Extension Lines

No supplementary references are needed for this task. 1. Introduction

In the previous three tasks, we covered orthographic projection, freehand drafting, drafting instruments, geometric construction, and pictorial drawings: oblique and isometric. In this task, we will identify shop terms, abbreviations, and dimensioning elements found on shop drawings. A shop drawing is the drawing one uses to work from, since it contains all of the information necessary to make an object.

2. Dimensions, Tolerances, and Allowances

In the following paragraphs, we will discuss dimensions, tolerances and allowances. The picture portion of a drawing defines the shape of the object, the dimensions define the size, and the tolerances define the amount of variance permitted in the size. All three pieces of information are needed to form a clear, understandable, manufacturable drawing.

a. Dimensions. Dimensions are placed on a drawing by using a system of extension lines, dimension lines, leader lines, and arrowheads. Figure 68 illustrates how these various kinds of lines are used for dimensioning.

Extension Lines Drafting

(1) Types of Lines. The lines are defined as follows:

(a) Extension lines are used to indicate the extension of an edge or point to a location outside the part outline.

(b) Dimension lines show the direction and extent of dimension.

(c) Leader lines are used to direct an expression, in note form, to the intended place on the drawing. The leader line should terminate in an arrowhead or dot.

(d) Arrowheads are used to indicate the ends of the dimension lines and the ends of some of the leader lines.

(2) Dimensioning Systems. Dimensions may be positioned on a drawing by using either the unidirectional or the aligned system. The unidirectional system is the preferred system. In the unidirectional system, all dimensions are placed so that they can be read from the bottom of the drawing, that is, with their guidelines horizontal. In the aligned system, dimensions are placed so that they may be read from either the bottom or the right side of the drawing, that is, with their guidelines parallel to the surface that they are defining. Figure 69 illustrates the difference between the two systems.

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