The guides here treated are those used for containing bearer-brasses and guide-blocks, which are connected to ends of crossheads belonging to piston-rods and pump-rods. A guide of some class is required for every engine whose to-and-fro motion is to be changed into a rotary one. When slotted guides are used, they are fixed so that the guide-slots are parallel with the direction of the piston-rod's motion ; and in order that each guide may be properly attached to its engine, it is provided with a broad base or foot which is solid with the other part, and is planed right-angular to the length of the guide-slot. A planed foot of this kind causes the slot to put into the desired parallelism with the piston-rod, through the plane surface of the foot being bolted in close contact with the cylinder of the engine, with its table, or with its base-plate, all of which portions should be furnished with surfaces which are planed right-angular to the length of the piston-rod.

Small guide-standards are often cast without any slot; and therefore require a number of holes to be drilled along the places for the intended slots; after which the portions remaining between the holes are removed by planing, shaping, or slotting, and the slots are thereby formed. The lining to show the proper place and position of the slot, is the same for a solid guide as for one in which the slot is roughly formed by casting, with a little difference in the methods of marking small guides and large ones.

The lining of a guide may be executed either on a lining-table, or on the table of the planing-machine with which the planing is to be performed; and commences by marking a centre line or periphery entirely around its narrow side. To do this the object may be placed upon a lining-table with the broad sides parallel with the face, and having parallel blocks in contact with the lower broad side of the guide; which will cause it to be supported a few inches above the table. A guide in this position is shown in Fig. 901. For a small guide, two parallel blocks are sometimes sufficient, one at each end; but three or four blocks are preferable, if the shape of the broad side will admit them. Each block is put into contact with a portion of the casting which is not to be reduced, or which may require only a small skim to be taken off to attain the specified thickness. If the blocks are in contact with such surfaces, these surfaces must necessarily be kept paralled with the table-face ; and any lines that may be marked upon the guide by means of a scriber-block on the table, must also be parallel to these surfaces. As soon as the object is properly placed upon the blocks, the point of a scriber-block is adjusted to somewhat near the middle of the narrow side, and a short line or two are marked. The precise place of such marks is of little consequence, and merely require to be within a sixteenth or an eighth of an inch from the centre. But after the marks are made, it is important to avoid shifting the scriber-point until the guide has been put upside-down, and one or two other marks have been scribed parallel with the first ones. At the time the object is put upside-down, it is placed with the parallel blocks in contact with those surfaces which are correspondent to the surfaces that rested on the blocks at the first marking. Therefore, the point which is midway between a couple of marks of this kind, is the centre of that part of the guide which touched the blocks; but is not necessarily the centre of the slotted part unless the two portions happen to be cast true with each other. By placing the parallel blocks to the proper portions, the centres of these portions are found, and when found, the scriber-point is raised or lowered thereto, and the block moved entirely around the standard to plainly and deeply scratch a line upon the entire narrow side, including the foot or base, the oil-cup portions, and all other-projections that may be in the path of the scriber as it is moved around. This scribing produces the centre periphery required, and it is now dotted to plainly indicate its place, and to provide dots into which a point of a compasses or divider can be put for measurement when requisite.

As soon as the periphery is scribed, it becomes a sort of primary line from which the intended thickness of the standard can be accurately marked in two directions. The ledges of the slotted part may, or may not be; equidistant from the centre line; but the exact distance to each ledge can be shown by measuring from the line, whether it is in the middle or elsewhere. By measuring with a compasses from the centre to show the intended extreme thickness of the slotted portion, the place for another gauge-line is shown, and this also is scribed by adjusting the scriber-point to the proper height and moving the block around as before. When one line is scribed the scriber is again adjusted to show another line at the other side of the centre, which two outer lines show the finished thickness of the guide, and also constitute gauge-lines to which the superfluous metal is to be cut off.

When the three lines on the narrow side or edge are marked, the object is ready for the scribing to indicate the place and shape of the slot. The primary lines marked for this purpose are straight lines along the broad sides of the guide showing the centre of the slot's intended width. To execute this scribing the object may be placed upon the lining-table with the broad sides at right angles to the table, as seen in Fig. 902, and therefore in a position which is right-angular to that in which it remained while scribing the narrow side. While the broad sides are square to the table the guide requires supporting by placing a few heavy blocks of sufficient weight at each side of the foot; or by lightly bolting the foot to the face of an el-chuck which is on the table. It is now necessary to determine which is to be the centre length of the slot, and a couple of dots are put at this place, one at each end of the slot. The centre length being thus shown, it is next needful to put the dots equidistant from the table-face, which is done by packing up the guide with blocks of suitable thickness. The broad sides must be also adjusted exactly square to the table, and an el-square's blade of suitable length is applied to the sides to indicate their position, and is also applied to the gauge-lines on the extremity of the foot which were scribed during the previous marking while the guide's broad sides were parallel with the table. When the sides are seen to be in a proper position, and the dots1 showing the centre length of the slot are seen to be parallel with the table, the guide is ready for being scribed upon both its broad sides. This marking commences by first adjusting the point of a scriber-block standing on the table to the same height as the centre-dots, and next scribing a centre-line to intersect the dots and thereby indicate the entire centre length of the slot. The line is also continued beyond both ends of the slot portion, and is therefore seen on the broad side; it is also dotted so that its place may be known for a considerable time and be used for reference. The scriber-block is now shifted to the opposite side of the guide, and another centre line is scribed which is exactly analogous to the previous one, the scriber-point being now at precisely the same height from the table as before. These two lines indicate the centre length of the slot on two sides of the guide; and from these the desired width to which the slot is to be finished, is scribed by means of two other straight lines on each broad side, one line being above the centre and the other fine below the centre. These are accurately marked parallel with the centre line by merely adjusting the scriber-point to the suitable heights and scribing as before.