thin shafts, where the amount to be removed is slight, this is a convenient and easily adjusted stay.

Large shafts are frequently turned by three cutting tools acting at equal distances around the shaft's periphery. The tools are carried in holders which are actuated independently; one tool roughs out the shaft, another reduces it nearly to size, while a final cut is taken by a scraping tool. The steel used is of the self-hardening class, which permits of a coarse traverse and much increased speed.

Since the head carrying the tools is secured to the saddle, it forms a compact mass and is capable of attacking the metal with much more ease than is the case when the cutting tools are simply carried in the slide rests. At a convenient distance from the cutter head a travelling stay fits the shaft, thus keeping it in a uniform position, as near the cutting tools as is possible.

Shaft-turning Rest.-A special rest for shaft turning is given in Fig. 158. This portable rest can be secured to any lathe of a given height of centre, and is fitted with three tools, each of which may be operated independently by a screw and slide.

Two Tools roughing down.--The two leading tools rough down the shaft, one of these being inverted to meet the cut.

This is obviously good, because where a single tool is employed the cutting force is all on one side of the lathe centres, but by the above method the cutting forces are balanced.

Finishing Tool-The "following" tool is made with a broad nose, to take just a scraping cut, which leaves the shaft beautifully smooth and true. The stay

Travelling Stay.-Forming a part of the rest is the travelling stay, which slides over the turned shaft close to the finishing tool. is important; by it the alignment of the shaft is maintained between the lathe centres, the shaft is held up to the tools, and the vibration is reduced to a minimum. See also "Back Stay," Fig. 119.

CHAPTER VIII.

BORING MACHINES.

Duplex Boring Machines.-Boring machines, like many other modern tools, have increased in variety, so that special forms of work may be easily set and tooled.

Two different types of machines are illustrated in Figs. 159-160, by Lee and Hunt, Nottingham. Taking Fig. 159, it will be seen that the boring bars have only a revolving movement, the cutter heads being stationary on the bars.

Here the work is mounted on the saddles, or carried in the two large cradles which are secured to each saddie. The cradles may be brought close together, or extended according to the length of the object to be bored, and the saddles traversed along the bed.

Each bar may be worked independently; for instance, one bar may be used to bore cylinder liners while held in the cradles, while the other bar may be engaged in boring the cylinders.

The cylinder liners held as above are thus self-set. Referring to Fig. 160, it will be noticed that there is a special feature, in that the two boring bars are placed at right angles to each other, the machine being designed to bore at the same time the large round bearing for the cylinder and the two bearings for the crank shaft in the bed of a gas or oil engine.

The large cast-iron boring bar revolves in an adjustable bearing in the headstock, and carries an automatic boring head, shown near extending end of the bar. The boring head is traversed by means of a screw actuated by the differential gear located at the opposite end of the bar.

The bar is driven by a worm and wheel, and is fitted with a face plate carrying a facing slide, which is used when machining the seating of cylinders. The steel boring bar, which is carried in the two upright bearings, is directly at right angles to the large bar, and has a similar feed motion to it.

The driving-cone shaft gives motion to both bars; but either bar may be set to work independently as required, and in this capacity resolves itself into a machine for an increased variety of boring work of both large and small dimensions.

Horizontal Boring Machine.-Figs 161-162 represents a cylinderboring machine by Messrs. William Muir & Co., Manchester. In this machine the headstock containing the driving gear is mounted and

bolted to the bed. The boring bar is fixed to a flange on the driving spindle, and revolves with it.

When a cylinder is to be bored and faced, it is securely clamped to

FIG. 161.-Horizontal boring and facing machine.

the machine bed, and the bar is passed through and secured in position. Then the "boring head," carrying the cutters, is traversed by means

of the long feed screw through the cored hole where the piston is to work.

The facing arms are of the double-ended type, each being made