is not, however, held stationary, since that would only result in cutting as many grooves around the work as the tool had teeth; but it is pressed against the work, and traversed or pushed along the rest at a speed depending on the speed of the lathe and the pitch of the screw. To traverse the tool thus appears to be very easy, but it must be considered that for every fractional part of a revolution of the work, the same fractional part of the pitch of the screw must have been traversed by the tool, otherwise the pitch is 'drunken,' and the screw is useless.

Beginners will find it easier to take the graver, Fig. 84, and first strike a deep scratch with the point on the work, and at about the inclination of the thread. This scratch then acts as a guide for the chasing tool, and makes it far easier to 'catch' the thread of the screw.

With bad iron no workman, however good, can make sure of getting a true and good thread, as the chaser cuts deeper into the soft places than it does into the hard; but, with good metal, threads can be chased by hand almost as correctly as by any other method.

For chasing brass the tools should be thinner than for iron. Chasers which have been used until ground too thin for iron, are better than new tools for brass.

When cutting a thread on wrought iron or steel, the surface of the metal should be kept moist by soapsuds, or oil and soda-water; but brass and cast iron do not require this treatment. Screw threads are cut internally, by means of the other chasing tool, Fig. 110. The same rest is used, but it is fixed across the line of centres at the same height as before; and the work being put

in motion, the chasing tool is pushed along the surface inside the hole, until the thread is properly cut. The hole to receive a thread should be turned out rather smaller than the thread is required to be, so as to allow of the removal of any scratches made when starting the chaser.

Both of these chasing tools are ground on the face, so that the correct shape of the thread is always retained, and the tools are only made somewhat thinner; this is easily compensated for by raising the tool-rest rather higher.

DRILLING AND BORING.

When holes are required through various materials, it is frequently more convenient to cut them out with special tools than with the turning tools; indeed, with hand-tools it is difficult to cut out holes or recesses of any great depth, but this may very easily be done by means of tools called drills or boring tools.

Sometimes these operations are performed with the work in motion, and the tool still; and sometimes with the tool in motion, and the work a fixture. Whatever the material of the article through which the hole is to be made, the mode of making the hole is very similar; but the tools differ, as in the operation of turning. The tools for woods are shown at Figs. 111, 112, and 113. Fig. 111 is the gouge, Fig. 112 the centre bit, and Fig. 113 the spiral bit.

The tools for metal are shown at Figs. 114 to 120; Fig. 114 is the common drill, Fig. 115 the spiral drill, Fig. 116 the pin drill, Fig. 117 the cutter drill, Fig. 118 the countersink drill, and Figs. 119, 120 are rhymers

or broaches. All these drills are made with a tapering square end, as will be seen, to fit the hole of the drillchuck, already described and used for other purposes.

FIG. 111.

FIG. 112.

FIG. 113.

FIG. 114.

FIG. 115.

The spiral drills are sold in sets, under the name of American drills, and are accompanied by a special holder or chuck. They are used much more in America than here, but are now coming into very general use.

FIG. 116.

FIG. 117.

FIG. 118.

FIG. 119.

FIG. 120.

They are not, however, an American invention, although

most are made there. The spiral is made with a gra

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dually-increasing pitch or rise. All the other drills can also be bought at the tool maker's; but it will frequently be found convenient to make them, as they are not difficult to manufacture. The spiral drill with its chuck is shown at Fig. 121.

FIG. 121.

The article through which the hole is to be made is centered, as for turning, or, at any rate, a centre mark knocked into the place where the hole is to be made; the drill for use is then put into the drill-chuck, on the lathespindle, and set into rapid motion; one centre-mark is then put against the point of the drill, and the other against the centre of the moving headstock. This headstock being fastened down conveniently on the bed, the hand-wheel is moved round so as to force the article against the point of the revolving drill. It will be understood that the article itself must be prevented from revolving with the drill, otherwise, of course, no hole will be made.

For holes of the ordinary size, the article is held stationary by the hands, and then the workman is enabled to ascertain the amount of strain there is upon the drill, by the amount of force he has to exert in keeping the work still. When the strain is too much for the drill, the workman must not screw up so fast, or the drill will be broken off in the hole.

With wood, the strain is seldom sufficient to damage a drill, and therefore it does not require such careful management; but with metal the case is different. The strain must be carefully watched; and it will require

!

some little practice and a breakage or two before the beginner will have learned to judge the strain that drills of certain size will bear with safety. Should the operator be so unfortunate as to break a drill in the hole, unless it will fall out by shaking, the article must be put in the fire and annealed, so as to soften the steel drill, before the work can be proceeded with. Spiral drills clear their own holes from shavings, the spirals being made for that purpose; but with other drills, whether for wood or metal, it is advisable to frequently withdraw the work from the drill, and clear out the hole with a wire or otherwise.

Wrought iron and steel require lubrication; and, therefore, either soda-water or oil must be poured into the hole so as to keep the point of the drill moistened.

Spiral drills have this disadvantage-they withdraw the shavings, but they also prevent the lubricant from reaching the drill's point; these drills are, therefore, better adapted to brass and cast iron than to wrought iron or steel. They are not used for drilling holes of large diameter, since in these there is plenty of room for the shavings, which do not consequently impede the action of the drill. For very small holes spiral drills are capital instruments, especially when the holes are rather long, as they very seldom become choked, but, for drilling even the smallest hole through sheet metal the ordinary drill properly made is as good as the spiral drill.

The amateur will probably consider it rather a feat to take an ordinary good-sized pin, and send a drill through it, leaving it a complete tube, without any external evidence of having done so except at the ends.