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This chapter describes the basic symbology used in the dimensions and tolerances of engineering fabrication, construction, and architectural drawings. Knowledge of this information will make these types of prints easier to read and understand.

This chapter will describe engineering fabrication, construction, and architectural drawings. These three types of drawings represent the category of drawings commonly referred to as blueprints. Fabrication, construction, and architectural drawings differ from P & IDs, electrical prints, and logic diagrams in that they are drawn to scale and provide the component's physical dimensions so that the part, component, or structure can be manufactured or assembled. Although fabrication and construction drawings are presented as separate categories, both supply information about the manufacture or assembly of a component or structure. The only real difference between the two is the subject matter. A fabrication drawing provides information on how a single part is machined or fabricated in a machine shop, whereas a construction drawing provides the construction or assembly of large multi-component structures or systems.

Fabrication drawings, also called machine drawings, are principally found in and around machine and fabrication shops where the actual machine work is performed. The drawing usually depicts the part or component as an orthographic projection (see module 1 for definition) with each view containing the necessary dimensions. Figure 1 is an example of a fabrication drawing. In this case, the drawing is a centering rest that is used to support material as it is being machined.

Figure 1 Example of a Fabrication Drawing

Construction drawings are found principally at sites where the construction of a structure or system is being performed. These drawings usually depict each structure/system or portion of a structure/system as an orthographic projection with each view containing the necessary dimensions required for assembly. Figure 2 provides an example of a construction print for a section of a steel roof truss.

Figure 2 Example of a Construction Drawing

Architectural drawings are used by architects in the conceptual design of buildings and structures. These drawings do not provide detailed information on how the structure or building is to be built, but rather they provide information on how the designer wants the building to appear and how it will function. Examples of this are location-size-type of doors, windows, rooms, flow of people, storage areas, and location of equipment. These drawings can be presented in several formats, including orthographic, isometric, plan, elevation, or perspective. Figure 3 provides an example of an architectural drawing, of a county library.

Figure 3 Example of an Architectural Drawing


Dimensioning Drawings

For any engineering fabrication, construction, or architectural drawing to be of value, exact information concerning the various dimensions and their tolerances must be provided by the drawing. Drawings usually denote dimensions and tolerances per the American National Standards Institute (ANSI) standards. These standards are explained in detail in Dimensioning and Tolerancing, ANSI Y14.5M - 1982. This section will review the basic methods of denoting dimensions and tolerances on drawings per the ANSI standards.

Dimensions on a drawing can be expressed in one of two ways. In the first method, the drawing is drafted to scale and any measurement is obtained by measuring the drawing and correcting for the scale. In the second method, the actual dimensions of the component are specified on the drawing. The second method is the preferred method because it reduces the chances of error and allows greater accuracy and drawing flexibility. Because even the simplest component has several dimensions that must be stated (and each dimension must have a tolerance), a drawing can quickly become cluttered with dimensions. To reduce this problem, the ANSI standards provide rules and conventions for dimensioning a drawing. The basic rules and conventions must be understood before a dimensioned drawing can be correctly read.

Dimensioning and Tolerance Symbology, Rules, and Conventions

When actual dimensions are specified on a print, the basic line symbols that are illustrated by Figure 4 are used.

Figure 4 Types of Dimensioning Lines

Figure 5 provides examples of the various methods used on drawings to indicate linear, circular and angular dimensions.

Figure 5 Example of Dimensioning Notation

When a drawing is dimensioned, each dimension must have a tolerance. In many cases, the tolerance is not stated, but is set to an implied standard. An example is the blueprint for a house. The measurements are not usually given stated tolerances, but it is implied that the carpenter will build the building to the normal tolerances of his trade (1/8-1/4 inch), and the design and use of the blueprints allow for this kind of error. Another method of expressing tolerances on a drawing is to state in the title block, or in a note, a global tolerance for all measurements on the drawing.

The last method is to state the tolerance for a specified dimension with the measurement. This method is usually used in conjunction with one of the other two tolerancing methods. This type of notation is commonly used for a dimension that requires a higher level of accuracy than the remainder of the drawing. Figure 6 provides several examples of how this type of tolerancing notation can appear on a drawing.

Tolerances are applied to more than just linear dimensions, such as 1 + 0.1 inches. They can apply to any dimension, including the radius, the degree of out-of-round, the allowable out-of-square, the surface condition, or any other parameter that effects the shape and size of the object. These types of tolerances are called geometric tolerances. Geometric tolerances state the maximum allowable variation of a form or its position from the perfect geometry implied on the drawing. The term geometry refers to various forms, such as a plane, a cylinder, a cone, a square, or a hexagon. Theoretically these are perfect forms, but because it is impossible to produce perfect forms, it may be necessary to specify the amount of variation permitted. These tolerances specify either the diameter or the width of a tolerance zone within which a surface or the axis of a cylinder or a hole must be if the part is to meet the required accuracy for proper function and fit. The methods of indicating geometric tolerances by means of geometric characteristic symbols are shown in Figure 6. Examples of tolerance symbology are shown in Figure 7.

Figure 6 Symbology Used in Tolerancing Drawings

Figure 7 Examples of Tolerance Symbology

Because tolerances allow a part or the placement of a part or feature to vary or have a range, all of an object's dimensions can not be specified. This allows the unspecified, and therefore non-toleranced, dimension to absorb the errors in the critical dimensions. As illustrated in Figure 8 (A) for example, all of the internal dimensions plus each dimension's maximum tolerance adds up to more than the specified overall dimension and its maximum tolerance. In this case the length of each step plus its maximum tolerance is 1 1/10 inches, for a maximum object length of 3 3/10 inches. However the drawing also specifies that the total length of the object cannot exceed 3 1/10 inches. A drawing dimensioned in this manner is not correct, and one of the following changes must be made if the part is to be correctly manufactured.

To prevent this type of conflict, the designer must either specify different tolerances for each of the dimensions so that the length of each smaller dimension plus its maximum error adds up to a value within the overall dimension plus its tolerance, or leave one of the dimensions off, as illustrated in Figure 8 (B) (the preferred method).

Figure 8 Example of Tolerancing