QMT News: March 2008
Editor's comment

“Be careful what you wish for -it may come true” - an old Chinese saying, laced with a hint of menace and ruefulness. As with these old sayings, there is more than a grain of truth in them.
That truth often repeats itself in many different ways. Take metrology. We all work on the maxim, “if you can’t measure it, you can’t improve it and without measurement, our knowledge is of an unsatisfactory and meagre kind.”  Ah yes, but how many shopfloors have successfully managed to manufacture a product or a part without really knowing the exact dimension or shape of that product - sometimes for years? The design  department or the customer will merrily have been supplying the CAD data with all the carefully marked dimensions and tolerances to production who don’t have the metrology to verify conformance. But it all fits together quite nicely when passed to the customer for integrating into its assembly. No problem. The system works fine. Here another old saying might  come into play - “If it ain't broke, don’t fix it!” However, our keen metrology manager has finally persuaded the powers that be in purchasing that the very latest in coordinate measuring machines is the best thing to help the company move forward. It will finally be able to check that the parts are within tolerance and conform to the design specification, down to the last minute detail. Indeed it does. The CMM provides irrefutable proof that the part does not meet the specification - that the part is mis-shapen in some respect. On investigation, it is found that production has been making the mis-shapen part fit all these years. Worse, that the reason why it fits is that the assembly itself is not as per design specification. If the part is then made correctly, the customer would have to undertake an expensive  redesign and remachining of the final assembly so that it marries up. What to do?

Brendan Coyne

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