You get what you ask for, but also what you didn’t ask for!
For many years, I have challenged the automatic assumption that suppliers are wrong. Many times I have been involved in emergency meetings to address a Quality escapes or manufacturing stoppages caused by components that cannot be used for their chosen or designed purpose. The starting assumption is often that the supplier has made the component incorrectly. Suppliers do often get it wrong. But more often, what they have been asked to produce is incorrect. Or even more often they have been given a specification and a target price and told to build. This is a sure way to receive ‘faulty’ product. As you read below, I suspect many people will say “well of course”, but don’t assume to quickly. In many situations, the work described below, does not occur in the way it should.
Tolerance
No, I’m not suggesting that purchasing managers just tolerate defective parts. One of the key issues is assigning tolerance to dimensions on components. (The word dimensions here could refer to the size of a metal or plastic component, the layout on a PCBA or even, dare I say it, the efficiency in software code) This sounds simple enough, but can be surprisingly difficult. But just because it is difficult, does not mean it can be avoided. Having reviewed more engineering drawings that I can count, when engineers are asked to provide comprehensive tolerancing, there is typically a proliferation of dimensions. Every nook and cranny of the part is now given a dimension and tolerance, whether needed or not. This is also a recipe for disaster. When design engineers then deal directly with the supplier and present these drawings one of two things occur. Either the supplier haemorrhages and raises the price to ensure all these dimensions and tolerances are achieved, or the supplier nods in agreement, and internally just ignores most of the detail and does what they think is correct. If the design engineer did not know which dimensions were important, isn’t it rather a risk to let the supplier guess? Neither situation is a great outcome.
System
Instead, when considering tolerance requirements engineers should determine and consider the critical dimensions. Those dimensions whereby a small change has a large impact on the product, and those dimensions that don’t. This simplifies the task of tolerance analysis by reducing the number of dimensions requiring attention. So how does an engineer know what dimensions are critical, and thus require control via tolerance analysis, and those dimensions that really don’t matter too much? It requires an understanding of the system, and the risks within the design. There are numerous system modelling and design risk evaluation tools, and this is a topic worthy of its own. But suffice to say that once critical dimensions are known, the job of tolerance analysis becomes much easier for the engineer.
Manage the process first, the product second
Once the few critical dimensions have been identified and determined, suppliers can then be informed as to what it is that they are supplying. It is tempting at this stage to leave them at it! Let them build and inspect. At least now they are inspecting the correct dimensions perhaps. However, while this is better, defects will still arrive and the greatest cost saving benefits have not been realised. Quality or Production Engineers can then work with suppliers to understand how easily the suppliers’ processes can achieve these critical dimensions. Then, work with the supplier to reduce the variability within the manufacturing process so that it inherently produces products within the required tolerances of the critical dimensions. So, if a given dimension requires +/- 1.0mm, then design the manufacturing process to be within +/-0.5mm. Then continue to monitor the process for variation. As the process starts to drift out to +/-0.7mm (say), then these parts can still be used, while the cause of the drift can be addressed. No more need to inspect components, and no more need to reject components. By design. This is where the real cost improvement arises and in many cases the piece part savings are substantial, but the supply chain resilience this creates is even greeter. Further, critical components are no longer at the edge of their acceptable tolerance and are therefore more reliable when in use throughout the products lifecycle.
This is not new, so why not use it?
Anyone from the automotive industry will recognise the concepts within this document as being reflective of Production Part Approval Process (PPAP) and the like, and of course it is. In UK Rail, this is similarly called Part Approval Process (PAP), however, lack of systems engineering thinking, modelling and integration often limits it’s benefits within UK Rail. As identified within the RSSB’s Rail Technical Strategy for CP6, reliability is a key objective. Many smaller businesses in general engineering, keen to push their product into the market as quickly as possible, tend to bypass these steps and instead push as much inspection as possible under the misgiving that this will increase quality.
Let us help you
Product Governance Ltd can assist your company to reduce costs designed into your products, increasing supply chain rigour and increase product reliability. We can help your company establish the processes above or we can review your existing products, while contributing to the development of your new products.
For further information, please contact Dr Adam Palm, Managing Director, Product Governance Ltd 07 391 407 068.
