Analysis Of3D Deviations In Design Of Geometric Procision For Mechanical Products

Geomemetric precision design plays an important role in guaranteeing quality ofmechanical products through improving functional performance and manufacturingefficiency significantly during design of structure, assembly and process. With morecomplicated structures on parts, increasing precision for satisfying assembly requirementsand wider applications on3D digital design and manufacturing, analysis of3D deviationshas become key and difficult points in development of products. Therefore, in order toprovide theoretical foundation and practical technology for increasing assembly precisionand success rate, analysis of3D deviations in design of geometric precision of mechanicalproducts is carried out in the thesis.The main contents of the thesis are concluded below:(1) The background of research and state-of-the-art of studies on tolerance modeling,tolerance analysis and tolerance optimization are introduced with comments on existingproblems. The purpose and significance of the research is presented as well.(2) For avoiding improper assignment of tolerance specifications, a method used toreason and verify specifications of geometric tolerances based on polychromatic sets theoryis proposed. A hierarchic structure is set up through establishing the ontology model oftolerance and summarizing constraint relationship among geometric information, locationinformation, constraint information and tolerance information. Reasoning algorithms oftolerance types and additional symbols are presented to verify specification of geometrictolerances based on polychromatic sets model of the structure and contour matrixes of theconstraint relationship after datum reference frame is proved to be reasonable.(3) In order to improve accuaracy of assembly success rates, an accurate computationmodel which relies on contact condition of fitting and takes location priority intoconsideration is proposed. Through variation analysis under constraint of location priority,constraint relationships among variations of assembly features are extracted according tocontacting conditions of fitting between opposed parallel planes, cyliners, cones andspheres. Assembly success rate between two parts located by several couples of assemblyfeatures is computed via random simulation. (4) For searching propagation paths determined by geometric tolerances, a searchingmethod based on deviation vector spaces is studied. Actual constraint directions betweenfeatures are derived through summarization of possible constraint directions under differentspatial relationship and linear dependence analysis. Deviation vector spaces are constructedto reflect ranges of actual constraint directions and employed to develop searchingalgorithems. With given algorithms and flowchart, propagation paths of3D deviations canbe searched under constraint of geometric tolerances and assembly fittings.(5) In order to improve effieciency of geometric tolerances anlalysis, an analyticalmodel used to predict assembly precision is proposed based on distributions of multi-DOFdeviations. Based on relationship between geometric errors and multi-DOF deviations,distributions of multi-DOF deviations constrained by geometric tolerances and fitting gapsare analyzed comprehensively for planes, straight lines and points. Combining withcovariance analysis of assembly variation accumulation, assembly precision and assemblycapability index can be computed efficiently.(6) For obtaining machining tolerances due to datum transfer in manufacturing process,a method used to transfer tolerances based on Tolerance-Maps is proposed. With directedgraph and mathematical model of tolerance transfer, Tolerance-Maps of target features bymachining tolerances and datum shift are constructed through variation analysis. Throughcomparing them with T-Maps of target features by design tolerances, constraint conditionsamong machining tolerances and design tolerances can be derived based on fittingrelationship between their cross-sections.(7) A software system named―Tolerance Information Management and analysissoftware‖system is developed based on outcomes of the thesis and related reasearcrhprojects. After introduceing sructures, funcations and interfaces of the system, an assemblyof several cabins and devices on an aircraft is taken as an example to prove feasibility andefficiency of the system.