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Ch09_Outline

# Ch09_Outline - CHAPTER 9 Dimensioning Tolerancing Practices...

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CHAPTER 9 Dimensioning & Tolerancing Practices INTRODUCTION Before an object can be built, complete information about both the size and shape of the object must be available. The exact shape of an object is communicated through orthographic drawings, which are developed following standard drawing practices. The process of adding size information to a drawing is known as dimensioning the drawing. In order that size information is communicated as clearly as possible, standard dimension practices have been established. 9.1 DIMENSIONING Geometrics is the science of specifying and tolerancing the shapes and locations of features on objects. Once the shape of a part is defined with an orthographic drawing, the size information is added also in the form of dimensions . Dimensioning a drawing also identifies the tolerance (or accuracy) required for each dimension. If a part is dimensioned properly, then the intent of the designer is clear to both the person making the part and the inspector checking the part. A fully defined part has three elements: graphics, dimensions, and words (notes). 9.2 SIZE AND LOCATION DIMENSIONS A well-dimensioned part will communicate the size and location requirements for each feature. Communications is the fundamental purpose of dimensions. Parts are dimensioned based on two criteria: Basic size and location of the features. Details of a part's construction, for manufacturing. On a drawing used in American industry, all dimensions are in inches, unless otherwise stated. Most countries outside of the United States use the metric system of measure, or the international system of units (SI), which is based on the meter. The SI system is being used more in the United States because of global trade and multinational company affiliations. Occasionally, a company will used dual dimensioning, that is, both metric and English measurements on a drawing. Angular dimensions are shown either in decimal degrees or in degrees, minutes, and seconds. 9.2.1 TERMINOLOGY There are a number of terms important to the understanding of dimensioning practices. A dimension is the numerical value that defines the size or geometric characteristic of a feature. A basic dimension is the numerical value defining the theoretically exact size of a feature. A reference dimension is the numerical value enclosed in parentheses provided for information only and is not used in the fabrication of the part.

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A dimension line is the thin solid line that shows the extent and direction of a dimension. Dimension lines are broken for insertion of dimension numbers. Arrows are placed at the ends of dimension lines to show the limits of the dimension. Arrows are uniform in size and style no matter what the size of the drawing. An extension line is the thin solid line perpendicular to a dimension line indicating which feature is associated with the dimension. There is a visible gap between the feature and the end of an extension line.
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