Owing to the increased strain and friction in metal, compared with wood turning, considerably reduced speed in the revolution of the work is generally necessary, this is requisite, to allow sufficient time for the severance of the turnings, to avoid the risk of tearing the surface of the work, breaking the point of the tool, or unduly heating and softening the temper of its cutting edge, or of damage to the lathe itself. The particular speed most suitable to any one. metal and set of circumstances, also slightly varies as explained in wood turning, in proportion to the diameter upon which the tool is cutting.
In turning cylindrical work of iron" or steel from about two to four inches diameter in the foot lathes described, the driving band is usually placed to run from one of the grooves of smaller diameter of the slow motion or second bevil of the fly wheel, to one of the larger, upon the mandrel pulley. The diameter of the former, not generally much exceeding twice that of the latter. This proportion is frequently reduced with advantage, the fly wheel being often provided with a third bevil of much less size, fig. 110, to be used for turning large work; the mandrel may then revolve almost turn for turn with the fly wheel. The speed in all cases being again somewhat susceptible of variation, by the slower or quicker use of the treadle. Iron and steel work of less diameter may be driven at a little increase of speed, either by increasing the pace of the treadle, or more usually, by altering the position of the driving band. That less than one inch in diameter, especially during the lighter finishing cuts, may be driven nearly as fast as in turning hardwood. In turning brass and gunmetal, the lathe may be driven at nearly the same pace as for hardwood, subject in like manner to variation of speed according to the diameter of the work. Lead and pewter are driven at about the same speed as softwood.
In surface turning, the driving band is placed at about the medium speed required by the diameter of the work. The foot lathe may then be made to revolve a little faster or slower, as the tool, travelling at a nearly uniform pace, approaches the center or the margin of the surface; the aim being to arrive at equality of cutting action, by causing the face of the work always to pass the tool at a nearly uniform surface velocity. This, may also be assisted, if the pace at which the tool travels be slightly retarded towards the margin of the work. The speed of lathes driven by power, in which the slow motion is generally obtained by back gearing, is increased or diminished by shifting the driving band upon the steps of the driving pulley and upon those of its countershaft; but the velocity chosen at which the mandrel revolves, is necessarily constant. Therefore to maintain the cut upon the surface at a uniform surface velocity, the pace at which the tool moves has to be decreased, as it travels from the center to the margin, and increased in the reverse direction, readily enough attained by hand, or with the slide rest; while in the larger power lathes, mechanical means are sometimes adopted to ensure this result.
Only a very small shaving or quantity, either in breadth or thickness, is removed from the harder metals by every separate cut of the hand tool; while the cutting traverse of the tool, which may often proceed uninterruptedly along the wood surface or cylinder, occurs to a less extent upon brass, and far less so, in turning iron or steel. Its limited application to these materials only occurs also, when the rough surface has been both reduced and then turned fairly level, by a series of small separate cuts made in close juxtaposition with the appropriate roughing and smoothing tools.
As previously referred to in wood turning, so in turning the metals, it is by no means easy to give definite instructions as to the amount or thickness of material that the tool may safely be permitted to remove at every separate cut. The quantity depends upon many factors; among the more important of which are the hardness and characteristics of the material, the magnitude and form of the work, and the strength and size of the -lathe and tool; all or some of which it is apparent, will cause what is reasonable in one case to be excess in another. Excess tears and damages the surfaces produced, frequently also breaking the edges of the tools; good turning being only attainable, when all the factors referred to, are employed well within their limits of strength. On the other hand, it is possible to observe too great caution in the latter direction, causing unnecessary expenditure of time. Unavoidably upon these points, much must be left to the judgment of the individual, but that may be rapidly matured by experience, for provided the work be securely and correctly chucked, the effects produced upon it and the tool surely indicate whether too severe a cut has been made, or if a deeper may be safely attempted.
The small inequalities, almost inseparable from the most carefully placed separate cuts, even when these have been connected by subsequently traversing the tool, are conveniently corrected, and the metal superficies reduced to one true line, with a file, applied to the work while that is in revolution. The flat file, as will be shown, is of great assistance in finishing cylindrical, surface and other turned works in metal, and its flat face is employed a second time, with fine emery paper wrapped around it, to polish and finish the surface it has previously corrected. The file used in this manner, may then be followed by the further methods of polishing steel, iron and brass, detailed in the Descriptive Catalogue of Apparatus, Materials and Processes for grinding and polishing, Vol. III. The processes for lackering turned and other brass work, for the prevention of subsequent surface tarnish, are also given in the same volume.
The graver and other tools for rough turning wrought iron and steel, require to be constantly supplied with, or to be dipped into water for lubrication, to prevent the metals being torn and to allay the heat set up by the friction in both work and tool. Oil, or sometimes soap and water, are used with the finishing tools and also in cutting screws for the same purpose. Cast iron, brass, gunmetal, and similar alloys, are turned dry; copper, with water, oil, or soap and water. Lead is turned with great facility with the softwood tools, provided they are frequently lubricated with any of the above, milk, or even dirty water; for the purpose of constantly soiling the chemical cleanliness of the turnings and the surface produced. Otherwise, these are liable to partially adhere again to each other as they are turned, and with so remarkable facility and tenacity, as sometimes to require the use of a cutting tool to separate their accidental adhesion. The turnings from copper, tin, and some others of the metals, adhere in like manner, although to a less degree.