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Avoiding Customer Complaints with GD&T

November 3, 2012

Statement:  We don’t need GD&T (Geometric Dimensioning and Tolerancing) on that part because it’s good enough.

Answer:  Really?!?  Then, why doesn’t it work?

Part Interchangeability has been a necessity that has been around since WWII.  They discovered that when parts were installed, or damaged parts needed to be replaced, a part should be able to be assembled, or replaced … without being customized.  Military production suppliers realized this decades ago, but some commercial outfits, think that a custom job is needed because “we deliver a high-end product.”  Nothing could be further from the truth.

Once you are in a production run (making more than one of something), you need to think about installation, interchangeability, and spares.  If your parts are customized, you run the higher than average risk of first-time parts, as well as replacement parts, not working.

Having (allowing) a supplier tailor a part to the high, or low side of a ‘sloppy’ drawing tolerance (such as +/-.030) will solve your temporary problem of meeting your customer’s needs (wants), but will create greater problems down-stream.

  • What if your supplier changes?
  • What if your machinist (with his / her tribal knowledge) is no longer making your specialized part?
  • What if another similar company program steals your production design (assuming it is a viable design) to make their assembly work?

That is why your parts need to be dimensioned (toleranced) properly to meet your design needs.  This results in a part that can be fabricated anywhere in the world.  Just stating you want a particular end product, doesn’t mean you can get there with your current design.  An assembly is a collection of parts with their own set of allowable shape envelopes.  Proper GD&T dimensioning allows for the variation of all of your assembly parts to be made, and fit … all of the time, and not just some of the time.

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