The adaptation of the plain lathe to the production of long screws or spirals, has been tried and arranged in many different ways. One early method of copying and varying the pitch of a guide or pattern screw, is shown by fig. 593, page 616, Vol. II., another mode by fig. 594, and others might be cited. The connection of the revolution of the mandrel, with the parallel traverse of the tool in some kind of slide, by means of bands running upon pulleys of relative proportions, has also received many forms; and this arrangement is referred to, as being both early and as still frequently attempted.
The employment of bands and pulleys, has the one advantage of simplicity in the apparatus required, which may be thus roughly described. One band may be led from a small pulley upon the crank, to a pulley of the same diameter upon the mandrel axis, thus making the revolutions of the crank and mandrel equal, or turn for turn. A second band from the largest groove of the foot wheel, may be led to a pulley, say one sixth of that diameter, upon the one end of an overhead shaft, which shaft also carries a second pulley, connected by a third band, with a pulley on the end of the slide rest screw. The second pulley on the shaft being, say, four times the diameter of that upon the screw. These diameters give a velocity of 24 turns of the screw to 1 of the mandrel, and, if the screw be supposed to be of one quarter inch pitch, a spiral of one turn in six inches, or of six inches rise, would result upon the work, about that usually seen upon spiral balusters. The proportions named may be reduced or exceeded, and if all the pulleys be made with coned grooves, a sufficient variety of pitch may be obtained.
Unfortunately, the practical value of this simple arrangement for producing the spiral, is but little; from the circumstance that the truth of the result is liable to constant interference, either from the elongation, or from the slipping of the bands. The equal tension of the bands, not altogether easily attained from their unequal lengths, is not permanent and is subject to daily variation, both from wear and barometric influence. Elongation may be somewhat mitigated by carrying the bands around adjustable stretching pulleys, or by the use of chains with pins inserted in the pulleys; but these contrivances also have their inconveniences. A less tractable interference with accurate results, lies in the material under operation; variations in its density, disturbing the equal traverse of the tool, hard places retarding the advance of the latter and causing some of the bands to slip. From these disadvantages, screw cutting by connection by bands and graduated pulleys, may be said to be restricted to some few spirals in soft wood; although as will be seen, analogous arrangements are sometimes usefully employed to give a self acting motion to the slide rest, for light cutting in plain turning in both wood and metal.
The foregoing apparatus, when employed for the ornamental spirals of the softwood turner, was usually home made and of the most primitive description, the shaft, pulleys and frequently the slide rest, being constructed of wood. The spirals required no particular accuracy, so long as they were fairly alike, the strands being usually finished by hand, after being cut or roughed out in the lathe. The cutting tool was a drill or revolving cutter, driven by a band from an independent fly wheel.
The more modern arrangement for spiral turning, now generally in use by the balustrade turners, consists of a rough copy of the slide lathe. A main screw of metal, or frequently of wood with the ends in metal sockets, is attached in bearings parallel to, and usually outside the wooden bearers; the latter being sometimes faced with iron plates. The main screw passes through a corresponding tapped hole in a l 2 wooden or metal block, lying on the face, and also embracing the sides of the bearers; a slide for advancing the tool for the depth of cut and the tool carriage, being carried on the upper surface of the block. The mandrel end of the main screw carries a metal toothed wheel and then terminates in a winch handle, by which the apparatus is set in motion. The mandrel also carries a toothed wheel which is connected in gear with that on the screw, by one or more other toothed wheels, used both to vary the pitch, and also as the spiral may be required right or left handed in thread. These wheels are carried upon arbors, fixing in slots in a bracket or a radial arm, attached to the lathe head or to the bearers. The action of these various parts is explained by that of the spiral apparatus in a later portion of this section.
A band passes from the fly wheel of the lathe for driving the overhead motion, a spindle provided with a long drum, from whence a second band, is led to the revolving drill or cutter in the tool carriage. The work is supported against the thrust of the tool by some form of backstay, usually a wooden bar running the whole length of the bearers at about the level of the work, and upon which other narrow pieces of wood are fixed at suitable distances by thumb screws; these pieces having angular notches to bear against the work. Multiplex threads, are obtained by a plain metal chuck, such as fig. 256, which is fitted with a cylindrical wooden stopper, having a central square aperture to carry the end of the square baluster, the opposite end of which is supported by the point of the popit head. The cylindrical edge of the chuck and that of the stopper, are pierced with four and six equidistant holes, a screw pin passing through the one into the other. One strand having been cut upon the work, the pin is removed, and replaced after the stopper has been turned round the required interval within the chuck. The popit head sometimes also has a small power of traverse, transversely to the lathe bearers, to produce the spiral upon shafts of a moderate degree of taper. The wheel on the mandrel can usually be replaced by a division plate, to arrest the work in different positions for plain fluting; the main screw then serves only to traverse the tool. The various portions of these additions to the lathe, are usually of home manufacture, of a rough and strong character; and it should be remarked, that the spirals as before, generally require to be smoothed and finished by hand.
Various arrangements of bands and pulleys attached to the screw of the slide rest to render the latter partially self acting, are to be met with. A worm wheel with a tangent screw provided with a pulley, driven by a band from the overhead motion, is sometimes attached to the main screw of the slide rest, to obtain a regular traverse of the tool for plain turning and fluting. The band may be thrown off the pulley, or the tangent screw out of gear, when the tool arrives at the end of its traverse; or, the revolution of the fly wheel may be gradually checked and finally brought to rest, at the same moment that the traverse of the tool carriage is arrested by contact with a stop, fixing upon the main slide of the rest or otherwise arranged.
A makeshift mode of communication a self acting or feed motion to the slide rest for plain turning, sometimes used by the engineer, consists of a pin or star wheel fixed upon the end of the screw, having a series of about eight projecting pins. Every time the mandrel revolves, an arm, temporarily fixed at any available position, either on the work itself or on the chuck, comes in contact with one of the pins or points of the star, and moves the wheel and the screw through a corresponding portion of a revolution. A better method is afforded by an eccentric cam fixed to the mandrel, which raises a lever, that is returned to its position by a spiral spring, once in every revolution of the mandrel. A line or chain from the lever passes over two guide pulleys, sliding on a rod suspended from the ceiling of the shop, and descends to a short weighted arm carrying a detent, that engages in a ratchet wheel on the main screw of the slide rest.
More complete apparatus is sometimes employed to give a continuous motion to the slide rest screw of the plain lathe. In one plan, a band from a small wheel upon the lathe crank, drives a short horizontal shaft placed at a little distance behind the bearers, carrying two bevil pinions and possessing a slight power of traverse, so that either pinion, may be made to engage in the opposite sides of a bevil wheel, fixed to the lower end of a short vertical shaft; the upper end of which carries a band pulley. From this latter and horizontal pulley, a long band is led around two guide pulleys fixed upon either side, about the center of a second and long vertical shaft; which shaft is removable, and may be fixed at different positions along the length of, and behind the lathe bearers, so as to regulate the tension. The band passes downwards to two corresponding pulleys attached to a weight, sliding upon this second vertical shaft, and then again upwards, around a third pair of guide pulleys, adjustable vertically upon the upper end of the same shaft, and from thence, to the pulley on the end of the slide rest screw. Placing the one or the other of the bevil pinions in gear, by shifting the horizontal shaft, traverses the tool in the one or other direction.
In an arrangement contrived by the late Charles Holtzapffel, and used in the author's workshops, one band is led from the mandrel pulley to two guide pulleys overhead, and then around a plain wheel or band pulley, revolving at a small distance from, and at right angles to the mandrel. This wheel carries a pinion upon its axis, which gears into a similar pinion supported at one side, the whole being mounted together on an eccentric. By this, the revolution may be conducted through the pinion upon the axis of the band wheel, or, through both pinions, to a toothed wheel affixed to the face of a second band pulley, contiguous to the first; a second band is led from this second pulley, to that affixed to the slide rest screw. The second band, which is also provided with a counterpoise stretching pulley, is reeved over guide pulleys mounted on a jointed swing arm, that traverses horizontally to suit the position of the rest, whether for turning surfaces, cylinders or cones. When one pinion alone is in gear with the toothed wheel, the screw is moved in the one direction, when both are in gear, it is moved in the opposite direction, and in the intermediate position, both pinions may be placed out of gear, when no motion is communicated to the band driving the screw. The change in the direction of the traverse is readily made while the work is in progress, and this arrangement to give a continuous motion to the slide rest screw, although perhaps rather elaborate, has proved both efficient and convenient for plain turning.