When DaimlerChrysler decided to go after the growing entry-level sport utility market segment, the company designed a brand new vehicle and built a brand new plant in which to produce it. From the get-go, the company wanted to achieve high quality and to minimize the effect of unforeseen errors that occur in the complex automotive assembly process.

Today, after a successful, on-time launch, the Jeep Liberty is in full production at Daimler-Chrysler's new Toledo North Assembly Plant (TNAP). This plant uses the latest technology in terms of tooling, production processes and quality control. Among the state-of-the-art equipment in DaimlerChrysler's body shop are five systems that perform in-process dimensional gaging on various subassemblies.

Dimensional control strategy
The company first developed a dimensional control strategy for the Liberty.

"As the Process Reliability Manager for TNAP's body shop, it was my job to develop a dimensional control strategy that includes the use of our new coordinate measuring machine (CMM) room for daily sampling," says Alex Yeh. "But we also wanted the ability to monitor and confirm dimensions on every vehicle in the line, while it was being built.

"To do this, we chose Perceptron in-process gages," Yeh says.

The Perceptron AutoGauge stations use laser triangulation sensors to measure part locating points (PLP) and other dimensional features at five locations in the process-right hand body side (11 checkpoints), left hand body side (11 checkpoints), underbody (60 checkpoints), full body (47 checkpoints) and pallet check (10 points). Because the AutoGauge systems are built into the assembly line, they measure these points on every vehicle produced at full line rates.

Each checkpoint is assigned measurement limits based on its design tolerance and/or functional build requirements. If reject limits are exceeded, the system is programmed to trigger an alarm that sends a pager message to the appropriate plant personnel. The procedure at TNAP regarding these out-of-tolerance conditions is rather strict. There must be a response to every alarm and certain alarms will stop the production line until the appropriate action is taken. This procedure can grab the attention of upper management when a production process is stopped. But when this procedure is implemented well, as it is at the Liberty plant, the result is world-class dimensional control.

There are various ways to utilize in-line gaging stations that yield different levels of benefit. At TNAP, Daimler-Chrysler uses them as containment and process control devices.

At the most basic level, in-line gaging stations can serve as containment devices. These stations check every part and if a problem is detected, they prevent the manufacturer from adding any more value to the part. They also ensure the part does not reach a customer.

An example of this application occurred at DaimlerChrysler's body side station. During the build-up of the Liberty's body side subassembly, a center pillar reinforcement gets welded to the outer body side. Occasionally, the pillar reinforcement was welded off-location. When this happened, it caused a "winked hole" in which the hole in the reinforcement is not aligned with the corresponding hole in the outer body side. The real problem was that when this condition was present it would not be detected until after the body was framed, painted and the roof rack was being attached.

"When this problem first occurred, this misalignment within the body side subassembly resulted in scrapping some nearly completed vehicles. The proximity switch, which is used to prevent improper nesting, cannot detect this problem, so we were not able to generate the same condition to understand the root cause," says Yeh. "To combat the problem, we added a sensor to the body side Perceptron station to check the presence, position and size of the hole in the body side subassembly. If the part is a reject, we automatically remove it from production without adding any more value to a bad part. This simple act of containment prevents us from scrapping more vehicles due to this condition, saving many thousands of dollars apiece each time."

A second way DaimlerChrysler utilizes in-line measurement to benefit quality is for process control. This application of the technology involves the proactive efforts taken to keep the process under control. A relatively simple example of this application is the tight control of tooling dimensions at the pallet check station.

In the pallet-check station, the company measures the tooling itself because a palletized build process is used to carry an underbody through the build process. The AutoGauge sensors mea-sure the pins and pads on every pallet so the pallet will correctly hold the next subassembly that will make its way through the build process. Because the pallet is critical to body dimension, DaimlerChrysler monitors this measurement with a stringent dimensional control. Any pallet that fails the tight tolerance is rejected to a repair station and adjustments are made to ensure pallet dimensional integrity.

"Since we utilize 100% data, we can keep our process in control and only resort to correction and containment when the inevitable sudden problems arise," says Yeh. "The same method can be used to tackle areas of highest variation for continuous improvement."

"By paying close attention to the data we collect, we have even reached beyond the doors of our plant to control the process further upstream," Yeh continues. "In some cases we have detected problems in parts produced by outside suppliers. We had a problem with our front and rear control arm cups because some parts were being damaged in transportation or at the supplier factory. We notified our supplier about the problem and they made corrections to eliminate it."

Plant personnel have also investigated the benefits from several instances in which the in-process measurement systems prevented potential problems. These case studies have shown substantial savings. In many of the cases, the cost avoidance from one prevented problem justified the entire cost of installing the in-process measurement equipment.

The people who work at Toledo North Assembly Plant are proud of the Jeep Liberty. They talk seriously about quality and back up their words with actions and procedures.

"Through the utilization of the vast amounts of data collected by our in-process measurement systems, we work to keep our manufacturing processes under control and to react quickly when unforeseen disturbances occur," says Yeh. "We hope that this relentless pursuit of quality will help the Liberty have as long and successful a life as its predecessor, the Jeep Cherokee, that it has replaced."

Daimler Chrysler's Toledo North Assembly Plant (Jeep Liberty) can now perform 100% inspection.
Saved thousands of dollars per error by identifying and containing problem parts before more value was added in the production process.
Tight control on pallet dimensions keeps the production process in control, running at peak efficiency and reduces future errors.
The data collected helps control the process further upstream. Problems have been detected in parts produced by outside suppliers.