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6. Dimensions and Those All-Important Notes
- Southern Illinois University Press
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60 6.1 The Need for Dimensions A drawing is of little use if it does not contain complete dimensions and adequate notation. Unfortunately, this requirement seems to create real hardships for many drafters. Too often, tight schedules “force” the drafter to take shortcuts, leaving many dimensions missing and assembly choices left up to the option of the carpenter. When drawings reach the studio in this condition, the resulting situation is typically time-consuming for the craftsperson and problematic for the design, because now there is no system of checkpoints and subsequently quality control is lost. This chapter addresses most of the major themes regarding dimensioning and notation practice. Keep in mind though that despite the volume of material covered, not all situations can be addressed by a formula. Where no established convention applies, focus on the basic question of dimensioning and notation : Can the specifications provided be interpreted in only one way and are they complete enough to build and finish the project without additional information? To ensure this is the case, the drafter must mentally walk through the layout or construction process. Almost certainly the answer can be “discovered” from within this framework. This axiom also points out the importance of practical experience for a drafter. Put quite simply, a drafter must be familiar with the processes and personnel employed in the studio if he/she is to be effective. The dimensioning guidelines that are addressed in this chapter are presented under several headings as a way to provide some focus to what is a lengthy and sometimes bewildering list of rules. These rules, however , are applications of the dimensioning conventions (USITT Graphic Standards) and should be observed, not simply because they are rules, but because they assure that information is presented in a form that is both thorough and familiar to drafters and readers alike. These same conventions apply to both CAD and drafting by hand. Just as none of us would question the importance of following the “rules of the road,” a similar observance of dimensioning practices is beneficial to everyone working on a project. 6.2 Orientation of Dimensions The American National Standards Institute (ANSI) approves two orientations for the reading of dimension figures (fig. 6.1): unidirectional and aligned. Either is acceptable, but drafters are encouraged to adopt one system or the other. Regardless of the choice made, use one system exclusively on any one drawing sheet or within a given project. 6. Dimensions and Those All-Important Notes 6.1 Unidirectional and aligned dimensioning 61 In the unidirectional system, both notes and dimension figures are lettered horizontally on the sheet and are read from the bottom of the drawing (which does not necessarily correspond to the bottom of the sheet). The unidirectional system has been adopted by many industries because it is easy to use and read, especially on large drawings. In the aligned system, notes are still lettered horizontally on the sheet, but all dimension figures are aligned with the dimension lines so that they are read from the bottom or from the righthand side of the drawing. In CAD, the drafter will chose default settings for dimensioning that establish a consistent style, including such qualities as orientation, spacing, font, and layout. Whether using pencil or CAD, an effort should be made to avoid locating dimensions in the shaded area of the quadrant shown in figure 6.2. Lettering in this position is awkward because it forces the reader to read upside down. 6.2 Direction of dimensions 6.3 Dimensioning Principles and Techniques Extension Lines 1. Run extension lines (fig. 6.3) perpendicular to visible profile lines. Allow a 1 /16″ gap of white space between them. 2.Attach dimensions to one view only; do not connect two views with extension lines. 3.A center line may be extended and used as an extension line, in which case it is still drawn as a center line. 4.Extension lines may cross each other; however, do not cross dimension lines and extension lines if avoidable. 5.When extension lines must cross another extension line or a visible line, do not break either line. 6.3 Extension line conventions 6.Do not use a profile line of the object as an extension line. Dimension Lines 1. Dimension lines (fig. 6.4) run between extension lines and should be spaced uniformly. Allow ½″ (full size, not scaled) between the object profile and the closest dimension line; allow ¼″ to ⅜″ between dimension lines. (Using increments of ⅛″ makes...