Assessing the capabilities of patternshop measurement systems Page: 4 of 31
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measurement system analysis methods used throughout this effort
can and should be used by patternshops to quantify measurement
system errors. This is critically important for patterns meant
to produce close tolerance castings.
CASTING DIMENSIONAL VARIABILITY
There are many individual processing steps needed to produce
a metal casting in a sand mold. Such steps include sand
preparation, core making, mold making, mold and core assembly,
and finishing. Each of these steps, as well as other factors
such as pattern wear, will contribute to the overall dimensional
variability of casting features. Figure 1 displays the major
categories of casting production variables influencing the
dimensional integrity of casting features.
The resultant size of every casting feature will not be the
same for each casting produced, but will be distributed over a
range of values. This variation of feature dimensions from
casting to casting is commonly expressed in terms of standard
deviations about the mean. A total of six standard deviations
(66) centered on the mean accounts for 99.7% of all observations
made, for normally distributed data. This means that for any
particular feature, the feature dimension will be expected to be
within the 6a range for 997 out of 1000 castings. As the process
variability decreases, the probability of a dimensional casting
error also decreases. An example of dimensional variability and
its relationship to the customer specified feature tolerance is
shown in Figure 2.
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Peters, F.E. & Voigt, R.C. Assessing the capabilities of patternshop measurement systems, article, December 1, 1995; University Park, Pennsylvania. (https://digital.library.unt.edu/ark:/67531/metadc619314/m1/4/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.