Again, when the swivel tool-holders were first used in cutting square-threaded screws, the utmost the lathe could do with forged tools was to take 4 degrees of feed at each cut, as indicated by the micrometer feed-wheel. The tool-holder on the other hand took 7 degrees of feed in the same lathe, doing the same work, and producing quite as good or a better finish with the same expenditure of steam power.

The cutters for the swivel tool-holders can not only be made at the outset, but also constantly maintained, at the best and most efficient angles which practice can teach; it therefore follows that a very much better class of machine work can be produced. The finished surfaces obtained from the tool-holders show a striking superiority over those from forged tools, especially when in the latter the angles are ground by hand by each man or boy working a machine. The tendency then is to grind the cutters to all sorts of incorrect forms, which more or less tear the surfaces of the machined work, and leave bad finishes, such as require a considerable amount of hand labour bestowed upon them afterwards, in filing, scraping, and polishing.

Again, the tool-holders have led up to a considerable extension of what is called broad-finishing, in planing, turning, shaping, slotting, etc.

Broad-cutting feeds, varying from 1/2 in. to 1 1/2 in. in width, are very commonly taken by the swivel tool-holders and more accurate surfaces produced than with finer feeds. The advantages in point of time saved are very great; the time occupied in finishing by broad-cutting being 1/12 to 1/20 of that consumed by finishing with ordinary feeds and in the usual manner. The width of broad-cutting can be increased to any desired limit, and there have been special cases where it has been advantageous to take thin shavings 3 in. to 6 in. in width.

The principal limits to broad-cutting are as follows : -

1. The power of grinding the cutting tool to a sufficiently straight or true cutting edge; the best plan, of course, being to do this by mechanical means.

2. The securing a sufficient stability in the machine tool to hold the broad-cutter bo rigidly up to its work that neither the cutter itself nor the work may spring away, and that no jarring or injurious vibration may be produced, and impart its evil effect to the finished surface.

3. The securing of sufficiently accurate work to answer the purpose for which it may be required : for instance, the piece of work planed or turned by this process may be a portion of a large railway bridge, where absolute accuracy is not required, or it may be some portion of a machine tool, where the utmost accuracy is needed; or, again, some portion of an engine, where the builder is anxious to obtain all the accuracy which can possibly be produced direct from the machine tool.

During the last 30 years many attempts have been made to introduce a better system of drilling and boring. Many engineers have used square bar steel, which the blacksmith has twisted, and then flattened at one end to form a drill. The object of the twisted stem was to screw the cuttings out of the hole, and to some extent this succeeded, but not perfectly. The twisted square section revolving in the round hole had a tendency to crush or grind up the cuttings; and if they were once reduced to powder it was difficult (especially in drilling vertically) for the drill to lift the powdered metal out of the hole. In most cases the lips of the drills were of such form that the cutting angle, or face of each lip, which ought to have been about CO", Fig. 1258, was 90°, or even still more obtuse; this being an angle which would scrape only, but could hardly be expected to cut sweetly or rapidly.

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Again, there were attempts to make the cutting angles of the 2 lips of much the same number of degrees as that given by the twist itself in a good twist-drill. This was done by forging or filing a semicircular or curved groove on the lower face F of each lip, Figs. 1250, 1251. For a short time lips thus formed cut fairly well, but a very small amount of regrinding soon put them out of shape, and made them of such obtuse cutting angles that good results could no longer be expected from them; and to be constantly sending such drills to the jobbing or tool smith, and then to the fitter to file into form again before they were rehardened, was found to be too tedious and too expensive. Again, to arrive at the best results in drilling, each of the cutting lips should make the same angle with a central line taken through the body of the drill; in other words, the angles A and B, Fig. 1245, should each have exactly the same number of degrees, say 60°. The clearance angles also should be identical, and the leading point P should form the exact centre point of the drill.

From practice it is found that if these proportions are not correct, the drill cannot pierce the metal it is drilling at more than about half the proper speed, and the hole produced will also be larger than the drill itself, as will be exemplified a little later on. To give an idea of the excessive accuracy which must be imparted to a twist-drill, we must bear in mind that even a good feed is only 1/100 in. to each revolution; and as two lips are employed to remove this thickness of metal, each lip has only half that quantity to cut, or 1/200 in. This 1/200 in. is as much as can be taken in practice by each lip in drills of ordinary sizes.