In some few cases the long metal bars are dispensed with; iron ears or plates, for two of the centers are then fixed to the wooden fence or rail, and the back centers are similarly attached to the platform itself, through which a circular mortise, parallel with the paths of the radius bars, is sometimes made for the clamping screw that fixes the rule. It is, however, better the rule should be constructed as in the figure 786, and quite independently of the platform, to admit of ready detachment. The long back rod is then essential, and also a fixing bar, placed as a chord to the arc described by the radius bars, and retained by a screw and nut passing through a mortise in the bar.
In the above construction, the long fence moves in an arc, like those described by the radius bars, and shown by the dotted lines, but the three-bar parallel rule is sometimes employed, because it may be opened in a right line, and therefore moves simply sideways to the saw; its path is directed by a pin in the long bar or fence, which enters a straight groove made transversely in the platform. The construction of the three-bar parallel rule is nearly a duplication of the former, and as it is equally important that the centers of the similar parts should be equidistant, the four radius bars are drilled together, to ensure their similitude, and so are also the three long bars. In the two and three-bar parallel rules, two slit clamping bars are occasionally used, which entirely restrain any wriggle, as they secure both ends of the fence; the perpendicular height of which varies from two to ten inches, according to the nature of the work to be sawn.
6. Sawing the sides of rectangular pieces. - In both small and large sawing machines, the work is applied much in the name manner; hut in saw-mills two individuals are commonly employed, one to hand up and thrust forward the work, and another to assist by dragging and afterwards removing the work from the bench. When the pieces arc short, the person who pulls commonly uses a tomahawk, which is like the half of a small pickaxe, the point of which is struck into the wood to serve as a handle.
When a log or round piece of wood is applied by the hands alone to the circular saw, it is difficult to get the first cut exactly true, as the wood is apt to roll on the two or three points at which it may touch the platform; but when the saw has penetrated a little way, the blade itself materially assists the holding of the work. One cut having been made, the flat side is placed downwards, and a second cut is made from either of the edges, and provided the first side is moderately true, the second will become at right angles to the first; the third and fourth sides will be found to present no difficulty.
As a ready means of adapting the parallel guide to works of different widths, a parallel piece of wood is often placed alongside the object to be sawn. Thus in cutting the blocks for wood-paving, the round larch timber is first cut into pieces about 3 feet 6 inches long, and these are, for the most part, sawn into pieces six inches square; but should any of them fail to hold that size, a parallel hoard half an inch thick, is placed alongside the work, which is then reduced to the next following size, or 5 1/2 inches square. And in the same manner, pieces of two dimensions, as of 2 by 1 inch in section, are in some cases cut by setting the parallel rule to 2 inches, and packing the work the thin way, with a piece 1 inch thick.*
* In reality, the standard size of the squared timber for the blocks of the Metropolitan Wood-Paving Company, is 5 1/2 by 6 inches; but the round logs are cut as large as they will respectively hold, the one measure being always half an inch more than the other. The wood is used very soon after it is felled, and is so wet, that the men find it needful to suspend a board over the saw and at right angles to it; this arrests the saw-dust, which if allowed to drive against the attendant, soon wets him to the skin.
In some woodcutting processes, a screen of wire-gauze is placed between the work and the workman, that he may be enabled closely to watch the operation without risk of the shavings entering his eyes.