The true cylinder or surface, is produced with sufficient facility with the tool under the guidance of the unassisted hand, in the manner described in the chapters on elementary turning. The process however is often of rather a tentative character, and the work requires constant testing with the callipers and straight-edge during its progress, to prevent the surface or cylinder being turned concave or convex, and to ascertain the parts in excess that require reduction to the general level. Moreover when apparently complete, it may require repetition, should the measurements show the tool to have made too deep an incision at any one spot; in which case the entire superficies has to be reduced to this new level.
The slide rest supplies mechanical guidance to carry the tool in a true straight line along the work, at once producing the true surface, cylinder, or cone, separate or in combination; with the material advantages of certainty and rapidity of result, economy of material and physical labor; which conduce to its general adoption on the lathe in some one of its forms. It is a necessary part of the slide or traversing lathe and is largely used upon all others, from those of the watchmaker to the largest steam power lathes, both for plain turning and for many purposes yet to be referred to.
The slides of the slide rest, along which the tool carriage is made to travel, give the path of the tool: so that the resulting k 2 accuracy or otherwise of the work, depends entirely upon that of the slide rest. The slides and plates of which it is composed, therefore, should be flat as surfaces and also true as straight-edges in the direction of their length. The upper and under surfaces of every piece, should be precisely parallel to one another, that when the several parts are superposed, the tool may travel in an absolute straight line, and also remain at precisely the same vertical height at all parts of its traverse. The main screws employed also require considerable accuracy, to move the tool a definite distance and for other purposes. Indeed it may be said, in view of the valuable assistance rendered by the slide rest, that it is hardly possible to bestow too great care upon its construction : when correctly made, both its screws and slides are prepared by those methods which obtain the greatest accuracy, processes, already fully described in the second volume of this work.
Slide rests employed for plain turning with the ordinary foot lathe, are usually made with either two, or three slides. Fig. 145, is one variety of the former, fig. 146, and fig. 599, Vol II., are different forms of the latter. The lower slide in fig. 145, which traverses the tool for turning the surface, stands at right angles to the lathe bearers, its correct position being ensured by a tenon cast upon it in the solid. The tenon, which is placed between the lathe bearers, is rather less in thickness than the width of their opening, and its more distant face, which should be exactly at right angles to the lower slide, is brought up against the further side of the interval of the bearers by the tail screw shown. When thus adjusted, the rest is fixed by a bolt screwing into the tenon, with nut and washer beneath the bearers. The lower slide is provided with a main screw of some definite number of threads to the inch, working in a nut attached to the under side of, and traversing its top plate. The top plate, exactly fits the surface and chamfered edges of the lower slide, and is furnished on one edge with suitable adjustments to allow for wear; it carries the upper slide upon its surface, upon a stud or pivot about the center of its length. The upper slide, which traverses the tool for turning the cylinder, is of the same general construction as the lower, and turns on the pivot as on a center; it is attached to the plate beneath it, by two bolts working in semicircular mortises, to permit the upper slide to be fixed at horizontal angles to the lower, for turning cones. The main screw of the upper slide agrees in pitch with that of the lower, both screws being actuated by winch handles fitting upon their projecting ends.
The toolholder of fig. 145, mounted on the top plate of the upper slide, is that contrived by Professor Willis; clamping tools of various sized shafts, even when these are irregular in thickness. It allows the tool to be presented to the work at all horizontal angles, so that a tool with a straight stem can be applied as well to undercut work as to the surface or cylinder; it also serves to carry various revolving tools applied to metal, as for wheel cutting and slotting. The construction of Professor Willis' tool holder, is described, and shown by figs. 1007, 1008, Vol. II.
The slide rest with three slides, fig. 599, Vol. II., is similar in construction to that just described, in addition it carries a third or tool slide, without the circular motion, upon the top plate of the second, which thus becomes the middle of the three slides. These are arranged as in fig. 145, the lowest across the bearers, the middle parallel with them, and the top slide again across. In this rest, the three slides are principally required to connect it with the mandrel for screw cutting, and when the lowest has placed the middle slide in its position, at the distance from the axis of the mandrel required by the diameter of the work and the train of wheels in use, the tool is advanced to the cut by the top slide.
In fig. 146, the lowest or third, is a plain slide, traversing in a gun-metal cradle. The sides of the cradle stand square across the bearers, being retained in position by its tenon and a tail screw, the latter not seen in the figure. The plain slide is represented as drawn out nearly to its fullest extent, and when it is adjusted for distance, it is first fixed by a set screw at the side; the edge of the slide being protected from injury, by the interposition of a thin slip of steel attached to the inner side of the cradle. The plain slide lies directly on the bearers, with which it is brought into close contact, when the rest is secured for turning, by the bolt screwing into the tenon of the cradle from beneath. The middle or second slide, traversing the tool for turning the cylinder, is provided with a screw and is of the same general construction as those previously described; it is carried on a pivot at the end of the plain side, to which it is fixed by bolts working in circular mortises, placed at the center of its length. The first or top slide, turning the surface, carries the tool holder.
The plain third slide of fig. 146, rapidly adjusts the position of the middle slide, to accommodate the diameter of the work in plain turning, or the length of the train of wheels in screw cutting; and while sufficiently strong, occupies much less vertical height than the bottom slide of either fig. 599, or fig. 145. The screw slides of fig. 146, may stand either directly over the bearers, for work of small diameter, or quite clear of them, as drawn. In which latter position, the small height occupied by the third slide allows the turning of cylindrical work of much larger diameter, than can be accomplished with either of the others; the relative difference in diameter, being nearly that of 3 to 2.
In traversing lathes, the lathe bearers, main screw and saddle, may be compared with the lowest slide of a large slide rest; upon the saddle or top plate of which, the remaining parts are carried. In the lathe fig. 114, these will be seen to consist of two slides, similar in construction to those already described; the lower, standing across the bearers and fixed by bolts in undercut grooves, giving it a power of adjustment on the saddle; the upper slide, is attached to the plate of the lower, by the circular motion and fixing bolts, for horizontal angles.
The mutual wear, caused by the friction between the screw and its nut in the slide rest, claims a few words. Absolute or perfect contact between the two is inadmissible, as were it to exist they would be immovable; in addition to which, the original extremely slight, but necessary freedom, to allow motion between the two, gradually increases with use.
Wear always takes place upon the one side of the threads of each that are in contact, when the screw is turned in the one direction, and upon the opposite sides of each, when the screw is turned in the reverse direction. Both nut and screw, are thus worn on both sides of their threads, the hollow or internal thread in the nut, becoming wider, and the solid or external thread of the screw, becoming narrower; by which process the shake or endlong motion between the two, is gradually and after long use, greatly increased; being also the most, about the center of the length of the screw. The deterioration allows a more or less considerable motion of the screw, when that is reversed, before the contact is transferred to the respective opposite sides of the threads.
The break of contact, called loss of time, is visible on reversing the motion of the winch handle, which turns through a portion of its circle, according to the wear, before the screw produces any effect in reversing the traverse of the tool. Loss of time in the slide rest screw is of little importance in plain turning, but it interferes when the rest is employed for screw cutting or for ornamental turning; yet as will be seen, its effects are easily neutralized by correct manipulation. Adjustable nuts, referred to, page 664, Vol. II., and shown by figs. 622 to 625, are occasionally employed to compensate this wear, when necessary, in delicate machinery, but such contrivances are hardly requisite for the slide rest screw. The main screws of slide rests are sometimes protected by thin steel covers passing through their nuts above them, or by other arrangements; this is desirable to prevent the access of dirt or turnings, particles of which, finding their way between the threads of the screw and the nut, greatly accelerate their mutual wear.
The strong slide rests shown by figs. 145 and 146 are employed for plain turning in wood and metal, for accuracy of result and to save expenditure of time and exertion; for roughing out or the preparation of materials, and for turning work requiring greater strength than is afforded by the unassisted hands. The slide rests employed for ornamental turning may also generally be used for light plain turning, especially for that, requisite to reduce to precise truth the different portions of the work about to be ornamented; but, it is good practice to first prepare the work true or concentric and nearly to the required size, with the stronger slide rest, or by hand turning. The description of slide rests for ornamental turning will occur in a later volume; but, it may be mentioned here, that it is advisable to limit as far as possible the extent of their application to the purposes of plain turning, to avoid all risk of impairing their superior delicacy of construction.