Modern Machine-Shop Practice Part 61

Fig. 829 also represents a two-jawed chuck, the body being cylindrical, and having a [V]-groove at A to receive the work. The screws C, D may act independently of each other, or a continuous screw may be used, having, as in the figure, a left-hand thread at C, and a right-hand one at D, so that the jaws move simultaneously when the screw is operated.

The difference between these two methods being as follows:--

When one screw is used the jaws will hold the work so that the centre of rotation will be midway between the points of contact of the jaws of the chuck and the work, hence work cannot be set eccentrically, unless pieces of iron are inserted between it and one of the jaws. When two screws are used the jaws may be operated separately, and one jaw may be set to such distance from the centre of rotation as the necessities of the work may require; but in this case more adjustment is required to set either square or cylindrical work to rotate on its axis than when the jaws operate simultaneously as with a right and left-hand screw. It is obvious that the axial line of the screw or screws must stand parallel with the plane of the face F. It will be observed that the back of each jaw is cut away at B: this serves two purposes, first it permits of a piece of work having a small f.l.a.n.g.e, head or projection being held in the [V]s of the jaws; and secondly, it equalizes the wear on the jaws of the chuck, because in jaw chucks generally there is more wear at the outer than at the inner end of the jaws, because work shorter than the length of the jaws, or requiring to be held as far out from the jaws as possible, does not have contact at the back end of the work holding jaw faces, hence the jaws are apt to wear, in course of time, taper. By cutting away the jaws at the back, the tendency to unequal wear is greatly reduced, hence this plan is adopted to a more or less degree in the dogs or jaws of all chucks, being in many cases merely a small recess from 1/16 to 1/8 inch deep only.

[Ill.u.s.tration: Fig. 828.]

When the jaws have a [V]-groove as in the cut, the face F of the chuck does not form a guide in setting the work, the truth of the [V]-grooves being solely relied upon for that purpose.

[Ill.u.s.tration: Fig. 829.]

[Ill.u.s.tration: Fig. 830.]

The form of two-jawed chuck shown in Fig. 830 is intended for square or rectangular work, and is mainly used by wood workers. It may be operated by a right and left-hand screw, but is generally preferred with independent screws. The face F of the chuck may be employed to serve as a guide in setting the work as shown in the cut, in which W represents a piece of work held between the jaws A, A, and resting against the face F, which therefore serves as a guide against which to set the work to insure that its axial line shall stand parallel with the face F, or in other words at a right angle to the line of centres of the lathe.

[Ill.u.s.tration: Fig. 831.]

In Fig. 831 is an example of a machinist's two-jawed chuck. The jaws are operated simultaneously by a right and left-hand screw. The jaws are provided with slides to receive the two separate pieces shown in figure, which may be made to suit the form of special work. The two screws shown on each side of the chuck face are to support a piece of work that is too large to be otherwise held firmly by the chuck. These screws may be operated by screw-driver wrench, to enable the face of the work to rest on them, and therefore be supported parallel or true with the chuck face. The jaws may be turned end for end in their slide ways as shown in Fig. 833, to enable them to grip work of small diameter, the separate pieces shown in Fig. 832, being placed on the jaws for such small pieces as drills, &c.

[Ill.u.s.tration: Fig. 832.]

In the larger sizes, lathe chucks are provided with either three or four jaws, which are caused to operate either independently or simultaneously, and in some cases the construction is such that the same chuck may be used as an independent or as a universal one at will, in which case they are termed combination chucks. Concerning the number of jaws it may be observed that a three-jawed chuck will hold the work with an equal pressure on all three jaws, whether it be cylindrical or not, but in a four-jawed chuck the jaws will not have an equal grip upon the work, unless the same be either cylindrically true or square, hence it is obvious that a three-jawed chuck is less liable to wear out of true, and is also preferable for holding unturned cylindrical work, while it is equal to a four-jawed one for true, but unsuitable for square work.

[Ill.u.s.tration: Fig. 833.]

[Ill.u.s.tration: Fig. 834.]

Fig. 834 represents the construction of the Horton chuck. Upon the screws that operate the jaws are placed pinions that gear into a circular rack, so that by operating one jaw with a wrench the rack is revolved and the remaining jaws are operated simultaneously. The chuck being constructed in two halves, the rack may be removed and the jaws operated separately, or independently as it is termed.

[Ill.u.s.tration: Fig. 835.]

Fig. 835 represents one of the jaws with its operating screw and pinion removed from the chuck. The gripping surfaces of the steps in the jaws are serrated to increase their grip upon the work, and the nuts A, A, against which the works rests, are ground true with the face of the chuck. The corner between the faces A and the bite or gripping surfaces of the jaws are recessed so that the work cannot bind in them, but will bed fairly against the faces A, A, which serve to set the work against and hold it true instead of the face of the chuck.

[Ill.u.s.tration: Fig. 836.]

Fig. 836 represents a Horton chuck for work up to four inches diameter.

[Ill.u.s.tration: Fig. 837.]

Fig. 837 represents a similar chuck for all sizes between 4 and 15 inches, the designated sizes of the chuck being 6, 9, and 12 inches, these diameters being the largest the chucks will take in.

[Ill.u.s.tration: Fig. 838.]

Fig. 838 represents a Horton chuck with outside bites for opening out to grip the bores of rings or other hollow work.

The term scroll chuck is applied to universal chucks in which the jaws are operated throughout their full range by means of a scroll thread such as was shown in Fig. 817. The objection to this form is that the threads on the jaws cannot be made to have a full bearing in the scroll thread.

[Ill.u.s.tration: Fig. 839.]

In Fig. 839, for example, let A A and B B represent grooves between the scroll threads, and if the thread on the jaws be made to the curve and width of A A, it would not pa.s.s in that of B B, and _vice-versa_, and it would take but five revolutions of the thread to pa.s.s a nut thread from A to B. To overcome this difficulty the jaw threads are not made correct to either curvature but so formed as to fit at points C, D, E, when in the groove A and at points F, G, H, when in groove B. This obviously reduces their bearing area and therefore their durability. To avoid this defect the jaws of many universal chucks are operated by screws in the same way as independent jaw chucks, but provision is made whereby the operation of any one of the jaw screws will simultaneously operate all the others, so that all the jaws are moved by the operation of one screw.

Thus in the following figures is shown the Sweetland chuck.

[Ill.u.s.tration: Fig. 840.]

Fig. 840 represents the chuck partly cut away to show the mechanism, which consists of a pinion on each jaw screw, and a circular rack beneath. The rack is shown in gear with a pinion at O, and out of gear with a pinion at C, which is effected as follows:--

The rack is stepped, being thicker at its outer diameter, and the thin part forms a recess and the shoulder between the thick and thin part forms a bevel or cone. Between this circular rack and the face of the plate at the back of the chuck is placed, beneath each jaw, a cam block bevelled to correspond with the bevelled edge of the recess in the ring.

The cam block stem pa.s.ses through radial slots in the face of the chuck, so that it can be moved to and from the centre of the chuck. When it is moved in, its cam head pa.s.ses into the recess in the ring rack, which then falls out of gear with the jaw screw pinion; but when it is moved outward the cam head slides (on account of the bevelled edges) under the ring rack and puts it in gear with the jaw screw pinion. Thus, to change the chuck from an independent one to a universal one all that is necessary is to push out the bolt heads on the cam block stems, the said heads being outside the chuck. The washers beneath these heads are dished to give them elasticity and enable them to steady the cams without undue friction.

[Ill.u.s.tration: Fig. 841.]

To enable the setting of the jaws true for using the chuck as a universal one, after it has been used as an independent one, a ring is marked on the face, and to this ring the edges of all the jaws must be set before operating the cams radially to put the rack ring in gear. In Fig. 841 a three-jawed chuck on this principle is shown acting as an independent one to hold an eccentric. On account of the spring of the parts, which occurs when the strain is transmitted from one part to another, it is desirable when using the chuck as a universal one to first operate one screw to grip the work and then pa.s.s to the others and operate them so that they may receive the pressure direct from the screw head and not entirely through the medium of the rack, and there will be found enough movement of the screws when thus operated to effect the object of relieving the rack to some extent from strain.

[Ill.u.s.tration: Fig. 842.]

[Ill.u.s.tration: Fig. 843.]

[Ill.u.s.tration: Fig. 844.]

Figs. 842, 843, 844, and 845 represent Cushman's patent combination chuck, in which each jaw may be operated independently by means of its screw thread, or a circular rack may be made to engage with the respective pinions, as shown in Fig. 844, in which case operating any one of the screws operates simultaneously all the jaws. The method of engaging and disengaging is shown in Fig. 845. C represents the circular rack and D a circular ring beneath it. This ring is threaded on its circ.u.mference, s.c.r.e.w.i.n.g into the body of the chuck, so that revolving it in one direction moves the circular rack forward and into mesh with the pinions, while revolving it backward causes the rack to recede from the pinions. To operate this ring the lug shown near the top of the chuck in figure is simply pushed in the required direction, while to lock the ring when out of gear with the pinions the spring catch shown on the left of that figure is moved radially. When the rack is in gear, the chuck is a universal one, all the jaws moving simultaneously and equally, whether they be set in such position in their slots as may be necessary to grip an oval or round piece of work; when the rack is out of gear the jaws may be moved by their respective screws so as to run true as for round work, or to hold the work to any degree of eccentricity required.

[Ill.u.s.tration: Fig. 845.]

The jaws may be reversed in their slots and operated simultaneously as a universal chuck, or independently as a simple jaw chuck.

It is obvious that the truth of the jaws for concentricity may be adjusted within the degree of accuracy due to the number of teeth in one pinion divided into the pitch of the jaw operating screw, because each screw may be revolved separately to bring each successive tooth into mesh until the greatest obtainable jaw truth is secured.

[Ill.u.s.tration: Fig. 846.]

Fig. 846 represents a front, and Fig. 847 a sectional view, of the Westcott combination chuck. F is the main body of the chuck s.c.r.e.w.i.n.g on to the lathe spindle. F carries the annular ring D, which has a thread on its face, as shown. D is kept in place by the ring E, which screws in an annular recess provided in the back of the chuck. C is a box fitting in the radial slots of the chuck. The back of the box C meshes into the radial thread on D, hence, when D is revolved, the boxes C move radially in the slots. Now the boxes C afford journal bearing to, and carry the worm or screws B as well as the chuck jaws A, hence revolving D operates the jaws simultaneously and concentrically as in a scroll or universal chuck. By means of the screws B, the jaws may be operated individually (the boxes C and ring D remaining stationary) as in an independent jaw chuck.

[Ill.u.s.tration: Fig. 847.]

Suppose, now, the jaws to have been used independently, and that they require to be set to work simultaneously and concentric to the centre of the chuck, then the screws B may be operated until the jaws at their outer edge are even with the circ.u.mference of the chuck (or, if the jaws are nearer the centre of the chuck, they may be set true with a pointer), and the ring D may be operated. In like manner, if a number of pieces of work are eccentric, the screws B may be used to chuck the work to the required eccentricity, and when the next piece is to be chucked the ring D may be operated, and the chuck will be used as a universal one, although the shape of the work be irregular, all that is necessary being to place the same part of the work to the same jaw on each occasion.

[Ill.u.s.tration: Fig. 848.]

The faces of the jaws of jaw chucks when they are true with the face of the chuck (or what is the same thing, run true, and are at a right angle to the axial line of the lathe centres), form guides wherefrom to set the work true, but this will only be the case when they remain true, notwithstanding the pressure of the jaws upon the work. Their truth, however, is often impaired by their wear in the chuck slots which gives them play and permits them to cant over. Thus in Fig. 848 is shown a chuck gripping a piece of work W, and it is obvious that to whatever extent the jaws may spring, or have lost motion in the ways or slots in the chucks, the jaws will move in the direction of the dotted lines A A, the face of the jaw then standing in the direction of dotted lines B B, instead of being parallel to the chuck face. If the spring or wear of the mechanism were equal for each jaw, the work would be held true, notwithstanding that the jaws be out of line, but such is not found to be the case, and as a result the work cannot be set quite true.

[Ill.u.s.tration: Fig. 849.]

When the jaws are applied within the work, as in Fig. 849 (representing the jaws of the chuck within the bore of a ring or piece of work W), the jaws spring in the opposite direction as denoted by dotted lines C, C, and when the jaws are locked to the work the latter moves in the direction of D and away from the chuck face. It will be observed that there is no true surface to put the face of the work against in either case.

[Ill.u.s.tration: Fig. 850.]

This is remedied in independent dog chucks by the construction shown in Fig. 850, in which each jaw has a square A, fitting in the grooves of the chuck, and a nut and washer at B secure the jaw to the face of the chuck so that the lost motion due to wear of the parts may be taken up.

[Ill.u.s.tration: Fig. 851.]