The Calculation Method Of Shift Tolerance Based On Linkage Mechanism Model

When the datum features apply the maximum or least material condition(MMC/LMC),the shift tolerance,because of the departure of the actual mating size of the datum features from their extreme condition,can be added to the stated tolerance of the related feature,thereby improving the acceptance rate of the parts.However,it is difficult to calculate the shift tolerance when more than one datum feature applies MMC/LMC,and few calculation lethods are developed for shift tolerance,therefore causing limitation in actual production process.This paper proposed a general calculation method of shift tolerance based on linkage eechanisl model,and a program for shift tolerance calculation based on SolidWorks is developed.This paper consists of five parts as follows:1 Basic concept of tolerance specification and research status of conditional tolerance is introduced.Calculation lodel of shift tolerance,technique of measurement and the composition principle of datums are reviewed.Prospect and significance of this reasearch are discussed.2 Technological background for calculation of shift tolerance is introduced,which includes concept of Datum Feature Simulator(DFS)and its construction process,research of the concept of constraining degrees of freedom,concept of Datum Reference Frame(DRF)and its lethod of establishment and research of combination of datums.3 Working process of shift tolerance is illustrated.The Datum Feature Simulators(DFSs)at actual virtual condition are used to construct the measurement coordinate system,and the DFSsat the stated MMC/LMC condition are used to construct the design coordinate system.According to the Type,size and location of Datum Features,relative position between the design coordinate system and the measurement coordinate system is simulated by series combination of two linkage mechanisms.Structural parameters and kinematic parameters of the mechanism are used for calculation of shift tolerance and the kinematic envelope of the design tolerance zone on the linkage curve corresponds to the ITZ.4 The possible combinations of datum features when three datum features apply MMC/LMC are listed,and according to the types,orientations and locations of the datum features,these combinations are divided into three classes.For each class,relative position between the design coordinate system and the measurement coordinate system is simulated by the series combination of linkage mechanisms.Structural parameter and kinematic parameter of the mechanism are used for calculation of shift tolerance.5.A program for shift tolerance calculation based on SolidWorks is developed.By extracting the three-dimensional annotation information of parts,datums of the measured feature are automatically identified.A lot of annotations are judged,including the geometric tolerance type and value of the measured feature,the tolerance specification of the datum features,the position-size relationship between datum features and the position-size relationship between datum features and the measured feature.Geometric tolerance annotations are used to calculate shift tolerance and inspection tolerance zone is delivered in sketch environment.The application method of program is introduced and illustrated through examples.