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Use Those Schedules of Tolerances as Design Guides
(In accordance with Y14.5M-1994 standard)

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Schedules of tolerance can be found for just about any process. This one is for diecasting.

They are very helpful as a guide. The problems arise when they appear on a drawing as a general tolerance. Take the above table as an example. The values shown have to be increased if the dimension is across a parting line. Also, there is no way of knowing the quality level. A much better way to assign tolerances is to determine the particular process standard deviation or expected Cpk based on historical data. For an aluminum gravity diecasting, as illustrated here, a tolerance of ±0.13 over the 40mm dimension in the plan view will predictably result in a Cpk of 1.53. At many companies striving for 6 Sigma quality, this would be acceptable. The 40mm dimension in the left side view is across a parting line and the Cpk drops to 0.33 and the probability of making parts out of spec is 0.1611. In other words, there is a 16.11% chance of creating bad parts. The tolerance would have to be greater than ±0.22 to be considered a 6 Sigma design. The 40mm across the parting line is the most difficult dimension to hold. That is why the general profile tolerance is 0.44. In the absence of historical data from the manufacturer, use these tables as design guides to determine a reasonable/producible tolerance. Hopefully, this will be expressed as a general profile tolerance relative to a datum reference frame. Then state exceptions to this general tolerance on the field of the drawing.

Back to Tips Tip added Apr 2004