The Viaduct Works' Handbook

88 purpose of drilling the rivet holes in the girders of a bridge over the river Thames at Blackfriars, described in the preceding part of this book. These machines are collectively capable of drilling S52 holes at once, and one of them is constructed for drilling a straight row 26 feet in length, shown by the accompanying illustrations, Figs. 1 and 2. In this machine the drills are all placed at a dis- tance apart suitable for ordinary work, namely, of 4 inches from centre to centre; but in otlier of their machines, arrangement is made by which this distance can be adjusted to suit special work. The plates or bars to be drilled are placed upon a table beneath the drills, and this table is raised by means of hydraulic pressure. The arrangement of the presses is shown in section, Fig. 2. As the pressure is put on, the table rises, and forces the work against the drills. This force is regulated by a valve which gives perfect con- trol, and all the holes are drilled thereby at one operation, with great accuracy and speed. The hydraulic pressure is conveyed from a force pump into an accumulator, which consists of a vertical cylinder fitted with a piston, which is weighted to give the requisite pressure, and from this accumulator to the hydraulic presses of the various drilling machines. The working pressure is about 336 lbs. per square inch, and produces a pressure of about 6 cwt. on each drill. In punching holes through a plate, a certain amount of distortion takes place in the iron surrounding the holes, leaving the upper surface slightly concave and a nearly corresponding amount of con- vexity on the lower surface, so that after punching, when two or more plates are put together, they do not lie close to each other, and require flattening by hammer or some other means. This disturbance of the fibre of the iron necessarily weakens it, and experiments have shown where holes have been drilled and others punched in similar plates, which were afterwards torn asunder in tension, that drilled bars broke with 31 tons where punched bars broke with 26 tons. It is difficult to give any accurate idea of the cost of drilled work as compared with punched; so much depends upon the arrangement of the detailed drawings, and it is desirable to consult the parties who are to carry out the work as to these details. The extra cost is, however, preponderated by the superior work it makes, the truth of which is so complete, that of a number of plates placed together indiscriminately, the holes fit so accurately that a turned pin can be