Statistical tolerancing

A process for establishing valid statistical detail part dimensional tolerance limits that will enable accurate prediction of an economically acceptable degree of non-conformance of a large flexible assembly made from said parts, comprising:

• a) establishing dimensional tolerances for said assembly;
• b) allocating initial said assembly dimensional tolerance among said detail parts to establish a first estimate of detail part tolerances based on reliable detail part fabrication processes that are capable and controlled;
• c) selecting and validating a preferred assembly sequence for assembling said parts into said assembly;
• d) determining whether said first estimate of said detail part tolerances for said selected assembly sequence are producible within acceptable cost limits, and if not, selecting one or more of the following actions:
• i. improve or select alternate detail part fabrication process,
• ii. revise assembly design or sequence,
• iii. revising said individual part tolerances,
• iv. use assembly tool with hard locators;
• e) reperforming steps b, c, and d until a set of detail part tolerances and assembly sequence is selected, use of which enables said parts to be produced and assembled into said assembly within acceptable cost limits.

A process of assembling large flexible structural assemblies having predetermined assembly dimensional tolerances from a plurality of individual parts, some of which are flexible, while remaining within established preload stress limits of said parts, comprising:

• establishing nominal dimensions and predetermined statistical dimensional tolerances for locations of coordination features in said parts;
• machining said coordination features in said parts;
• locating said parts together relative to each other in desired relative positions primarily using said coordination features as the way of locating the parts in relation to each other instead of hard tooling;
• and
• fastening said parts together in said desired relative positions;
• whereby said predetermined statistical dimensional tolerances in said parts are determined as a fabrication requirement for said parts that enables said parts to be economically produced and assembled into assemblies that meet said predetermined assembly dimensional tolerances, said individual part statistical dimensional tolerances being broader than conventional tolerances that are established to ensure acceptable assemblies when parts are assembled at the extremes of their tolerances, stacked in a worst case manner.

A process of assembling a large flexible structure, having a set of predetermined dimensional tolerances, from a plurality of individual parts, some of which are flexible, while remaining within preestablished stress limits of said parts, comprising:

• selecting and validating a preferred assembly sequence for assembling said parts into said assembly;
• selecting locations, numbers and size of coordination features to be machined in said detail parts by which said parts are located relative to each other and fastened together to form said assembly;
• establishing individual part statistical dimensional tolerances as a fabrication requirement for said parts that enables said parts to be economically produced and assembled into assemblies that meet said predetermined assembly dimensional tolerances, said individual part statistical dimensional tolerances being broader than conventional tolerances that are established to ensure acceptable assemblies when parts are assembled at the extremes of their tolerances, stacked in a worst case manner;
• producing said parts to said individual statistical dimensional tolerances in a capable process, having a Cpk equal to at least 1.0, and assembling said parts in accordance with said preferred assembly sequence by locating said parts relative to each other by reference to said coordination features and fastening said parts together as located, using said coordination features as the primary determinator of assembly configuration.

A process of assembling a large flexible assembly from a plurality of individual parts, some of which are flexible, while remaining within predetermined dimensional tolerances of said structure and within preestablished stress limits of said parts, comprising:

• selecting and validating a preferred assembly sequence for assembling said parts into said assembly;
• selecting locations, numbers and sizes of coordination features to be machined in said detail parts by which said parts are located relative to each other and fastened together to form said assembly;
• establishing statistical dimensional tolerances for individual parts as a fabrication requirement for said individual parts that enables said parts to be economically produced and assembled into assemblies that meet said predetermined dimensional assembly tolerances, said statistical dimensional tolerances for individual parts being broader than conventional tolerances that are established to ensure acceptable assemblies when parts are assembled at the extremes of their tolerances, stacked in a worst case manner;
• producing a lot of one of said individual parts by an uncontrolled process, and measuring a sample of said parts from said lot to a confidence level acceptable for calculating a lot quality index and mean shift for said lot;
• calculating said lot quality index and said means shift mean for said part in said lot based on measurement of said sample;
• and
• removing enough of said parts from said lot near the limit of said tolerance to bring the mean dimension of parts remaining in said group to within a predetermined percentage of the prescribed mean of the tolerance for that dimension and bring said remaining parts in the lot to an acceptable level of short term capability.

A process of ensuring an accurate assembly made from parts assembled with minimal use of hard tooling, at least some of which parts are flexible, comprising:

• establishing dimensions and valid tolerance requirements for said assembly;
• using said established assembly tolerance requirements, allocate detail part tolerances to limit dimensional non-conformance of said assembly to an economically acceptable probability;
• applying a set of drawing requirements necessary to ensure compliance with statistical limits established by analysis;
• applying a detail part verification process to assure compliance to said limits, wherein said part verification process includes collecting and analyzing selected part dimensional data from populations of parts for use in said assembly to determine a Cpk associated with said populations, and to determine a shift of a mean value of said dimensional data from nominal dimensions of said parts established in part designs to which said parts are made;
• and
• accepting for use in said assembly a first set of parts populations which have a Cpk at least equal to 1.0 and a mean shift at most equal to 20%.