7.0 ASSEMBLY TOLERANCE SPECIFICATIONS
An engineering design must perform properly in spite of dimensional variation. To achieve this, engineering design requirements must be expressed as assembly tolerance limits. The designer must assign limits to the gaps, clearances and overall dimensions of an assembly which are critical to performance. Assembly tolerance limits are applied to the statistical distribution of the assembly variations predicted by tolerance analysis to estimate the number of assemblies which will be within the specifications. Designers need to control more than just gaps and clearances in assemblies. Orientation and position of features may also be important to performance. To be a comprehensive design tool, a tolerance analysis system must provide a set of assembly tolerance specifications which covers a wide range of common design requirements. A system of assembly tolerance specifications patterned after ANSI Y14.5 has been proposed [Carr 93]. Those ANSI Y14.5 feature controls which require a datum appear to be useful as assembly controls. However, there is a distinct difference between component tolerance and assembly tolerance specifications, as seen in Fig. 9. In the component tolerance specification shown, the parallelism tolerance zone is defined as parallel to datum A, a reference surface on the same part. By contrast, the assembly parallelism tolerance defines a tolerance zone on one part in the assembly which is parallel to a datum on another part. In order to distinguish an assembly tolerance specification from a component specification, new symbols have been proposed. The feature control block and the assembly datum have been enclosed in double boxes.
Fig. 9 Comparison of component and assembly tolerance specifications.
8.0 MODELING PROCEDURES AND RULES
The ability to model a system is a fundamental skill for effective engineering design or manufacturing systems analysis. Unfortunately, few engineers know how to construct variational models of assemblies beyond a 1-D stack. This is primarily because the methods have not been established. There is little treatment of assembly modeling for tolerance analysis in engineering schools or texts. Until engineers learn how to model, tolerance analysis will never become widely used as have other CAD/CAE tools. A consistent set of modeling procedures, with some guiding rules for creating vector assembly models, allows for a systematic approach which can be applied to virtually any assembly. The steps in creating a model are: 1. Identify the assembly features critical to the assembly. Locate and orient each feature and specify the assembly tolerances. 2. Locate a datum reference frame (DRF) for each part. All model features will be located relative to the DRFs. 3. Place kinematic joints at the points of contact between each pair of mating parts. Define the joint type and orient the joint axes. These are the assembly constraints. 4. Create vector paths from the DRF on each part to each joint on the part. The paths, called datum paths, must follow feature dimensions until arriving at the joint. Thus, each joint may be located relative to the DRF by controlled engineering dimensions. 5. Define the closed vector loops which hold the assembly together. The datum paths defined in Step 2
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A Comprehensive System for Computer-Aided Tolerance Analysis of 2-D...
become segments of the vector loop. A vector loop must enter a part through a joint and leave through another joint, passing through the DRF along the way. Thus, the vector path across a part follows the datum path from the incoming joint to the DRF and follows another datum path from the DRF to the outgoing joint. 6. Define open vector loops to describe each...