says. "That kind of data is very handy for stamping-die rework."

For competitive analysis, team members may touch-probe and partially digitize portions of a vehicle. They recently performed such an inspection on the front passenger compartment of a European-made luxury car.

"We used the laser scanner to handle volumetric data generated from scanning hundreds of points all over the front of the passenger compartment, including the dashboard, steering wheel, and the reach to the controls," explains team member Greg McDonald.



Advanced Laser Boosts Cutting Productivity


Michel AG (Herzogenhuchsee, Switzerland) is a 20-employee company that manufactures precision sheetmetal parts using laser cutting, bending, boring, threading, welding, and painting operations. In 2003, the company invested two million Swiss francs into new production units, and the plan for this year is continuing expansion.

With products ranging from housings and machine parts to snowplow components and drawers for furniture manufacturers, Michel buys more than 1000 tons (907 t) of steel annually. Typical material thickness is 3­8 mm, but the company can process up to 25 mm of steel, 20 mm of stainless, and 12 mm of aluminum.

Michel bought its first laser cutting machine 10 years ago from Bystronic (Hauppauge, NY). The CNC system with integrated material handling can cut flat sheet to 25-mm thick as well as tubes and profiles. Two storage towers, each containing 40 shelves from which sheets are automatically transported to the laser cutter, expedite processing and reduce labor costs.

In 2003, Michel became the first user of Bystronic's new 5200-W Bylaser 5200 ARC laser system. "With the new system, we were able to raise cutting speed compared with our 4000-W laser by 30%," says owner René Michel. To date, the installation is up and running and is absolutely problem-free, he reports.

The new laser features integrated Adaptive Radius Control (ARC), which optimizes the beam for all sheetmetal thicknesses and materials. Precise beam optimization enables extremely fast processing changes for micro-stages, flying-point entry, and scanning (position optimization). High-performance optics ensure consistent beam quality, while an accelerated entry-point directly on the part contour saves time. According to Bystronic, the system's semiconductor stimulation guarantees minimal current consumption and saves users considerable maintenance costs.

Michel says the technology provides significantly higher cutting speeds than previous systems, especially in thicker materials. Edge quality is improved through optimized laser pulsing.       




EDM, Automation Speed Nuclear Fuel Production

The package met MEAA's requirement for wireless connections between system hardware and production equipment, and came with optional modules that helped management track the goals of the manufacturing department. Three important capabilities were:   

Sometimes, an automation project turns out even better than expected. Case in point: the AREVA nuclear fuel manufacturing plant in Richland, WA, which processes uranium into fuel rods and manufactures fuel rod bundle assemblies.

At the plant's Component Fabrication Center, skilled technicians produce the tie plates that hold the fuel rods in a precise and critical alignment. Tolerances are strict--±0.0005" (0.013 mm) on the diameter of holes that hold the rods in the tie plates, and 0.005" (0.13-mm) true positional tolerance on hole location for some designs. The entire brazed assembly must hold overall tolerance of 0.015" (0.38 mm). Most components are Type 304L stainless steel castings. The center runs two shifts and operates seven days a week.

AREVA's Mitsubishi sinker EDM in action, and a machined tie plate
for nuclear fuel rods.

Until last year, this critical work was outsourced to suppliers who did all the brazing. That changed when center technical staff used lean manufacturing principles and practices to set up a state-of-the-art machining process based on a ram EDM machine from Mitsubishi EDM (Wood Dale, IL) and a robot supplied by System 3R USA Inc. (Elk Grove Village, IL).

The Mitsubishi EA12E, brought into the plant in July 2003, produces 1" (25.4-mm) deep holes in the tie plate components. The CNC machine cuts at least 10 different hole patterns and easily holds required tolerances. The robot loads both tooling fixtures and the EDM electrodes. The system can produce two parts at the same time. Loaded with 24 different parts, it can run 20 hr straight with no human interaction in a two-shift "lights out" operation.

"We evaluated waterjet cutting, wire EDM, lasers, and more for the cutting operation," recalls Center Manager Gary Sanders. "We tested at a Mitsubishi dealer over a period of a year and determined that ram EDM was the best process for our particular needs."

"The dealer made recommendations on the type and size of EDM machine, types of electrodes, fixtures, and more based on the process development at their facility," he continues. "The Component Center team made the decision to go to a machine with 100-A current capability instead of 60-A. Then we got more input from both suppliers before bringing the process to Richland."

AREVA had initially decided not to go with a robot, but the dealer made a strong case for it. "That had a huge impact," Sanders says. "Without it we would have had to do all tool and part changes by hand with the machine not cutting."

The automated system cut labor costs for the operation by at least two-thirds, Sanders reports. Other benefits include:

  • Elimination of batch processing of brazed tie plate assemblies, reducing cycle time and eliminating work-in-process inventory.
  • Support for a company-wide push for lean manufacturing.
  • Making the Fabrication Center the "center of expertise" for brazing at AREVA.


In-House Machining = More Profits


Tom Armfield, owner of MasterWorks Machining (Taneytown, MD), was subcontracting turning work out to suppliers. But Armfield felt that, with the right turning center, he could serve customers better, reduce inventories, and speed part turnaround--and make a comfortable profit in the bargain.

In business since 1997, MasterWorks is a small job shop with 14 people and 6000 ft2 (558 m2) of floor space. The company produces some unique products, and does machining work for Honeywell's instrumentation division.

Masterworks also produces parts for the aerospace industry from Hastelloy, beryllium alloys, titanium alloys, various steel grades, stainless materials, and even plastic. Average runs can be several parts to roughly 2000. Tolerances on many of the parts for Honeywell and aerospace customers are usually in the ±0.0005" (0.013-mm) range.

"We were starting to do a lot of parts for instruments," Armfield recalls. "It got to the point where I was subcontracting a lot of the turning work out. But then I wasn't able to get the part delivery or control that I needed. I figured I might as well bite the bullet and buy a CNC turning center and bring the work in-house, because I couldn't service the customer the way I wanted to."   

After looking at various potential suppliers, Masterworks eventually purchased a VT15 CNC turning center from Fortune Machine (Somerset, NJ). According to Armfield, delivery and cost played a large part in the selection.

"I looked at how much the machine weighed, its CNC controls, and other features," he says. "It had everything the high-end machines did, even down to the encoders and other features. I ended up getting what I considered a small, high-end machine but for not quite as much money as most other competitors. They also had units available, with plenty of equipment to look at on the floor. After I made my decision, they had the machine running in the shop three days later."

Armfield uses the machine to cut parts with diameters to 8" (204 mm) and lengths to 15" (380 mm). A bar puller handles 4" (1.2-m) lengths' of bar up to 1.5" (38 mm) in diameter. The first job he put on it was "another one of these runs from China that they weren't getting done, and what was being done was wrong," he recalls. "I asked the customer, 'Do you want me to do it?' They said, 'If you can have it done in a week.' So I stepped up the pace and made thousands of dollars in parts in just the first few days I had the machine."


Software Improves Die Design, Maintenance


Over the past few years, Nissan's Sunderland, UK, plant has become known as the most productive car plant in Europe. The plant's Press Engineering and Die Maintenance sections use software to improve die development and keep press tools in top shape.

All the plant's press tools are supplied from Japan, and it would clearly be too expensive to return them there for design modifications. So, Sunderland set up an NC facility based around an Okuma five-axis machining center. The investment was made to reduce new model introduction time by completing detailed tool changes in the UK rather than Japan, and to allow more efficient repair of trimming tools.

Previously, design changes and damaged cutting surfaces on the tools were built up with weld and then ground into shape by hand.

Now, a profile is digitized from the trim line on the press tool by fitting a Renishaw probe onto the Okuma machine. The captured data is then used in a combination of software packages supplied by Delcam Ltd. (Birmingham, England). Digitized geometry data is fed into both CopyCAD reverse engineering software and PowerShape hybrid modeler to recreate the trimming edges of the tool. A model of the trim edges is then passed to a PowerMill CAM package that cranks out machining data for the Okuma machine.

Initially, the new process was used simply to duplicate the manual operation in carrying out local repairs to each damaged area. More recently, it has been expanded to cover machining of all the welded sections and replacement of the complete trimming edge.

"Replacing the whole edge doesn't take that much more time," says Steve Easter of the plant's Press Engineering section. "Plus, it gives a much better result with a longer time between subsequent repairs."

The next application to be developed was replacement of sections in draw tools, especially where laser welding is used to join sheets of different grades of metal together. These areas wear more quickly because of the effects of the laser weld, so there is a need to replace them with harder material to extend the overall lifetime of the tool.

Again, surface data from the affected area is collected with the Renishaw probe. The replacement section is modeled with CopyCAD and PowerShape, then machined on the Okuma with toolpaths generated in PowerMill.

Technicians also developed a database containing PowerShape models of the various press tools as they were repaired, improving Sunderland's quality control. If inaccuracies are found in panels from a particular die set, the tool can be scanned and the results checked against the database. It is then both quick and easy to see if the problem is with the die surface or whether it is from another cause.


Software Boosts Lean Initiative

Obtaining shop management software that went beyond factory-floor data collection was essential to supporting lean manufacturing at Mitsubishi Electric Automotive America (MEAA; Maysville, KY), a manufacturer of automotive electrical and electronic components such as starters, alternators, and ignition coils. Traditional manufacturing execution system (MES) software incorporates scheduling and data collection/reporting, but MEAA managers wanted a package that would support their lean production philosophy.

That philosophy is based on four core principles: Communication, Teamwork, Elimination of Waste, and Continuous Improvement. Looking for software to help them quantify and develop these principles, managers contracted with Production IQ LLC (Rocky River, OH) to supply production monitoring and downtime tracking software for the plant's eight ignition coil assembly lines.

  • Calculation and display of real-time production target quantities based on process cycle time
  • Broadcast of real-time production performance data to the factory floor
  • Real-time network access to data for reporting and monitoring in a visual format via a web browser.

Based on process cycle time, the system calculates a running real-time production target so line workers and managers know instantly whether they are ahead or behind. "This allows manual assembly areas to see a target and to see where they stand compared to that target," says assistant engineering manager David Maher. "It helps us react to problem areas faster to keep our efficiency high."

The ability to visually communicate current production data and status was essential to the system, Maher continues. "I believe we pay more attention to our progress throughout the shift," he says. The data is broadcast using large LED alphanumeric displays to show quantity produced, target quantity, and current efficiencies. The same display also indicates any downtime in progress and its duration.

Maher believes the displays go beyond traditional andon boards. "They support continuous improvement by allowing us to see the results of any process improvement we make," he says. "All our associates want to meet the goals and work hard to do so, and this feature gives them the feedback they need."

The software's Webview module makes real-time system data available to approved network users. Users highlight specific areas of the plant to see relevant data and calculations. "We have it set up so other managers can view the data," Maher explains. "This lets them track production volumes throughout the day and see where they need to follow up. It also gives us the ability to chart past data for comparison."

MEAA's lean initiative appears to be paying off. A 40,000 ft2 expansion is under way, and is expected to create 60 new jobs that will boost the plant's workforce to nearly 400.


This article was first published in the September 2004 edition of Manfacturing Engineering magazine. 

Published Date : 9/1/2004

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