June 26, 2003
Market pressures to reduce tooling costs are pressing the tool and die industry to seek lower-cost tooling solutions. This column discusses different build approaches and the merits of an integrated build for trying out stamping dies (and molds) as part of the manufacturing validation process.
North American tool buyers are pushing to lower tool costs by:
Many of these activities are beyond the scope of small tool shops, which is a reason that North American tooling coalitions are forming (see January/February issue, p. 68, and March/April issue, p. 44).
A strategy recommended for domestic suppliers is to develop high-value engineering and system capabilities that are not readily available in developing countries. System support in the form of functional and integrated build approaches can give domestic suppliers a competitive advantage. The functional build (FB) approach is conducive to developing tooling coalitions that can work together to apply low-cost solutions when assembled parts have dimensional discrepancies.
An integrated build (IB) approach goes a step farther. The IB approach is a more holistic strategy that incorporates both product design and manufacturing capability.
While it deals with the complexities involved with sheet metal stamping, such as compliance, measurement system repeatability, and part distortion during assembly, an IB also encourages interaction with the customer, which is a competitive advantage. The value of IB is enhanced as tool complexity increases, which makes the long-term viability of this philosophy attractive to suppliers.
Traditional tooling tryout requirements in North America and Europe often involve the net build concept. Net build means that the quality of an assembly of sheet metal parts is directly correlated with the quality of each part that comprises the assembly. In other words, the assembly reflects a stackup of individual part tolerances. Typical metrics used to measure net build quality are Cp and Cpk, which often are set to 1.33 or greater as the minimum requirement for tool approval.
Figure 1illustrates how the net build premise is flawed based on an analysis of 13 independent sheet metal door assembly processes. The door inner panel, a main component in a door assembly, shifts on average 40 percent closer to specification during assembly. The deviation from the normal specification is reduced by 40 percent. At the same time, however, the variation in the door dimensions increases 29 percent, reflecting some stackup of component variances.
The mean bias graph (left) shows that, on average, the mean bias (average deviation from nominal specification) dimensions of the door inner panel are improved by 40 percent when the panel is assembled into the door. The Six Sigma graph (right) shows that although the average dimension is improved, the variation from door to door increases an average of 29 percent.
The variance in measurements from door to door increases by an average of 29 percent. The increased variation (the slope of the lines in Figure 1) does not appear to be related to the amount of variation in the door inner panel. The resulting variation is unrelated to the initial inner panel variation. Challenges to the net build paradigm for assembly quality in sheet metal are widely noted.1,2,3,4,5
Functional build tooling tryout methods were developed in response to the problems associated with the net build premise.6,7,8These methods focus on getting stamped parts close to specifications and assembling them so that assembly can be used to determine if a part (stamping tool) needs to be changed. Figure 2 contrasts the sequential net build approach, which is more data-dependent, with the iterative, experience-based functional build approach.
Net build is a sequential process that focuses on the part (achieving Cpk on all major check points) and then the assembly. The functional build approach first tries to get parts close to specification, then the assembly close to specification. The assembly must eventually achieve a high Cpk. Functional builds are more iterative and require closer coordination among tooling suppliers.
The Cpk index usually is not used to determine part acceptance with a functional build. Once the FB assembly is evaluated, individual tool adjustments can be made. The FB approach can result in improved assembly quality with less tooling rework costs over the net build approach, but management of the process, which is less data-driven, is a concern.
The following are challenges associated with the FB approach:
These challenges are why many stampers do not use the FB approach. Consequently, this is one area of opportunity for a coalition of suppliers working on related tools to band together and make the process work.
The IB approach encompasses functional build, but extends farther into the manufacturing process. While the FB approach focuses on achieving a dimensionally acceptable subassembly, IB strives to establish meaningful part tolerances, process setup parameters, and material specifications.
Part Tolerances. Because of sheet metal assembly characteristics such as compliant parts and distortion during assembly as many as 50 to 80 percent of original part specifications and tolerances are modified by the time production begins because of difficulties and prohibitive costs in meeting many overly stringent tolerances. Original tolerances are product designers' best guesses regardless of process capability and what the cause-and-effect stackup result is likely to be during assembly. Even material specifications are estimates based on past capabilities; little is known about how variations in material properties affect part variation.
Material Specifications. With the introduction of new materials such as high-strength steels, little is known about what suppliers can achieve and what material property variations will have on production. Tooling tryout is the best time to challenge initial design objectives and material specifications, because this is when information about the capability and its effect on processing and assembly can be ascertained through multiple tryout iterations.
Setup Parameters. Several well-known Japanese automotive manufacturers practice IB by "perturbing" the stamping process (by using material from multiple coils, tweaking process setup parameters, and running multiple die sets) to introduce variation and then evaluate its effect on the assembly. The outcome of this phase can be proper setup parameters that, if ignored, will produce defective parts and assemblies. This results in the desired effect of controlling the process by controlling the input parameters (material specifications and setup conditions), eliminating the need for output measurement and reactive control.
X-bar and R control charts routinely indicate that stamping processes are out of control but are still allowed to run because they are producing acceptable assemblies. The IB process can generate acceptance control charts that reflect a true range of acceptable variation before an undesirable result occurs in assembly.
IB goes beyond deriving manufacturing tolerances and validating a process for production. For example, IB can include the assembly of modules, such as the integration of a door assembly with a body side assembly. Data has shown that achieving a dimensionally accurate door assembly does not guarantee high performance once it's married to the body sides.9
This aspect of IB is beyond the reach of the average tool supplier, which is yet another argument for a collaborative business model that includes stamping tool suppliers, welding tool suppliers, and product engineers.
Domestic suppliers need to learn how to exploit their advantages of proximity and intellectual capabilities to retain their competitiveness. An opportunity exists by helping customers better understand and validate their manufacturing processes as an integral step with buying-off their tools. The functional build and integrated build approaches are ways to accomplish this.