Making Complex Parts That Fit Together
Cars and trucks do more than just transport people, they control their environment, entertain the passengers and serve as mobile offices. Rear view mirrors are self-dimming, engines are self-monitoring and brakes are self-applying. Modern vehicles are electronic marvels.
But all of these devices require power, and power requires wiring. And with each new function, more wiring is needed. In the past, the architecture required a spaghetti-plate of wiring. And the individual wires, secured in solder, tended to fray, exposing bare metal through the insulation layers, sometimes causing shorts.
The solution is a distribution center that eliminates the multiple wiring circuits and interconnects that had increased the probability of a quality problem. At Delphi Automotive Systems' Brookhaven, MS, plant, the solution is called the Bussed Electrical Center (BEC), which houses a vehicle's fuses and relay centers, and harness-to-harness interconnects in a centralized, adaptable package that reduces the complexity of the wiring system.
Completing a puzzle
The BECs are produced for use in the GMT 800 pickup truck and sport utility vehicles, which are known to consumers as the Chevrolet Silverado, Tahoe, Yukon and Avalanche. These popular-selling trucks use four BECs, one under the hood and three others in the cab. The company makes enough BECs for as many as 6,500 vehicles a day.
While the BECs reduce the complexity of the wiring system, they also create complex parts with individual components that must fit together like jigsaw puzzles. And, if the pieces aren't made accurately, then the puzzles won't fit together well.
Each BEC is enclosed in injection-molded upper and lower housings. In between these housings are the guts of the BEC. "The BEC will have a press-fit layer, it will have a routed wire plate, and in some cases may have an additional spacer plate," says Jim Petyak, senior quality engineer and Metrology Lab Supervisor in Brookhaven.
Metal terminals stand up in rows across one layer of the BEC and these must be able to fit through small rectangular holes created during the molding process. After this process, the female ends of the part are stitched using the inserter machine. The BECs are built up one layer at a time in employee-designed U-shaped assembly stations. The BEC moves from one machine to the next, and each time another plastic component is added, it must fit within a 0.3 millimeter tolerance.
"If the key characteristics are dimensionally out of tolerance, then problems will occur with part assembly and part functionality," says David Knox, quality control engineer.
Or perhaps even worse than this, assembly operation would slow. In the past, every part that came in may have been slightly different than the last and the assemblers would have to continually readjust the part, maneuvering it back and forth so that the terminals would line up with the holes.
So dimensional accuracy is key, but considering that as many as 26,000 BECs may be built in a day, accuracy maybe difficult to achieve. To solve this problem, Delphi spent nearly $200,000 to place vision inspection equipment on the shop floor for random sampling tests and inspection. It integrated statistical process control into its processes and developed measurement techniques that used part fixturing that complies with measurement system analysis standards.
In the past, "we had no understanding of the variation in the process and how it was affecting the actual part that we were producing," says Petyak. "We were concerned about trying to define what the process was doing dimensionally to the plastic components. We determined it was important to get the inspection process close to the components, so we centralized the measurement systems in conjunction with the molding operation."
On the shop floor, in a central location close to the battery of Battenfeld injection mold presses, is a vision inspection station. Every two hours, the technicians who work here, Richard Panzica and Malena Britt, randomly select a housing that has come off the molding machine and measure the part. "If it fails here," says Britt, "then it won't work on the assembly line."
Seated next to each other, Panzica and Britt test any parts from the molding machine that are out of tolerance. One such part had a problem with terminal stitching--the hole and the terminal were not dimensionally aligned causing an interference condition.
Suspect parts are fitted into a fixture and staged on a vision inspection system called the Smartscope Flash from Optical Gaging Products Inc. (OGP, Rochester, NY). Delphi uses the noncontact system because the parts have features too small for most probes, explains Knox. The Flash, which features a 12X zoom lens and a backlighting system that relies on LED illuminators located beneath the stage glass, follows an inspection routine programmed by the SPC technicians. It finds the holes and checks them against the control limits.
Delphi uses a system of flags to alert technicians of problems. Britt or Panzica will place a green acrylic sign onto the molder when a tested part is within tolerance. When software integrated into the vision system detects that the process is going out of tolerance, a yellow flag is posted, alerting the molding machine operators to take appropriate corrective action. When the vision system reports a "red" result, it means the inspected part is out of tolerance, and the operator places a red flag on the molder. When this occurs, production is stopped until the problem is corrected.
Production line stops happen infrequently, as the company's use of statistical process control has begun to pay off. SPC was put into place for BEC production in March 2001, and since then, the company has used the software, QC-Calc from OGP, to do live calculations of Cpk, Cp, averages and automatic real-time statistical reports. The software can do a range of control charts and process charts.
"We went through a process of elimination. We started out with about 10 different dimensions for each part and we have eliminated dimensions that won't work in the production process," says Petyak. "We correlated what dimensions our assembly lines can use."
When the SPC program was put in place, parts were randomly tested once an hour, a frequency that has since been reduced to once every two hours. "Once we get good Cpk values, the inspection frequency is reduced," Knox explains. "We want to prevent problems instead of trying to react to them."
Now that the Brookhaven plant has successfully integrated vision inspection and statistical process control, the company is looking ahead. "We are working on virtual inspection using [our] Unigraphics [computer-aided design system] to develop the inspection program. In an effort to save time, we are using solid modeling to develop an inspection routing before the tool is built. We should have that up and running in a year."