The first centring of a two piston-rod crosshead, is conducted with regard to its planing; and the lining can therefore be executed while the article is on a planing-table. The position in which the object is first placed, if on a planing-machine, is that indicated in Fig. 1115, its length being across the length of the table, and the intended flat sides of the arms situate upwards. In this condition, without any lining, the crosshead is ready for commencing the planing, supposing it to have an abundance of superfluous metal. But if it is forged somewhat near the dimensions, it requires to be now lined, in order that the upper surfaces may be planed exactly to the lines, and thereby finished at the first planing, so as not to require any additional fixing in the same position at some future time in course of shaping.

To properly mark the lines for planing, a primary centre line is first scribed around the sides, and this line is that which is to be put parallel with the table during the first fixing. The primary line or periphery is analogous to the dots which were stated to be necessary for adjusting an ordinary straight crosshead; but while scribing the crosshead now in hand, it is necessary to plainly show the entire line by scribing with a scriber-block, and to dot the line afterwards. A crosshead of this class is usually forged with a large amount of superfluous metal, being but very little bent, and in some cases, not at all, the object consisting of a piece whose shape resembles that in the Fig. 1115. Consequently, it is seldom necessary to scribe the first line with regard either to the mid-portion, or to the ends for the piston-rods; but, if the line is marked, it becomes a means of planing the arms to a proper thickness.

In order to adjust the crosshead into the proper position for scribing, it is raised gradually with plates and wedges, the wedges not being, in any case, in contact with a table, and therefore not capable of injuring its plane surface. This wedging-up is continued until the surfaces of the parts to be planed are somewhat near to parallelism with the table, which condition is known by means of a tall scriber-block having a scriber with a bent end. The block, while on the table, is put at various sides of the crosshead, and the bent end of the scriber is placed and fixed at a suitable height above the rough surface, to enable the operator to see whether the scriber's point is at about the same height above the crosshead while standing at any side.

The article being now put parallel, is ready for marking the centre line. The place for this line is found with a calliper, which is used in about the same manner as described for finding centres along the arras of a straight crosshead. The marks made are seen in Fig. 1115, and are easily made while the article is on the packing-pieces, because its lower side is then a few inches above the table, and therefore allows the end of one calliper-leg to be put close to the lower surface, as required while marking. These centres or middle points being shown, the scriber-block's point is next adjusted to the mean height of all the dots, and the periphery is marked around. From this line the specified thickness to which the arms are to be planed can now be marked, upwards and downwards, which completes the lining for this occasion. The crosshead is now ready to be fixed with plates and bolts; and if it does not greatly alter its position while tightening the plates, it is ready for planing as soon as fixed. If it is specially desired to fix it with regard to the lines, the wedges are adjusted accordingly, previous to finally tightening the bolts.

The first planing of the crosshead consists in reducing it to the upper gauge-line, and planing one face of each of the two bosses for the rods ; after which it is put upside-down, and reduced to the opposite gauge-line, and the other two faces of the bosses are planed. This reduces the arms to the proper thickness, and the bosses to the proper length. At the fixing for the second planing no wedging-up is required, because the already planed surfaces are put upon parallel blocks, which immediately places the planed surfaces parallel with the table.

When two opposite sides have been planed the crosshead is ready to be fixed against an el-chuck, which is bolted to the planing-table in order to plane the two sides of the arms not yet planed. These are the crooked or curved surfaces of the arms, and because one of the previously planed surfaces is against the el-chuck, the curved side will be planed square to the flat side or edge. To indicate the intended shape another lining is now necessary ; and during this planing the crosshead is shifted about four times to obtain the proper form. For a large crosshead it may be preferable to use two narrow el-chucks instead of only one comparative wide one; these can be so situate on the table that the middle boss can be put between the two chucks, as denoted in Fig. 1116, which arrangement will obviate the necessity of placing parallel blocks between the chuck-face and the arms, to keep the middle boss from touching the chuck.

After the four sides of each arm have been planed the centring for lathe-turning can be properly executed. This also can be done on the planing-table by the aid of parallel blocks, el-chucks, and a tall scriber-block. The centring is partly effected while the object yet remains against the el-chucks, in somewhat near the position it occupied during planing, but with the arms properly inclined to the table, the exact position being shown in Fig. 1116. While in this condition the faces of the piston-rod bosses can be also lined, to show the centres for the holes. By this mode the centres for the axis of the large boss in the middle, and the centres for the holes in the end bosses, can be accurately marked at the proper distances from each other. The first scribing should consist in making a short line upon each of the superfluous centre-pieces, which are seen to project from the arms in the Figure (1116), and are allowed to remain solid with the arms until lathe-turning is finished. To adjust the scriber-block to the proper height for this marking it is first adjusted to the top and bottom surfaces of the planed junctions of the arms, and a couple of lines are marked upon the thick mid-portion, which lines are equidistant from the centre or axis required; the middle point is therefore found with a calliper, and the scriber-block's point is adjusted thereto, for marking the centre-pieces or ledges, as intended.

The lines which are now scribed on the ledges indicate the exact height of the axis for the middle boss; and from this, above and below it, the vertical distance to the intended centre ot either face of the end bosses can be scribed. For this purpose a three or four feet measure is employed, and stood upon the table to measure the exact height of the axis, or the height of the scriber-point from the table. We now suppose that this height is twenty-four inches, and that the vertical distance between the centre of the middle boss and the centre of either end boss is specified in the drawing to be fourteen inches. This fourteen inches is to be added to the twenty-four inches, making three feet two inches, which is the height above the table to which

2z2 the scriber's point is to be adjusted for scribing the two faces of the highest boss. To mark the faces of the lowest boss, the fourteen inches is subtracted from the twenty-four inches, which gives ten inches as the height above the table at which the scriber is to stand for scribing.

When the crosshead has been thus partly scribed, it is removed from the el-chuck, or chucks, if two were used, and it is put upon parallel blocks to complete the marking for the centre-recesses. The blocks are placed beneath the planed flat surfaces of the arms, and are of sufficient height to keep the mid-part from touching the table. Each of the centre-pieces or pivot-pieces is to be now marked with a couple more lines to complete the indication of the places for the centre-recesses, one line of each couple being marked while the crosshead is in one position, and the other line being marked after the crosshead has been put upside-down. But for a heavy crosshead, this reversing should be avoided, and it is preferable to accurately measure from the edge of the planed surface to find the place for the centre, and adjust the scriber-point thereto. By this method only one line is scribed upon each extremity, and the object is not reversed.

The centring of a two-piston-rod crosshead, can be done also while it is on a slotting-table, if it is large enough in diameter. This plan is to be adopted for a crosshead which is very crooked, in which case the crooked surfaces of the arms are obtained with the slotting-tool instead of by planing; the planing of such surfaces is more suitable to a crosshead having comparative long arms, and which is nearly straight. Previous to placing the crosshead upon a slotting-machine, it should have been planed on a planing-machine to make its two flat sides parallel with each other, and to make the arms of a proper thickness. It is next lifted up and put against an el-chuck, and lined while on the planing-table, or on the slotting-table, if large enough.

When the centres of the end bosses have been shown, the outside of the bosses can be reduced to circular lines scribed thereon; the curved junctions also can be shaped ; and the straight mid-parts of the arms also shaped; all these operations being effected with slotting-tools while the crosshead remains on the slotting-table with its planed surface situate parallel with the table - in the same position as that which it occupied while it was on the planing-machine.