Manufactured parts are always plus/minus so many thousandths. Sure, these parts typically fit together, but Jaguar Cars wanted to know what affect so many thousandths on every part would have on the fit and finish of its new Jaguar S-Type luxury sports sedan.
Dimensional variability, even in thousandths, is an anathema to assembly operations. The stackup in dimensional variability can make cars difficult to build and costly to rework. Fit and finish usually suffers.
Manufacturing can do just so much to manage dimensional variability. It can spend more time manufacturing parts. However, the closer manufacturing gets to nominal, especially where small part tolerances are involved, the higher the costs to manufacture those parts. It can spend more time or add more labor during assembly to ensure fit and finish, but that slows production and adds costs. It can fix product after production. That just won't do, especially at high production runs. Plus, while rework can make cosmetic appearances acceptable, it often fails at fully eliminating the functional problems, such as wind noises, water leaks, increased squeaks, rattles, and doors that don't close properly.
Now comes the S-Type, announced on October 20, 1998, which competes against the BMW 5 series, Audi A6, and theMercedes E Class. The car has traditional Jaguar styling. Striking, yes, but that styling is also difficult to manufacture because of its complex curves and surfaces. True, like all other automakers, Jaguar has consistently increased the quality of its cars. "But if the entire vehicle isn't designed from the beginning with variation reduction in mind, then there's going to be a limit to how much better we can make the car after the fact," says Antony Harper, principal engineer, at Jaguar's Whitley Engineering Centre (Coventry, England).
"We needed an engineering process and accompanying analysis in place to design this car right from scratch to be resistant to variation and easy to put together consistently. By managing dimensional variation, our customers' quality requirements can be met with a minimum of rework." Helping Jaguar to do just that is VSA-3D, a dimensional management software application from Engineering Animation, Inc. (EAI; Ames, Iowa).
The Conventional Approach Doesn't Work
Analyzing part variation and tolerance stacking used to "fairly simple, fairly two-dimensional, and fairly localized," explains Harper. Automotive design and manufacturing engineers typically analyzed a handful of the major components that comprised the systems they designed. But there were at least three problems with this approach. First, the analysis was not a whole-vehicle approach. Second, it did not typically start at the beginning of the design program. Last, it typically did not involve more than a calculator. In fact, conventional computer-aided design (CAD), visualization, and prototyping software applications simply do not account for the variations in the manufacture and assembly of parts.
The problem with analyzing dimensional variation is that it requires a statistically significant number of representative parts built into assemblies. Unfortunately, continues Harper, "Unlike the nominal design, where a model can be cut and inspected as soon as CAD data is available, variation cannot be validated until very late in the vehicle development cycle."
In 1994, Jaguar started a different approach. It used VSA-3D to shift the dimensional validation process early in the development cycle. VSA-3D is a software package that simulates manufacturing and assembly variations in a digital environment. It lets engineers determine the maximum variation possible in the size of a part or several parts before the function of a finished assembly is affected.
Jaguar first used VSA-3D in a pilot program for the Jaguar XK8 models. In 1995, the automaker used the application for all of the dimensional management throughout the S-Type development program.
VSA-3D simulates component part variation, assembly sequences, and assembly method variations to predict how much build variation will occur in a final assembly. The VSA-3D model of a car body requires four data sets:
- Geometry and tolerances on all of the parts
- Assembly sequence of the parts
- Assembly methods, for example, what part features attach to other part features or to a fixture
- Desired measurements and locations on the car, such as the gaps between doors.
For the S-Type, Jaguar modeled virtually every component or system that attaches to the body. This included the chassis, powertrain, interior trim, (instrument panel, floor console, steering wheel, headliner, and moonroof), exterior trim (including bumpers, grille, glazing, and lamps), and hardware (such as door latches, hinges, and windshield wipers).
In operation, VSA-3D varies the physical dimensions of each part within its allowable tolerance in manufacturing. In the physical world, this variance is not purely random; it's biased towards nominal. "Each part is slightly different from the other, even though they are all `good' parts," explains Harper.
VSA-3D "pulls" these parts out at random and digitally "assembles" them as specified by the S-Type assembly procedures. As required, parts are digitally fixtured, bolted, or welded together. Once the final assembly is digitally complete, VSA-3D measures the variation, that is, the tolerance buildup.
Then VSA-3D does this again. And again. Thousands and thousands of times again, just like in a production line.
What results is a statistical picture of Jaguar's build capability given the parts as designed. This picture helps predict build variation and flag the build problems early in the design cycle, before they occur in production and well before committing to any type of hard tooling, says Marty Vanderploeg, EAI's executive vice president & chief technology officer.
For the S-Type design program, Jaguar was using the standalone version of VSA-3D, which generated statistical representations detailing the variations of the parts of interest. These representations included bell curves and a ranking of the contributors to the variations. The output is a true analysis of the causes, rather than the symptoms, of tolerance buildup. An analyst picking through these data can determine the effects of an obscure part on an assembly, such as how an underbody component can be a major contributor to the gap variation at the top of a door. Seeing this enables the automotive designers to concentrate their efforts and control resources on that underbody component.
A typical VSA-3D run on a localized problem takes minutes. Investigating an automotive interior can take longer—hours—because VSA-3D has to first run an exterior build to determine the variation in exterior components, which is what the interior components attach to.
Jaguar is running VSA-3D on Unix workstations from Sun Microsystems and Hewlett-Packard. The application costs between $22,000 and $28,000, depending on the CAD system it is to be integrated with. At Jaguar, VSA-3D is now directly embedded within I-DEAS Master Series from Structural Dynamics Research Corp. "Looking at the numbers without a graphical view can lead you to a skewed opinion about part variation," admits Vanderploeg. "The graphical representation helps drive you to the right numbers to analyze in more detail. You really need both."
Jaguar's S-Type Is Not Your Father's Car
|Jaguar Car's new S-Type luxury sports sedan is the company's first release of the 2000 model year. Available this spring in North America, this S-Type sedan is Jaguar's first step in attracting a new generation of customers to Jaguar.
There's a lot to be attracted to. Outside, the S-Type's styling pays tribute to Jaguar's 3.8 S-Type of the 1960s. Inside the passenger compartment, the sedan is strictly up-to-date. Voice-activated controls (continuous-speech, speaker-independent) operate the climate control, audio system, and telephone. An optional in-dash GPS navigation system can be enhanced with Jaguar Assist, an emergency messaging system that provides automated response when an air bag is deployed. Other features include dual-zone automatic climate control, a dashboard-mounted ignition lock, a heated windshield-wiper, Dynamic Stability Control, rain-sensitive windshield wipers, and an electronic obstacle-detection system called Reverse Park Control. A six-disc CD can be mounted in the glove box.
The S-Type comes with either the new Jaguar-engineered 3.0-liter AJ-V6 engine or Jaguar's 4.0-liter AJ-V8 engine. Both aluminum engines are paired with a new five-speed automatic transmission. A double-wishbone suspension and a speed-sensitive, variable-ratio rack-and-pinion steering system gives a sporty feel to the sedan's rear wheel drive. Sports car enthusiasts may also want the optional Sport Package, which adds Jaguar's adaptive Computer Active Technology Suspension (CATS) and 17-inch wheels with Z-rated tires.
EAI has big plans for VSA-3D. First, it is integrating VSA-3D with EAI's VisMockUp digital prototyping software. VisMockUp adds real-time motion, collision detection, and analysis functions that can help engineers identify design flaws before a physical mockup is created. Together, these two products will dynamically reflect dimensional variations in computer-generated animations of fully assembled products—entire automobiles, if desired.
Second, EAI is adding functionality to VSA-3D to automatically optimize the allocation of part tolerances across an entire structure once the base model is available. That structure can be an individual assembly or a finished product. At Chrysler Corporation, beta versions of this functionality are already optimizing the reallocation of tolerances according to cost.
No "Panic Stations"
Because the S-Type is the first design program at Jaguar using VSA-3D, and production is just beginning to ramp up, actual cost savings from VSA-3D are hard to come by.
In theory, making part design and assembly adjustments in the computer, rather than on tools that have already been paid for, should result in higher quality automobiles, smoother production, and lower-cost products. So far, VSA-3D has helped Jaguar at least in selecting locator features on components to ensure that body components are properly positioned in space during production. "Locators both influence and are dependent on the assembly processes," explains Harper. A good locator strategy helps manufacturing meet design targets by helping to minimize the appearance of gaps, provide for the variation in gaps, and let production reliably and repeatably measure parts and subassemblies on fixtures.
According to Harper, most automakers admit that their prototypes, even late prototypes, suffer dramatically from build quality issues. Unfortunately, the automakers cannot know before actual production whether these quality issues are real or a result of prototype part quality. Not so with VSA-3D. VSA-3D separates the part-design issues from the part-quality issues. Before the first S-Type hit the assembly line in Jaguar's new production complex at Castle Bromwich, near Birmingham, England, VSA-3D had built over a million virtual cars. These cars have been thoroughly examined for where even the smallest reductions in variation could be made. As a result, explains Harper, "We know that our prototypes are coming out extraordinarily well. When we were about five months away from job one, the factory was really quiet, whereas you'd normally expect it to be complete panic stations!"
With the announcement in October of the S-Type model are the expectations that Jaguar has a winner on its hands. So instead of typically manufacturing between 40,000 and 50,000 units a year, Jaguar is planning to double that production run. "We are confident that we will be able to make 80,000 of these cars in a year and meet all of our variation targets," says Harper. "We're also confident that we can increase production on this vehicle without any detriment to build variation."