Figure 1 is a solid model of a gyroscope housing that represents typical complex surface geometries. The features on this part represent a collection of small arc radii and are used within this article to demonstrate the historical and ongoing weaknesses of linear (plus/minus) tolerancing and the tremendous value and precision of profile tolerancing.
This article will make visible common mechanical specification requirements that do not optimally represent the designer’s true intent. It will also cover associated measurement errors and biases which lead to incorrect decisions, as well as solutions for optimizing specification requirements and associated measurement results, providing precise analysis to optimized design specifications. Finally it will close with recommendations to aid companies in their design, manufacturing and measurement optimization initiatives
International Design Standards
Mechanical components and assemblies throughout the world are defined with engineering standards, such as ASME Y14.5M-1994: Dimensioning and Tolerancing and ASME Y14.5.1M-1994: Mathematical Definition of Dimensioning and Tolerancing Principles. The ASME Y14.5M-1994 Standard is a core foundational standard however there are two principle methods of tolerancing specified within this standard: linear and geometric tolerancing.
Risk to Industry
Linear tolerancing results in product specifications which frequently fail to analyze the size and location of features. On the one hand, inspection results can look good and the product still fails, or on the other, the results look bad and the product can still work. In either situation the features specified with linear tolerancing do not address true design functionality of the component.
Physical geometry of any 3-D part is controlled by four primary elements: size, form, orientation, and location. For example, size controls how large and how small individual features can be; form controls how cylindrical or flat a feature can be; orientation controls how perpendicular or parallel features can be to each other; and location controls the distance from the specified datum reference frame and between each of the features. Without getting down to controlling elements such as surface finish parameters, material properties and other considerations, these are the only four elements a designer is attempting to control per the ASME Y14.5M-1994 Standard. For information on surface finish requirements, see ASME B46.1-2010: Surface Texture (Surface Roughness, Waviness and Lay).
Product Development Cycle
The following steps are outlined as a potential sequence of events that take place in a manufacturing environment throughout a product’s development cycle:
1. Define engineering intent on mechanical drawings/specifications or other digital media.
2. Manufacture components to engineering specifications.
3. Measure components to determine compliance to specifications, provide feedback to manufacturing engineering for process optimization and provide feedback to design engineering for tolerancing optimization.
4. Optimize design specification based on measurement and process feedback.
5. Optimize manufacturing processes.
6. Optimize measurement programs and measure optimized components.
Ship components and assemblies.
7. Steps 1, 3, 4 and 6 will be used to highlight the downside of linear tolerancing using Figures 2-5. Then Figures 6-9 will show the upside of using the optimized methods resulting from the more robust tolerancing methods of profile of a surface. Figures 10-12 will represent expanded implications for linear analysis of the small arc radii intended to solidify negative methods of definition and analysis. A 2-D drawing example will be used initially to show a simplified case study and then the gyroscope housing component shown in Figure 1 will be used to fully describe the robust 3-D application and solution using Figures 13-17.
Figure 2 is a simple 2-D drawing example with engineering specifications commonly used by designers, showing multiple small arc radii defined as diameters. The size and form of the features are controlled using linear tolerancing while the orientation and location are controlled using position tolerancing. To minimize clutter on the drawings, consider all position callouts having associated BASIC dimensions defined in the CAD model and constrained to a standard A, B, C datum reference frame.
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