Geometric Dimensioning and Tolerancing for Mechanical Design Part 12 pptx

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Chapter

13

Graphic Analysis

Graphic analysis, sometimes referred to as paper gaging, is a technique that

effectively translates coordinate measurements into positional tolerance geom￾etry that can easily be analyzed. It provides the benefit of functional gaging

without the time and expense required to design and manufacture a close￾tolerance, hardened-metal functional gage.

Chapter Objectives

After completing this chapter, you will be able to

Identify the advantages of graphic analysis

Explain the accuracy of graphic analysis

Perform inspection analysis of a composite geometric tolerance

Perform inspection analysis of a pattern of features controlled to a datum

feature of size

Advantages of Graphic Analysis

The graphic analysis approach to gaging has many advantages compared to

gaging with traditional functional gages. A partial list of advantages would

include the following:

Provides functional acceptance: Most hardware is designed to provide inter￾changeability of parts. As machined features depart from their maximum

material condition (MMC) size, location tolerance of the features can be in￾creased while maintaining functional interchangeability. The graphic anal￾ysis technique provides an evaluation of these added functional tolerances

in the acceptance process. It also shows how an unacceptable part can be

reworked.

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208 Chapter Thirteen

Reduces cost and time: The high cost and long lead time required for the

design and manufacture of a functional gage can be eliminated in favor of

graphic analysis. Inspectors can conduct an immediate, inexpensive func￾tional inspection at their workstations.

Eliminates gage tolerance and wear allowance: Functional gage design allows

10 percent of the tolerance assigned to the part to be used for gage tolerance.

Often, an additional wear allowance of up to 5 percent will be designed into

the functional gage. This could allow up to 15 percent of the part’s tolerance

to be assigned to the functional gage. The graphic analysis technique does not

require any portion of the product tolerance to be assigned to the verification

process. Graphic analysis does not require a wear allowance since there is no

wear.

Allows functional verification of MMC, RFS, and LMC: Functional gages are

primarily designed to verify parts toleranced with the MMC modifier. In most

instances, it is not practical to design functional gages to verify parts specified

at RFS or LMC. With the graphic analysis technique, features specified with

any one of these material condition modifiers can be verified with equal ease.

Allows verification of a tolerance zone of any shape: Virtually a tolerance

zone of any shape (round, square, rectangular, etc.) can easily be constructed

with graphic analysis methods. On the other hand, hardened-steel functional

gaging elements of nonconventional configurations are difficult and expensive

to produce.

Provides a visual record for the material review board: Material review board

meetings are postmortems that examine rejected parts. Decisions on the dis￾position of nonconforming parts are usually influenced by what the most se￾nior engineer thinks or the notions of the most vocal member present rather

than the engineering information available. On the other hand, graphic anal￾ysis can provide a visual record of the part data and the extent that it is out

of compliance.

Minimizes storage required: Inventory and storage of functional gages can

be a problem. Functional gages can corrode if they are not properly stored.

Graphic analysis graphs and overlays can easily be stored in drawing files or

drawers.

The Accuracy of Graphic Analysis

The overall accuracy of graphic analysis is affected by such factors as the ac￾curacy of the graph and overlay gage, the accuracy of the inspection data, the

completeness of the inspection process, and the ability of the drawing to provide

common drawing interpretations.

An error equal to the difference in the coefficient of thermal expansion of

the materials used to generate the data graph and the tolerance zone overlay

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Graphic Analysis