White Paper on Geometric Dimensioning & Tolerancing with Tolerance Stack-Up Analysis

EGS Computers India Private LimitedChennai, Coimbatore, Trichy

Web: www.egsindia.com info@egs.co.in Tel: 044-2480 3370 / 2372 0265

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SummaryProfitability and Cost Reduction objectives of Manufacturing organizations are directly influenced by Drawings. Completeness and Correctness of Drawings are essential to avoid Re-work and ambiguity. Effect of part tolerances on assembly build objectives and allocation of Tolerances using Least-Cost approach for achieving assembly functionality, form the heart of any product development process. It is necessary that Tolerance Studies are conducted to understand product variations that result in rejections. Such Tolerance analyses would be useful in identifying Process deviations that culminate in assemblies going out of specification. GD & T with Tolerance Analysis would go a long way in helping organizations stay profitable while striving to achieve 0 PPM. In today's economic scenario, this approach is certain to ensure Profitability and improve Reliability without imposing a cost penalty. This is a necessity involved in manufacturing products ranging from limited volume to high volume production.

Importance of GD & TASME Y 14.5 has evolved into a Standard on Geometric Dimensioning and Tolerancing, accepted by Manufacturing organizations world-wide. While providing a structured approach to part dimensioning, The Standard, emphasizes on the correctness and completeness of developing drawings for Parts and assemblies. With part drawings adhering to Standard, it ensures that the Manufacturing Team and Inspection Team speak the same language and understand the Design Specifications that the Design Team intended to convey.

By eliminating ambiguity, following advantages are imminent:1. Reduction in art to part transformation time2. Elimination of Re-work3. Elimination of Assembly Build Issues4. Elimination of incorrect interpretations and assumptions5. Lowering of product cost6. Improvement in Product reliability

3D CAD and GD & T:

There is no better place than 3D CAD to start implementing GD & T. Software such as SolidWorks have 3D Annotation capabilities that are compliant with Y14.41-2003: Digital Product Data Definition Practices. By offering a choice between Plus / Minus Tolerancing and GD & T based Tolerancing, SolidWorks enables the users to adhere to the ANSI/ ISO Standards while developing drawings. DimXpert, inside SolidWorks, helps the designers to workout the Dimensional schematic early in the design process, thereby ensuring correctness of representation.

Optional settings for Fine, Coarse, Medium tolerance specifications assists the designer in providing tolerance values and ranges based on Industry Best Practices. Additionally, default tolerance values can be logically set based on feature representation to aid in adhering to Company standards for acceptance tolerance levels for least cost.

Redundant Dimensions and Tolerances are eliminated. Adhering to ASME/ ISO standards also ensures that Material Modifiers, their applicability based on Geometric Characteristic Symbols employed – either in the Feature or as applied to Datum Features of Sizes, are incorporated for least cost tolerancing strategies.

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Fig. 1: 3D Part Design Fig. 2: DimXpert – Visual Detection ofCompleteness of Part Dimensioning

Fig. 3: 3D Annotations for complete Product Definition using GD & T

In the example shown above, the visual color coding (Green – Complete, Yellow – Under defined, Red – Over Defined or In Conflict) of Features as shown in Fig. 2, helps ensuring completeness of representation. 3D Annotations developed using DimXpert are shown in Fig. 3. These annotations can be directly imported into Drawing views for rapid Drawing development. In addition to reducing time and effort, this practice ensures that downstream errors during Drawing development are avoided.

DimXpert also provides Warnings and insights into Lack of definition or Inappropriate Definition of Feature control frames, size definitions, Datum definitions among other parameters to ensure correctness of drawings at every stage of the design process. This helps streamline drawing generation procedure without being person specific and goes a long way in achieving standardization of Best Practices in Design and Drawing Development.

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Tolerance AnalysisGD & T Drawings convey Design Specifications evolved from Design Intent in an unambiguous manner to the Manufacturing and Inspection Teams. Often the Designer is beset with the problem of allocating appropriate tolerances at the part definition stage. The implications of the tolerances specified at part level, in assembly build is never known. This affects assembly build, assembly functionality as well as increases rejections that affect profitability of the organization. Viewed in a different way, if assembly level tolerances for functional requirements are known, range and contribution of individual part dimensions need to be evolved with emphasis on low-cost tolerancing strategies.

SigmundWorks, integrated inside SolidWorks, performs just that to help designers arrive at optimal tolerances to ensure assemblies work all the time. Benefits of Tolerance analysis using SigmundWorks are many-fold:

1. Understanding how part level tolerances affect Assembly Build2. Allocating part level tolerances for required Assembly Build3. Determining part level tolerances that influence Assembly Build in terms of cost4. Influence of dimensional changes such as thermal expansion in functionality5. Interaction with parametric SolidWorks Dimensional scheme to identify contributors as well

as updating tolerances to achieve Assembly objectives6. Least cost tolerancing based on weighing factors7. Designing for Six Sigma without compromising on Cost8. Identifying causes for parameters that affect Process deviations resulting in rejections9. Estimate rejections based on RSS, Worst-Case, Process Centred RSS, Monte-Carlo

simulations to achieve 0 PPM

Tolerance Analysis by example

Fig. 4: Assembly Clearance Study

In the assembly shown, the clearance needs to be maintained within a specified range to meet functional requirements.

First step towards performing Tolerance Analysis would be to identify the vector loop of dimensional chain that would affect assembly build. This can be done manually, by selecting Dimensions defined using DimXpert, inside SolidWorks, or automatically allow SigmundWorks to identify and create the loop. Either way, this vector loop is based on the GD & T Scheme employed during the part dimensional definition process.

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Fig. 5: Vector Loop Diagram of Dimensional Chain

Fig. 6: Tree definition for Vector Loop based on nodes

The vector loop logic and the Nodal tree map with appropriate sign (additive or subtractive depending on the direction of the vector loop entities) are shown. Note that the Cap sub-assembly forms a separate sub-node with associated dimensions represented underneath. This is a powerful method to employ SigmundWorks to a complex Dimensional chain involving multiple sub-assemblies or vector loop diagrams that affect assembly build directly or indirectly. A complex dimensional chain involving multiple loops can be assimilated in this manner in a very intuitive way, resulting in simplified approach to developing large systems.

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Roll-up method of evaluating assembly build clearance from part tolerances using Worst Case Method (results in conservative dimensional management approach) is used to arrive at the clearance range.

Fig. 7: Roll-up clearance values based on individual part tolerances using WC Method

Sensitivity Report highlighting the contribution of individual dimensions and their tolerances to the overall assembly build helps designers obtain an insight into the interaction between different tolerance zones. This helps in identifying key contributors that would solve dimensional management issues.

Fig. 8: Sensitivity Graph of contributing dimensions

Finally, PPM calculations for assemblies that would be out of specification is estimated based on Roll-Down approach wherein the overall clearance is specified based on a Six Sigma approach.

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Fig. 9: Statistical PPM Report of Distribution of assemblies out of specification

The Statistics report plots the calculated distribution over the specified control limits and calculates the percent out of spec for the calculated distribution. In this example, 13.36% of assemblies will be built out of spec for the Worst Case analysis. These out of specification calculations are performed under normal distribution, an assumption for the resultant build objective.

Summary of Benefits:

Following benefits are evident in using SolidWorks with SigmundWorks for Design and Development of Drawings for achieving required assembly build.

1. Correctness and Completeness of Drawings are assured using DimXpert inside SolidWorks2. Automatic and Manual method of dimensioning based on Datum Selections is available in

DimXpert to accelerate the drawing development process3. Visual method of checking completeness of drawings avoids errors and ambiguities.4. Integrated tolerance analysis helps assimilate dimensional loops that change automatically

when designs undergo revisions5. Process centring of manufacturing processes are possible based on tolerance reports and

deviation sensitivities6. Lowers cost of production and eliminates rejections7. Powerful tool in the hands of the user to give the right amount of tolerances, taking into

account process capability, at part level for required assembly build specifications

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