Function structures

In the DMAIC model, the process flowchart is an indispensable tool in identifying the "as is" process, as well as the "should be" process. Of course, the idea is to identify the value added steps and eliminate the hidden factory. In the DFSS approach we are still interested in function structures for the same reason. However, the structures in the DFSS are process mapping and datum flow chain.

Process mapping

Process mapping in the DFSS methodology is used as a process flow indicator and as a project status indicator. In the case of the first application, no special consideration is given. In fact, it is the traditional process flowchart. In the case of the project status, however, it attempts to provide a uniform, well-defined measure of the project activity currently underway. Using the activity underway—as opposed to the most recent activity completed or the next activity to be undertaken—as the project status reference point seems appropriate for most projects we have had experience with thus far.

The values in the diagram can help to identify bottlenecks in the flow of work through the organization and may validate project flows that are working well. (It is imperative for the black belt to note that if none of the status values is appropriate, to describe the project status in the comments section of the project. Of course, the status "on hold" may be set at any point in a project.) A typical sample of project status values is shown in Table 6.3.

Table 6.3: Typical project status values

Project status	Description
Potential	An idea for a project has shown merit, but no resources have been allocated,
Preplanning (1)	An initial project description is being created.
Business case preparation	Preliminary Investigation is going on as to whether there is a valid business case for the project.
Conceptual design (2)	The basic idea of what the project will do, including what technology might be appropriate, is being fleshed out.
Seeking commitment	The business case and conceptual design are being presented to sponsors—in an effort to seek sponsor commitment to go forward.
Pending	Sponsor has committed to implementing the project, but the project has not yet gotten underway.
Requirements analysis	Project team Is gathering and reviewing customer requirements.
Prototype	Project team is building and reviewing a prototype. For example, a low-fidelity prototype may be built to validate customer expectations.
Design	Team is figuring out how to accomplish the implementation.
Buy (5)	Implementation by purchase is underway. This stage may include related development, documentation, training, and other activities. Some projects involve both buy and build activities, with the "build" usually depending on the "buy" activity.
Build (3), (5)	Building the implementation is underway. This stage may include related development, documentation, training, and other activities. Some projects involve both buy and build activities, with the "build" usually depending on the "buy" activity, "Build" includes development, documentation, training preparation, and other activities.
Internal test	Implementation is functionally complete and is being tested internally by the project team.
External test	After passing the Internal testing stage and correcting all the show-stopper bugs, the implementation has been given to testers outside the project team.
Handoff/service review	People with expertise in service and support processes are working with the project team to determine whether the implementation is serviceable and supportable. This review may begin during internal testing—to permit service and support perspectives to influence the modifications undertaken as a result of testing. An alternative view of this stage, appropriate for some projects, is to make this a final gate, which is imposed after testing and rework is complete.
Ready (or operational)	Internal and external testing and handoff/service review are complete. All necessary modifications have been made. The implementation is ready for production use and has been accepted as such by the service and support teams. Its release may depend on scheduling and timing, as determined by the service and support teams.
Completed on hold	Project ended after successfully meeting its objectives. Work on the project has been suspended either temporarily or indefinitely. This status may be set at any point in the process.
Terminated (4)	The project was abandoned with significant objectives incomplete.
Comments	If none of the current status values apply, describe project status in the comments section for the project. "On hold" status may be set at any point in the process. "Build" includes development, documentation, training preparation, and other activities. "Terminated" indicates that the project has ended with significant objectives incomplete. The red line from "buy" to "build" represents those projects that involve both activities. For these projects, the "build" generally depends on the "buy" activity.

Datum flow chain

The datum flow chain (DFC) is a directed acyclic graph (a graph with no cycles) representation of assembly with nodes representing parts, as well as assembly fixtures and arc representing mates. The direction of the arc represents the direction of the content. Every arc constrains certain degrees of freedom depending upon the mating conditions. The sum of the degrees of freedom constrained by all incoming arcs to a node should be equal to six, unless motion is part of the function. That is, each part must be fully constrained; in each DFC there is only one root node with only outgoing arcs; every joint where the DFC passes is a mate.

The significance of the DFC to DFSS is that DFC directly controls the delivery of significant and critical to quality (STQ and CTQ) characteristic. For example, in a nominal analysis of assembly we may begin the process by:

• Identifying CTQ characteristics.

• Identifying the types of assembly (part-defined or assembly-defined).

• Identifying constraints and fixtures.

• Defining DFC, features, mates and contacts for each CTQ characteristic.

• Checking for proper constraint (six degrees of freedom, unless motion is required by function).

• Finding feasible assembly sequence with shorter CTQ chain of delivery.

• For multiple CTQ characteristics, checking if there is conflict in delivering CTQs. If there is, revise the assembly sequence, parameters or product architecture to remove the conflict.