A great number of the engines for cutting screws, and also of the other shaping and cutting engines now commonly used, are clearly to be traced to a remote date, so far as their principles are concerned.
For instance, the germs of many of these cutting machines, in which the principles are well developed, will be found in the primitive rose engine machinery with coarse wooden frames, and arms, shaper plates, cords, pulleys, and weights, described in the earliest works on the lathe, and referred to in pages 4 to 8 of Vol. I.; whilst many others are as distinctly but more carefully modelled in metal, in the tools used in clock and watchmaking, many of which have also been published.
The principles of these machines being generally few and simple, admit of but little change; but the structures, which are most diversified, nay almost endless, have followed the degrees of excellence of the constructive arts at the periods at which they have been severally made, combined with the inventive talent of their projectors.
In most of the screw-cutting machines a previously-formed screw is employed to give the traverse, such are copying machines, will form the subject of the present section; and a few other engines serve to originate screws, by the direct employment of an inclined plane, or the composition of a rectilinear and a circular motion; the notice of this kind of screw niacin -nery will be deferred until the next section.
The earliest screw-lathe known to the author, bears the date of 1569, and this curious machine, which is represented in fig. 593, is thus described by its inventor Besson; "Esplce de Tour en nulle part encore veiie et qui nest sans subtilite, pour engraver petit a petit la Vis a Ventour de touie Figure ronde et solide, voire mesmes ovale." *
The tool is traversed alongside the work by means of a guide-screw, which is moved simultaneously with the work to be operated upon, by an arrangement of pulleys and cords too obvious to require explanation. It is however worthy of remark, that bad and imperfect as the constructive arrangement is, this early machine is capable of cutting screws of any pitch, by the use of pulleys of different diameters; and right and left hand screws at pleasure, by crossing or uncrossing the cord; and also that in this first machine the inventor was aware that a screw-cutting-lathe might be used upon elliptical, conical, and other solids.
The next illustration, fig. 594, represents a machine described as "A Lathe in which without the common art all sorts of screws and other curved lines can be made;" this was invented by
* The figure is copied half size from plate 9 of the work entitled "Des Instruments Mathematiques et Michaniques, etc, Inventees par Jaques Besson." First Latin and French Edit., fol. 1569. Second Edit., Lyons, 1578; also a Latin Edit., Lyons, 1582. The same copper plates are used throughout.
M. Grandjean prior to 1729. * The constructive details of this machine, which are also sufficiently apparent, are in some respects superior to those m Besson's; but the two are alike open to the imperfection due to the transmissions of motion by cords; and Grand jean's is additionally imperfect as the scheme represented, will fail to produce an equable traverse of the mandrel compared with its revolution, owing to the continual change in the angular relations between the arms of the bent lever, and the mandrel and cord respectively. Sometimes the spiral board or templet s, is attached to the bent lever, to act upon the end of the mandrel; this also is insufficient to produce a true screw in the manner proposed.
Several of the engines for cutting screws, appear to be derived from those used for cutting fusees, or the short screws of hyperbolical section, upon which the chains of clocks and watches are wound, in order to counteract the unequal strength of the different coils of the spiral springs. The fusee engines, which are very numerous, have in general a guide-screw from which the traverse of the tool is derived, and the illustration fig. 6 selected from an old work published in 1741, is not only one of earliest, but also of the most exact of this kind; and it exhibits likewise the primitive application of change wheels, for producing screws of varied coarseness from one original.
* Communicated to the "Academie Royale," in 1729, and printed in the "Mackines approuvies," tome v. 1735. As a matter of arrangement, this figure belongs to Section VI, but as a specimen of early mechanism, its present place eems more appropriate.
This instrument is nearly thus described by Thiout. "A lathe which carries at its extremity two toothed wheels; the upper is attached to the arbor, the clamp at the end of which holds the axis of the fusee to be cut, the opposite extremity is retained by the center; the fusee and arbor constitute one piece, and are turned by the winch handle. The lower wheel is put in movement by the upper, and turns the screw which is fixed in its center: the nut can traverse the entire length of the screw, and to the nut is strongly hinged the lever that holds the graver or cutter, and which is pressed up by the hand of the workman. Several pairs of wheels are required, and the smaller the size of that upon the mandrel, the less is the interval between the threads of the fusee" *.
In the general construction of the fusee engine, the guide-screw and the fusee are connected together on one axis, and are moved by the same winch handle: the degree of fineness of the thread on the fusee is then determined by the intervention of a lever generally of the first order; a great variety of constructions have been made on this principle†, the mode of action will be more clearly seen in the next figure, wherein precisely the same movements are applied to the lathe for the purpose of cutting ordinary screws.