Augmenting DFC method to support the top-down design process for mechanical assemblies

Abstract

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2001.Includes bibliographical references (p. 181-187).The aim of this thesis is present tools which support the top-down design process for assemblies by analyzing the locating scheme or constraint structure of assemblies in absence of detailed level part geometry. The top-down design process has received attention both in academia and industry. However, there have been few analytical tools to support it. The bottom-up approach supported by CAD systems is good for detailed level design of a single part. The representation and manipulation of assemblies involves structural and spatial relationships between individual parts at a higher level of abstraction than the representation of single parts. This thesis uses the Datum Flow Chain (DFC) for symbolic representation of mechanical assemblies and screw theory for representation of constraints between two parts. DFC captures the design intent by recording location scheme of assemblies. Screw theory can represent constraints in three dimensions. This thesis presents the design steps and corresponding analytical tools for a top-down design process in a logical progressive way. The approach of bottom-up process supported by CAD systems is compared all along the presentation. A method to generate the screw theory representation of relative constraints between two arbitrary contacting surfaces is presented first. A procedure has been outlined to generate the screw representation of an assembly feature constructed by several contacting surface pairs. These tools can be used to construct screw theory representation of an arbitrarily complex assembly feature. A method of finding the constraint properties of assemblies, which uses screw theory, is presented next. The method of motion analysis can find under-constraints for all assemblies. This can be used for analysis of instantaneous kinematics of a general mechanism as well. Finding over-constraints in an assembly is a separate problem and it requires different procedure of analysis than motion analysis. This thesis presents a method of finding over-constraints of assemblies. Quantitative information about over-constraint of all assemblies may not be found in cross-coupled assemblies. Motion and constraint analyses can help assembly designers in evaluating the nominal design. A method to calculate the sensitivity of the location of a part due to variation in the location of an assembly feature is presented next. This method uses the screw theory representation of constraints and information about location of assembly features. Clearance is introduced on bi-directional assembly features to reduce the probability of interference but it introduces uncertainty in the location of parts. A method is proposed to analyze uncertainty in the location of parts due to clearance on the size dimensions of assembly features. These analysis tools can be used to check robustness of the nominal design. A classification of assemblies based upon constraint properties is presented next. This classification relates properties of constraint structure of assemblies to design context. Finally, this thesis lays out a coherent scheme of design steps forming a procedure for designing mechanical assemblies in a top-down fashion.by Gaurav Shukla.S.M