In [Fig. 2411] is shown the forms commonly given, the requisite form of liner being shown beneath each. Form 1 will bed very firmly to its seat, but it will be observed that its liner is a difficult one to make, the bottom section a being thicker than the sides or wings b. This is a troublesome form of liner to fit as well as to make. If it be made of wrought iron, the wings b must be either forged or filed to their reduced thickness.
In the form at 2 in the figure we have the same defect, while in addition the liner will not adjust itself so readily in position to its bed.
This latter is an easier form to make in the moulding pattern, and easier to mould, and somewhat easier to fit, but it is not so firm as the first. To cause this form of brass to bed easily to its proper position it is sometimes given a lug on the bottom, as at 3 in the figure, the lug extending part of the width across only, because if it extended fully across, the liner would require to be in two pieces, causing trouble both in fitting them and in getting them into their places. When the lug extends partly across, the liner must have a slot to pass over and admit the lug, and this causes trouble in bending the liner to the required curve.
In the form shown at 4 in the figure all these difficulties are avoided, while, if the lower corners are made square instead of rounding, a simple piece of sheet metal will serve as a liner requiring but little fitting and bedding if it be of the proper thickness.
Fig. 2412.
To fit up a link motion, assuming the machine work to be done, the first thing to do is to face up the side faces of the links, making them parallel, and true to a surface plate. The slot is then filed out square to the side faces, its curve being filed to a template t, [Fig. 2412], which is provided with a piece of wire for a handle. It is supplied with red marking, and is rubbed upon the slot to mark the high spots. The same template may be used to prepare the link block or die; but as soon as the block can be moved in the slot with slight hammer blows (using a mallet or a block of wood) it should be used instead of the template, the bearing marks serving to correct and finish the block as well as the slot. In filing up the block care should be taken to make it of even thickness on each side of its hole and with its sides parallel to the hole, the latter being of great importance. When the block is a sufficiently easy fit in the slot to permit it, a round stick of wood may be put through it and used to move it up and down the link slot for the marking process.
The next operation is to fit the eccentric rod eyes to the link, and to then ream out the holes in both the link and the eyes while they are put together. The block may then be placed in the link, and the rocker pin passed through the block and into the rocker arm, so that the working fit of these parts when put together may be tested and adjusted if necessary. The link hanger may then be fitted to the saddle pin, when the whole will be ready for the file finishing and polishing, after which it may be case-hardened.
Case-hardening.—Case-hardening consists in the conversion of the surface of wrought iron into steel, or in converting the grade of a low steel into a sufficiently high grade to render it capable of hardening. The depth to which this conversion occurs depends upon the material used to produce it, and the length of time the process is continued, varying from 1⁄64 inch under the prussiate of potash process to 1⁄16 or 1⁄8 inch in the case of long-continued box case-hardening.