speed within the range of the wheels; usually, however, it will be driven at the same speed and in the same direction as the lathe-spindle itself, by means of a 40tooth wheel on both the spindle end and the back shaft, through an intermediate wheel of seventy or other convenient number of teeth. The intermediate wheels are carried on a radial arm in the usual manner; but the amateur may need a word of explanation to enable him to understand the arrangement.

The lug or bracket carrying the shaft has a circular flange, to which is bolted the radial arm в by means of two small bolts passing through curved slots cut in the arm, and screwing into the metal of the bracket. These curved slots allow the arm to be shifted some distance radially around the centre of the shaft A and thus to accommodate change-wheels of different sizes. This arm has also two straight parallel slots, as seen; one of these is used to fasten a spindle a to the arm by means of a nut on the other side, and the other slot is occasionally used to carry another spindle or stud when double intermediate wheels are necessary. This second stud is usually made of the full size of the holes in the change-wheels, which revolve directly upon it, but the stud a is made much smaller, and the wheels do not revolve upon it, but are fastened upon a socket b which fits the stud a and runs upon it. The socket is made wide enough to receive two wheels, the wheels being held upon it and prevented from revolving by a key and nut.

It will be seen, therefore, that the radial arm can be shifted round the shaft to bring the slots nearer to, or farther from, the end of the lathe-spindle, and the

stud carrying the wheel-socket can be shifted anywhere

along the slots in the arm.

The uses of these powers

of adjustment will be more specially obvious when screw-cutting is treated of.

А

The shaft A is supported at the other end by another bracket springing from the lathe-bed, and at the extreme end of the shaft there is provision for taking change-wheels for driving the leading-screw. By referring to Fig. 140, which is the end elevation of the lathe, this arrangement of wheel gearing will be understood. The intermediate wheels are carried by a radial arm in the same manner as at the end already described. The wheels shown in place are those used for ordinary traversing. A 20-tooth pinion on the shaft A drives a wheel of 95 teeth on the socket b, and a 21-tooth pinion on that socket drives a wheel of 100 teeth on the end of the leading screw.

There being an equal-sized wheel on both the lathespindle and the shaft A, it is clear that this shaft must make the same number of revolutions as the spindle.. The intermediate socket b, of Fig. 140, being driven from the shaft A by a pinion of 20-teeth gearing into a wheel of 95 teeth, it is equally clear that for every revolution of the shaft, the socket b will only make g of a revolution; and the socket driving the leadingscrew by a pinion of 21-teeth gearing into a wheel of 100 teeth, it will be seen that for every revolution of the socket, the leading-screw will only make 2 of a revolution, and for every revolution of the shaft A, the leading screw will make but 2 of 28 of a revolution. The shaft A must therefore revolve 22 times to cause the leading-screw to reyolve once. The leading-screw

has four threads to the inch, so that for one revolution of the screw, the slide-rest carrying the tool is moved along the lathe bed one-fourth of an inch. It will therefore require four revolutions of the leading-screw to traverse the tool one inch, and to cause the four revolutions, the lathe spindle must rotate 22×4-901 times. Work turned with these wheels would consequently have 90 cuts to the inch; and by altering the wheels, almost any required speed of traverse may be obtained, as will be hereafter explained.

The slide-rest is perhaps the most important part of a screw-cutting lathe, and it is in this that there is the greatest extent of variation in design. In the lathe now under notice, the saddle c of the rest is in one casting; its shape will be best seen at Fig. 137, which is a side elevation of the slide-rest and a section of the bed.

The front part of the saddle carries the mechanism for gearing with the leading-screw. This consists of two half nuts c, which are brought together to clasp the leading-screw, or are separated, by a curved slotplate d, the action of which is better seen in the front elevation. The half nuts are so shaped as to work in slides attached to the saddle; the two screws-the heads only of which are seen-go through the slot holes and screw into the metal of these half nuts. The screw at the centre is merely for the purpose of keeping the slot-plate in proper position.

On the other side of this plate there is a horizontal spindle passing through the front flange of the rest, and having on its inside end a worm-pinion gearing with the leading-screw, and on its outside end a handle.

This pinion and spindle are for traversing the saddle along the bed by hand, which, of course, can only be done when the half nuts are withdrawn from the screw. To traverse the saddle in this manner, the handle is quickly turned round by hand, when the threads of the leading-screw act as the teeth of a rack. Besides this, it is convenient in other ways; as when the screw is in motion, this spindle is necessarily also in slow rotation, so that if it be required to move the rest a very short distance at the usual speed of traverse, it is much more convenient to place the hand or knee against the handle and prevent it rotating, than to throw the half nuts into contact with the leading-screw. By preventing the handle rotating, the rest may be started at any point; but when the leading-screw is used, either the rest must be moved by hand until the threads of the half nuts coincide with the spaces of the leadingscrew, or the operator must wait, with the rest stationary, until the spaces of the screw come round to coincide with the nut threads.

The saddle-slide D has a long movement, exactly at right angles to the bed. It has also three dovetail grooves planed in it, to receive similar shape bolt-heads for fastening other mechanism or work to the slide.

The movement of the slide D is obtained from a screw passing through the top part of the saddle. This screw e has 10 threads to the inch. In front it is fitted to receive a handle for hand rotation, and at the back it has a worm-wheel of 21 teeth, driven by a threethreaded worm on the shaft A.

For every revolution of the back-shaft the wormwheel will make one seventh of a revolution, and

during one-tenth of a revolution of the screw e the slide D will traverse one-tenth of an inch. For every revolution of the back-shaft, it is plain that the slide D will move one-seventieth of an inch. When, therefore, the speeds of the lathe-spindle and shaft ▲ are alike, the surface traverse will have 70 cuts to the inch.

The worm-wheel is not directly connected to the screw e, but is free to revolve upon it without communicating its motion to the screw, so that the slide may be traversed by hand or not moved at all. The worm-wheel is kept in place by the tail plate g; but when it is wished to traverse the slide D automatically, the sliding clutch f is pushed forwards to gear with a corresponding clutch on the boss of the wormwheel. The clutch f is only capable of a sliding motion on the screw, but it cannot be revolved without the screw revolving also. When not gearing with the clutch on the worm-wheel, the clutch f partakes of the motion of the screw, but when in gear with the worm-wheel, the motion of that wheel, derived from the worm of the back-shaft, is communicated to the sliding clutch, and necessarily also to the screw itself, and the slide D is consequently caused

to traverse.

There is other mechanism attached to the back of the saddle, but that will be described when treating of its uses.

The thread of the screw e is square. It begins just behind the worm-wheel, and continues the whole length of the spindle. The bearing for the screw e in the front part of the saddle is not in the metal of the