The tools for making taper holes are much less varied than the drills and boring tools for cylindrical holes. Thus the carpenter employs only the rimer, which is a fluted tool like the generality of his bits; it is sharpened from within, as shown in fig. 518, so as to act like a paring tool. Flutes and clarionets are first perforated with the nose-bit, and then broached with taper holes, by means of tools of this kind, which are very carefully graduated as to their dimensions. Fig. 519 represents a German rimer, used by wheelwrights for inlaying the boxes of axletrees; the loose blade is separated from the shell of the instrument, by introducing slips of leather or wood between the two; the detached cutter fits on a pin at the front, and is fixed by a ring or collar against the shaft.
A curious rimer for the use of wine-coopers, was invented by the late Mr. John Hilton, by which the holes were made more truly circular, and the shavings were prevented from dropping into the cask. The stock of the instrument consisted of a hollow brass cone, seen in section in fig. 520; down one side there was a slit for containing a narrow blade or cutter, fixed by three or four screws placed diametrically. The tube was thus converted into a conical plane; the shavings entered within the tube, and were removed by taking out a cork from the small end of the cone.*
The broaches for metal are made solid, and of various sections; as half-round, like fig. 521, the edges are then rectan-guar, but more commonly the broaches are polygonal, as in fig. 522, except that they have 3, 4, 5, 6, and 8 sides, and their edges measure respectively 60, 90,108, 120, and 135 degrees. The four, five, and six-sided broaches are the most general, and the watchmakers employ a round broach in which no angle exists, and the tool is therefore only a buruisher, which com-sses the metal and rounds the hole.
Ordinary broaches are very acute, and fig. 528 may be considered to represent the general angle at which their sides meet, namely, less than one or two degrees; the end is usually chamfered off with as many facets as there are sides, to make a penetrating point, and the opposite extremity ends in a square tang, or shank, by which the instrument is worked.
Square broaches, after having been filed up, are sometimes twisted whilst red hot; fig. 527, shows one of these, the rectangular section is but little disturbed, although the faces become slightly concave. The advantage of the tool appears to exist in its screw form: when it is turned in the direction of the spiral, it cuts with avidity and requires but little pressure, as it is almost disposed to dig too forcibly into the metal: when turned the reverse way, as in unscrewing, it requires as much or more pressure than similar broaches not twisted. This instrument, if bent in the direction of its length, either in the act of twisting or hardening, does not admit of correction by grinding, like those broaches having plane faces. It is not much used, and is almost restricted to wrought iron and steel.
See Trans. Soc of Arts. 1880, vol. xlviii. page 125.
Large countersinks that do not terminate in a point, are sometimes made as solid cones; a groove is then formed up one side, and deepest towards the base of the cone, for the insertion of a cutter, see fig. 523. As the blade is narrowed by sharpening, it is set a little forward in the direction of its length, to cause its edge to continue slightly in advance of the general surface, like the iron of a plane for cutting metal.
Fig. 529 represents Mr. Richard Roberts' broach, in which four detached blades are introduced, for the sake of retaining the cone or angle of the broach with greater facility. The bar or stock has four shallow longitudinal grooves, which are nearly radial on the cutting face, and slightly undercut on the other. The grooves are also rather deeper behind, and the blades are a little wedge-form both in section and in length, to constitute the cone, and the cutting edges. In restoring the edges of the blades, they are removed from the stock, and their angles are then more easily tested: when replaced, they are set nearer to the point, to compensate for their loss of thickness.
Broaches are also used for perfecting cylindrical holes, as well as for making those which are taper. The broaches are then made almost parallel, or a very little the highest in the middle; they are filed, with two or three planes at angles of 90 degrees, as in figs. 524 or 525. The circular part not being able to cut, serves as a more certain base or foundation, than when the tool is a complete polygon; and the stems are commonly made small enough to pass entirely through the holes, which then agree very exactly as to size. Such tools are therefore rather entitled to the name of finishing drills, than broaches.
The size of the parallel broaches is often slightly increased, by placing a piece or two of paper at the convex part; leather and thin metal are also used for the same purpose. Gun-barrels are broached with square broaches, the cutting parts of which about eight to ten inches long; they arc packed on the four sides with slips or spills of wood, to complete the circle, as in fig. 520, in which the tool is supposed to be at work. The size of the bit is progressively enlarged by introducing slips of thin paper, piece by piece between two of the spills of wood and the broach; the paper throws the one angle more towards the center of the hole, and causes a corresponding advance in the opposite or the cutting angle. Sometimes, however, only one spill of wood is employed.
A broach used by the philosophical instrument makers in finishing the barrels of air pumps, consisted of a thin plate of steel inserted diametrically between two blocks of wood, the whole constituting a cylinder with a scraping edge slightly in advance of the wood; slips of paper were also added.
According to the size of the broaches, they are fixed in handles like brad-awls, they are used in the brace, or the tap wrench, namely, a double-ended lever with square central holes. Sometimes, also, broaches are used in the lathe just like drills, and for large works, broaching machines are employed; these little more than driving gear terminating in a simple kind of universal joint, to lead the power of the steam-engine to the tool, which is generally left under the guidance of its own edges, according to the common principle of the instrument.
In drills and broaches, the penetrating angles are commonly more obtuse than in turning tools; thus in drills of limited dimensions, the hook-form of the turning tool for iron is inapplicable, and in the larger examples, the permanence of the tool is of more consequence than the increased fiction, lint on account of the additional friction excited by the nearly rectangular edges, it is commonly necessary to employ a smaller velocity in boring than in turning corresponding diameters, in order to avoid softening the tool by the heat generated; and in the ductile fibrous metals, as wrought iron, steel, copper and others, lubrication with oil, water, etc, becomes more necessary than in turning.
The drills and broaches form together a complete series. First the cylinder bit, the pin-drills, and others with blunt sides, produce cylindrical holes by means of cutters at right angles to the axis; then the cutter becomes inclined at about 45 degrees, as in the common piercing-drill and cone countersink; the angle becomes much less in the common taper broaches; and finally, disappears in the parallel broaches, by which we again produce the cylindrical hole, but with cutters parallel with the axis of the hole.
Still considering the drills and broaches as one group, the drills have comparatively thin edges, always less than 90 degrees, yet they require to be urged forward by a screw or otherwise, the resistance being sustained in the line of their axes. The broaches have much more obtuse edges, never less than 90, and sometimes extending to 135 degrees; and yet the greater force required to cause the penetration of their obtuse edges into the material, is supplied without any screw, because the pressure in all these varied tools is at right angles to the cutting edge.
Thus, supposing the sides of the broach extended until they meet in a point, as in fig. 528, we shall find the length will very many times exceed the diameter, and by that number will the force employed to thrust forward the tool be multiplied, the same as in the wedge, whether employed in splitting timber or otherwise; and the broach being confined in a hole, it cannot make its escape, but acts with great lateral pressure, directed radially from each cutting edge; and the broach under proper management leaves the holes very smooth and of true figure.