Fig. 8
We must now make the crank-shaft. This can be made with bent wire one-eighth of an inch thick. But when made in this way it very seldom works steadily and true. The best way is to build it up. You must get a piece of iron wire a quarter of an inch in diameter and about two feet long. Part of this will be required for the screw-shaft; straighten and smooth the wire and polish it up. Cut from the end three pieces, one an inch long, the second two inches long, and the third one and seven-eighths of an inch long. Next get four pieces of flat iron plate one-eighth of an inch thick, one inch long, and half an inch wide. Cut them into the shape shown in [Fig. 7]. The distance between the centres of the holes is to be a little less than three-quarters of an inch. The largest hole is one-eighth of an inch square, and the smallest hole a little less. The metal is to be left one-eighth of an inch wide round the holes. Take the shortest piece of iron wire and cut one end of it away, leaving a square pin and shoulder; the pin is to be three-sixteenths of an inch long, and one-eighth of an inch square ([Fig. 8]). Cut both ends of the two-inch piece and one end of the remaining piece in the same way. Counter-sink the largest holes in the plates ([Fig. 7]) and rivet them on the pins of the portions of the shaft, being careful that they are at right angles to the rods. The plates on the two-inch piece must be at right angles to each other. The pins should fit very tightly in the holes, to make them firm when riveted.
Fig. 9
Cut two pieces of iron wire one-eighth of an inch in diameter and five-eighths of an inch long, and at each end of each piece make a pin and shoulder to fit the small holes in the plates, leaving a full quarter of an inch of the wire between the pins untouched. Join the cranks together in pairs by riveting in these wires, being careful to keep the cranks at right angles to the shaft, and also to keep the several pieces of the shaft in the same straight line. Place the shaft on the bearing-blocks in the position it will occupy, with the cranks over the holes in the bed-plate and with the longest end to the after end of it. Mark on the shaft the position and thickness of the bearing-blocks, and cut the metal of the rods away in these places till it is reduced to one-eighth of an inch in thickness, so that it will work freely in the bearing-holes. The crank will now look like [Fig. 9].
Fig. 10
The next step is to connect the top and bed-plates by four pillars. The length of these will depend on the length of the piston-rod. They must be made of four pieces of brass wire a quarter of an inch thick. Take one of the cylinders and a sheet of paper; on this paper draw a line about six inches long, and at one end mark the point A ([Fig. 10]). Push the piston-rod in as far as it will go, and push the pivot of the cylinder through the point A, and mark on the line the point B, exactly under the hole in the crosshead of the piston-rod. Now draw out the piston-rod as far as it will go, and mark the point C exactly under the hole as before. Bisect the portion of the line between B C in the point D, and measure the distance between A and D. Reduce this length by the distance the centre hole in the steam-blocks is from the lower edge, and add to it a quarter of an inch for the height of the centre of the bearing-block from the upper surface of the bed-plates, one-eighth of an inch for the thickness of the bed-plate, and a quarter of an inch for riveting.
Fig. 11