Fig. 247.
Fig. 248.
The double eccentric is represented in [Fig. 248]. The part A is the foundation plate, with a projection at the back, tapped to fit the mandrel. B, B, the lower guide bars; K, K, the lower sliding plate. All the above parts are precisely similar to those of the simple eccentric chuck. Upon the face of this lower slide are attached two chamfered guides, C, C, at right angles to the first. They are kept in place by screws passing through oval holes on their faces, and tightened when required by screws, tapped into four little square blocks, D, D. Between these guides slides the upper plate, which carries the screw for chucks, and the dividing wheel as before worked by a tangent screw, G; to either end of which a key is fitted. The leading screws, E and F, which move the two slides, have squared ends projecting both ways, so that the plates can be made to work eccentrically in either direction, which is sometimes an advantage. The chucks do not screw down upon the face of the division plate, on account of their projecting parts at the back; and very commonly a round plate, O, somewhat smaller than the wheel and about ¼ in. thick is attached to the face of the latter to raise the work still higher, so that the dividing plate can be readily seen. The more compactly, however, the parts of this chuck are made, and the less the work projects from its face the better; as there will be the less strain upon the central pin, and upon the plates and their guide bars when the tools are applied to the work.
To be able thus to place in the centre of rotation any given point in a piece of work, whatever may be the form of its boundary lines, is of immeasurable advantage, even though the capabilities of this chuck are confined to objects of plane superficies, it being impossible to reach by its means the side of a cylinder, or the surface of a sphere or spheroid. It is evident that any line upon the face of a box, for instance, whether the latter be square, round, octagonal, or of any other form, may be followed with two movements of the slides, combined with the rotatory movement of the dividing wheel. Thus, a border of interlacing circles may be carried round the edge of such a box, [Fig. 248]; or, a series of such circles forming constantly diminishing octagons, hexagons, squares, &c., may be thus readily executed. Nevertheless, what was said of the simple eccentric chuck, applies with even greater force to the compound eccentric. It is a heavy piece of apparatus, requiring a lathe with substantial poppets and bed; the whole well braced to the floor and wall, to withstand the excessive vibration caused by the revolution of the apparatus. It was, indeed, in view of this and similar appliances that we insisted in our initiatory paper upon the great importance to the workman, of adequate strength and solidity in the various parts of the lathe itself.
In [Fig. 249], we give a simple specimen of work to be executed by the compound eccentric chuck.
Fig. 249.