Six Sigma and Beyond: Design for Six Sigma, Volume VI

GEOMETRIC DIMENSIONING AND TOLERANCING (GD&T)

GD&T is an engineering product definition standard that geometrically describes design intent. It also provides the documentation base for the design of quality and production systems. Used for communication between product engineers and manufacturing engineers , it promotes a uniform interpretation of a component's production requirements.

This interpretation and communication are of interest to those who are about to undertake the DFSS baton. Without the appropriate and applicable interpretation of the design and without the appropriate communication of that design to manufacturing, problems will definitely occur.

Therefore, in this section we will address some of the key aspects of GD&T in a cursory manner. We will touch on some of the definitions and principles of general tolerancing as applied to conventional dimensioning practices. The term conventional dimensioning as used here implies dimensioning without the use of geometric tolerancing. Conventional tolerancing applies a degree of form and location control by increasing or decreasing the tolerance.

Conventional dimensioning methods provide the necessary basic background to begin a study of geometric tolerancing. It is important that you completely understand conventional tolerancing before you begin the study of geometric tolerancing.

When mass production methods began , interchangeability of parts was important. However, many times parts had to be "hand selected for fitting." Today, industry has faced the reality that in a technological environment, there is no time to do unnecessary individual fitting of parts. Geometric tolerancing helps ensure inter-changeability of parts . The function and relationship of a particular feature on a part dictates the use of geometric tolerancing.

Geometric tolerancing does not take the place of conventional tolerancing. However, geometric tolerancing specifies requirements more precisely than conventional tolerancing does, leaving no doubts as to the intended definition. This precision may not be the case when conventional tolerancing is used, and notes on the drawing may become ambiguous.

 
Table 10.10: GD&T Characteristics and Symbols

When dealing with technology, a drafter needs to know how to properly represent conventional dimensioning and geometric tolerancing. Also, a technician must be able to accurately read dimensioning and geometric tolerancing. Generally , the drafter converts engineering sketches or instructions into formal drawings using proper standards and techniques. After acquiring adequate experience, a design drafter, designer, or engineer begins implementing geometric dimensioning and tolerancing on the research and development of new products or the revision of existing products.

Most dimensions in this text are in metric. Therefore, a 0 precedes decimal dimensions less than one millimeter, as in 0.25. When inch dimensions are used, a 0 will not precede a decimal dimension that is less than one inch. For review of decimals and their operations, refer to Volume II of this series.

Most dimensions in this text are in the metric International System of Units (SI). The common SI unit of measure used on engineering drawings is the millimeter. The common U.S. unit used on engineering drawings is the inch. (The reader may want to review the discussion and conversions of the SI system in Chapter 20 of Volume II of this series.) The actual units used on your engineering drawings will be determined by the policy of your company. The general note "UNLESS OTHERWISE SPECIFIED, ALL DIMENSIONS ARE IN MILLIMETERS" (or "INCHES") should be placed on the drawing when all dimensions are in either millimeters or inches. When some inch dimensions are placed on a metric drawing, the abbreviation "IN." should follow the inch dimensions. The abbreviation "mm" should follow any millimeter dimensions on a predominantly inch-dimensioned drawing. Angular dimensions are established in degrees ( °) and decimal degrees (X.X °), or in degrees ( °) minutes (') and seconds (").

The following are some rules for metric and inch dimension units (for a more detailed discussion see Volume II of this series):

Millimeters

Inches

The following rules are summarized from ASME Y14.5M. These rules are intended to give you an understanding of the purpose for standardized dimensioning practices. Short definitions are given in some cases:

To appreciate GD&T, the following definitions must be understood to be successful in your dimensioning practices:

A tolerance is the total amount that a specific dimension is permitted to vary. A tolerance (not to be confused with TOLERANSING) is not given to values that are identified as reference, maximum, minimum, or stock sizes. The tolerance may be applied directly to the dimension, indicated by a general note, or identified in the drawing title block.

The limits of a dimension are the largest and smallest numerical value that the feature can be. For example: A dimension is stated as 12.50 ± 0.25. This is referred to as plus-minus dimensioning. The tolerance of this dimension is the difference between the maximum and minimum limits. The upper limit is 12.50 + 0.25 = 12.75 and the lower limit is 12.50 - 0.25 = 12.25. So, if you take the upper limit and subtract the lower limit you have the tolerance: 12.75 - 12.25 = 0.50.

The specified dimension is the part of the dimension from which the limits are calculated. The specified dimension of the example above is 12.5. A dimension on a drawing may be displayed with plus-minus dimensioning, or the limits may be specified as 12.75 and 12.25. Many companies prefer this second method because the limits are shown and calculations are not required. This is called limits dimensioning.

A bilateral tolerance is permitted to vary in both the + and the - directions from the specified dimension. An equal bilateral tolerance is where the variation from the specified dimension is the same in both directions. An unequal bilateral tolerance is where the variation from the specified dimension is not the same in both directions.

A unilateral tolerance is permitted to increase or decrease in only one direction from the specified dimension.

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