This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The following notes treat of some of the processes of cutting metals adopted by W. F. Smith, Salford, and described by him in a paper read before the Institution of Mechanical Engineers. In a former paper, the author described mainly what have since become known as right- and left-hand round tool-holders. They are used in different machine tools principally for " roughing out," or, in other words, for rapidly reducing castings, forgings, etc, from their rough state nearly to their finished forms and dimensions. The tool-holders are so called from their cutters being made of round steel cut from the bar. Notwithstanding that they are very widely applicable, take heavy cuts, and do the bulk of all machine work in lathes, and in planing, shaping, and slotting machines, it was soon found that they could not compass the whole of the work required in the shops; and it was, therefore, necessary still to allow the use of some of the common forged tools in conjunction with the round tool-holders. This, however, was objectionable, as no positive rule could then be laid down to define what number of forged tools should be allowed to each workman; and it became apparent that the tool-holder system, in order to reach the highest degree of efficiency, must be made complete and independent in itself.
This led to the designing of another tool-holder of the most general kind the writer could possibly devise, in the hope thereby to complete the system.
With this object in view, all the remaining forged tools then in use were collected together, and the swivel tool-holder (Figs. 1234, 1235) was schemed, with cutters so adjustable that they could not only be swivelled round and then fixed to any desired angle, but could be made to project at pleasure to any required distance in order to reach and cut into all sorts of difficult and awkward corners; in fact, to machine any work which the round tool-holder could not finish. Two of the principal objects aimed at were to devise a system of cutters which should not require any forging or smithing, and yet should be capable of being adapted by the simplest possible means, and by grinding the ends only, to all forms which the round cutters would not admit. The special section of steel decided upon was a sort of deep V section, the lower part of which is slightly rounded, as shown in Fig. 1235. The angles of the sides give the same amount of clearance (1 in 8) as that given in the round tool-holders, and this same angle of clearance is given to the ground parts. The section of the swivel cutter is very deep, in order to obtain ample strength in the direction of the pressure it has to support when cutting.
The angle in Fig. 1234 is common to every swivel tool-holder. In the cutter for the round tool-holder two angles had been fixed upon as standards, one to cut all kinds of wrought metals, the other all cast metals. To avoid complication, however, in the swivel tool-holders one cutting angle was fixed upon for all metals, and applied to all cutters. The angle selected is one slightly differing from that of the round cutters, but is that which worked the best in practice. The cutters of the round tool-holder system are found most advantageous in producing and finishing standard-size round corners in journals of shafts, etc, and in other cases, where the engineer of the present day is anxious to preserve all the strength he can in the parts ho is constructing; but there are still cases where square, angular, or undercut surfaces must be produced, as illustrated by Figs. 1236 to 1241. These are front views showing the tool-holders at work planing or shaping. They are supposed to be travelling forward, or the work to be moving in the opposite direction; and the arrows in each figure indicate the direction in which the tool-holder is being fed at each stroke of the machine, to take the next cut.
Fig. 1237 shows the mode of planing the under horizontal surface of a lathe-bed. The cutter shown in use is ground to an angle of 86°, or 4° less than a right angle, and thus has a clearance of 2° at each side when cutting either horizontally or vertically
This cutter is very general in its applicability, and is devised so as to finish with one setting, both the vertical surface A, and the horizontal surface H, without the necessity for disturbing the cutter in any way. The ordinary system is to use, at least, 2 tools for roughing out, and 2 for finishing, on 2 surfaces right angles with each other.
Fig. 123G shows the method of planing in a very limited space the under horizontal surface S; the corresponding surface is planed afterwards, without disturbing the tool-holder, by simply swivelling the cutter half-way round in the holder and securing it there by the nut N.
Fig. 1240 shows a swivel tool-holder clearing without difficulty a boss which projects, and would be very much in the way of an ordinary tool. The cutter in this case planes not only the horizontal surface but the vertical surface also, with one setting and without being disturbed in the tool-box.
Fig. 1238 shows the method of cutting a vertical slot in a horizontal surface of metal. The cutter in this case is called a parting tool.
Fig. 1242 is a side elevation of this same cutter, showing the cutting angle.
Figs. 1239 and 1241 are tool-holders with cutters of rather special forms. The former is shown planing out or undercutting a T-shaped slot, and the latter is planing out a small rectangular clearance corner.
Figs. 1243 and 1244 show a swivel tool-holder with a round shank, such as is used on the slide-rest of a screw-cutting lathe, for cutting square threads. It is carried on a wrought-iron or steel block, provided with a groove, semicircular in section, in which the round shank of the tool-holders lies, and is clamped down in the usual way. The cutters for cutting out the spaces between the square threads are of a very simple form, and by aid of this tool-holder any tool of the correct width of the space will cut either right-hand or left-hand screws, no matter whether they are single threads, double threads, or any other. To cover the same ground with forged tools, no less than 6 expensive cutters would be required, each forged from square steel, and carefully filed up and hardened. With the tool-holder only one cutter is required, and it costs, probably, not more than 10 per cent. of one of the 6 forged tools, while it maintains its size much better, and, consequently, lasts much longer. It also takes about twice the weight of cuttings per hour as compared with an ordinary forged tool.