Page:The American Cyclopædia (1879) Volume XVI.djvu/85

 TURNING of curved lines upon plane or cylindrical sur- faces, by means of appropriate tools and a machine called a lathe. Theodore of Samos, named by Pliny as the inventor of turning, may have originated the application of the process to the shaping of hard substances. The principle of turning is simple. A piece of wood or other hard substance being so fixed in a horizontal position by pivots or oth- erwise at its two ends as to be allowed to revolve freely about an axis in the direction of its length, and caused to turn rapidly in this manner, while a chisel or other cutting tool is approached to the piece so as to meet it as it advances on one side, and held firmly to it, the tool will cut away from the piece at that place all the material outside of a cir- cle whose radius is the distance of its point from the axis of motion ; thus it will give to the part a circular outline, and will reduce the diameter of this circle as its point is advanced further into the material. The tool being gradually moved along the length of the turn- ing piece, it can thus be made to reduce suc- cessively the entire length to the circular out- line, and, by cutting to different depths in dif- ferent parts, to produce a turned piece marked with circular grooves, or other forms of curved surface. Outside circular turning is the most common, and is known as " centre work." With lathes of peculiar construction the work may be turned hollow, or bored or reamed, or turned both inside and out. The most sim- ple lathe for wood turning is called the pole lathe, made of two horizontal and parallel planks or beams, having a narrow space be- tween them in which there are two uprights or puppets, one stationary, at the end of the bed, the other having a tenon passing through the space and secured by a wedge beneath, so as to be movable along the bed, to accommo- date it to the length of the work. Near the upper end of each puppet, on the sides facing each other, is a conical iron or steel point, the two being in the same line. The piece is placed between the points, and the movable puppet brought up so that both points are pressed slightly into its ends ; the line between these is the axis about which the piece will revolve. The fixed rest is placed convenient- ly for steadying the tool ; while to turn the work, a groove being cut about it, usually at the left end, a stout cord or catgut is passed twice or oftener about the piece, then drawn straight and attached below to a treadle to which the workman's foot is applied, and above to an elastic pole or lath fixed at one end to the ceiling overhead, whence, probably, the name pole lath, or simply lath, or lathe. The workman holds the gouge or chisel to the work, and pressing down the treadle with his foot, the work is caused to spin rapidly round toward the point of the tool ; and so long the latter takes effect. When the treadle, having come quite down, is released by the foot, the recoil of the lath carries back the cord and work in the reverse direction, and the tool does not cut. If it is required to turn the entire length of the piece, the cord must be shifted after a time to the finished part. This contrivance serves for ordinary wood turning ; but on account of its imperfections, and es- pecially the loss of time during the return or rising of the treadle, it is now little used. For light or fine work, the pole is often replaced by an elastic bow and string overhead, the cord giving the revolution being attached to the middle of the string. But the forms now more usual, and especially for heavy work, are : 1, the foot lathe, in which the treadle is by a link made to give motion to a crank, from a larger grooved wheel upon which a cord crosses in form of an 8 to a smaller grooved wheel or pulley fixed upon an axis at one end of the work, and giving motion to it ; 2, the hand-wheel lathe, in which the power is applied by the action of one or two persons in turning a wheel, from which a band communicates movement to the axis and work ; and 3, the power lathe for the heaviest work, moved by horses, water, or steam. Any wood- en lathe, such as is used by turners in wood, is also distinguished as a bed lathe ; while those of iron, for the best work in metals, are called bar lathes. In any form of lathe such as those now considered, the turning axis at one end of the work, to receive the power and give motion to the piece, in place of the simple point which can be used in the pole lathe, becomes indispensable. This axis is called the mandrel, and sometimes the " live spindle." The chucks, or contrivances fixed upon the end of the mandrel, are of various forms and construction, according to the kind of work they are intended to secure; the most common being the screw chuck, the hollow, drill, uni- versal, concentric or die, and ring chuck, and the carrier and driver. The crowning improve- ment in the adaptation of the lathe to accuracy of work, and to that of all varieties, is attained through the invention of the slide rest, a car- riage upon which the tool is supported^ and by the construction of which it can be moved along the work by the machine or by hand, and at the same time advanced toward it, or set at any angle, as the character of the work may require. In the cases thus far considered, there is but one axis or centre line of the work ; that is, the centre line is a fixed direction throughout the process of cutting. But it is desirable to execute work in which the cutting in different parts or moments shall be in refer- ence to two or more different axes through the solid or surface acted upon. A simple mode of effecting this is that of fixing the work suc- cessively with the different axes, and turning it at two or more operations. But practically, a far more complete and satisfactory result is attained, the forms executed being variable at the pleasure of the workman, by incorporating into the lathe devices which shift the place or direction of the single axis of motion, and in a