(4) °Templet and Pattern Shop,' in which the templets for striking the positions of the rivet and pin holes and the mold patterns for iron and steel castings are made.
A templet is usually a board or framework of boards, one side of which is a representation of one of the sides of the metal shape or piece required to be made. On it the positions for all the rivet holes, pin holes, bevels and notches are accurately marked, and these are subse quently transferred to the metal piece by punch ing, scratching or other means, preparatory to the final operations of punching, drilling, ream ing, beveling, shearing, etc. Templets are usually made of soft white pine boards 7A inch in thickness. When very accurate fitting is required, they are laid out full size on the floor of the shop; but, for small and general work, they are made in separate pieces from the detail drawings. As a rule, as many parts as possible of a structure are designed to be in duplicate so that the number of templets required will be as small as possible, and the final fitting and assembling of the work facilitated and simpli fied.
(5) °Laying-out Shop,' where each indi vidual metal shape, plate and piece is accurately marked in accordance with the templets.
(6) °Punch and Shear Shop,' where the various plates, shapes and pieces are sheared or cut to the required length and bevel, and the holes marked from the templets are punched by suitable machines. See article under title METAL WORKING MACHINERY.
When the thickness of the pieces is greater than the diameter of the punch, punching will unduly strain the material, and the holes will not be straight and smooth. The holes in such plates and shapes are usually drilled, although the process is much slower and more expensive. Single and gang drills are used for this purpose.
Since the punching of a series of holes in a line tends to distort or twist the steel, it is good practice to stagger the holes, also to punch every other hole first, and then the intermedi ate holes, which permits gang-punching.
Wrought iron and soft and medium steel when of the proper thinness may be easily punched; but hard steel must always be drilled as it cannot be punched without cracking.
Since the diameters of the edges of a punched hole correspond to the diameters of the punch and die respectively, and gives the hole a tapering instead of a cylindrical form, the practice is to require all important holes, and especially those in the tension members, to be reamed to a diameter 1/2 inch larger than -that of the punch, so as to remove all of the material injured by the process of punching.
After passing through this shop, all the material is completely ready to be fitted together and riveted to form the final bridge members for which they were designed to form a part. Each piece when completed is marked with a number or letter designating its proper position in the finished member, and all the pieces be longing to the same structure are marked by a job number so that they may be easily identified in the assembling shop.
(7) °The Assembling Shop? This is usually arranged so that all the heavy work is handled on one side, and the light work on the other. All the different pieces which make up the various members are assembled, fitted and bolted together, and then turned over to the (8) °Riveting Shop° where the riveting is accomplished by various forms of compressed air, steam and hydraulic riveters.
When a bridge-member exceeds 10 tons in weight, it is more economical to use portable riveters than stationary riveting machines. As a rule, hydraulic riveters are the most efficient. They do not get out of order easily, and always exert their full pressure upon the rivets; but in their portable form they are very cumber some, and, therefore, the pneumatic portable riveters are found to be the most convenient in such cases.
Rivets are made of soft steel, and their lengths ought to be such that when put into the holes they will project enough to furnish a sufficient amount of metal to fill the holes com pletely when they are compressed. Before in sertion they are heated to a cherry red, great care being taken, however, not to burn the metal in the heating, and thereby render them hard and brittle. On the other hand, if driven when they are too cold they will not fill the holes completely, and although apparently all right will be weak in the head. All rivet heads ought to be smooth and free from cracks, and con centric with their holes.