Six Sigma- DFSS
Introduction
Conventional Six Sigma follows the Define, Measure, Analyse, Improve, Control (DMAIC) methodology. This has proven to be well suited to problem solving and process improvement projects. DFSS uses the Identify, Define, Design, Optimise, Validate, Evaluate (ID DOVE) methodology.

Implementation of the DFSS methodology requires that the purpose of the project be suitably defined before the product requirements are identified. Several design concepts are then proposed and suitable decision making processes are used to select the option that is most appropriate.

The following steps are taken in a DFSS project.

Identify
At this stage the requirement for the project is justified in terms of strategic and customer requirements. The scope of the project is also defined.
Define
The key customer needs are defined and the critical design requirements necessary to meet customer needs are determined. The link between the need and the requirement should be documented and preliminary target values set where possible. Key tools at this stage are Quality Function Deployment (QFD) and decision analysis methods.
Design
Several design concepts are generated and these are assessed against the criteria formulated in the ‘Define’ stage. Following thorough analysis the best concept is selected. The QFD analysis may be extended to help with this process or other decision analysis processes may be used such as Pugh Concept selection. The decision criteria and weightings used should be documented along with any assumptions that have been made. Potential risks should be identified at this stage and a plan developed to mitigate these. Capability analysis should be employed at this stage, if possible, and DOE may also be used if appropriate.
Optimize
Having chosen the best design concept, the opportunities for defects should be minimised. All risks that have been identified in the Design stage must be addressed. Several tools may be applicable at this stage including Finite Element Analysis (FEA), Computational Fluid Dynamics (CFD), Design For Manufacture and Assembly (DFMA), Failure Mode Effect Analysis (FMEA) and DOE. The key to design optimisation is in understanding which of the design features are critical parameters and then to understand the inherent variability of these features. This will usually require the use of inferential statistical techniques. By setting a parameter target value we understand that we are setting a target mean. Then, when setting tolerances, this is done with an understanding of the process capability and with reference to Six Sigma quality standards. This will be covered in greater detail in Section 4 below, ‘Achieving Robust Designs through the us of DFSS’.
Validate
The design is tested to evaluate how well it meets the design requirements identified at the ‘Define’ stage. Any assumptions should also be validated at this stage. Test results should be analysed with reference to any Capability and Reliability requirements that have been stated. The FMEA and risk analysis should also be updated at this stage. If applicable the need for Statistical Process Control (SPC) should be identified and this should be included as Manufacturing procedures are updated.
Evaluate
The project should be reviewed and documentation should be completed. This should include an assessment of how well the project has delivered in terms of the requirements defined at the ‘Define’ stage.
DFSS- Short Description

1. Identify

  • Customer Requirements,
  • Critical-to-Quality (CTQs) Parameters,
  • Set Technical Requirements & Quality Targets

2. Design

  • Concept Design
  • Develop Transfer Functions between CTQ’s
    and Parts&Process Design Parameters

3. Optimise

  • Analyze and Optimize for Robust Performance,
    Predictive Manufacturability and Reliability

4. Validate

  • Test and Validate Predictions
  • Assess Performance for Robustness
  • Compile and Maintain Control Plan for
    Critical Parameters (parts and processes)
DFSS- Main Activity and Tools

Step

Main activities

Tools

Identify

  • Identify what is important to the Customer
  • Translate Customer Requirements to CTQ’s
  • Determine how to measure CTQ’s
  • Flow down product CTQ’s to sub-systems
  • VOC practices (Customer Research)
  • System Engineering
  • QFD
  • FMEA
  • Gage R&R

Design

  • Formulate Concept Designs
  • Identify Potential Design/Program risks
  • Concept selection
  • For each sub-system CTQ identify design
    parameters
  • Brainstorming
  • Creative Group Method
  • TRIZ
  • DFMA
  • Pugh Matrix

Optimise

  • Determine Process Capability for each design parameter
  • Optimise design to minimise sensitivity of CTQ’s to design parameters
  • Allocate and set Tolerances
  • Flow-up process capability
  • Estimate defect rate and costs
  • Zst , Zlt
  • DOE
  • Sensitivity Analysis
  • Statistical tolerancing
  • Transfer Functions
  • DFMA
  • Design Verification Test Plan
  • Reliability Analysis

Validate

  • Test and validate predictions
  • Assess performance, Reliability and Risks
  • Compile and Maintain DFSS scorecard for critical parameters
  • DFSS scorecard for
  • product CTQ’s
    • internal critical characteristics
    • external (supplier) critical charact.
  • SPC

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