Design for Quality – A Perspective
Increasingly a lot of attention is being paid to Quality in Product Engineering with emphasis on Quality By Design. Prof. Genichi Taguchi famously defined Quality as ‘Financial Loss to Society AFTER an Article is shipped.’ While 80% of the product cost is determined by design, the fate of a product is determined by its quality. If the concept of Quality is instilled in the design process, then it is bound to be a win-win situation. Today a Customer prefers a good quality product at an AFFORDABLE cost. It is not cheap product any more. This is more so driven by the cost incurred by the Customer AFTER he/she buys the product. If the user experience is bad, the Customer is lost forever.
With ever growing emphasis on quality products to market faster, the process of Design Engineering is getting more focus. After all, in design lies the heart of the product fit, form and function definition. A design fix costs a dollar. The same, when fixed at tooled up stage may cost an order more. After the product is shipped, the same fix would come at monumental costs, that includes the loss of business opportunity, due to mistrust gained.
Design for Quality Process
With Emphasis on Quality the DFQ Process starts with the definition of the DFQ Objectives
Guidelines for selecting DFQ Objectives are:
- Directly control Fit, Form and Functional requirements
If the DFQ Objective cannot be measured or quantified, it cannot be incorporated. Critical To Quality (CTQ) parameters are measurable. They should be driven by DFQ Objectives having Clear Definition.
Once the DFQ Objectives are enumerated, they are to be classified as Assembly and Performance Objectives. Necessarily the Design Failure Modes and Analysis (DFMEA) has to contain the DFQ objectives. The Risk Priority Number (RPN) drives the Quality Objective Priority. This enables quality assessment in an objective manner that is not person specific, but process centric.
9 out of 10 issues in product quality arises out of Assembly. When components transgress each others’ space, Quality issues arise. By emphasizing on Design for Assembly as the next priority, the Quality issues get addressed upfront. It is not simplified part design that ensures success. It is simplified assemblies. One may have many simple parts that are cheaper to produce. However, with more simple parts, the process of assembly and its build variation, compounds quality issues that, many times, lead to line stoppages. Simplified assemblies, sometimes with integrated parts that are complex, eliminate Quality issues. It is the cost of the Assembly that finally matters and not the cost of parts.
Assembly of parts and Sequence in assembly influence the selection of Datums and their precedence on the drawings. With the ASME Y 14.5 (2009) Standard mandating GD&T practices for Product definition, this step is all the more important. Choice of wrong, inappropriate or flimsy datums have proved to be disastrous. The Automotive and Medical Industry knows this better than others, on account of painful recalls and liabilities.
Allocation of Tolerances (or Specifying Tolerances) on drawings is no more an art or based on EXPERIENCE. The Tolerance specification has to be based on assessment of the build up of the tolerances in assembly and their DIRECT influence on the DFQ Objectives. Both Tolerance Analysis and Tolerance Synthesis (Tolerance Optimization) are the next steps that need to be performed to ensure that the DFQ Objectives are satisfied. It is not merely enough to ‘check and see’ if the tolerances adding up are passing the requirements. It is compulsively important that the Tolerances are Cheaper to achieve. In other words, the mantra is to Maximize Tolerances and not be satisfied with a worst-case verification alone.
This leads us to the next question. What tolerances to specify?
Tolerance specification is an optimization problem. The measurable DFQ Objective serves as the objective function. The dimensions and their tolerances serve as the variables. Cost of Precision, Cost of Poor Quality, Process Capability (Cp, Cpk) form the constraints for the optimization problem. Least Cost Optimization is the rule. Optimization of tolerances across multiple DFQ objectives is prioritized by the Risk Priority Number (RPN) arrived using DFMEA. Herein lies the success of the Product in terms of its Design, Cost and Quality.
DFX – Other Considerations
Design for Life (not Factor of Safety based designs), Design for Performance, Design for Service, Design for Ergonomics, Design for Safety, Design for Packaging & Transportation are the other items on the checklist.
When will I finish my Design with this Process?
Though the process looks long requiring more efforts, upfront engineering with these considerations are a necessity today, and not a luxury. Revisiting these items on the checklist, AFTER a product is launched, is more costly. Surgical corrections, after tooling, is always painful and wrought with constraints. Industry has long suffered as a result of this experience. It is time that Quality takes the Prime Time in Deliberations in Design, for producing a Good Quality Product at Affordable Cost, consistently.
Otherwise, as Prof. Genichi Taguchi put it, quality will be a financial loss to society after the article is shipped.
The ill effects of the same cannot be ignored.