Boots – shoes – and leggings
Patent
1997-12-04
1999-09-21
Grant, William
Boots, shoes, and leggings
36446801, 702 81, 702179, G05B 19418, G07C 314
Patent
active
059562510
DESCRIPTION:
BRIEF SUMMARY
This invention relates to methods for meeting end item/assembly tolerance criteria for large flexible parts, and for identifying the tolerance path starting with the end item/assembly feature through all detail parts in the path, and for selecting tolerances of detail part locating features in the tolerance path. It also considers the relationship of part tolerances to tool tolerances, and the use of a modifying factor to account for detail part process mean shifts.
BACKGROUND OF THE INVENTION
Traditional arithmetic tolerancing simply adds all the tolerances in a tolerance stack-up at the extremes of the drawing tolerances to predict a "worst case" assembly variation. It is important to note that if parts are built within tolerance and the assembly was correctly analyzed, a worst case approach assures 100% good assemblies.
Statistical tolerancing takes advantage of the fact that assemblies rarely or never stack in a worst case manner, and accepts the possibility that a small percentage of assemblies will fail to meet tolerance requirements. Under this approach, the tolerances of the detail parts can be increased because it can be shown that the statistical chance of worst case tolerance accumulation is small. Analysis indicates that the economic advantage accruing from the use of statistical tolerancing and the larger detail tolerances they make possible exceeds the cost of reworking or even scrapping the few assemblies that fail to meet the tolerance requirements. When statistical tolerancing is used to develop drawing requirements, both the design calculations and part inspection plans are more involved so normally only critical dimensions will be statistically controlled.
An assembly method known as "determinant assembly" is an approach to the production of large flexible parts and assemblies, such as airplanes, that eliminates the use of most traditional "hard tooling." An example of "determinant assembly" used to make airplane fuselage panels and fuselages is disclosed in U.S. patent application Ser. No. 07/964,533, now U.S. Pat. No. 5,560,102 entitled "Panel and Fuselage Assembly" filed on Oct. 13, 1992, by Micale and Strand. Another example of "determinant assembly" used in the airplane industry, this time to make airplane wings, is disclosed in U.S. Provisional Application 60/013,986 entitled "Determinant Wing Assembly" filed on Mar. 22, 1996, by Munk and Strand. To ensure that the assemblies, designed using the determinant assembly method, can be assembled successfully, tolerances should be analyzed to insure that the specified drawing tolerances will be producible and will support the preferred manufacturing plan/assembly sequence. Typical tolerance stack-ups for airplane assemblies require that a statistical tolerance analysis be performed in order to predict good assemblies made with producible detail part tolerances.
The "population" of manufactured parts, as used herein, is a term used to describe sets of numbers or values, consisting of measurements or observations about those parts. Populations of parts and the measurements thereof are described herein by distributions of these values. Such a description is usually given in terms of a frequency distribution, a probability distribution, or a density function with values given by f(x). Two parameters used to describe a population are its mean .mu. and its standard deviation .sigma., wherein .sigma..sup.2 called the population variance. These parameters characterize the center or location of a population and the variation around the center. More specifically, these parameters are defined in terms of f(x) by ##EQU1## In the discrete case the population consists of many finite values and in the continuous case the population is so large that it is more conveniently represented by a continuum of values and the distribution of values is described by a density function f(x). If the population is normally distributed, part measurements will distribute and divide approximately in the proportions as shown in FIG. 1.
It is often impractical or uneco
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patent: 5337462 (1994-08-01), Hedman
patent: 5581466 (1996-12-01), Van Wyk et al.
"Review of Statistical Approaches to Tolerance Analysis" Nigam & Turner, Computer Aided Design, Jan. 95, pp. 6-15.
"Six Sigma Design and Statistical Tolerance Analysis" Sehlhorst, TI Technical Journal, Dec. 1995, pp. 54-63.
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"Case Study in Statistical Tolerancing" Altschul & Scholz, Manufacturing Review Mar. 1994, pp. 52-56.
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"Chemical Engineers' Handbook" Fifth Edition, Perry & Chilton, pp. 1-38 thru 1-40 and 2-62 thru 2-76, 1973.
Atkinson Robert E.
Miller Teresa S.
Scholz Friedrich-Wilhelm
Garland Steven R.
Grant William
Neary J. Michael
The Boeing Company
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