WHAT IS DESIGN FOR MANUFACTURE AND ASSEMBLY?

In this text we shall assume that "to manufacture" refers to the manufacturing of the individual component parts of a product or assembly and that "to assemble" refers to the addition or joining of parts to form the completed product. This means that for the purposes of this text, assembly will not be considered a manufacturing process in the same sense that machining, molding, etc., are manufacturing processes. Hence, the term "design for manufacture" (or DFM) means the design for ease of manufacture of the collection of parts that will form the product after assembly and "design for assembly" (or DFA) means the design of the product for ease of assembly. Thus, "design for manufacture and assembly" (DFMA) is a combination of DFA and DFM. DFMA is used for three main activities: 1. As the basis for concurrent engineering studies to provide guidance to the design team in simplifying the product structure, to reduce manufacturing and assembly costs, and to quantify the improvements. 2. As a benchmarking tool to study competitors' products and quantify manufacturing and assembly difficulties. 3. As a should-cost tool to help negotiate suppliers contracts. The development of the original DFA method stemmed from earlier work in the 1960s on automatic handling [1]. A group technology classification system was developed to catalogue automatic handling solutions for small parts [2]. It became apparent that the classification system could also help designers to design parts that would be easy to handle automatically. In the mid-1970s the U.S. National Science Foundation (NSF) awarded a substantial grant to extend this approach to the general areas of DFM and DFA. Essentially, this meant classifying product design features that significantly effect assembly times and manufacturing costs and quantifying these effects. At the same time, the University of Salford in England was awarded a government grant to study product design for automatic assembly. As part of the study, various designs of domestic gas flow meters were compared. These meters all worked on the same principal and had the same basic components. However, it was found that their manufacturability varied widely and that the least manufacturable design had six times the labor content of the best design. Figure 1.1 shows five different solutions for the same attachment problem taken from the gas flow meters studied. It can be seen that, on the left, the simplest method for securing the housing consisted of a simple snap fit. In the examples on the right, not only does the assembly time increase, but both the number and cost of parts increases. This illustrates the two basic principles of design for ease of assembly of a product: reduce the number of assembly operations by reducing the number of parts and make the assembly operations easier to perform. The DFA time standards for small mechanical products resulting from the NSF-supported research were first published in handbook form in the late 1970s, and the first successes resulting from the application of DFA in industry were reported in an article in Assembly Engineering [3] .In the article, Sidney Liebson, corporate director of manufacturing for Xerox and a long-time supporter of our research, suggested that "DFA would save his company hundreds of millions of dollars over the next ten years." The article generated intense interest in U.S. industry. 

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Sectioning or cross-hatching lines

When you go to a museum, you often see artefacts that have been cut up. For example, to illustrate how a petrol engine works, the cylinder block can be cut in half and the cut faces are invariably painted red. In engineering drawing, cross-hatching is the equivalent of painting something red. It is used to show the internal details of parts which otherwise would become too complex to show or dimension. The cross-hatch lines are usually equi-spaced and, for small parts, cover the whole of the 'red' cut area. They are normally positioned at 45 ~ but if this is awkward because the part itself or a surface of it is at 45 ~ , the hatching lines can be at another angle. Logical angles like 0 ~ 30 ~ 60 ~ or 90 ~ are to be preferred to peculiar ones like 18 ~ (say). If sectioned parts are adjacent to each other, it is normal to cross hatch in different orientations (+ and -45 ~ or if the same orientation is used, to use double lines or to stagger the lines. Examples of single and double + and --45 ~ cross-hatching lines are shown in the vice assembly drawing in Figure 3.1. An example of staggered cross-hatching is shown in the inverted plan drawing of the movable jaw in Figure 3.2. If large areas are to be sectioned, there is no particular need to have the cross-hatching lines covering the whole of the component but rather the outside regions and those regions which contain details. When sections are taken of long parts such as ribs, webs, spokes of wheels and the like, it is normally the convention to leave them unsectioned and therefore no cross-hatch lines are used. The reason for this is that the section is usually of a long form such that if it were hatched it would give a false impression of rigidity and strength. In the same way it is not normal to cross hatch parts like nuts and bolts and washers when they are sectioned. These are normally shown in their full view form unless, for example, a bolt has some specially machined internal features such that it is not an off-the-shelf item. Example of threads that are not cross-hatched can be seen in the vice assembly drawing in Figure 3.1.

Engineering Drawing for Manufacture
by Brian Griffiths
· ISBN: 185718033X
· Pub. Date: February 2003
· Publisher: Elsevier Science & Technology Books

Cams and gears

A cam is generally a disc or a cylinder mounted on a rotating shaft, and it gives a special motion to a follower, by direct contact. The cam profile is determined by the required follower motion and the design of the type of follower. The motions of cams can be considered to some extent as alternatives to motions obtained from linkages, but they are generally easier to design, and the resulting actions can be accurately predicted. If, for example, a follower is required to remain stationary, then this is achieved by a concentric circular arc on the cam. For a specified velocity or acceleration, the displacement of the follower can easily be calculated, but these motions are very difficult to arrange precisely with linkages. Specialist cam-manufacturers computerize design data and, for a given requirement, would provide a read-out with cam dimensions for each degree, minute, and second of camshaft rotation. When used in high-speed machinery, cams may require to be balanced, and this becomes easier to perform if the cam is basically as small as possible. A well-designed cam system will involve not only consideration of velocity and acceleration but also the effects of out-of-balance forces, and vibrations. Suitable materials must be selected to withstand wear and the effect of surface stresses. Probably the most widely used cam is the plate cam, with its contour around the circumference. The line of action of the follower is usually either vertical or parallel to the camshaft, and Fig. 24.1 shows several examples. Examples are given later of a cylindrical or drum cam, where the cam groove is machined around the circumference, and also a face cam, where the cam groove is machined on a flat surface. 

Manual of
Engineering Drawing
Second edition
Colin H Simmons
I.Eng, FIED, Mem ASME.
Engineering Standards Consultant
Member of BS. & ISO Committees dealing with
Technical Product Documentation specifications
Formerly Standards Engineer, Lucas CAV.
Dennis E Maguire
CEng. MIMechE, Mem ASME, R.Eng.Des, MIED
Design Consultant
Formerly Senior Lecturer, Mechanical and
Production Engineering Department, Southall College
of Technology
City & Guilds International Chief Examiner in
Engineering Drawing