The object of this paper is not to supply patterns of tools, as the best form will be no better than the worst unless properly applied; but to set forth those general principles, which may enable the workman to distinguish between forms which are accidental and those which are essential, and thus to make the shape of any tool his servant rather than his guide.
Whatever the shape or purpose of any acute-edged tool may be, its action will always depend on the manner in which the extreme edge is applied to the surface acted upon; and as the same laws govern the action of every acute edge, whether formed on a razor or a tool for cast iron, it will assist a clear comprehension of this subject to consider first the action of edges generally, without reference to any particular tool.
The same edge may be made to act in four different ways, viz.: to cut, dig, chatter or scrape. Digging and chattering are intermediate stages between cutting and scraping, and are fatal to good work. Thus cutting and scraping remain the two standard principles, on one of which every tool should be made to act; and while cutting depends on the penetration of the edge, scraping results from using an edge so that it cannot penetrate. Consequently, the conditions most favourable to cutting will give the key to both principles of action.
Every cutting edge is simply a wedge, keen enough to guide its own path without depending on the grain or other accidental line of separation in the material on which it is employed; and when such a wedge is forced into any substance, it will show a constant tendency to penetrate in a line with that face which receives most opposition. The comparative amount of opposition which each face receives, will be determined either by one having more of its surface in contact with the material than the other as in [Fig. 2], or by the material giving way on one side, as in [Fig. 1] and [3]. These last two figures illustrate the action of all paring tools, to which class cutting lathe tools belong. The dotted lines are added in [Fig. 2], to show that the action of the edge is the same, whether it be formed by one or two bevels.
Illustration No. 1.(1, 2, 3)
Thus in all cases,—except when an edge is applied so that the pressure is equal on both faces,—one face will guide the course of the edge, and in paring tools this will always be the lower face, or that next the surface of the work.
The first consideration in placing any paring tool must therefore always be that, the lower face of the edge should lie as nearly as possible in a line with the direction the cut is intended to follow, so as to place the whole edge in its natural wedge-like position: for when any edge is compelled to act in a manner contrary to this, it will assuredly assert its natural tendency by digging and chattering in the direction of its lower face. But when the action of the tool is continuous as in turning, planing, or boring, care must be taken that this face of the edge does not actually rub against that of the work; and, to avoid this, Nasmyth recommends that the face of the edge should be inclined from the surface of the work at an angle of 3°. Babbage calls this angle "the angle of relief," because it relieves the friction; and to show how little variation is admissible in this angle, Holtzapffel places its maximum at 6°. In cylindrical work the angle of relief is estimated from a tangent to the circumference. Thus, in Figs. [4 and 7], the lines C, D, may represent plane surfaces or tangents at pleasure, and in either case the lower face of each edge is supposed to make an angle of 3° with these lines respectively.
Illustration No. 2.(4, 5, 6, 7)