Experience has shown that cast steel is the best material for making saws, as well as most other tools, on account of the greater uniformity of its structure, which is not lost by the subsequent operations of rendering it malleable and elastic. To prepare this material, the liquid metal is poured into a cast-iron mould, out of which the casting, when cooled, is taken, in the form of a small slab, about le inch thick. This slab is next laminated between rollers until It is extended to the required dimensions. If intended for the larger kind of saws, as mill or pit-saws, the whole piece may be required, in which case it is clipped -by shears to the required shape ; but if for smaller articles, it is cut up mto suitable pieces; the edges are • next perfected by filing, and holding the flat ide, of the plates against large grindstones, which process prepares them for the ending of the teeth. This operation is usually performed by a die-cutter ht a fly-press, the motion of the saw-plate being duly replated, so that the teeth shall be uniform ; the large teeth being cut one at a time ; and the smaller, two, three, or more at a time, according to circumstances. The wire edges left on the teeth of the plates by the cutting-out press, are next removed by filing:. The operations of hardening and tempering succeed, which require considerable care and attention on the part of the operator. A variety of ditty compositions have been recommended for this purpose, as possessing peculiar efficacy in hardening, amongst which we may instance that recommended by Mr. Gill, who appears to have had considerable experience in matters of the kind, and to be somewhat acquainted with chemical science ; we should, otherwise, have taken an exception to the variety of similar ingredients in his caldron. He desires us to melt together 3lbs of black rosin and 1lb of pitch, and to these (when melted) one gallon of neatsfoot oil, 20Ibe of beef suet, rendered, and twenty gallons of neatsfoot oil. All these are to be heated together in an iron vessel until the aqueous vapour is driven off, and the composition will take fire by the applica tion of flame to the surface, which is then to be extinguished by placing on the cover of the vessel. The saw-plates being now heated in a reverberating or other suitable furnace to a cherry red, are precipitated edgewise into the liquid mixture just mentioned, contained in a vessel of a proper figure for that and when sufficiently cooled therein to be handled, they are taken out and e found to be extremely hard and brittle. 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 abort.
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 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. be becomes -twisted with alums sludge, formed by the attri
tion against the stove an operation apparent* so dangerous and dinar/amble, as to give pain to the .spectator, and make him wish to see a machine supply ing the place of the operator.
The grinding of thesaw.plates materially impairs their previous fatness sad . elasticity; they are, therefore, submitted to a by the • planishers, and are afterwards heated over a coke fire until they attain a faint straw colour, which restores, to them their elasticity. The surfacesare 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 reqnired in the market, for which the saws are intended. For which puree the glazing wheel of buff leather and emery, or the " bard 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 at 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 appe rancs. 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 let, 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 off, 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 improve ments; we shall, therefore, simply notice, in a brief matter, 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 re presented 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 hold fast; d d two pits; e the lever; f a double arch, pierced with holes, the lever , working between the two parts of the arch; the saw can beheld to any degree of tight neas by a small peg, fastened by a chain to the end of the lever. TO shift this saw, press the lever, takeout 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.