Edited by Martha K. Raymond

THOMAS DOERR
Variation Systems Analysis Inc.
St. Clair Shores, Mich.

Every designer’s goal is to create a product that goes together right the first time, not just because it’s an element of a successful design but also because it eliminates the headaches of redrafting and testing more prototypes. For example, elegantly designed parts will always need to fit together without placing too much strain or preload on their joints. The practical tests and checks, however, can seem a hang-up to creativity.

One way to check the assembly-ease of a product is to perform a tolerance analysis. Designer’s have only three choices when implementing a tolerance analysis: make several thousand assembles, take an educated guess, or perform a comprehensive simulation of the product build. The practical choice is a simulation and, when implemented early in the design cycle, typically improves quality and reduces cost.

Tolerance Tools
A method to perform this type of check is with a comprehensive 3D Tolerance Analysis. The analysis establishes a design and process that allows the largest amount of variation without negatively affecting product requirements and quality. The analysis should reflect real-world tolerances, such as those governed by predetermined Geometric Dimensioning and Tolerancing (GD&T) zones, as well as actual assembly methods and sequences. And, it should also account for assembly measurements that accurately predict production variation. One software package that encompasses all parameters for tolerance analysis is VSA-3D from Variation Systems Analysis Inc. Examining the steps in the comprehensive simulation demonstrates how it helps designers create a streamlined product from concept to assembly.

Building Blocks
The first step is to develop a concept design in CAD software, such as Pro/Engineer or Unigraphics, along with GD&T schemes. Since VSA’s software is directly integrated with the CAD system, the part model is easily organized into functional features. Examples include holes, planes, slots, tabs, and patterns of holes. They are defined and related to one another according to GD&T references and feature control constraints, such as form, orientation, location, and size.

The core of the tolerance software is a math-rule-based program derived from ASME/ANSI and ISO standards that verifies and analyzes GD&T schemes. In total, VSA-GDT software analyzes the tolerancing system and consistently applies over 290 GD&T rules to identify conflicts between features and their tolerances.

Warning Signs The VSA-GDT software produces warning messages and identifies corrective actions if the feature is incorrectly controlled by the assigned GD&T. The same holds if multiple, redundant featur
es overconstrain form, orientation, location, or size which require unnecessary gages or inspection processes. It also checks for features not fully constrained in form, orientation, location, and size which leads to incomplete measurements.

The tolerance notations are easy to interpret because they are concise mathematical descriptions instead of the sometimes vague or sketchy tolerance notations. VSA-GDT also automatically creates the component-level tolerance model analyzed in VSA-3D software.

Model Performance
The next part of the analysis applies the VSA-3D software within the CAD database to create a functional assembly model. The model contains mathematical descriptions of the component parts as well as a definition of assembly sequences, methods, and measurements. The model represents the between-component variation that results while assembling multicomponent products. It also simulates the variation associated with typical assembly methods, such as bolting. Assembly measurements, for example gaps, interferences, misalignments, and force variations, are also simulated.

The next stage focuses on 3D assembly variation that consists of a statistical simulation using the functional assembly model to provide a target list for statistical process control in production. The software analytically determines if the product and process, as specified by design, will meet final assembly build objectives. Another task is to optimally allocate all sources of variation, not just component tolerances, to minimize cost and allow maximum variation without negatively impacting the product. The software also identifies and ranks the effect critical characteristics will have on assembly requirements.

As for inspection, the software validates that each component is manufactured within specified tolerance zones. To guarantee accurate measurements, the inspection process uses the same tolerance zones as in the 3D tolerance analysis. Measurement data input in the VSA-3D model compares actual build capability with the final assembly requirements. When design changes are necessary, proposed production modifications are simulated through VSA-3D for verification, before being implemented in production.

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