The pit frame-taw, fig. 076, in commonly used for deals, and for such pieces of the foreign hard woods as are small enough to pass between its frame, which is about two feet wide.
The frame-saw blade has two holes above or at the wider end, and one below, and is attached to the wooden frame by two iron buckles or loops, which are split about half way round. The upper buckle fits squarely and firmly to the top head, and receives, above its lower side, two pins passing through the holes in the saw. The lower buckle is similarly cleft, and receives one pin only; this buckle is drawn tight by a pair of equal or folding wedges, beneath the bottom transverse piece.
The blade is usually five or six feet long, and thinner than that of the whip saw, which latter although it may be used for the widest timbers, is more wasteful. Insome few cases, where the double frame, fig. 676, is inapplicable, as in removing a plank from outside a very large log, the single frame, 677, is used; but this latter is generally narrow, and employed alone for small curvilinear works.
It is now proposed to give some few particulars of the sawpit, and the modes employed by the sawyers in marking out the timber preparatory to sawing.
The sawpit varies from about twenty to fifty feet in length, four to six feet in width, and five to six feet in depth; it has two stout timbers running the whole length, called side strakes, and transverse pieces at each end, called head sills, upon which the one end of the timber rests, whilst the other end is supported on a transome, or a joist lying transversely upon the strakes: a second transome, is used in case of the first breaking; this is called a trap transome.
Sometimes holdfasts, or L-formed iron brackets, are added to the head-sills, by which thick pieces of plank are fixed horizontally; screw chops are also used for fixing short pieces of hardwood vertically or edgeways, for slitting them.
In cutting deals into thin boards, three deals, which from being as many as the frame of the saw will include, are called a pit-full, are placed vertically against the stake, and are securely attached to it by a rope passed once round the deals and the lower end of the stake, and strained by a binding-stick.
Foreign timbers and hard woods are mostly squared with the axe or adze, for the convenience of transport and close stowage on shipboard, and such square pieces are readily marked out with the chalk line into the scantling, or the planks and boards required. More skill is called for in setting out the lines upon our native timbers, which are mostly converted into plank, or the various pieces, without being previously chopped square.
The converter determines in which direction the tree can be cut most profitably into plank, and the section chosen is usually that, which when opened, shows the greatest curvature or irregularity; this section is supposed to be shown longitudinally by a, b, c, d, fig. 678, and, on a larger scale and transversely, by e' e, fig. 679; the central points a and b, and the line b c, being given by the converter, who also gives instructions as to the thicknesses desiredin the planks. The sawyer's firstobject is accurately to mark the margins of the irregular central plane, a b c d, so truly, that when the lines are followed with the saw, the surface shall be true and thoroughly out of winding or twist.
The sawyer gets the timber on the sawpit, with the hollow side upwards: that being always first marked: it is plumbed upright, or, so that the plumb-line, suspended by the hand at z, exactly intersects the line b c, which has been marked on the end. The butt is then secured from rotating, by dogs or staples, s s, fig. 679, driven both into the end of the timber and into the vertical face of the head-sill; for which purpose the two ends of the dogs are bent at right angles', both to each other and to the intermediate part of the dog, the extremities of which are pointed with steel, made chisel-form, and hardened.
A chalk-line is now stretched in the dotted line from a to b, end pulled vertically upwards, exactly in the plane in which it is desired to act; the string is then let go, as in discharging an arrow, and striking the timber, it leaves thereupon a portion of the white or black chalk with which the line was rubbed.
Should the curvature of the timber be such that, as in the example, the chalk-line would scarcely reach the hollow, it is strained on the dotted line a, b, and left there; the plumb-line is held in the hand at z, and an assistant holds a piece of chalk on the top of the timber at the point e. The principal then observes, in the same glance, that the plumb-line z, intersects the string a b, the line b c, and also the point of the chalk, showing them all to be in the plane of vision; a mark is then made at e. Marks arc similarly made at f and g, or as many places as may be required; and, lastly, the points a g,g f,f e, and e b, are connected by short lines struck with the chalk-line around the curve.
The required thickness of the planks is then taken in the compasses, with a little excess for the waste of the saw, and two, three or more planks are pricked off on each side the center e' e, fig. 679; until, from the circular section of the timber, its surface becomes so inclined, that the compasses would measure a slanting instead of a horizontal distance, end which would diminish the thickness assigned to the boards.
The sawyer then holds the compasses as at y, and fixing his eye on the part of the wood perpendicularly beneath the off leg of the compasses, he removes the instrument and pricks a mark therewith; after which the compasses are replaced as at y, to see that the mark is correct. This is repeated at different points in the length, and the chalk-line is stretched from point to point thus set out with the compasses, and marks the edges of the intended saw cuts with sufficient certainty.