Saws And Saw-Mills. A saw is a cutting instrument, with a serrated edge; a saw-mill, a machine or building, wherein several or many of these instruments are actuated by horse, wind, steam, water, or other power.
It was not until the seventeenth century that saw-mills were introduced into England, attended with the most violent opposition from the sawyers, who apprehended they would be the means of depriving them of their subsistence. Some that were undertaken were abandoned at the outset, and others were destroyed by the populace.
The saw-mills of the present day are of two distinct kinds; the circular, those that cut by a continuous rotatory motion, and the reciprocating, which operate as the common pit or frame-saw. The circular saw-mills are for the most part used for cutting up timber of small dimensions; and the reciprocating for large timber, in forming beams, rafters, planks, etc, out of large timber. The most important machinery of the kind was erected by Mr. Brunei, at Portsmouth, to whom the mechanical world is indebted for many important inventions and improvements.
Saws are made of a great variety of forms and sizes, to adapt them to the materials on which they are designed to operate. The most common are those used by carpenters, who require in ordinary no less than ten different saws; namely, a cross-cut saw, for dividing a tree or log transversely, by means of two workmen, one on each side, who alternately pull the saw towards them, the teeth being made to cut equally in each direction; a pit-saw, for sawing the logs up into planks or scantlings, the operation being performed in a pit by a vertical motion of the saw, and usually by a class of workmen called sawyers; a large frame-saw, which is a saw-plate five, six, or seven feet long, stretched in a frame, and used to cut timber longitudinally with greater nicety than the pit-saw; a ripping-saw, which is a hand-saw, with a blade twenty-eight .or thirty inches long, and having large teeth for ripping, or cutting out stuff coarsely and quickly; a. hand-saw (peculiarly so denominated), usually provided with a twenty-six inch blade, and angular teeth, five to the inch; a panel-saw is the same as the hand saw, but with finer teeth, (seven or eight to the inch,) for cutting stuff very clean, and for the more delicate or exact species of work.
SaW8 with very fine teeth, and very thin blades, stiffened with stout pieces of iron or brass, riveted to the back edge, are also used, of several kinds, which are distinguished by the several terms, dovetail, sash, carcase, and tenon, indicative of their uses, and also of their sizes, which vary from six to twenty inches in length; several very narrow saws, indifferently called lock, compass, key-hole, and turning saws, for cutting out small pieces, and rounding work: small frame-saws, six or eight inches long, are sometimes required by the carpenter for cutting both wood and iron; the teeth of the latter being smaller and more obtuse than the former. There are many saws used by other mechanics, which differ from the carpenter's, the details of which would be uninteresting; we shall therefore proceed to take a brief notice of the process of manufacturing saws, as practised at Sheffield, from whence three-fourths of the inhabitants of the globe are supplied.
The very commonest kind of saws are made of iron plates, hammer-hardened, and planished upon an anvil, to give them some degree of stiffness and elasticity. Such instruments are, of course, spurned by the workmen; nevertheless, as their cost is but trifling, they are purchased in great quantities by those who consider any saw to be better than no saw at all.
The more useful saws which workmen employ are made, nominally, of either shear or cast steel; but the quality of these materials may differ, as well as the saws made of them, in every possible degree. The common test of a good saw, that of bending it into a bow, and letting it spring again into a straight line, is considered by some persons as a fallacious and unnecessary test, and that it sometimes spoils a saw, possessing in other respects all the properties of a valuable tool. A dispute has been raised on this point, and ably advocated on both sides. For our own parts, we would simply observe, that such process of springing infallibly proves two of the essential properties of a good saw, namely, uniformity of thickness in the blade, and perfect elasticity.
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 1 1/2 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 into suitable pieces; the edges are next perfected by filing, and holding the flat side of the plates against large grindstones, which process prepares them,for the cutting of the teeth. This operation is usually performed by a die-cutter in a fly-press, the motion of the saw-plate being duly regulated, 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 fatty 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 l lb. of pitch, and to these (when melted) one gallon of neatsfoot oil, 20lbs. of beef suet, rendered, and twenty gallons of olive 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 application 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 purpose, and when sufficiently cooled therein to be handled, they are taken out and are found to be extremely hard and brittle.