The elasticity of turned works that are long compared with their diameter, causes them to spring or bend away from the cut of the turning tool. The tendency to spring is also variable upon every piece, being less when the tool is near the ends, or points of support, and most, when it is in operation about the center of their length. This elasticity interferes with the true revolution of the work, and tends to produce a varying and irregular oval instead of a true circular section.
Various contrivances simple and complex, have been employed as guides for slender turning, to prevent the work springing away from the thrust of the tool. The hand alone, affords the readiest assistance for those long works, in which much accuracy is not required. The fingers of the left hand are placed around the work from above, to support it and oppose the tool; the left thumb being pressed upon the face of the tool, both, to assist its guidance by the right hand and also, that the supporting hand may travel with it along the work. Very delicate, slender works mounted between centers, or when fixed in a chuck by one end only, can be supported by the left forefinger alone, placed around them from below; the inner side of the first joint, or sometimes the tip of the finger pressing the work against the tool. This requires a little practice, the thrust of the tool on the one side having to be met by only just sufficient pressure by the finger on the other ; the eye and the sense of touch modulating the force of each, so as to keep the work in true revolution between them. Nevertheless, it is frequently the only method available and when skilfully managed is very successful.
The simplest mechanical support, is that afforded by a stick fig. 132, placed between the bearers, and forced against the work by a wedge. The stick is moved from place to place as the work progresses, that it may follow the tool, and the pressure from the wedge is lessened, as the work becomes reduced in diameter. This method is that employed, but with the stick horizontal instead of vertical, by the Arab turner and others, using the bow lathes already described. The Arab uses either a plain stick, or for boring and internal turning, two jointed together with a semicircular recess in each, forming a hole to embrace the work, fig. 8; the same contrivance roughly serving, both as guide for slender turning and as boring collar.
Fig. 132. 133. 134. 135. 136. 137. 138.
The small block of wood or metal fig. 133, filled out with a rectangular notch, touches the work at two points instead of one. It is attached to a pedestal fixed on the bearers, by a bolt passing through an elongated hole, longer and rather wider than the diameter of the bolt; permitting the block a small adjustment on its support. This arrangement is employed for wood and metal turning, and may be sometimes mounted on the base of the handrest. The two pieces jointed to the base, fig. 134, are similar in their action, but more convenient from additional power of adjustment.
Fig. 135, is an early arrangement of two independent slides, each having a vertical traverse by screws fixed in the pedestal, and forming a square opening adjustable to the size of the work. Fig. 136, has one slide forming a triangular opening; it is attached by a bolt and nut to an independent head, usually that of the boring collar. The circular divided collars in fig. 137, are made of wood, tin or brass, and are carried in an iron frame with a pressure screw; the frame has a limited power of adjustment upon the head by which it stands on the bearers, and several pairs of collars are required to suit the sizes of different works. This guide is principally used for metal turning.
Fig. 138, copied from Bergerons work, is used for supporting very light works in wood and ivory, that are too delicate to bear even their own weight in the horizontal position; these slender specimens, turned as examples of dexterity, being generally of an ornamental character and of great length compared with their diameter. The portions of the work most distant from the chuck, while the remainder of the material is still strong, are first reduced and then completely finished, a small length at a time, commencing at the extreme end. So soon as the length of the attenuated portion renders it necessary, it is supported while in revolution and its consequent gyrations restrained, by fig. 138. The adjacent succeeding portions of the work are then reduced and finished, and so on, proceeding gradually onwards towards the base of the work in the chuck; two or more of the supports being used, according to the length of the piece. Fig. 138, is formed by stretching double silk threads across a wood frame, the work being in the interstice between the four.
The guide for slender turning figs. 136, and 139, is perhaps the most generally useful. The external surface plates are pierced by a central, equilateral, triangular opening, point downwards. Contained between them, and adjustable vertically by a thumb screw, is a third or central plate, of which the lower edge forms the upper side of the triangle, and by its depression diminishes the size of the opening. The apparatus is carried upon the head of the boring collar fig. 128, by a slot in the lower half, which permits its vertical and radial adjustment upon the fixing bolt or pivot, as with the boring collar.
To adjust the guide for use, the pivot is slackened and the apparatus placed in position upon the bearers; the aperture is then closed upon the work by the thumb screw, the vertical and radial adjustments being found, as with the boring collar, by putting the work itself in gentle revolution, and when the latter is found to run truly without undue friction, the pivot is refixed. The work is usually supported by the popit head, the sliding guide being placed close to the hand rest, near to the tool, and always if possible, on a portion of the work in a still unfinished state, in order that any marks it may leave on the surface may be ultimately removed.