In cutting screws in the turning lathe the tool only cuts as it traverses in the one direction; therefore whilst the cutter is moved backwards, or in the reverse direction, for the succeeding cut, it must be withdrawn from the work. Sometimes the tool is traversed backwards by reversing the motion of the lathe; and in lathes driven by power, the back motionis frequently more rapid than the cutting motion, to expedite the process: at other times the lathe is brought to rest, the nut is opened as a hinge, so as to become disengaged from the screw, and the slide-rest is traversed backwards by hand, or by a pinion movement, and the nut is again closed on the screw, prior to the succeeding cut. This mode answers perfectly for screws of the same thread as the guide, and for those of 2, 4, 6, 8 times as coarse or as fine; but for those of 2 1/4, 4 1/2, or any fractional times the value of the guide screw, the clasp nut cannot in general be employed advantageously.
The progressive advance of the tool between each cut, is commonly regulated by a circle of divisions or a micrometer on the slide-rest screw, which should always correspond with the decimal division of the inch. The substance of the shaving may be pretty considerable after the first entry is made, but it should dwindle away to a very small quantity, towards the conclusion of the screw. To avoid the necessity for taxing the memory with the graduation at which the tool stood when it was withdrawn for the back stroke, the author has been in the habit of employing a micrometer exactly like that on the screw, which is set to the same graduation, and serves as a remembrancer; another method is to employ an arm or stop, which fits on the axis of the screw or handle with stiff friction, but nevertheless allows the tool to be shifted the two or three divisions required for each cut.
In Mr. Roberts's screw lathe, the nut of the slide screw, instead of being a fixture, is made with two tails as a fork, which embraces an eccentric spindle; by the half rotation of which spindle, the nut, together with the adjusting screw, the slide, and the tool, are shifted, as one mass, a fixed distance to and from the center, between each cut; so as first to withdraw and then to replace the tool. Whilst the tool is running back, the screw is moved by its adjusting screw and divisions, the minute quantity to set in the tool for the succeeding cut, and the continual wear upon the adjusting screw, as well as the uncertainty of its being correctly moved to and fro by the individual, are each avoided.
Sometimes, with the view of saving the time lost in running back, two tools are used, so that the one may cut as the tool slide traverses towards the mandrel, the other in the contrary direction. Mr. Shanks' arrangement for this purpose, as ap-plied to the screwing of bolts in the lathe, is shown in fig. 609; f represents the front, and b the back tool, which arc mounted on the one slide ss,
Fig. 609 and all three are moved as one piece by the handle h, which does not require any micrometer.
In the first adjustment, the wedge w, is thrust to the bottom of the corresponding angular notch in the slide s, and the two tools are placed in contact with the cylinder to be screwed. For the first cut, the wedge is slightly withdrawn to allow the tool f, to be advanced towards the work; and for the return stroke, the wedge is again shifted under the observation of its divisions, and the slide ss, is brought forwards, towards the workman, up to the wedge; this relieves the tool f, and projects b, which is then in adjustment for the second cut; and so on alternately. The command of the two tools is accurately given by the wedge, which is moved a small quantity by its screw and micrometer, between every alternation of the pair of tools, by the screw h.
In cutting very long screws, the same as in turning long cylindrical shafts, the object becomes so slender, that the contrivance called a backstay, is always required for supporting the work in the immediate neighbourhood of the tool. The backstay is fixed to the slide plate, or the saddle of the lathe which carries the tool, and is brought as near to the tool as possible; sometimes the dies or bearings are circular, and fit around the screw; at other times they touch the same at two, three, or four parts of the circle only. Some of the numerous forms of this indispensable guide or backstay, will be hereafter shown.
In using the screw-lathe with a backstay for long screws, it is a valuable and important method, just at the conclusion, to employ a pair of dies in the place usually occupied by the tool; as they are a satisfactory test for exact diameter, and they remove trifling errors attributable to veins and irregularities of the material, which the fixed tool sometimes fails entirely to reduce to the general surface. The tool and backstay may be each considered to be built on the tops of pedestals more or less lofty, and therefore, more susceptible of separation by elasticity, than the pair of dies fixed in a small square frame. Sir John Robison has judiciously proposed, in effect, to link the backstay and turning tool together, by the employment of a small frame carrying a semicircular die of lignum-vitae, and a fixed turning tool, adjusted by a pressure screw; the frame to be applied cither in the hand alone or in the slide rest, and to be inverted, so that the shavings may fall away without clogging the cutter.