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. |