Modern Machine-Shop Practice Part 60

The set-screws are so adjusted upon the work, that the outside runs quite true from end to end. The jaws of the steady rest are then set to just touch the circ.u.mference of the sleeve, care being taken that their pressure does not spring the axial line of the work out of its normal straight line. If the shaft is to be turned from end to end, the cat head should be placed sufficiently to one side of the centre of the length of the work and nearer the live centre, that the lathe tool may turn up the work for a distance of at least half its length, or slightly more than half. One half of the work being turned, the shaft is reversed end for end in the lathe, when the cat head may be moved to envelop the turned part, and again set true, or the jaws of the steady rest may be set direct upon the work; in this latter case, however, the friction between the jaws and the work will be apt to leave rings or marks upon the latter.

If the cat head is not set to run quite true upon the work, the latter will not run true when the steady rest is removed, and if the jaws of the steady rest spring the axial line of the work out of its normal straightness, the work will be turned either larger or smaller in diameter in the middle of its length, according to the direction in which the work is sprung.

Suppose, for example, that the work is sprung laterally towards the tool point, then the work will be turned smaller in the middle, or if the work were sprung laterally in the opposite direction, it would be turned larger in the middle than at the ends. If the work is sprung vertically so as to approach or recede from the lathe bed, the amount of the error will be less than if it were sprung laterally, and the nature of the error will depend upon the height of the cutting tool with relation to the work. If, for example, the point is above the centre of the work, and the latter is sprung towards the lathe bed, the work will turn of largest diameter in the middle of its length; or with the tool point placed at the centre of the work, the same result will follow, whether the work be sprung up or down; but if the work be sprung up or away from the lathe bed, and the tool point be placed above the centre, the diameter of the work will be turned smaller than that at the ends.

[Ill.u.s.tration: Fig. 810.]

When the work is to be turned from end to end or for a considerable distance, a follower rest such as shown in Fig. 810 should be employed, being similar to the steady rest shown in Fig. 802, except that it is open in front, and being fastened to the slide rest carriage, of course travels with the tool; hence the plates P may be either directly in front of the tool or following it, but if the work W has been turned true and parallel, the plates P may be in front of the tool, or rather may lead it.

[Ill.u.s.tration: Fig. 811.]

[Ill.u.s.tration: Fig. 812.]

The follower rest should always be set to the work when as near as practicable to the dead centre, in which case it will be easier to set it without springing the work.

For work of small diameter for which the plates P would be too large, and therefore in the way, the plate P, Fig. 811, may be used, being bolted to the follower rest. For work of larger diameter the device shown in Fig. 812 is sometimes used. It consists of a plate P with a cap C, and bolts for holding the bearings B, B. These bearings are bored slightly larger in diameter than the finished diameter of the work.

The advantage of the use of this device is that bearings of the requisite bore having been selected they may be inserted and adjusted a proper fit to the work before P is fastened to the follower rest, thus avoiding the liability of being either too tight or too loose as may happen when the plates cannot be moved or rotated to test the fit.

Another and great advantage is that if after the adjustment of the bearings B, B to the work, the plate P is carefully bolted to the follower rest, the liability of springing the work is eliminated, hence truer work will be produced.

[Ill.u.s.tration: Fig. 813.]

A representative of another cla.s.s of follower rest is shown in Fig. 813, the hub H is accurately bored to receive collars or rings of various diameters of bore to suit the work. The bore of H may be made to stand axially true with the lathe centres, and thus avoid the trouble of setting, by employing the steady pin S, which, being a close fit in the follower rest and in the lathe carriage will bring the rest to its proper distance from the lathe centres, where it may be secured by the bolt B, which may screw into the metal of the carriage or operate to lift a wedge or guide slip so as to grip the [V]-slide of the carriage and take up any lost motion between the slide in the rest and that in the lathe carriage.

[Ill.u.s.tration: Fig. 814.]

Fig. 814 shows a follower rest in position on the cross slide of a lathe.

CHUCKS AND CHUCKING.

There is a large cla.s.s of small work that could be held between the lathe centres, but that can be more conveniently held in chucks. Chucks are devices for holding work to the live spindle, and may be divided into cla.s.ses as follows:

1st. Those in which the work is secured by a simple set-screw.

2nd. Drill chucks, which are applied mainly to drive drills, but which may also be used to drive very small work to be operated upon by cutting tools, the mechanism causing the jaws to move simultaneously to grip or release the work.

3rd. Independent chucks, in which the jaws are operated separately.

4th. Universal chucks, which are larger than drill chucks, and in which the jaws operate simultaneously.

5th. Combination chucks, in which the jaws may be operated either separately or simultaneously as may be required.

[Ill.u.s.tration: Fig. 815.]

Referring to the first, Fig. 815 represents a simple form of set-screw chuck, the stem S fitting into the live centre hole, and the outer end being pierced to receive a drill shank, and the iron from which a piece of work may require to be turned, which is secured in the chuck by the set-screw B. In the case of drill or other cutting tools, however, it is better that they be provided with a flat place A, to receive the set-screw pressure, and enable it to hold them more securely. The objections to this cla.s.s of chuck are threefold: First, each chuck is suitable for one diameter of work only; secondly the screw head B is in the way; and thirdly, the set-screw pressure is in a direction to set the work out of true, which it will do unless the work is a tight fit to the bore of the chuck. In this case, however, it is troublesome to insert and remove the drill, unless the bore of the socket is relieved on the half circ.u.mference nearest to the set-screw, as shown at C in the end view, in which case the efficiency of the chuck is greatly enhanced.

[Ill.u.s.tration: Fig. 816.]

Referring to the second cla.s.s they are made to contain either two or three jaws.

[Ill.u.s.tration: Fig. 817.]

When two jaws are employed they are made to slide in one slideway, and are operated therein by a right and left-handed screw, causing them to simultaneously advance or recede from the chuck axis. Fig. 816 represents a chuck of this cla.s.s, the jaws fitting one into the other to maintain each other in line, and prevent their tilting over from the pressure.

In scroll chucks the mechanism for operating the jaws is constructed upon two general principles. The first may be understood from Fig. 817, in which the body of the chuck is provided upon its end face with a scroll C, with which the ends of the jaws A engage. These jaws fit into radial slots in the sh.e.l.l E, which is capable of rotation upon B and is held thereto by the cap D; hence rotating E carries around the jaws A, and the thread C causes them to approach or recede from the chuck axis, according to their direction of rotation.

[Ill.u.s.tration: Fig. 818.]

The second general principle upon which small drill chucks are constructed may be understood from Fig. 818, in which C may be taken to represent the end of a lathe spindle or a stem fitting into the live centre hole in the same. At the other end it is to receive the sh.e.l.l D which screws upon it. D is coned at the outer end of its bore, and the jaws E are made to fit the cone, and it is obvious that if D be rotated to screw farther upon C, the coned bore of D will act to force the jaws E nearer to the chuck axis and cause them to close upon and grip the work. To operate D it is knurled or milled at G, or it may have pin spanner holes as at H. In this cla.s.s of chuck it is essential that the direction of rotation of D to close the jaws must be opposite to that in which the drill rotates, otherwise the resistance of the work against the jaws would cause D to rotate upon C, and the work to become released from the jaw grip. Furthermore, as the larger the work the more severe the duty in driving it, it is usually provided by the construction of such chucks that the jaws shall be opened to their maximum when at their nearest approach to the body (as C) of the chuck, and shall close as they move outward or away from the same. This principle of moving the jaws radially by means of a cone sliding upon a cone is applied in numerous ways, thus sometimes the jaws are provided with wings that slide upon a cone or in slide ways that are at an angle to the chuck axis.

[Ill.u.s.tration: Fig. 819.]

[Ill.u.s.tration: Fig. 820.]

[Ill.u.s.tration: Fig. 821.]

Figs. 819, 820, and 821 represent Gage's patent chuck, in which the gripping surfaces of the jaws are serrated to increase the grip, and to further secure the same object the jaws move at an angle instead of in a radial line, so that the body of the jaws is more directly in the line of strain, and therefore resists it better. The serrations are left-handed, so that the tendency is to force the drill forward and toward the cut, supposing them to act as a nut and screw upon the drill shank. The jaws are supported by the central cylindrical piece that contains them out to the extreme end, and have in addition a lug which slides in radial grooves. Fig. 819 is a side elevation, with a piece of the sh.e.l.l removed to show the jaw and its slide way, and an end view showing the arrangement of the jaws. Fig. 820 is a sectional side elevation, and Fig. 821, two views of the jaws removed from the chuck; A represents the jaws with the lug E to slide in the radial slots provided in B. The wings A' of the jaws slide in the ways in B, the ways pa.s.sing through the opening F in Fig. 821; C is the cone for causing the jaws to open and close radially. The driving piece H has A left-hand thread operating in B. It also has a collar ab.u.t.ting over one side against the end of B, and secured on the other by the cap I, which threads into the sh.e.l.l G. A pin in C secures it to the cap I, so that if rotated both move together. On the other hand, if H be rotated and G is held stationary, the thread on H operates on B as a nut, causing it to slide, carrying the jaws with it, and the jaws are simultaneously opened or closed according to the direction of rotation of H. Fig. 819 shows the jaws screwed partly out, and therefore partially closed, while in Fig.

820 the jaws are shown within the chuck, and therefore opened to their fullest extent.

[Ill.u.s.tration: Fig. 822.]

Figs. 822 and 823 represent a chuck employed by the Hanc.o.c.k Inspirator Co., of Boston, for very true work. This chuck will not get out of true by wear, and holds bra.s.s work against a good lathe-cut without indenting it.

Fig. 822 shows the chuck complete. Fig. 823 is a mid-section of chuck complete. Fig. 824 is a side and an end of the work-gripping piece. The chuck is composed of three pieces, A, B and C. Piece A screws upon the lathe spindle and is bored to receive C; piece B screws upon A and receives the outer end of C, which is provided with a double cone D E, and is split nearly its full length at three places, one of which is shown at F, so that when B is screwed upon A the two cones upon A, B compress C, and cause the diameter of its bore to decrease and grip the work. The splits F are made long, so that C shall not close at its outer end only, but on both sides of the cones, and thus grip the work parallel.

There are several advantages in this form of construction; thus the parallel bore of A, in which C fits, is not subject to strain or wear, and therefore remains true and holds C true. Furthermore, B has no tendency to wear out of true, because it fits upon A at the part G, as well as at its threaded end, while the cone E of C also acts to keep it true. As B is screwed up with a wrench fitting its hexagon exterior, the work can be held against any amount of cut that the lathe will drive.

[Ill.u.s.tration: Fig. 823.]

It is obvious that the capacity of the chuck, so far as taking in range of different diameters, is quite limited, but the excellence of its execution far more than compensates for this when work is to be turned out true and correct to standard gauge.

To increase the range of capacity of the chuck, the split piece only needs to be changed. Before hardening the split piece the jaws should be sprung well apart, so that they will spring open when released by uns.c.r.e.w.i.n.g the outside sh.e.l.l to release the work and insert another piece.

[Ill.u.s.tration: Fig. 824.]

In proportion as the diameter of the work is increased it requires to be more firmly held, and the chucks are made with jaws moved by screws operated by wrench power. These chucks are made with two, three, or four jaws, and the bite of the jaw is shaped to suit the nature of the work, the gripping area being reduced for very small work, and serrated parallel to the chuck axis so as to form gripping teeth for firmly gripping rough work, as shown in some of the following examples:--

[Ill.u.s.tration: Fig. 825.]

[Ill.u.s.tration: Fig. 826.]

Figs. 825 and 826 represent the Horton two-jawed chucks with false or slip jaws, which are removable so that jaws of various shapes in the bore may be fitted to the same chuck, thus enabling the jaws to be varied to suit the shape of the work to be held. The jaws are secured in place by the pins shown.

[Ill.u.s.tration: Fig. 827.]

Fig. 827 shows a two-jawed solid jaw chuck, the bite of the jaws being made hollow, so as not to mark the surface of the work, while they will hold it very firmly.

In Fig. 828 is shown what is termed a box-body two-jawed chuck, which is mainly used by bra.s.s turners. The object of this form of body is to permit the f.l.a.n.g.es, &c., of castings escaping the face of the chuck.