The construction of the slide rest ensures the tool travelling forward in either direction, in one horizontal plane ; but, it is also essential that the face or cutting edge of the tool, should be at precisely the same height as that of the axis of the mandrel; that the tool may travel radially and horizontally, between the circumference and center of the work.

For example, when turning a surface, if the tool be supposed to be a quarter of an inch below the height of center of the lathe, it would cut towards the margin, but on arriving towards the center, it would cease cutting and pass below it; leaving a space at the center half an inch in diameter, untouched. A tool used below the center, would still sooner cease cutting and rub in turning the cylinder, which latter as it became reduced in diameter would soon overhang the edge of the tool, which would probably break from the strain, page 528, Vol. II. If the tool be above the correct height, it will in like manner, leave a corresponding portion untouched at the center, when used upon the surface. While upon the cylinder, the cutting contact between the edge of the tool and the work is gradually transferred, by the reduction in diameter of the latter, to contact between the work and the back or non-cutting portion of the edge, the tool ceasing to cut and being arrested by friction. The height of center of the edge of the tool is therefore first adjusted, and with greater care for works of small diameter, or for internal turning, in which latter the tool still sooner rubs instead of cutting, when incorrectly placed in this particular.

The top plate of the slide rest upon which the tool lies when clamped, is below the center, and the stems of the tools may be made of a corresponding thickness, to cause their faces to lie sufficiently nearly central. More usually the stems of the tools are made of less depth and thin parallel pieces of metal, of a length equal to that of the stem, are placed beneath them to pack the tool to the required height; such lifting pieces being always required with those tools in which the cutting edge has been so far ground away by sharpening, as to stand beneath the correct level. The tool is usually adjusted or packed to the height of center, by approaching its cutting edge as it lies on the top plate of the rest, either to a pointed center chuck on the mandrel, or to the point of the popit head, and then, one or more of the thin lifting pieces are placed beneath it, as may be necessary.

The production of smooth well finished turning with the slide rest, is greatly assisted by preventing, or at least diminishing, any vibration in the tool. With this view, the shaft of the tool is supported as much as possible when clamped in the holder, the cutting end being allowed to project no further beyond the edge of the top plate than is absolutely requisite. Regularity in the traverse of the tool, whether that has to be equal, increasing or diminishing, is also necessary, the winch handle upon the screw moving the tool is therefore always turned in a careful regular manner, but, with greater or less speed according to circumstances.

Should the tool be traversed too rapidly for the diameter or material of the particular cylinder upon which it may be engaged, it cannot remain a sufficient time at all parts of its transit, to remove the shaving equally from all along the entire superficies; and the cut produced then has the appearance of an irregular screw line, from having passed over or escaped portions of the original surface. On the other hand, if the tool be made to linger too long, the unnecessary friction with the work assists to more rapidly blunt the cutting edge. The requisite pace of traverse can hardly be precisely stated, as it depends upon the material under operation, its diameter, the speed at which it revolves, and the amount or depth determined to be removed at every cut; the last factor, sometimes with the larger lathes, depending mainly on the strength of the work and the apparatus. But, it may be said generally, that the larger the diameter of the work, the harder the material, or the heavier the cut, the slower of necessity must be the traverse of the tool.

The pace of the tool is the most rapid, and the depth of cut the greatest, in turning softwood; both being principally limited by the tendency of the tool to cut roughly and bury itself in the work, when either is in excess. The hardwoods, more rapidly deteriorate the edges of the tools, with these therefore, the pace of the traverse is reduced about one-half, and the depth of cut still more. Brass, may be turned at nearly the same rate as the hardwoods, the softer metals more quickly. Iron and steel, which are principally turned with narrow pointed tools, require again a much slower rate in the traverse, with diminished depth of cut, for all ordinary work; the depth of cut in heavy iron turning however, being determined conjointly by the strength and size of the work, the strength of the lathe, and the power available for driving it. Lastly, the surface velocity of the work has also to be taken into account, this, requires a slower pace for work of large than for that of small diameter. Various diameters requiring different rates, occur in the various portions of the same solids, or combined, as in the surface; upon which the tool has to be gradually accelerated, as it advances from the circumference to the center, and retarded, when travelling in the opposite direction; so as to equalize the progressive surface velocity at the point of the tool, and to maintain a nearly constant amount in the cut.

The traverse of the tool being guided by these generalities, the amount or depth of cut taken by every traverse upon either a large or small diameter, with a properly constructed tool, ground to the correct angle for the particular material being turned; should be limited to, and is pointed out by, its fair cutting action. The fair use of the tool, is rather within its full powers; and this is shown by the regularity in the appearance of the cut, the continuity of the shaving and the smoothness of the result. It also has the advantage of causing the cutting edge to last longer, and is therefore eventually the more economical in point of expenditure of time. When, on the other hand the depth of cut is in excess, the undue strain or overloading of the tool, results in a rough or torn surface upon the work; which is also frequently accompanied by the fracture of the cutting edge. The rough or torn surface thus established is at all times difficult to obliterate, having a tendency to perpetuate itself through the subsequent lighter finishing cuts. The work should therefore be first reduced nearly to size by a series of moderate cuts, and subsequently finished or turned smooth by a few fine cuts of less depth.

In first turning cylindrical works, the tool is made to travel from end to end, by reversing the direction in which the screw of the traversing slide is moved. The tool being set in at each end to an increased depth of cut, at the termination of every trip, by the slide that is at right angles to that by which it is traversed. The increased depth for the finishing cuts is given at one end only, as the tool starts in the one direction, towards the lathe head; on its return, and at all times that the tool is not intended to cut, it is withdrawn just clear of contact with the work, during its traverse back to its starting point. Upon the surface, the occupations of the two slides are reversed, that which on the cylinder gave the depth, now giving the traverse, and in turning forms in which the cylindrical and surface portions meet, the tool may be led along the one and withdrawn along the other, cutting upon either, indifferently.

The winch handle is commonly employed to regulate the depth allowed for the advance of the tool; thus, if the main screw of the slide rest be supposed to have ten threads to the inch, one complete turn of the winch handle advances the tool to the depth of one tenth of an inch, frequently a suitable quantity for wood turning. For finer cutting and for turning metal, the half, quarter, eighth of a turn, or less, is given to the winch handle, and this manner of advancing the tool, is sufficiently definite for most purposes of plain turning. For a more delicate advance, and as described in the chapter on screw cutting, the main screws or the winch handles sometimes carry a circular plate or micrometer head, divided into a number of equal parts, with a fixed reading point, by which arrangement the most minute advance may be regulated and imparted to the tool. An adjustable stop is sometimes employed to determine the extent of the depth or forward traverse of the tool slide, used in turning several duplicate pieces. This is either a simple piece with a power of traverse affixed to the slide, so as to abut against the top plate, or a fixed piece, carrying an adjusting screw, fulfilling the same purpose. The arrangement aids in attaining a fair agreement in first turning down the work nearly to size, but in metal turning, from the sensible wear of the point of the tool, and also on account of its frequent removal for grinding, the work still requires careful measurement towards its completion; the stop may then be removed out of contact, or gradually withdrawn, to permit a further small advance to be given to the tool for finishing the work to its exact diameter.