Principles of assembly drawings
Introduction
An assembly drawing shows how each of the different parts of a finished product fit together. Sectional views may be drawn for the purpose of making each part on the drawing as clearly identifiable as possible.
In most cases dimensions are entirely omitted on assembly drawings. Some machines or structures, however, require a few assembly dimensions for important center distances, or for location of valves, switches, and other features.
Assembly dimensions are occasionally needed to guide the shopman in properly installing the various parts in their correct locations.
Assembly drawings are drawn full size whenever practical. For this reason, large sheet sizes are often used for assembly drawings. Sheet sizes as large as 34 inches by 5 feet, and larger, are frequently used for assembly drawings.
(See Table 7, "Standard Sizes of Drawing Sheets.")
Kinds of assembly drawings
Layout or design assembly drawings
One of the first drawings to be made to describe an idea, a layout or design assembly drawing, is shown in Fig. 17-1.
Fig. 17-1. A layout or design assembly drawing
This drawing is usually prepared by the design engineer, together with any necessary calculations and freehand sketches. It is drawn to scale, usually full size, depending upon the size of the structure.
A layout assembly drawing generally shows only the important features of the product. Most of the minor details such as nuts, bolts, screws, and cotter pins are omitted.
Essentially, the layout is a graphic model of the assembled parts. We might think of this drawing as a kind of a test. By drawing the views of the various parts, each fitting with one another, the engineer can test the appropriateness or function of each individual part.
Also, he can determine the general appearance and size of each part as he draws it in its proper location.
The engineer may also indicate other important information, either on the drawing or on a separate sheet. Such information may include a listing of the required materials; notations for clearances and interferences; and additional descriptions of specific parts which may be required, such as keys, bushings, and fasteners.
Data of this nature are very necessary to the machine draftsman who works from the layout or design assembly as he prepares the detail drawings. The shopman manufactures the parts by using the detail drawings. The assembly drawings are used as a guide in assembling the various parts in their proper positions.
Working assembly drawings
These assembly drawings, illustrated in Fig. 17-2, can be used to a good advantage when the product contains a limited number of parts.
Fig. 17-2. A working assembly drawing
In this case, all of the detail description applying to the parts, such as dimensions, notes, and symbols, may be placed directly on the assembly views.
Thus, the need for making a separate set of detail drawings is usually eliminated. The single drawing is in itself complete and usually requires no additional drawings to describe the product.
Industries are using this kind of assembly drawing more and more today. There is a saving of time in placing the specifications for all the parts directly on the views of the assembly drawing.
Also, it is considered an advantage to group all information on one sheet rather than to use two or more sheets for the same purpose.
It must be understood, however, that only products which consist of reasonably few and simple parts are prepared in this manner. Because of possible confusion and misunderstanding, a working assembly drawing would not be prepared for products consisting of many and complex parts.
Diagram assembly drawings
An assembly drawing such as the one shown in Fig. 17-3 is prepared for piping layouts, structural work, electrical diagrams, and industrial floor plan layouts.
Fig. 17-3. A diagram assembly drawing
Most features are shown symbolically or are represented by as few lines as possible. The chief requirement of such a drawing is to show the general shape and location of the various parts.
Due to the large sizes frequently involved (on a floor plan, for example), diagram assembly drawings are usually made to a reduced scale.
Diagram assembly drawings may also be prepared pictorially. Since these drawings are general in nature, only the key parts of features are usually labeled.
Final or erection assembly drawings
The assembly drawing shown in Fig. 17-4 is drawn to an accurate scale. Such a drawing is usually made by tracing a previously prepared layout or design assembly drawing; if this is done, previously omitted parts are drawn in their proper positions.
Fig. 17-4. A final or erection assembly drawing
The erection assembly drawing may also be prepared by tracing the views of the parts from the detail drawings. The draftsman begins by examining each part to observe its function when assembled with the other parts.
The parts are next carefully drawn or traced on the assembly drawing; this step serves as a check of one part to another. Necessary clearances and interferences of the mating parts may be conveniently examined.
The final assembly drawing is
also used for planning methods of assembling and dismantling the various parts.
Hidden lines are kept to an absolute minimum. Only those hidden lines which are
absolutely necessary for a complete understanding are drawn. Sectional views are
frequently used to show interior construction.
Unlike the design assembly drawing, this type of drawing is made as complete as possible. The final or erection assembly drawing is used as a reference throughout the manufacture, assembly, and inspection of the product.
Outline assembly drawings
Figure 17-5 shows an assembly drawing in which only the exterior features and the general appearance and arrangement of certain parts of the product are shown. Certain dimensions may be given, such as center distances, overall sizes, and mounting hole spacings.
Fig. 17-5. An outline assembly drawing
This kind of assembly drawing is commonly used for sales and catalog material, where only the general size and appearance of a product are required.
Dimensions are often listed in columns or tables on the drawing, especially when one illustration is used to describe several available sizes, as in Fig. 17-6. In general, each part of the product is drawn proportionately in size to each of the other parts, but the drawing is usually not made to an accurate scale.
Fig. 17-6. An outline assembly drawing
Parts are generally represented by as few lines as possible. Complicated detail of each part is omitted for clarity.
Subassembly or unit assembly drawings
Figure 17-7 is a drawing of a product which shows only a limited group of parts belonging to an entire assembly of parts.
Fig. 17-7. A subassembly or unit assembly drawing
It is used for complicated products when it would be confusing to show all of the parts assembled on one drawing. Subassembly drawings represent a breakdown of the assembled product into small groups of parts.
For example, a set of drawings for a machine lathe might consist of separate subassembly drawings of four groups of parts: gear box, headstock, tailstock, and carriage.
Pictorial assembly drawings
The assembly drawing shown in Fig. 17-8, usually referred to as an exploded drawing, is used principally for catalog or display purposes.
Fig. 17-8. A pictorial assembly drawing
Some companies also use such drawings to instruct trainees in assembly line techniques in their training programs. Parts lists and other descriptive material may be placed on the drawing or may accompany it on other sheets. Companies have found pictorial assembly drawings to be more easily understood than any other type of drawing, especially for personnel with limited technical training.
Part numbers