THE TEMPERING DEVICE.
The tempering portion of the machine (Fig. 6i) consists of a strong cast-iron conical case, in which revolves a horizontal shaft into which are set spirally, strong tempering knives, or blades of wrought iron or steel (see Fig. 62), so that, as they pass through the clay, they move it forward. The clay being stiff, and not having much water on it, is not liable to slip be fore the knives, but is cut through and through, and thoroughly tempered, the air escaping back through the untempered clay, so that by the time the clay reaches the small end of the tem pering case it is ready to be formed into brick.
On the end of the tempering shaft is secured a conical screw of hard iron, which revolves in a hard iron conical case, the inside of which is ribbed or fluted lengthwise, so as to prevent the clay revolving in it, and is hard, to prevent wearing (see Fig. 61 ).
The screw being smooth and very hard, the clay slides on the screw, thus becoming, as it were, a nut; the screw revolving and not being allowed to move backward, the clay must go forward, sliding within the screw-case. This case is heated by steam, which facilitates the sliding of the clay and saves con siderable power.
This operation further tempers the clay, and delivers it in a solid round column to the forming-die.
Plastic materials, moving under pressure, follow the laws of fluids.
The great difficulty heretofore experienced in machines ex pressing plastic materials has been to make the flowing mass move with uniform velocity through all its parts. As the channel of a river flows faster than the shallow portions, or those near the banks, so does clay move through a die, the friction of the corners holding them back, while the centre moves more freely. This difficulty is overcome by the pecu liar form of the "former," which is so shaped as to facilitate the flow of the clay to the corners, and by this means the angles of the bar of clay are re-enforced and made very solid and sharp, thus insuring perfectly square and well-defined corners to the brick.
The "former" is secured to the screw-case by a hinge and swinging bolt, so that it may be quickly swung open for the removal of stones. This swinging bolt is secured to the case
by a pin of just sufficient strength to hold under normal con ditions, and when undue strain comes from hard clay, etc., it yields, thus forming a perfect safeguard against accidents aris ing from improper feeding. This " former" is also heated by steam, to facilitate the forming and sliding of the clay.
The forming and finishing part of the die (which determines the exact breadth and thickness of the bar of clay or the brick) is a hard iron lining, that can be removed and renewed in a few minutes and at trifling cost, thus enabling us to always keep our dies (or moulds), and consequently the brick, to standard size.
As the bar of clay issues from the forming-die, it passes through a small chamber filled with fine, dry sand, which ad heres to the surface of the brick. The surplus sand is kept back in the chamber by swinging elastic scrapers, which allow the bar to escape with its adhering sand.
This sanded surface of the clay bar renders the brick, when green, much nicer to handle, prevents them from sticking together on the barrows or in the hacks, or on the drying cars, and much improves them in color when burnt.
This continuous rectangular bar is then cut into brick lengths by automatic devices which are under the control of the issuing clay bar. One of these devices is known as the spiral cut-off, and consists of a thin blade of tempered steel, secured to the periphery of a drum, in the form of a spiral, the distance be tween the blades of which is that required for the length of a brick, and the projection of which gradually increases from nothing at its first end to the full width of the widest brick to be cut.
This spiral knife runs perpendicularly in openings in the links of an endless chain, supported upon rollers, the chain being so formed as to support the bar of clay from the bottom and one edge ; the clay is thus fully supported while being slowly cut off by the long drawing cut of the spiral blades in passing through the openings in the chain.