These are very much used for engine-making, and require to be made very compact by proper steam-hammering. If the iron should be too small to admit of being welded and hammered sufficient to destroy the appearances of any plates or layers that may be in it, these plates should always be parallel to the two sides of the key that sustain the strain while in use.

The term, square key, is applied to any four-sided key whose width is nearly the same dimension as its thickness: the geometer's idea of a square is not signified; but the idea of something resembling a rectangle is that which is implied when speaking of a square key.

While drawing a rectangular key upon the anvil, it is necessary to watch the form that is produced by presenting each side to the hammer. If the key is improperly placed, the form of a rhombus will be produced ; and if the width of the iron or key greatly exceeds its thickness, a rhomboid will result. Both these forms result from the blows being delivered at the wrong angle to the side of the work, either by the hand-hammer or steam-hammer. To produce again the proper form is always easy, by presenting the longest diameter of the rhomboid or rhombus to the direction of the hammer's blow, which may be either vertical or horizontal; consequently, under the vertical steam-hammer, the proper position in which to place the longest diameter of the piece of work in hand is vertical, as shown by Fig. 81. By placing the work in this position, the two prominent corners or projections are battered in by the hammer, after which the work is drawn down by hammering the four sides that are required.

All kinds of rectangular keys are flattened by a flatter, shown by Fig. 79, or by a short thick set-hammer (Fig. 80). By careful flattening, keys may be nicely reduced to the finished width and thickness, when it may be necessary to avoid the planing process. The amount of time saved by a careful use of the flatter and several pairs of callipers is very great. Previous to adjusting the callipers, the finished width and thickness of the keys required are ascertained by measuring the key-beds or key-ways, and not by referring to any kind of sketch or drawing, which would be useless in such cases. The callipers are adjusted to allow only sufficient to fit the key to its place by filing; and when convenient, the key is also put into its key-way a few times, previous to being finished by the flatter. During the final reducing by the flatter, the square is used to ascertain if the work is rectangular and straight.