FIG. 169.-12-ft. Vertical turning and boring mill.

support of the bar.

It is only necessary to remove the cylinder cover and piston, and the cylinder is bridged across the face, by the large plate, which also serves as a bearing for the boring bar.

The cutter heads are fed by a screw in one side of the bar, and are

operated by the feed-casing on the end that contains the gearing, by altering the position of which two changes can be made.

The boring bar is driven by a train of gears, and can be worked by belt or hand power. At the opposite end of the bar, a self-centering chuck guides the path truly concentric with the stuffing box, while the plate is rigidly held at the other end by the clips and bolts shown. In those cases where there is no support for the chuck a second cross head is used.

Vertical Turning and Boring Mill.-Fig. 168 illustrates a vertical turning and boring mill.

These upright lathes are specially powerful and compact. The design is to give solidity to the table and tool, and to insure a steady but positive drive. The table of the machine is supported by an annular bearing, which extends to the circumference. In this respect an ordinary lathe, with a face plate of similar dimensions, is not so well supported.

The tool bar is massive, and is housed in two bearings which are adjustable. The bar has an automatic vertical traverse by means of a rack and pinion, while the transverse slide is fed by a screw.

By reference to the figure, it will be seen that a large disc is located on a pinion shaft near the base of machine. In contact with the disc is a leather-faced wheel; this is known as a frictional feed motion, and may be instantly released or reversed as required.

These mills are used in boring and turning cylinders, valves, pulleys, wheels, etc. Deep cuts and coarse feeds can be taken at high speeds without causing chatter.

12-ft. Boring and Turning Mill.-Heavy work of large diameter is turned and bored in a special boring and turning mill (in place of the facing lathes) in the American and in some British workshops.

A machine of the type illustrated in Fig. 169, made by Wm. Sellers & Co., Philadelphia, has capacity to turn a wheel or plate 16 ft. diameter. There are three saddles, each being provided with an independent feed movement. By this arrangement a wheel can be bored with a tool supported in the table bushing at one end, and carried by the central slide, while the two outer slides carry tools operating on the wheel's periphery. All the feeds are actuated by a mechanism from either end of the cross slide. The vertical slides, which are made of steel, have a stroke of 4 ft. When very large pieces are to be turned, the upright standards can be worked back automatically. Castings 9 ft. 6 in. deep can be admitted beneath the cross head.

CHAPTER IX.

LATHE WORK.

Screw Cutting Iron and Steel with Hand-Tools.-The art of chasing screws in iron or steel by means of hand tools can be acquired only after considerable practice. Fig. 170 represents defective screw threads which are sure to be imitated by a beginner.

Drunken Threads.-At A are shown threads almost like parallel rings, having a sharp crook on each thread as if in some way to compensate for the lack of inclination in the remaining part of the threads. Threads of this description are produced either by sliding the chasing tool at an irregular rate on the tool rest of the lathe, or by a seam in the iron (this latter is always disastrous to a screw, even when the threads have first been partially cut in a lathe with the aid of change wheels and a guide screw).

A double-threaded screw is sometimes produced. This is done by sliding the chasing tool too rapidly. along the lathe rest, so that the B threads appear as shown in Fig. B.

A correctly finished stud is shown in Fig. D. Hand-chased screws and studs are being replaced by screws and studs made in capstan, or chasing lathes, or in special screwing machines. Fig. C shows a stud

FIG. 170.-Stud to be chased.

turned and finished, ready to be chased, to fit a g-in. Whitworth nut.

C

A

Tools required.-An elbow rest having a smooth and even surface ; a diamond-pointed tool, i.e. a graver ground keen; a chasing tool having eleven threads per inch, are the necessary tools for work.

The graver and the chasing tool should be securely placed into their respective handles; and in order that the tools may be firmly controlled, the distance from point of tool to end of handle should not be less than 12 in. The chasing rest should be fixed a little below the centre line, and parallel to the work.

Chasing Screw Threads. The first thing is to chamfer the end of the stud. Then look at the pitch of the chaser, and with the graver held hard on to the rest, try, by giving a slight but even twist to the

wrist, to produce a spiral cut on the revolving work, which resembles the pitch of the teeth of the chasing tool. To do this effectively the first time is scarcely to be expected; but, when properly experienced, a workman can produce a spiral so near and so true that the chaser will follow the groove, and so make a threaded screw from its guidance.

The workman's right hand should grasp the loose poppet, while the left hand holds the tool by the handle. The pressure is given from the workman's shoulder, which should well cover the extremity of the tool handle. By this means the depth of the cut is regulated; and to a

Outside chasers.

Inside chasers.

FIGS. 171-2.

certain extent, by a uniform movement of the body, the chaser is kept up to its cut as it traverses along. To assist the chaser, when making a start, a gentle pressure from the thumb of the right hand is often helpful. Good chasing depends entirely upon judging the sliding movement of the tool in combination with the rate at which the work revolves, and the inclination at which the cutting edges of the chaser is presented to the revolving work.

Each size of screw has a different number of threads per inch; therefore it is obvious that experience is necessary to enable one to chase screws creditably. Care must also be given to keep the tool