It may therefore be. safely said, that not less than. 200 sorts and sizes of blocks are constantly making, by these machines.
It will give some idea of the expedition of works, to state the number of block shells of different sizes, made by each set of machines in a clay.
The first set of machines Make blocks from 4• to 7 inches in length, at the rate of 700 per day. These have wooden pins. • The second set from 8 to 10 inches in length, at the rate of 520 per day. These have iron pins.
The third set from 11 to 18 inches in length, at the rate of 200 per day, with iron pins. So that all. the machines will m;ke 1420 per day.
The larger sizes, from 19 to 28 inches, arc not so. frequently. wanted as to employ the machines con stantly : indeed they cannot make these large blocks. by the machines alone, though they perform partieu larparts with them, as the sheaves, pin, and cheeks.
Having now enumerated the number and objects of these machines, we shall proceed to describe each of. them in the same order.
The straight cross cutting saw. The log is placed' horizontally on a very low bench, which is continued through the window of the mill into the yard.. An horizontal shaft, turned by the mill, is situated head, with a crank in it. The crank rod descends to horizontal arm of a bent lever, whose centre is. rather below the floor.. The upper end of the vertical arm .of this lever is jointed to the saw, which will; therefore rise and fall on its joint in a vertical plane. It. is a large pit saw without any frame, placed horizon tally, its teeth downwards, but sloped so as to when drawn towards the lever. Over the back of the saw a piece of wood is fixed, and when the saw is. lifted up upon the joint connected with the lever, so • as to be at its most elevated position, the .baek of its. blade is received into a kerf or cleft cut in this fixed' piece of wood. The end of the saw, opposite to that by which it is jointed to the lever, has a handle or spar fixed to it in a_straight line with the blade : this. spar, which may be called.•ts handle, is received be tween two vertical posts, which confine it to move: straight, hut allow it to rise or fall.. By a rope attach ed to this pole and conducted over a pulley, the at-.•
tendant lifts up the saw into the cleft, in the fixed piece or guide before mentioned, (we suppose the, saw.is not now in motion, the crank being cast off from the mill,) then by.a windlass and lever he draws. the log forward on the platform, till the end of it• (which we suppose has been cut off square by the • same process we are about to describe) comes in I contact with a lump of wood screwed .upon the plat.. form. The saw is now exactly over the place where the log is to be divided. It is let down and suffer ed to rest Aviticits teeth upon the log, the hack still, being in the cleft of the guide. The crank being set in motion, the saw reciprocates backwards and forwards' with exactly the same motion as if worked by a. carpenter, and quickly cuts through the tree. When it first begins to cut, its back is in the cleft in the, guide, and this causes it to move in .a straight .line ; but before it gets out of the guide, it is so deep in the wood as to guide itself : for in cutting across the: grain of the wood, it has no tendency to be diverted from its true line by the irregular.grain. When the saw has descended through the tree, its handle is. caught in a fixed stop, to prevent its • cutting the bench. The machine is cast off, the attendant lifts up , the saw by the rope before mentioned, removes the block cut off, and advances the tree forwards to take a , A' fresh cut. The lump of wood before mentioned can be fixed at any place along the platform, so as to cut any required length off the end of the tree. It should be mentioned, that a lever is placed across the end of the tree; one end turning on a pin fixed in a firm post, and the other so loaded, as to keep the piece steady upon the bench when it is nearly or quite cut through. The under side of the lever is fitted with large teeth, which penetrate into the tree and hold it. This is a very simple and effective machine ; but does not cut so quickly as the circular saw which follows, because it does not admit of such a rapid motion ; but it can be constructed to cut trees of very large di, mensions at a small expence.