The best rule for hand-turning is, therefore, to apply the tool to the work, with these ends in view before fixing the rest, and then to bring that up to the necessary position.
When the heel of any hand tool has a firm bearing on the rest, and the edge is applied in the wedge-like position, the preservation of this during the progress of the work depends on delicacy of touch rather than muscular power. But when the edges are applied out of their natural line, it puzzles a strong wrist to keep them to their cut at all without digging into the work. This affords the best practical illustration of the necessity of careful attention to the position of slide-rest tools, which are deprived of all power of accommodation to the sense of touch, and which therefore require accurate adjustment in the first instance.
For the motion of the tool is now confined to that of the rest, and as this moves in horizontal planes, the edge of the tool must be applied to the work on that parallel plane which passes through the lathe centres. The reason for this rule will be at once apparent, if the edge be not placed on this central line in facing up a plate—for then it will lose its cut before reaching the centre, leaving a core untouched. Now although it would require an exaggerated error in the position of the edge to lose cut altogether in turning a cylinder, yet this example proves that, unless the edge be applied exactly on the central line the relative position between it and the surface of the work, on which the cutting action depends, will imperceptibly change with the reduction of the work; and supposing this to vary much in diameter, the same tool may cut beautifully on one part and badly on another. [Fig. 4], which illustrates the cutting action of the edge, has been purposely placed on a part of the circle where a slide-rest tool could only act for a very short time, in order to draw attention to the difference between those conditions which govern the cutting action, and those which depend on the motion of the rest from which the tool is used. It is obvious that if [Fig. 4] were moved inwards on a horizontal line the edge would pass over the smaller circle without touching it. The illustration is of course exaggerated, but it proves that [Fig. 7] is the only position in which the tool will cut over varying diameters without some change in the relative positions of its lower face and that of the work. Hence the usual instructions to apply the edge about the centre of the work. But Babbage has observed, that however good this direction may be as far it goes, it is insufficient and liable to mislead when given alone. It is impossible to do away with elasticity when the tool is supported at some lateral distance from the line of strain, as in the slide rest or planing machine; and unless this elasticity is counteracted by the position of the tool, it may upset the best position of the edge. To meet this, Babbage gives the following rule—First, consider whereabouts the tool itself will bend under strain on its edge, when fixed in the rest; and then take care that this part of the tool, which Babbage calls "the centre of flexure,"—is placed above a line joining the centre of the work and the edge of the tool. [Fig. 7] will explain the reasons for this rule and the consequences of neglecting it when there is much strain put on the edge.
Let [E, I, F,] be the line joining the centre of the work and the edge of the tool. Then if G above this line be the centre of flexure, when the tool bends its edge must follow some part of the arc, H, I, J, from G as a centre, and will be thrown out of the work. But if K below the line, E, I, F, be the centre of flexure, then, under the same circumstances, the edge will follow some part of the arc L, I, M, from K as a centre, and must dig into the work. It is important to recognise this principle, because while it shows that every tool in which the top of the edge stands above the shaft must be liable to the evils resulting from elasticity, it shows also that even cranked tools may fail to obviate the danger, unless care is taken to place the weakest point in the shaft above the central line. Babbage remarks, that although it is not always possible to strengthen any part of a tool, it is always possible and sometimes desirable to make some particular point weaker than the rest, by cutting away a little where the weak point should be. [Fig. 9] shows that the crank principle may be applied in another form, and although the crank is upwards in this case, the same object is attained by making P the weakest point, and placing it above the central line, N, O. This form, however, is only used for light finishing cuts; for any unnecessary length of crank evidently adds elasticity, and Holtzapffel observes that, "in adopting the crank form tools the principle must not be carried to excess, as it must be remembered we can never expunge elasticity from our materials, whether viewed in relation to the machine, the tool, or the work." The crank, therefore, should only be just sufficient to give the edge the right direction if the tool should spring; and Holtzapffel remarks, that as a tool will generally bend somewhere in the central line of its shaft, it is sufficient if the top of the edge is kept on or just below this line, as in [Fig. 8]. Referring again to [Fig. 7], and looking at the line C, D, as a plane surface, and I, F, as a line perpendicular to that surface, the same arguments and illustrations apply to the form of a tool in the planing machine. The point at which the tool is now applied ceases to be of moment.
Illustration No. 3.(8, 9)
Having considered the conditions necessary to insure the best cutting action of acute edges and the preservation of that action during the progress of the work, it remains to treat of the edges most suitable to particular materials, the method of giving them any desired angle, and the manner of applying slide-rest tools so as to obtain the best work with the least expenditure of force and time.
Willis observes that different metals and qualities of the same metal require to be treated with edges differing in their degree of acuteness, and all the standard authorities concur in giving the following code as near enough for all practical purposes. The modification of these angles is ruled by the general principle that fibrous and cohesive materials require more acute edges than crystalline and granular substances, as will be apparent in the following code:—
| Wrought iron and steel | 60° |
| Cast do. do. | 70° |
| Roughing brass | 80° |
| Finishing do. | 90° |