Lathes for Irregular Forms.—In lathes for irregular forms (which are chiefly applied to wood and very rarely to metal turning), the work is performed by rotary cutting tools carried in a rapidly rotating head. The work itself is rotated slowly, and the carriage or frame carrying the cutting tools is caused to follow the outline of the pattern or former at every point in its circumference as well as in its length. The principle of action by means of which these ends are attained is represented in [Fig. 721], in which s represents a slide which carries the sliding head, affording journal bearing to the rotating head h, driven by the belt e, and carrying the cutters, and also the wheel w. f represents the pattern or former, and b a piece of wood requiring to be turned to the same form as that of f. Suppose then that f be slowly rotated by a and c, receiving rotary motion from a (through the medium of d), then the rotations of c will equal those of f, because the diameter of a is equal to that of c. The diameter of the circle described by the cutters at h is also equal to the diameter of w, hence the motion of the extremities of the cutters is precisely the same as that of the circumference of w, and as w receives its motion from f it is obvious that the cutters will reduce g to the same form and size as f, and if the head be traversed in the same direction as the axis of f, then the diameter and form of b will be made to correspond to that of f at every corresponding point throughout its length. Contact between w and f is maintained by means of a weight or spring, the rotation of f being sufficiently slow to insure its being continuous, while the necessary rapidity of cutting speed for the tools is attained by rotating h at the required speed of rotation.
This class of lathe is termed the “Blanchard” lathe from the name of the inventor, or “Lathe for irregular forms,” from the chief characteristic of the work, but is sometimes designated from the special article it is intended to turn, as “The Shoe-last lathe,” “Axe-handle lathe,” “Spoke lathe,” &c., &c.
Fig. 722.
Let [Fig. 722] represent a lathe of this kind provided with a frame a affording journal bearing to the shaft of the drum b, which is driven by the pulleys c. Let e represent a pulley receiving motion from b by the belt d. The cutting tools are carried by the head f which is rotated by pulley e. Let the carriage or frame carrying the shaft of e carry a dull pointed tracer, with continuous contact with the former h by means of a weight or spring, the carriage being so connected to the way n on which it traverses that it is capable of rocking motion, and if h be rotated the carriage will, by reason of the tracing point, have a motion (at a right angle to the axis of h) that will be governed by the shape of h; hence since g rotates equally with h, the form of the blank work g will be similar to that of h, but modified by reason of the tracing point being at a greater distance than f from the centre of rocking motion.
All that is necessary to render this motion positive throughout the lengths of g and h is to connect them together by gears of equal diameter, and traverse the carriage along n for the full length of the pieces. But the effect will be precisely the same if the frame carrying g and h be pivoted below, capable of a rocking motion, and h be kept against the tracing point by means of a spring or weight, in which case the carriage may travel in a straight line upon n and without any rocking motion. This would permit of the carriage operating in a slide way on n enabling it to traverse more steadily.
To maintain continuous contact between the tracing point and the former h, the rotations of h are slow, the necessary rapidity of tool cutting action being obtained by means of the rapid rotation of the head and cutters f.
Since motion from the line shaft to the machine is communicated at c it is obvious that the gears or devices for giving motion to h and g may be conveniently derived from the shaft carrying c and b, for which purpose it extends beyond the frame at one end as shown. Lathes of this kind are made in various forms, but the principles of action in all are based upon the principles above described.