Expanding Tolerance Analysis for a Robust Product Design Expanding Tolerance Analysis for a Robust Product | Page 8

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Expanding Tolerance Analysis for a Robust Product Design

functional limits, the dimensions contributing the most to the
current overall variation of the functional requirement, and the
impact that changing a dimensional value will have on the
functional requirement (i.e. the sensitivity of the result to each
dimension). When trying to optimize the allocation of
permissible variation of the functional requirement to each of
the contributing dimensions the engineer should also have an
understanding of the statistical capabilities of the various
manufacturing processes available as well as high-level
understanding of the relative cost differences. Typical sources
of such information include published industry references,
internal recommended tolerance databases gathered from
process capability studies, or from experienced manufacturing
personnel.
This case study shows the top three tolerance contributors
are: tolerances of (1) Distance of the top to the seating face on
valve. (2) Camshaft bore location on head. (3a) Counter bore
depth of shim on tappet. (3b) Depth of valve seat machining.

Fig. 18. Sensitivity Graph

If the precision or Cp is good, but accuracy or Cpk is not,
the engineer should shift the nominal value looking first at the
most sensitive dimensions to get the most impact for the least
amount of design change. (Fig19). In this study, decrease “(1)
the distance between top face and seating face on valve” by
0.1mm, then shift the nominal value by 0.1mm because of its
sensitivity
-1.0 mm/mm.

0.1

Fig. 17. Contribution graph

Fig. 12. Nominal Shift Example

4.6 Critical Dimensions Management
The sensitivity chart (Fig. 18) below shows the top five
most sensitive dimensions including: (1) Distance between top
face and seating face on valve. (2) Distance between camshaft
bore and mating face of valve seat on head. (3) Profile of base
circle on cam. (4) Distance between top face and mating face
of shim on tappet. (5) Location of seating face on valve seat.

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