This mode of action always cuts the key-way parallel and not taper as frequently wanted. From the subdivision of the work amongst the many cutters, the work is well done, and almost without injury to the cutters, which should be sufficiently close together, that the succeeding cutter may enter the groove, before the previous one has passed through the same; in other words, the interval between the cutters should be always less than the thickness through the boss of the wheel. The cutters after having been sharpened, are set forward by aid of little screws fitted in a thin bar, inlaid in a chamfered groove extending the whole length of the cutters.

Figs. 991 and 992, represent Mr. Babbage's tool-holder with many blades for the planing machine. This tool-holder consists of two parallel bars of gun-metal, united to cross pieces at the ends, so as to form a narrow central cleft; the side bars are pierced with several holes which receive as many pins, that constitute the centers upon which a series of short parallel blades are jointed to the holder. When in use, the blades are separated by parallel slips of brass, and at the left extremity is a block to which is given the inclination of 27°, and the end screw being fastened the whole of the blades are fixed at that angle; Mr. Babbage says in making another tool-holder of this kind he would cast the holder in one piece, and tighten the cutters by the method of the screw and wedge a, b, fig. 988. In order to sharpen the cutters, the brass separating pieces and the angle block at the end are removed, and all the flat pieces then fall down so that their chamfered ends lie in a straight line; when thus fixed by the end screw, their chamfers are all ground at once upon a lap; on the re-insertion of the brass plates, the tools bristle up like so many saw teeth after the manner shown. The tool is fixed in the planing machine at such an inclination, that the first cutter penetrates but a little way and every succeeding cutter penetrates more and more unto the required degree, owing to the inclined position of the tool-holder; the difference in elevation or projection of in two ends, being exactly equal to the intended thickness of the having to be removed, and the two tails of the tool-holder enable each end of the same to be securely grasped in the planing machine. (See first paragraph, page 982.)

Fig. 991.

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Fig. 992

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Fig. 993, a face-cutter for the lathe, is the last of there tools which Mr. Babbage's occupations have given him leisure to devise. The circular block is screwed to the lathe as an ordinary chuck, and on its cylindrical surface are cut 10 wedge-form grooves or notches, the one aide of every notch is exactly on a diameter, the other side of the notch is inclined a few degrees, and fitted with a parallel steel blade, and a gun metal wedge; the several wedges are sent forward by tail screws, tapped through a ring screwed on the back part of the chuck or otherwise attached.

To sharpen the blades they are removed from the chuck and placed in the rhom-boidal cavity of a tool-holder shown in perspective in fig. 995, and in plan in fig. 990; the sides of the cavity are parallel and in pairs, but inclined in both directions to the angles at which the cutters are ground upon a revolving lap; the horizontal angle seen in 996 is 24 degrees, the vertical is 16. By means of this holder the chamfered ends of the cutters ore all thrown into one plane, and the sides of the cotters into another plane, and secured by two equal or folding wedges, the ends and sides of all the cutters are then ground en matt.

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When replaced in the chuck a distance plate d with a central projection or boss is first fixed to the end of the chuck, the cutters are allowed to rest in contact with this plate, and on the screws being tightened, every cutter becomes fixed by its wedge, and the distance plate ensures the ends of the cutters lying on one plane, and as much in advance of the end of the chuck, as the space between the chuck and the reduced margin of the distance plate.

This circular cutter with removable blades, may be viewed as a miniature and refinement, of some of the largo boring tools and cutters with loose blades, figs. 516 and 517, pages 569 and 571; and the tool here shown has been extensively used by Mr. Babbage in facing all kinds of rectilinear pieces, which are at the time fixed in the slide rest, or in a universal chuck with screw jaws attached to the slide rest, by means of which the works are carried post the end or face of the slowly revolving cutter, which serves for several of the metals including steel, but the most effectively for brass and gun metal.

Note AU. - To follow note AT at the foot of page 588.

(Paper on the Principle of Tools for Turning and Planing Metals, by the Rer. Prof.

Willis of Cambridge, A. M., F. R. S., etc.)

Let FOHK, fig. 997, represents rough cylinder of metal running in a lathe between the centers M and L, and suppose that this is subjected to the action of the tool

DBE, which in the figure is supposed to have travelled from A to B, for the purpose of turning the surface of the cylinder. The tool is fixed in a slide rest by which it is carried in a direction parallel to the axis of the cylinder; it moves at such a rate that during each revolution of the cylinder, the point B of the tool is carried onwards through the space B b. The proportions of the figure are greatly exaggerated for the purpose of showing the effect proposed to be illustrated, for in practice, as is well known, although it is true that the effect produced by the tool in turning a cylinder, is to trace a screw upon the surface, yet the thread of that screw is either so fine from the slow motion of the tool, that it appears as a mere roughess of surface, or else it is so flattened as to disappear from sense.* In this figure the screw mustbe considered as an exaggeration for the sake of explanation.