SAW MILLS. Although saw mills have not been very generally introduced till within a few years, they are by no means of recent origin. They were used in England in the century, but not without great opposition. Many of the earlier saw mills were driven by and those of North America are still generally worked by that power.
The earliest kind of sawing-machinery was, in its essential features, the same as that still used for sawing logs of timber into planks. In this machine the saws are stretched in a frame which slides up and down on vertical guides; the reciprocating motion being im parted to the frame by a crank upon an axle turned by a connection with the water-wheel or other prime mover. The log is supported on a carriage resting upon rollers, and is made to advance a little at each stroke of the saws, which cut during their descent only. The saws are usually either six or eight in num ber. Attempts have been made to introduce sawing machines with two sets of saws, one of which should cut upwards ; but they do not appear to have succeeded. A similar effect is sometimes produced by connecting two machines with one axle; the cranks being so adjusted that one saw-frame descends while the other rises.
The balks of timber to be divided into planks, of which two are generally operated upon simultaneously, are supported by rollers, and secured at the ends by suitable fastenings to a long iron carriage. This carriage is so connected to the saw frame by wheel and pinion work, that the carriage and balks of timber are propelled forward as fast as the wood is cut. In order to keep the balks of timber steady during the cut, their inner sides slide against polished steel plates fixed to the frame work of the machine, against which they are pressed by rollers.
The loss of power occasioned by the reci procating motion of a long saw, has led to the adoption of Circular Saws, which, by revolv ing constantly in one direction, require less power, and may be driven with far greater speed than reciprocating saws ; while their continuous action not only expedites the ope ration of sawing, but also makes the motion of the machinery more uniform. Perhaps the most interesting kind of circular saw is that used for cutting logs of hard wood into veneers. The late Sir M. I. Brunel, to whomEngland is indebted for many valuable improvements in this class of machinery, took out a patent in 1808 for a method of constructing very large circular saws,by attaching several pieces of steel-plate to a flanch of iron turned per fectly true. In this way saws have been made
of as much as 18 feet diameter; but such large saws can only be used for cutting veneers or very thin boards, which will easily bend so as to pass the flanch of the saw, which is neces ' sarily of considerable thickness. In the prin cipal room for cutting veneers at the City Saw Mills, London, there are eight saws varying from 8 to 17 feet diameter, and revolving from - 70 to 00 times in a minute. By the largest saws logs of mahogany 3 feet square can be 'cut up into unbroken sheets of veneer, at the rate of about 10 to an inch, and so beau tifully smooth as to require scarcely any dressing.
For cutting thinner boards, the circular saw is usually mounted in a bench or table, under which the axle and having a slit or opening through which the upper part of the saw projects. The piece of wood to be cut is laid on the smooth surface of the table, and pushed towards the saw by hand. Small circular saws, so mounted, are often moved by means of a treadle and crank, and by a variety of ingenious modifications may be applied to many useful purposes.
In the beautiful and ingenious block machi nery erected at Portsmouth, by Brunel, cir cular saws are extensively applied. [Brom Maori] NtItY.
Mr. Eastman, of the United States, patented some curious modifications of sawing machi nery about 1824. He found that when a cir cular saw is propelled with great velocity, it will cut much more smoothly and easily if it have only a few teeth placed at equal distances round its circumference than if, as usual, its periphery be full of teeth. He made a saw which had four cutting instruments called section teeth, each consisting of two teeth re sembling a hawk's bill in form. The saving of labour as compared with a common saw is estimated at full three-fourths, and it is stated that, when driven at a proper speed (which is from a thousand to twelve hundred revolu tions per minute), it will cut nino or ten inches in depth into the hardest white oak timber with the greatest ease. Mr. Eastman contrived these saws for cutting up timber in Inr ususual way, not through the log, but from the cir cumference to the centre ; so that the cuts form the radii of a circle, and the planks or boards produced are thicker at the outer than the inner edge. The boards cut by this machinery are much used for covering build ings.