Modern Machine-Shop Practice Part 57

[Ill.u.s.tration: Fig. 746.]

Fig. 745 represents a lathe dog, driver, or carrier D, in position to drive a piece of work in the lathe. It is obvious that the work is secured within the carrier or driver by means of the set-screw shown.

The tail of the driver here shown is bent around to pa.s.s within the slot provided in the face plate, a plan which is convenient, but is objectionable, because in this manner of driving the work two improper strains are induced, both of which act to spring or bend the work. The first of these strains is caused by the carrier being driven at a leverage to the work, as shown at A in the figure, which causes the live centre to act as a fulcrum, from which the work may be bent by the strain caused by the cut.

[Ill.u.s.tration: Fig. 747.]

The second strain is caused by driving the carrier from one side or end only, and is shown in Fig. 746, where the dog receives the face-plate pressure at the point A, and the cut or resistance being on the opposite side of the work, the leverage of the driving point causes a tendency to lift the work in the direction of the arrow C. The direction of this latter strain, however, varies as the work revolves. For example, in Fig. 747 the dog is shown in position at another point in its revolution, and the point A, where the power is applied to the carrier, is here on the same side as the tool cut; hence there is less tendency to spring the work. It becomes obvious then, that work driven in this manner will be liable to be oval, or out of round, as it is commonly termed.

[Ill.u.s.tration: Fig. 748.]

The methods of overcoming these two sources of error are as follows: Instead of the end of the dog being bent around to pa.s.s within the slot in the face plate, as in Fig. 745, the leverage A in that figure may be avoided by the means shown in Fig. 748, in which a driver having straight ends is used, and a pin P is fastened to the face plate to drive the carrier. But this does not remove the tendency (shown in Fig.

746) acting to spring the work from the pressure of the cut; hence, to obviate this latter tendency, two driving-pins P P, in Fig. 749, are sometimes used with the idea of driving the work from both sides, and thus equalizing the strain. But this is effective only when each pin is in working contact with the dog. This condition is difficult to secure for several reasons. First, suppose the two ends of the carrier to be of equal thickness, and the driving-pins to be of equal diameter, while the work receiving hole of the carrier is quite central to these two ends, then the work also must be true, in order to cause the pins to act equally on the ends of the carrier. Hence, this method is only applicable, even if all the above conditions be fulfilled, to the finishing cuts, and these would have to be taken on work that had been sprung in the roughing cuts, so that it would be difficult to obtain accurate results. A nearer approach to correctness is therefore sought by various means. Thus, Fig. 750 represents a face plate provided with an annular [T]-groove, having a cut at H to admit two nuts into which the pins P are screwed. These pins may be tightened lightly, so that they will slip under the pressure of the roughing cut, and thus come to an equal bearing upon the carrier or work, as in case of the arms of a pulley where a carrier is not used. When the pins have adjusted themselves to have as near as may be an equal driving bearing, they may be tightened up. By this means the pins are compelled to act at an equal leverage upon the carrier or work, but there is no a.s.surance of an equal degree of pressure of the pins P.

Another method is shown in Fig. 751, in which a clamp in two parts is employed, the driving-pins P fitting into two holes equidistant from the lathe centre, while loosening one bolt, J or K, and tightening the other is resorted to, to equalize the driving contact on the two arms, but in this case again there is no certainty that the two pins will drive equally, and there is danger of drawing the work somewhat out of true.

Another form is shown in Fig. 752, the idea being to equalize the pressure of the driving pins, by means of the four screws, but here again, there is no means of knowing whether the driving pressure is equalized.

[Ill.u.s.tration: Fig. 749.]

The best form of driver is shown in Fig. 753, which represents a Clement's driver. The driving-plate F has four slots; two of them, A and B, pa.s.s entirely through this plate to admit bolts C D, which have a shoulder, so that they may be secured firmly to the lathe face plate, but which are an easy fit in the plate F, so as to permit it to move upon the lathe face plate. The other two are [T]-shaped slots to receive nuts, into which the pins P P are to be screwed. The bolts C D drive F, and the pins P drive the work, the freedom of the plate E to move upon the lathe face plate permitting this strain-equalizing action of the driving-plate and driving-pins.

[Ill.u.s.tration: Fig. 750.]

[Ill.u.s.tration: Fig. 751.]

[Ill.u.s.tration: Fig. 752.]

Sometimes, as in cutting screws, the work requires to be revolved backwards, without having any lost motion between the arm and carrier, or in other words, the carrier must revolve backwards as soon as the face plate does. To accomplish this, a common plan is to tie the driver or carrier to the driving-pin, but a better plan is to employ a bent tailed dog and secure its end in the face-plate slot. A convenient form of face plate for this purpose is shown in Fig. 754, A, B, C, and D, being slots, and E a set-screw for binding the dog as shown in Fig. 755.

[Ill.u.s.tration: Fig. 753.]

For special lathes in which the work is of uniform diameter, the driving pins P, Fig. 753, may be replaced by solid jaws, thus in Fig. 756 is a Clement driver, such as is used on axle lathes, C C being driving lugs in place of the pins P in figure.

To prevent the ends of the set-screw or screws of the driver from damaging the surface of finished work, the form of driver shown in Fig.

757 has been patented in England. It consists of a disc arched to receive a lever C, which is pivoted in the disc at D. A set-screw provided in the disc binds one end of the lever to the work, and as the pressure to drive the work is applied at the other end of the same lever, it serves to a.s.sist (to some extent) the set-screw in binding the lever to the work. The work is held between a [V] in the disc and one on the lever, the object being to provide a large area of contact, and thus prevent the damage to finished work which screw ends are apt to cause.

[Ill.u.s.tration: Fig. 754.]

The same end may be obtained for ordinary drivers by using a copper or bra.s.s ring, such as shown in Fig. 758, which may be opened or closed, within certain limits, to suit the diameter of the work, being placed on the end of the work, and within the dog, to receive the pressure of the set-screws.

[Ill.u.s.tration: Fig. 755.]

One such ring will serve for several diameters of work, springing open when forced, under hand pressure, upon the work, or closing upon the work as the pressure of the dog set-screw is received. It is obvious that the split of the ring should be placed diametrally opposite to the dog set-screw.

[Ill.u.s.tration: Fig. 756.]

In very small lathes the driver is sometimes driven by the device shown in Fig. 759, which consists of a small chuck, screwed on the live spindle, and containing the live centre and a driving arm B, which pa.s.ses through the chuck, and is set to any required distance out, by the set-screw C. The objection to this is, first, that either the live centre must be very short, or the arm B must be very long; and, second, if the chuck wears out of true, it carries the live centre also out of true; hence this cla.s.s of driver is but little used, even in foot lathes.

[Ill.u.s.tration: Fig. 757.]

[Ill.u.s.tration: Fig. 758.]

In small drivers of this kind it is sometimes the practice to cut away rather more than one quarter of the thread on each side of the live spindle as shown in Fig. 760 at A, and to then cut away one quarter of the thread on each side of the bore of the driver as at B in the figure.

This enables the driver to be pa.s.sed upon the spindle and screwed home with one quarter of a turn, thus saving time in putting on and taking off the driver.

Fig. 761 ill.u.s.trates a work driver very convenient for turning bolts. It consists of a piece of iron or plate P bolted to the lathe face plate F, and having jaws so as to fit to the sides of the bolt B and drive it.

This not only saves the time that would otherwise be required to put on a driver or carrier but leaves the underneath face of the bolt clear to be faced up by the turning tool, an example of its use being shown in connection with the knife tool or facing tool.

[Ill.u.s.tration: Fig. 759.]

[Ill.u.s.tration: Fig. 760.]

Fig. 762 represents a driver of this kind having a sliding jaw so that it may be set for different sizes of bolt heads. When the driving end of the work is threaded an ordinary dog or driver cannot be used because its screw would damage the thread on the work. A common method of overcoming this difficulty is to place over the ring a split ring of copper, or to place on it two nuts, putting a common dog on the end nut.

It is better, however, to use a driver, threaded part of the way through, as in figure 762 (from _The American Machinist_) and to screw it upon the work.

[Ill.u.s.tration: Fig. 761.]

Fig. 763 represents a very useful form of work driver designed by Mr.

William A. Lorenz. It consists of two jaws A, A held together by two screws, and threaded to receive two driving screws D, E in the figure, which enable it to be used to hold work to the live centre as is necessary when using the steady rest, as is shown in the figure, in which B represents the work and C the jaws of the steady rest. It is obvious that the dog may be thus employed to chuck work independently of the steady rest, because the live centre may be removed, and the face of the work held against the face of the chuck, the short screws H being used instead of the long ones D, E.

[Ill.u.s.tration: Fig. 762.]

If the carrier is used to simply drive the work without clamping it to the live centre or face plate, one or both of the screw pins J, K may be used in place of bolts D, E, the carrier being balanced when both are used.

[Ill.u.s.tration: Fig. 763.]

[Ill.u.s.tration: Fig. 764.]

Fig. 764 represents a driver, carrier, or dog threaded in its bore to drive threaded work, which the screw of the ordinary dog would obviously damage.

[Ill.u.s.tration: Fig. 765.]

[Ill.u.s.tration: Fig. 766.]

[Ill.u.s.tration: Fig. 767.]

Fig. 765 represents an excellent driver for cored work such as the piece W. Its hub A is screwed on the live spindle in place of the face plate, and carries the rods B, B', both of which are adjustable in the distance they stand out from A, so that B may be set to suit the work, and B' set out sufficiently to balance B and D. The driving arm D is adjustable along B, and by being bent to the form shown is more out of the way, and obviates the necessity of using a dog on many kinds of work. The other end of the work is shown supported by a cone centre C, whose construction is shown in Figs. 766 and 767. Its object is to avoid the wear that occurs at the mouth of the hole in cored work, when it is run on the dead centre, and to avoid the necessity of plugging the hole to provide a temporary centre. In the figures, A represents a stem (fitting into the tailstock spindle S, in place of the ordinary dead centre), having a collar B and carrying the cone C. The work is supported upon C, which revolves upon the stem of A. At E is a raw-hide washer, intended to prevent the abrasion which would occur on the faces of B and C. The pin F prevents C from coming off D, one half of its cross section being in C, and the other half in a semicircular groove running around D. An oil groove is provided through the collar B, and pa.s.ses along the stem D. This is an exceedingly handy device for cored work, and may also be used to sustain work against the lathe face plate, while chucking the work true by its bore.

[Ill.u.s.tration: Fig. 768.]

The work drivers employed by wood turners, for work held between the lathe centres, are as follows:--

Fig. 768 represents two views of a fork centre to be placed in the cone spindle of the lathe, and serve as a live centre, while also driving the work; C is a sharp conical point, which should run true, because it serves to centre the work; D, E are two wings which enter the wood to drive it. This device answers well for work that can be finished without taking it in and out of the lathe, it being difficult to place the work in the lathe so as to run true after removal therefrom; in case, however, that this should become necessary, the work should be replaced so that each wing falls into its original impression. For heavy work this device is unsuitable, hence the two plates shown in Fig. 769 are employed, being termed centre plates. They are composed of iron and are held to the work by screws pa.s.sing through the respective holes shown at the corners of the plates. The plate having the round centre hole is for the dead centre end of the work, while that having the rectangular slot is for the live centre end of the work. The rectangular slot is made a close fit to the wings of the fork centre shown in figure. Figs. 770 and 771 represent a spur centre designed to hold pieces of soft wood, that may be liable to split from the pressure of the centres. The spurs are made parallel on their outer surfaces, while the inner ones are at an angle, so as to close the wood around the central point, and not spread the wood outwards. The plate for the dead centre is formed on the same principle as is shown in figure 769.

[Ill.u.s.tration: Fig. 769.]

[Ill.u.s.tration: Fig. 770.]

Another form of chuck centre or driving centre for wood work is shown in Fig. 772, being especially useful when the work cannot be supported by the lathe dead centre. The body A screws on to the thread on the live spindle of the lathe, while the work screws on the pointed screw B, which will hold disc-shaped pieces of moderate diameter, as about 4 or 5 inches, leaving its face to be operated on as may be desired. To prevent B from splitting the work, or when hard wood is to be turned, a small hole may be bored up the work to permit B to enter sufficiently easily.