2. Clothe the mechanism decided upoti in metal by determining the general form of the elements and the materials best suited for their construction together with the allowable work ing stresses. Complete the sizes of such parts of the mechanism as a simple layout will per mit. Lay out the principal joints of the mech anism. When mechanisms are enclosed build from the inside out Don't design the frame first and try to put the mechanism inside, the frame. must conform to the mechanism.
In many cases the forces acting on the ma chine element are very complex, owing to any combination of the following conditions: (a) Transmission of force from one ele ment to another.
(b) Internal restraint due to circumstances in manufacture.
(c) Restraint due to energy transmission.
(d) Forces due to the inertia of moving parts.
Therefore the theoretical design is not al ways clear and our knowledge of materials and their laws is limited in many respects so that in many cases we must rely on judgment and em pirical data, the result of experiments. Even where the conditions are clear the design must always be controlled by practical modifications and construction considerations. The designer must therefore be a man of judgment, thor oughly familiar with the production of ma chinery, and the relation thereto of the allied commercial departments so that he may give each factor its proper weight and obtain the best results at the least possible cost.
The last step, specification and drawing, is a necessary and important supplement to the process of design. It is a powerful aid to the mental process of the designer and is the best and easiest way of showing the workmen how the machine is to be constructed, and also of making a record of what has been done. The drawing is not machine design itself, as ma chines may be designed and built without drawings. It is, however, an essential part of the designer's equipment because the mere act of putting the results of his thought on paper is one of the greatest helps to force systematic and definite thought.
Design which stops short of exact working drawing is only half done. In fact the crucial test of practicability of a piece as part of a machine is the possibility of its being exactly detailed. It is more easy to generalize than to
get down to exact detail and there is nothing more productive of trouble, delay and waste of time and money than poor detail drawings.
Very often written specifications to accom pany the drawings are not only useful but nec essary, and as the writing of these specifications presupposes a most intricate knowledge of de sign and selection of materials, the highest skill of a designer is often needed clearly and fully to specify in writing just what is to be done.
The actual process of drawing in the devel opment of a design may be outlined as follows: Rough sketches merely representing ideas, not drawn to scale, are made to determine the me chanical ideas concerned in the design. These are followed by layout drawings in which the design is developed and the general detail es tablished. This drawing contains the principal dimensions to fix and control the detailed de sign. Each part of the machine is then de tailed and a working drawing showing every small detail of its shape together with dimen sions and specifications is made so that the workman may be definitely informed as to its construction. Assembly drawings are then made and constitute the last step in the repre sentation. They show the parts assembled in the complete machine. The layout mentioned above may be finished up into an assembly drawing, but it is usually safer to make the lat ter a separate drawing, as changes in detail are often necessary, and it will serve a valuable aid to the draftsman in checking up the dimensions of his detail drawings.
Bibliography.---Bartlett, ing'; Cathcart, W. E. L., Design' French, Drawing' Hess, H. D., (Machine Design, Hoists, Derricks, Cranes' Jones, Forrest R., (Machine Design> •, McCord, 'Mechanical Drawing); Parshall 4 Hobatt, Machine Design); Reid & Reid, (A Text Book of Mechanical Drawing and Ele mentary Machine Design' • Spooner, Henry J., (Machine Design, Constntction and Draw Undwin, Wm. C., of Machine Design.>