The process of turning is accomplished with considerably more facility, truth, and expedition, than any other process requiring cutting tools, because in the most simple application of the art, the guide principle is always present, namely, that of rotation. The expedition of the process is due to its being uninterrupted or continuous, except as regards the progressive changes of the tool, and which is slowly traversed from part to part, so as to be nearly always in action.
To choose the most simple condition, let us suppose the material to be in rotation upon a fixed axis, and that a cutting tool is applied to its surface at fifty places. Provided the tool remain quiescent at one place, for the period of one revolution of the material, the parts acted upon will each become one circle; because the space between the tool and the axis is for a period constant, and the revolution of the material converts the distance of the tool from the center, into the radius of one circle; and the same is equally true of the fifty positions.
The fifty circles will be concentric, or parallel with each other, because the same axis extended, or continued as a line, remains constant, or is employed for each of them, and therefore conceiving the fifty circles to be as many parts of the outline of a vase or other object, simple or complex, it will be strictly symmetrical, or equidistant from the central line at corresponding parts.
Each of the fifty circles will also become the margin of a plane at right angles to the axis, and which axis being a straight line, the whole of the circles will be parallel, and therefore the top and bottom of the vase will be also exactly parallel. And yet all these accurate results must inevitably occur, and that without any measurement, provided the material revolve on one fixed axis, and the the tool is for ashort period constant or stationary at each part of the surface; conditions inseparable from the turner's art.
The principle of rotation upon a fixed axis, removes the necessity for many of the steps and measurements required to produce with accuracy the various angular solids employed in carpentry and many other arts. For example, at page 501 the methods were explained by which the joiner produces the three pairs of parallel surfaces A a, B b, C c, of fig. 855, and which are generally required in each separate piece of his work. And in Making a box he has to combine six such pieces with the same relations of parallelism, and therefore thirty-six various surfaces have to be operated upon, to obtain the hollow cube, or the carpenter's box.
The turner's box consists of two pieces, in place of six; as the bottom and its four sides arc resolved into one piece; when of wood, by nature in the forest; when of metal, by man in the crucible. The surfaces are therefore reduced from thirty-six to eight, namely, the inner and outer surfaces of the bottom and lid amounting to four, and the inner and outer sides or margins, amounting to four also, and the revolution of the work upon one axis, places the eight in exact and true relation with extreme rapidity.
For example, the ends or terminal planes of the box, are from necessity at right angles to the axis of rotation, and parallel with each other. In each of these superficies the question of being in or out of winding ceases; as if straight, they can only be planes or cones, and which the one straight-edge immediately points out.
The principle of rotation ensures circularity in the work, and perpendicularity or equality as regards the central line; it only remains, therefore, to attend to the outline or contour. The right line serves to produce the cylinder, which is a common outline for a box; and the employment of mixed, flowing, and arbitraly lines, produces vases and ornaments of all kinds, the beauty of which demands attention alone to one single element, or conception, namely, that of form; and in the choice and production of which a just appreciation of drawing and proportion greatly assist.
In the art of drawing, it is almost essential to the freedom of the result, that the lines should be delineated at once, and almost without after correction; in the art of turning, it is always desirable to copy a drawing or a sketch, but having nearly attained the end, the tool may be continually re-applied, partially to remove any portions which may appear redundant, until the most scrupulous eye is satisfied.
The combining of the several parts of turned objects, as the separate blocks of which a column or other work is composed, is greatly facilitated from the respective parallelism of the ends of the pieces of which turned objects consist; and the circular tenons and mortises, whether plain or screwed, place the different pieces perpendicular and central with very little trouble.
These several, and most important facilities in the art of turning, are some amongst the many reasons, for its having obtained so extensive and valuable an employment in the more indispensable arts of life, as well as in its elegances.
The relative advantages of the different sections of the tree, as regards the works of the turner and carpenter, were explained with figures in the fifth chapter of Vol. I., at pages 49 and 50, where it is shown that, from various reasons, the transverse section of the entire tree or branch is the most generally proper for the lathe; and therefore, in turning the tops and bottoms of works, as in figs. 13 and 14, page 49, Vol. I., we are cutting across the ends of the fibres, and in turning the sides of the same we are, as it were, proceeding across the width of a plank or board.
The tools used in turning the woods act much in the manner of the blades of the carpenter's planes; but as we have now, at all times, a circular guide in the lathe-mandrel, we do not require the stock of the plane or its rectilinear guide. Although if we conceive the sole of the plane applied as the tangent to the circle, the position it would give is nearly retained, but we are no longer encumbered with the stock or guide. In turning-tools for soft woods, the elevation of the tool, and the angle of its edge, are each of them less than in ordinary planes, and in those for the hard woods both angles are greater.
For example, the softest woods are turned with tools the acute edges of which measure about 20 to 30 degrees, and are applied nearly in coincidence with the tangent, as in fig. 360.
These tools closely assimilate to the spokeshave, which is the plane of the lowest pitch and keenest edge. On the contrary, the hardest woods may be turned with the above soft-wood tools. applied just as usual; but on the score of economy and general convenience, the edges are thickened to from 60 to 80 degrees, and the face of the tool is applied almost horizontally on the lathe-rest, or as a radius to the circle, as in fig. 361, thus agreeing with the opposite extreme of the planes, in which the cutter is perpendicular and much less acute, as in the scraping and toothing-planes, which are only intended to scrape and not to cut.
The hard-wood tools may be figured, and employed as scrapers in turning the members of the capital or the base of a column, or similar object in hard word or ivory; but if we try the same tools on deal, ash, and other soft woods, we shall in vain attempt to produce the capital of a column, or even its cylindrical shaft, with a thick horizontal tool as in hard wood; for the fibres would not be cut, but forcibly turn asunder, and the surface would be left coarse and ragged.
But a reference to the planes with which the joiner proceeds across the fibres of deal, will convey the particulars suited to the present case; the iron is always thin and sharp, and applied in an oblique manner, so as to attack the fibre from the one end, and virtually to remove it in the direction of its length.
It is proposed now to describe some of the more important of the turning-tools, commencing with those employed on the soft Rained woods, but it would be both hopeless and unnecessary to attempt the notice of all the varieties which are to be met with in the hands of different individuals; and as their practical applications will be entered upon in detail in the succeeding volume, only so much will be here advanced as, it is hoped, may serve to explain the modifications of the general principles of cutting tools, to some of the more usual purposes of turning. To avoid repetition, it may be observed, that in general the position of the tool for turning the cylinder, and secondly that for the flat surface or plane, will be alone described. For works of intermediate angles, whether curves or flowing lines, the position of the tool slides from that for the cylinder to that for the plane, or the reverse; and these changes will be readily made apparent, when the reader gradually moves either a tool or even a rod of wood, from the one to the other of the described positions.
It may be added, that most of the tools for metal are applied direct from the grindstone, the oilstone being used for such tools only as are employed for the more delicate metal works, or for the last finish of those of stronger kinds; all the tools for wood, ivory, and similar materials, are invariably sharpened on the oilstone. It may be desirable to remark, in addition, that the rough exterior faces of all works should be turned with narrow or pointed tools, and only a narrow portion at a time, until the surfaces are perfectly true or concentric; as wide flat tools, applied to rough irregular surfaces, especially of metal, would receive a vibratory, or rather an endlong motion, quite incompatible with truth of work.