Work of this character is first roughed concentric by series of separate cuts made with the router, figs. 450. 451. The ridges between the furrows are then reduced by the same tool, and the cylinder is finished with the flat tool, sometimes followed by the file. The roughing tool is supported on the rest with its shaft at about the same angle as the graver fig. 461, the under side of its round cutting edge indenting the face of the tee. For lighter cutting the under surface of this and the other turning tools for brass, may lie in contact with the edge of the tee, the cutting edge slightly overhanging it. In either position the tool is held very firmly, and the cut is obtained by pressure, or by gently lowering the handle. The irregularities in the line of the brass cylinder left by the roughing tool are reduced with the flat tool, first by separate cuts, a narrow portion at a time, and then by continued traverses of the tool; the tool being held and used much after the same manner as upon hardwood.
The flat and other finishing tools cut very readily upon brass and similar alloys, but their use presents a difficulty incidental to these materials, which are readily set in vibration by the process of turning. The vibration causes the work and the tool to "chatter" upon each other, and to leave the cylinder covered by numerous irregular, but parallel, fine lines or striae, also called " utters" by the brass turners; the latter term probably arising from the sound emitted by the work when in vibration against the tool. The formation of striae is still more frequent in turning the brass surface, and when they are once established upon this or the cylinder, they incite the work to increased vibration, and deepen and spread under the continued use of the turning tool; they can then only be obliterated by returning to the use of the roughing tool, and recommencing the entire process.
To avoid this latter inconvenience, which would probably also unduly diminish the size of the work, the formation of striae is carefully guarded against from the commencement. A rather narrow flat tool is chosen, and this is first employed to turn the cylinder smooth, only a small portion at a time. The tool is held with considerable firmness, with its shaft presented to the work at a small horizontal angle, so that in making the separate cuts only about half the width of the cutting edge penetrates the work. So soon as the line of the work becomes fairly straight, the separate cuts are connected by traversing the tool, during which time it is more liable to "chatter"; this may be avoided in some cases, by retaining the edge during its traverse still in the same position, one corner penetrating and the other just free of the work, when either the cutting or the disengaged corner may precede, as the tool travels along the cylinder. Many brass turners for the same purpose prefer to tilt the flat and other finishing tools upon one corner, and not at one constant angle as in turning hardwood, but continually moving the cutting edge up and down upon the corner on which it rests; so as to constantly vary its angular elevation or tilt, during the entire traverse. The impulses to vibration are then given to the work at slightly, but constantly, varying angles, when their effects cross, and to some extent neutralize each other.
The finishing shavings, which are thinner, may be removed with a wider tool, held also in the horizontal manner; the fingers of both hands close about the end of the tool and upon the rest, the degree of tilt being constantly varied as before. The diminished thickness of the cut sets up less vibration, while the fingers, being more or less interposed between the tool and the surface of the rest, act as a spring or cushion, and largely absorb or check the vibrations by associating them with the elastic frame of the operator.
The parallelism of the metal cylinder is ensured by frequently and carefully testing the work done by the tool, by the callipers or some form of gage. With long works, short portions at intervals and towards the two ends, are first turned to the same diameter and measured; the intermediate longer portions of the cylinder are then reduced to the same diameter, under the guidance of a steel straight edge and the callipers. Many long works, it should also be said, require support against the thrust of the tool, by some one of the back stays or guides already described. Fixed gages in place of the callipers, partly as avoiding risk of error from the elasticity of the latter, are very convenient for cylindrical works requiring exact turning to definite diameters. A square notch filed in a piece of sheet steel, similar to fig. 870 Vol. II., is very commonly employed; it is usual to file two notches in the same piece, one of the exact finished size required, the other slightly larger. Carefully bored ring gages are also used for works requiring a higher degree of accuracy.
After the best attainable surface has been produced on the cylinder by the turning tool, a greater degree of smoothness may be given to it with a flat file, its side made to lie exactly in contact with the revolving work. The guidance of the file largely depends upon the cylindrical correctness of the previous turning; whilst its use requires some care to avoid deteriorating angles or filing the work taper, but these results may be prevented by attentively distributing its strokes equally along the length of the work, and occasional measurement with the callipers or gages.
Most frequently, the handle is held in the right hand and the tip of the file between the thumb and two first fingers of the left, both hands exerting about equal pressure, but holding the file without stiffness to allow its flat surface to take a true bearing upon the revolving cylinder. While to maintain the true bearing without interruption, the file still rests upon the work during the back stroke, during which however, the pressure is almost entirely relieved. The reader is referred to the chapter on files in the second volume, for the correct positions of the operator, and the manipulation observed in delivering the strokes of the file upon a flat surface, most of which it may be said, apply with equal force to the employment of the file upon the revolving cylinder. Upon the more delicate cylindrical works the smaller files may be held in the right hand alone, guided and retained in contact with the work by the forefinger stretched out and resting along them. For the opposite extreme or heavy work, the palm of the left hand is placed on the end of a large flat file, both hands and arms exerting considerable pressure in the forward stroke. The work revolves always towards the operator, or meets the file as that is thrust steadily forward at about right angles to the mandrel; these straight forward strokes being occasionally varied by others, made at a horizontal angle of about 45°; in order that the individual strokes of the file, may be made occasionally to cross and thereby correct each other's imperfections.
Collars and other surface projections upon the cylinder, are protected from injury by the safe edge of the file being turned towards them. Smooth or finer files are used to follow the coarser; these smoother files are usually slightly oiled, or they may be rendered less active, by rubbing their faces with dry chalk, which partially fills and remains in their teeth ; when a still higher degree of surface smoothness is desirable, a strip of flour emery paper is neatly and tightly wrapped around a smooth flat file, which is then again applied to finish or polish the work. The emery paper covered file is shifted from place to place along the work, held square, and then occasionally at small angles to its axis, in order that the abrasions of the emery may cross, and leave less or no decided marks; the covered file is sometimes held quite still, at others it receives a gentle forward movement, but never approaching its previous liberal strokes. It is lifted and shifted forward from time to time, to use a fresh portion of the paper as that previously employed, becomes worn or overcharged with the dust it removes from the work. Moistening the emery paper with oil, improves the quality of the surface but leaves it dull; the latter when cleansed from all trace of the oil is rendered lustrous, by using nearly worn out paper that has received no oil. Less carefully finished turned works in iron and steel, may have their surfaces rendered bright and smooth with the emery stick, prepared in the manner described page 1057 Vol. III.; used after the file, and applied in a similar manner. Those in which mere surface smoothness is sought, as also grooves such as those turned in the edges of wheels, are finished with a strong deal stick and fine grinding emery; the end of the stick is chopped to an edge, and it is applied to the work supported on the rest, the end being from time to time dipped in oil and then in a vessel containing the dry emery powder; precautions being observed in all cases, that no particles of the emery may find their way to the mandrel or other working parts of the lathe.
The numerous cylinders that may be found forming portions of metal work, for machinery, implements, and tools; such as axles, arbors, levers, and pivots, rods, rollers, spindles, uprights, wheels and others; some of which are indicated by figs. 464 to 476, and offered as examples, that may be wrought by hand turning with the graver and foregoing tools in the foot-lathe ; the larger specimens, with greater ease with the fixed tool in the slide rest; the surfaces of many being finished with the flat file.
The various spindles and rods, figs. 464 to 469, or the uprights, figs. 474. 475 are first set or straightened and centered z 2 in the manner described pages 192 - 196; any difference in their subsequent treatment arises principally from their relative stability, and lies to a great extent in the manner in which they are chucked or driven between centers. One of the carriers can generally be attached to their ends, or sometimes a portion of the work will lie in contact with the driver of the running center chuck, and serve the purpose; as in turning the shaft of the small crow-bar, fig. 470; the tee of the hand rest, fig. 472; or the end of the rod, fig. 469; which last would be left solid to carry the centers, and be filed open, subsequently to the turning. A portion of the iron hand brace fig. 473, would in like manner lie in contact with the driver of the chuck.
Fig. 464. Fig. 470. Fig. 475.
A projection is left on the forging upon such forms as figs. 470. 471. 472, to receive one of the lathe centers, and is cut off or filed away, when the turning has been completed. The ends of spindles are sometimes forged with tails to serve as carriers, fig. 466; and the one end of light works, such as the blanks for screws, or the lever, fig. 465, may be left a little longer than required, and filed square to be driven by a square hole chuck; the portions in excess being afterwards nicked in with the graver, and cut or broken off. The flat plate carrying a spindle at right angles, fig. 476, would be bolted down upon a surface chuck to turn the cylindrical stem.