The unctuous matter which adheres to the plates being next partially removed, they are taken up successively by a pair of tongs, and passed backwards and forwards over a clear charcoal fire, so as to cause the unctuous matter to inflame, or "blaze off," as it is termed, which reduces the saws to the desired temper; and whilst the saw-plates remain hot, any warping they may have acquired in the process is removed by smart blows from a hammer, over an anvil strewed with sand, to prevent the article from slipping about.
The next operation is planishing by hammers, which renders them more even and equally elastic; and the dexterity and care with which this operation (so difficult and tedious to ordinary smiths) is performed, is a remarkable instance of what human art is capable of by long practice.
The saws are now ready for the grinder, who applies them to the circular face of a large grindstone, by an interposing board, against which he presses with all his force, so as to grind it as evenly as possible. Standing on tip-toes, he stretches himself over a large grindstone, which is revolving with great rapidity; his hands, arms, breast, and knees, being all brought into operation to produce the effect, while he becomes covered with ochrous sludge, formed by the attrition against the stone; an operation apparently so dangerous and disagreeable, as to give pain to the spectator, and make him wish to see a machine supplying the place of the operator.
The grinding of the saw-plates materially impairs their previous flatness and elasticity; they are, therefore, submitted to a second hammering by the planish-ers, and are afterwards heated over a coke fire until they attain a faint straw colour, which restores to them their elasticity. The surfaces are next lightly passed over a grindstone, to remove the appearances of the hammer, and next over a fine hard stone, to remove the scratches of the last, and give it the kind of polish required in the market for which the saws are intended. For which purpose the glazing wheel of buff leather and emery, or the "hard head," which is a wheel of hard wood, worked bare, are also used, as occasion may require. To correct any defects that the saws may have acquired during the processes described, they are next" blocked," that is, struck upon a post of hard wood, by means of a small polished hammer, by which the truth of the work is presumed to be perfected.
The saws are next "cleaned off" by women, by means of fine emery rubbed over them lengthways by a piece of cork-wood, which gives them an agreeable, even, white tint, and a very level appearance. They are next handed to the setter, who places each alternate tooth over the edge of a little anvil, in an angular direction, and strikes them so as to bend each uniformly into the required deviation from the plane of the saw; then turning over the saw, the setter strikes, in like manner, the alternate teeth, which he left untouched on the other side; in this manner each successive tooth is placed in opposite directions, at the desired set, to allow the blade of the saw to pass through the wood without resistance, while its breadth acts as a guide, and serves to give stability and effect to the operation of sawing. The teeth of the saw are again touched up with a file, to finish their sharpening; for which purpose they are fixed between two plates of lead, contained in the chaps of a vice; after which their handles are fixed by nuts and screws, cleaned oft", oiled, and packed in brown paper, for sale.
The form or mode of construction of the saws we have described, has been generally found so efficient and useful, as to have needed no material improvements; we shall, therefore, simply notice, in a brief manner, two or three matters of a subordinate character, connected with the subject, which may prove of service to the workman.
A frame-saw, it is well known, can be made thinner than a common pit-saw; and as it works in a smaller kerf, it would effect a considerable saving of timber if it could be employed in lieu of the pit-saw in cutting logs up into thin boards. To effect this object, an ingenious shipwright of Rotherhithe contrived the arrangement represented in the subjoined cut. a is the lower end of a frame-saw; b a section of the lower bar of the frame; c the holdfast; d d the two pins; e the lever; f a double arch, pierced with holes, the lever working between the two parts of the arch; the saw can be held to any degree of tightness by a small peg, fastened by a chain to the end of the lever. To shift this saw, press the lever, take out the peg, lift up the lever, take out the two pins d d, and the saw being lifted, and swung back, can be put in the next cut, and again fastened.
In operating with a common frame-saw, it would be necessary, at every successive cut, to shift all the transoms behind the saw to the end of the piece, or it would be necessary to take the saw out of the frame, when a difficulty would arise of fixing it again tightly. Both these objections are obviated by the plan we have described; and by which, long deals, planks, and bowls, may be cut with an important saving of material.
An expanding wedge for the use of sawyers, represented in the subjoined sketch, was invented by Mr. T. Griffiths, of the Royal Institution, and was deemed worthy of an honorary medal from the Society of Arts, a is the handle or centre-piece, to which is connected two springs c c, joined together at b; the handle also carries a cross piece d. This instrument is intended to save the time and trouble of shifting the common wedges, while sawing up balks of fir into deals. When the saw has cut two or three feet, the loose ends of the springs c c are to be brought by hand as near to the centre piece a as their elasticity will admit; the end b is then to be introduced into the cut, and the wedge is to be thrust up to the end of the spring, the cross piece d resting on the upper surface of the balk. The elasticity of the springs will then be continually opening the cut as the saw proceeds, to the length of about twelve feet; and the wedge, when at its utmost expansion, will be prevented, by the cross piece, from falling into the pit.