In turning the iron, or annealed steel cylinder, the graver rests on the tee by one of the lateral angles of its chamfer, figs 461. 462, the point being placed in contact with the work, with the shaft at a small horizontal angle. In cutting, the portion of the edge close to the point, which displaces the material to be removed, travels in a path combining two distinct movements. It is moved through a small lateral arc upon its supporting corner as on a center; until at the termination of the stroke, the shaft of the tool arrives at the dotted position fig. 462; and it is moved simultaneously through a small vertical arc, by slightly rotating the handle from right to left; until the point, which at the commencement was at about the level of a, fig. 463, at the termination of the cut, arrives at about the height of b, shown by the second section of the tool.
The graver is moved deliberately, that it may have sufficient time to reduce to concentric truth the entire width of the narrow portion of the work over which it travels, at one operation; the cut therefore is not deep, and the width, like all cuts made with the graver, varies from about one eighth to about a quarter of an inch, generally lying between the two.
When first applied to the work the graver is placed with its chamfer towards the left, and the first cut is made at the extreme right hand end of the cylinder. The tool is then shifted along the rest towards the left and a second cut made beside the first, and so on for the length required. Placing the first cut at the end of the cylinder, the corner of which has also been previously thoroughly cleaned by the file or on the grindstone, ensures that the edge of the graver attacks the hard exterior of the cylinder sideways; exaggerated for illustration fig. 462. The second cut being then placed beside the first, the point of the graver enters the clean metal exposed and also reduced to concentric truth by the first, and so on for every succeeding cut; with the effect, that the point, the weakest part of the tool, is greatly preserved from possible fracture by rarely coming into contact with the outer surface of the work. The triangular tool may be employed on the cylinder in exactly the same manner as the graver and is as efficient, but hardly as strong; the tool is supported upon one corner of its cutting facet.
Fig. 461. Fig. 462. Fig. 463.
The first series of roughing cuts with the graver leaves the cylinder concentric, but in a succession of short curves of irregular width, magnified in the line fig. 462. The tops of the ridges are reduced by a second or third series of cuts, the graver being allowed to travel along the work in either direction; after which the line may be corrected by separate cuts, made with one edge of the graver lying on its side, fig. 462, or more conveniently, with the flat tool fig. 444.
The graver is frequently dipped in water which somewhat reduces the friction, nevertheless, the heat set up by the removal of the turnings and in some cases but a small increase in the temperature of the work, will often expand its length; elongation taking place still more readily, in long works turned with the slide rest, from the more continuous cutting action. In the foot lathes this effect is at once felt by the increased labor in driving the treadle; the point of the popit head has then to be slightly withdrawn and refixed, and as the work cools, it may have to be readvanced, so as to maintain a nearly equal amount of pressure. Otherwise in the former case, the work especially when of comparatively small diameter, being bound between the centers, slightly bows and is liable to be turned untrue, while in the latter, the same result may obtain from its insufficient support.
The flat tool for iron is presented to the cylinder at about the same vertical angle as the graver, and usually touches and indents the rest, by the angle formed by its back and cutting bevil; it is made to cut by being pressed forward and by slightly lowering the handle, or for fine finishing cuts by simple pressure, the tool being usually replaced to make every separate cut. Some prefer to give the flat tool a moderate tilt upon one or other angle of its shaft, to disengage the corners, others, to slightly twist or rotate the tool upon its under corner while cutting; in either case the tool requires holding with increased firmness, and should the tilt or the twist be in excess or variable, the work is liable to be turned in hollows. A near approach to the cylinder may be obtained by the skilful application of the separate cuts, and as the work approaches completion and there remains but little material to be removed, these may be supplemented by short traverses of the tool; a higher degree of smooth finish may then be given with the flat file and emery stick.