The section now to be commenced, refers exclusively to the principles and construction of cutting tools, which will be considered in a general manner, and without reference to any particular branches of mechanical art, the tools and applications being selected by their characters and principles alone.
Some few tools are pointed, from the meeting of three or more planes or surfaces.
Occasionally, as in the hatchet, the chipping chisel, and the turner's chisel for soft wood, the tool is ground from both sides, or with two bevils or chamfers; at other times, as in the carpenter's chisels and plane irons, the tool is ground from one side only, and in such cases, the general surface or shaft of the i constitutes the second plane of the wedge; this difference does not affect the principle.
general characters of cutting tools, namely, their angles, and their relation to the surfaces to be produced, depend upon the hardness of the opposed substances, and the direction and nature of their fibres; these primary characters require especial consideration.
The particular or specific characters of cutting tools, namely, the forms of their blades, stocks, or haudles, are adapted to the convenience of the individual, or the structure of the machine by which they are guided; these secoudary characters, the less quire or admit of generalization.
1t will be now attempted to be shown that, granting the latitude usual in all classifications, cutting tools may be included in three groups, namely, Paring Tools, Scraping Tools, and Shearing Tools.
First - Paring or splitting tools, with thin edges, the angles of which do not exceed sixty degrees; one plane of the edge being nearly coincident with the plane of the work produced (or with the tangent, in circular work). These tools remove the fibres principally in the direction of their length, or longitudinally; and they produce large coarse chips or shavings, by acting like the common wedge applied as a mechanical power.
Secondly - Scraping tools with thick edges that measure from sixty to one hundred and twenty degrees. The planes of the edges form nearly equal angles with the surface produced; or else the one plane is nearly or quite perpendicular to the face of the work (or becomes as a radius to the circle). These tools remove the fibres in all directions with nearly equal facility, and they produce fine dust-like shavings by acting superficially.
Thirdly - Shearing, or separating tools, with edges of from sixty to ninety degrees, generally duplex, and then applied on opposite sides of the substances. One plane of each tool, or of the single tool, coincident with the plane produced.
In explanation of these views, the diagram, fig. 310, is supposed to represent seven different tools, the bevils or edges of which are all at the angle of sixty degrees, this may be considered as the medium angle of the paring, scraping, and shearing tools, The cutting and scraping tools are supposed to be moving from A to B, which line represents the face of the work; or the tools may be considered to be at rest, and the work to be moving from B to A.
Or, in turning, the tool may be supposed to remain fixed, and the circle to represent the moving surface of the work; one plane of the tool then becomes a tangent or radius.
The shearing tools, if in pairs, are proceeding towards each other on the line C D, whilst A B still represents the face of the work. The single tools act on the same principle, but the body of the material, or the surface of the bench or support, supplies the resistance otherwise offered by the second tool.
The tools a, c, f, are bevilled or chamfered on both sides, the others from one side only; in these latter, the general face of the tool forms the second side of the angle, and allowing for exaggeration, both as to excess and deficiency, the diagram may be considered to represent the edges of the following tools.
[a, b, c, d, Splitting and Paring Tools, proceeding from A to B.] a - The axe, or the cleaver for splitting.
b - The side hatchet, adze, paring and drawing knives, paring chisels, and gouges, the razor, pen-knife, spokeshave, the engraver's graver, and most of the engineer's cutting, turning and planing tools for metal.
c - The turning chisel, for soft wood; the chipping chisels, for iron, stone, etc.
d - The joiner's chisels, and carving tools, used with the bevils downwards, the joiner's planes, the cross-cut chisel for metal, and some other metal tools.
[e, f, Scraping Tools, proceeding from A to B.] e - When single, the scraping tools for turning the hardwoods, ivory, and brass, the hand-plane for metal, and when multiplied, the various saws, and files. f - When single, a triangular scraper for metal, and when multiplied, the cross-cut saw for wood, and also polygonal broaches or rimers with any number of sides, for metal.
[e,f, Shearing Tools, proceeding from C to D.] e - When duplex, shears with edges from eighty to ninety degrees, commencing with delicate lace scissors for single threads, and ending with the engineer's shears for cutting iron bars and plates upwards of two inches thick; also duplex punches with rectungular edges, for punching engines and fly-presses. e - When single, the carpenter's firmer and mortise-chisels, the paring-knife moving on a hinge, and cutting punches for gun wadding and thin materials.
f - When duplex, common nippers for wire; more generally, however, the blades arc inclined, so that one bevil of each blade in one and the same plane, and which is vertical to A B, as at g g. f - When single, the smith's cutting-oft" chisel.
In practice, the tools differ from the constant angle of sixty degrees assumed in the diagram for the convenience of explana-tion, as the angles of all tools are determined by the hardness, and the peculiarity of fibre or structure, of the several substances upon which they are employed. The woods and soft fibrous materials, require more acute angles than the metals and hard bodies; and the greater or less degree of violence to which the tools are subjected, greatly influences likewise the angles adopted for them.
Thus, under the guidance of a little mechanism, the thin edge of a razor, which is sharpened at an angle of about 15 degrees, is used to cut minute slices or sections of woods, in all directions of the grain, for the purpose of the microscope. But the carpenter and others require more expeditious practice, and the first change is to thicken the edges of the tools to range from about 20 to 45 degrees, to meet the rough usage to which they are then exposed, whether arising from the knots and hard places in the woods, or the violence applied.
In tools for iron and steel 1 from 60 to 70 will be found a very common angle, in those for brass 80 to 90, in hexagonal broaches for metal it increases to 120, and in the octagonal broach sometimes employed the angle is still greater; in the circular broach required by clock and watchmakers, the angle disappears and the tool ceases either to cut or scrape, it resolves itself into an instrument acting by pressure, or becomes a burnisher.
To a certain extent, every different material may be considered to demand tools of a particular angle, and again the angle is somewhat modified by the specific mode of employment: these conditions jointly determine the practical angles suited to every case, or the angles of greatest economy, or most productive effect.
The diagram shows that, independently of the measure of the angle of the tool, we have to consider its position as regards the surface of the work, the broad distinction being that, in the paring tools, the one face of the wedge or tool, is applied nearly parallel with the face of the work; and in the scraping tools, it is applied nearly at right angles, as explained in the foregoing definitions. Indeed the paring tools, if left to themselves, will in some cases assume the position named; thus, for example, if we place a penknife at an elevated angle upon a cedar pencil, and attempt to carry it along as a carpenter's plane, the penknife if held stiffly will follow the line of its lower side and dig into the wood; but if it be held slenderly, it will swing round in the hand until its blade lies flat on the pencil, and it will even require a little twisting or raising to cause it to penetrate the wood at all. This disposition appears to be equally true, in the thin edges of the penknife or razor, and in the thick edges of the strong paring tools for metal.
The action of a cutting tool in motion is twofold. The moving force is first exerted on the point of the wedge, to sever or divide the substance particle from particle; the cohesion of the mass now directly opposes the entry of the tool, and keeps it back. But the primary motion impressed on the tool having severed a shaving, proceeds to bend or curl it out of the way; the shaving ascends the slope of the wedge, and the elasticity of the shaving confines the tool in the cleft, presses it against the lower side, disposes it to pursue that line, and therefore to dig into the substance.
In pursuing the more detailed examination of different cutting tools employed in the mechanical arts, amongst the several classifications which might be adopted, it appears to the author to be the more generally useful to consider the various tools in separate chapters under the following heads, namely, Chisels and Planes - Turning tools - Boring tools - Screw-cutting tools - Saws - Files - Shears and Punches - as some of all these kinds of tools may be found in every work-room.
The several chapters and sections will be commenced with the tools for the woods, which are perhaps the more commonly used by he amateur, the corresponding tools for metal will generally be then considered, and lastly some illustrations will be given of the same tools applied to various machines, still further to prove uniformity of principle upon which they act, throughout these several circumstances.
These comparative views may serve to show the similitude of principle in tools for like purposes, whether the tools be large or small, whether they be used for wood or metal, and either by hand or machinery; and in cases of indecision or difficulty, a glance through any one section or chapter may denote, either the most appropriate of the ordinary tools, or may occasionally suggest some new modification to suit a particular case, in imitation of the numerous conversions which will be already found to exist amongst the tools used in the constructive arts.