Engineering drawing specifications are the rules for making and reading engineering drawings. The American National Standards Institute, or ANSI, and American Society of Mechanical Engineers, or ASME, have cooperated to determine these specifications. The purpose is to make engineering drawings universally readable. While most companies follow these standards, some make minor modifications to fit their own needs.
The standardization of paper sizes and nomenclature was one of the early ANSI standards. A, B, C, D, and E are the common names for paper sizes. The sizes are 8.5 inches by 11 inches for A; 11 inches by 17 inches for B; 17 inches by 22 inches for C; 22 inches by 34 inches for D; and 34 inches by 44 inches for E. The standard international metric sizes in millimeters are A4 210 millimeters by 297 millimeters; A3 297 millimeters by 420 millimeters; A2 420 millimeters by 594 millimeters; A1 594 millimeters by 841 millimeters; and A0 841 millimeters by 1189 millimeters. ANSI recommends standard tile blocks with company name, drawing title, drawing scale, drawn by, checked by and drawing number.
Orthographic projection is the drawing specification to show different views of the part clearly. Sections, or cutaways, are the method of portraying hidden areas in the part that is hard to view in other ways. Line thickness for object lines, centerlines, phantom, hidden, break and section are all standardized. Each line has its own uses and specifications as to how it is drawn and used. Bills of materials on assembly drawings have standards for location and balloon callouts. Notes provide information about how to process or manufacture the item. They have their own specifications as to location and format. Abbreviations used on drawings have standards.
Dimensioning and Text
Specifications control dimensions. This includes distances to draw the extension and dimension lines from the object, and arrowhead size for the dimension lines. It also covers distances from each dimension line. “Geometric Dimensioning and Tolerancing, Y14.5” is the internationally recognized book authored and published by ASME for dimension location, shape and form. It is the universal specification used for dimensioning and checking the part. Specifications for dimension and note text size vary, but the US Department of Defense specification for text is one standard.
Symbols such as welding and surface finish provide manufacturing information for the part. Welding symbols give information for the type, location and depth of the weld. Surface finish is a specification that dictates the smoothness or roughness allowed on the surface of a part. Symbol specifications include specialized areas such as piping and electronic drawings. The specifications for these operations and materials are standard and used throughout the industry.
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