Chapter VIII: Special Forms of the Lathe (1)

The lathe is made in many special or limited forms, to suit particular purposes, the object being to increase its efficiency for those purposes, which necessarily diminishes its capacity for general work.

In addition to this, however, there are machine tools whose construction varies considerably from the ordinary form of lathe, which nevertheless belong to the same family, and must, therefore, be classified with it, because they operate upon what is essentially lathe work. Thus boring and turning mills are essentially what may be termed horizontal lathes.

Figs. 650 to 655 inclusive, represent the American Watch Tool Company's special lathes for watch-makers, which occupy a prominent position in Europe, as well as in the United States.

In lathes of this class, refinement of fit, alignment, truth, and durability of parts are of the first importance, because of the smallness of the work they perform, and the accuracy to which that work must be made. Furthermore, such lathes must be constructed to hold and release the work as rapidly as possible, because in such small work the time occupied by the tools in cutting is less, while that occupied in the insertion and removal of it is greater in comparison than in larger jobs; it often takes longer to insert and remove the work than to perform it.

These facts apply with equal force to all such parts as require the removal to or from the lathe-bed, or frequent adjustment upon the same. Thus the devices for holding and releasing the tool post or hand rest and tailblock are each so constructed that they may be set without the use of detached wrenches.

Fig. 650 represents a general view of the lathe, while Fig. 651 represents a sectional view of the headstock. The live spindle consists of two parts, an outer sleeve A A, having journal bearing in the head, and an inner hollow spindle B B, threaded at its front end _e_, to receive the chucks. The main spindle at the front end works in a journal box _c_, that is cylindrical to fit the headstock, but double coned within to afford journal bearing to the spindle A. The inner step of this double cone is relied upon mainly to adjust the diametral fit of the bearing, while the outer step is relied upon mainly to adjust the end fit of the spindle; but it is obvious in both cases there is an action securing simultaneously the diametral and the end fit. In the back bearing there are two cones. The outer one _r_ is cylindrical outside where it fits into the head, and coned in its bore to receive the second cone _s_, which rotates with spindle A. The nut F is threaded upon A, so that by operating F, A is drawn within _c_, and S is simultaneously moved within _r_, so that both bearings are simultaneously adjusted. D D are _dust_ rings, being ring-caps which cover the ends of the bearings and the oil holes so as to prevent the ingress of dust.

The inner spindle B has a bearing in A at the back end to steady it, and a bearing at end _e_, and is provided with the hand wheel H, by which it may be rotated to attach the chucks which screw into its mouth at _e_. To rotate or drive the chucks there is in A a feather at _g_, the chucks having a groove to receive this feather and screwing into B at E, when B is rotated.

The mouth of A is coned, as shown at _h_, and the chucks are provided with a corresponding male cone, as shown at _h_ in Figs. 652 and 653, so that the chucks are supported and guided by the cone, and are therefore as close to the work as possible while having a bearing at _g_. But the cone on the chucks being split, (as is shown in Fig. 652), rotating B while holding A stationary (which may be done by means of the band pulley P), causes the chucks to move endwise in A, and if the motion is in the direction to draw the chuck within A, the cone _h_ causes the chuck to close upon and grip the work. Thus in Fig. 652 is shown a step chuck. The thread at J enters the end _e_ of B, in Fig. 651, which screws upon it. Cone _h_ fits mouth _h_ in Fig. 651, and _l_ represents the splits in the chuck, which enable it to close when the cone _h_ is drawn within the mouth _h_ of spindle A.

The chuck is employed to hold cylindrical plates or discs, such as wheels and barrels, and the various steps are to suit the varying diameters of these parts in different sizes of watches.

Fig. 653 represents a wire chuck, having the cone at _h_, and the three splits at _l_, as before, the cone-mouth _h_ closing the chuck as the latter is drawn within the spindle A.

In both the chucks thus far described, the construction has been arranged to close the splits and thus grip the circumferences of cylindrical bodies, but in Fig. 654 is shown the arrangement for enabling the chuck to expand and grip the bores of hollow work, such as rings, &c.

The outer spindle A corresponds to the outer spindle A in Fig. 651, and the inner one to spindle B in that figure. The chuck is here made in two separate parts, a sleeve V fitting in and driven by A, and a plug X fitting into a cone in the mouth of V, and screwing into the end of drawing spindle B. But while V is driven by and prevented from rotating within A by means of the feather at _g_, so likewise X is prevented from rotating within V by means of a feather _h_ fast in X and fitting into a groove or featherway in V. It follows then that when B is rotated X may be traversed endways in V, to open or close the steps Y according to the direction of rotation of B.

It will now be apparent that in the case of chucks requiring to grip external diameters, the gripping jaws of the chucks will, when out of the lathe, be at their largest diameter, the splits _l_ being open to their fullest, and that when by the action of the cones, they are closed to grip the work, such closure must be effected against a slight spring or resistance of the jaws, and this it is that enables and causes the chuck to open out of itself, when the enveloping cone permits it to do so.

But in the case of the opening or expanding chuck, the reverse is the case, and the chuck is at its smallest diameter (the splits _l_ being at their closest) when the chuck is removed from the lathe, as is obviously necessary. In reality the action is the same in both cases, for the chuck moves to grip the work under a slight resistance, and this it is that enables it to readily release the work when moved in the necessary endwise direction.

The band pulley P is fast upon A, and is provided with an index of 60 holes on its face G, and which are adjusted for any especial work by a pin Q, so that a piece of work may have marked on it either 60, 30, 20, 15, 12, 10, 6, 5, 4, 3, or 2 equidistant lines of division, each of those numbers being divisors of 60. In marking such lines of division upon the work a sharp point may be used, supported by the face of the hand rest as a guide; or a sharp-pointed tool may be placed in the slide rest to cut a deeper line upon the work. The index plates used for cutting wheels and pinions may be placed on the rear end of A, the pawl being secured to the work-bench. The wheel H is for rotating spindle B to screw the chucks on or off the same.

Fig. 650.

Fig. 651.

Fig. 652.

Fig. 653.

Fig. 654.]

Fig. 655.

Fig. 656.

Fig. 657.

Fig. 658.

Fig. 659.

Fig. 660.

Fig. 661.

Fig. 662.]

Fig. 655 represents an end view from the tailstock end of the lathe; A´ is the bed having the angles _a_ _a_ to align the heads and rests. The means of holding or releasing the tailstock, on the lathe-bed, is the same as that for holding the headstock, the construction being as follows: _b_ is the shoulder of a bolt through which passes the shaft _c_, with a lever _d_ to operate it. This shaft is eccentric where it passes through the bolt, so that by using the lever aforesaid the bolt secures or releases the head according to the direction in which it is moved. A very small amount of motion is needed for this. The standard for the hand rest is split, and a screw is used to tighten it in an obvious manner, the screw being operated by the handle _e´_. An end view of the rest, showing the device for securing the foot _h_ to the bed, is shown in Fig. 656, _f_ is a shoe spanning the bed and fitting to the bed angles _a_. Through _f_ passes the bolt _g_, its head passing into the [T]-shaped groove _h_; N´ is a hand wheel for operating bolt _g_. At S is a spiral spring, which by exerting an end pressure on washer _w_ and nut N´, pulls _g_ and the head _h_ down upon _f_, and therefore _f_ down upon the bed, whether the rest be locked to the bed or not; hence when N´ is released to remove or adjust the rest, neither dust nor fine cuttings can pass either between the rest and shoe or the shoe and the lathe-bed, and the abrasion that would otherwise occur is thus avoided.

Two qualities of these lathes are made: in the better quality all the working parts are hardened and afterwards ground true. In the other the parts are also ground true, but the parts (which in either case are of steel) are left soft for the sake of reducing the cost. In all, the parts are made to gauge and template, so that a new head, tailstock, or any other part in whole or in detail may be obtained from the factory, either to make additions to the lathe or to replace worn parts.

Two styles of slide rest are made with these lathes: in the first, shown in Fig. 657, the swivel for setting the top slide at an angle for taper turning is at the base of the top slide, hence the lower slide turns all radial faces at a right angle to the line of lathe centres. In the second, Fig. 658, there is a third slide added at the top, so that the bottom slide turns radial faces to a right angle with the line of lathe centres, the next slide turns the taper and the top slide may be used to turn a radial face at a right angle to the surface of the taper, and not at a right angle to the axis of the work. Both these rests are provided with tool post clamps, to hold tools made of round wire, such clamps being shown in position in figure 657.

Fig. 659 represents an additional tailstock for this lathe, the tail spindle lying in open bearings so that it can be laid in, which enables the rapid employment of several spindles holding tools for performing different duties, as drilling, counter-boring, chamfering, &c.

Fig. 660 represents a filing fixture to be attached to the bed in the same manner as the slide rest. It consists of a base supporting a link, carrying two hardened steel rolls, upon which the file may rest, the rolls rotating by friction during the file strokes, and serving to keep the file flat and fair upon the work.

Fig. 661 represents a fixture for wheel and pinion cutting; it is attached to the slide rest. When the cutter spindle is vertical the belt runs directly to it from the overhead counter shaft, but when it is horizontal the belt passes over idler pulleys, held above the lathe. The cutter spindle is carried on a frame, pivoted to the sliding piece on the vertical slide, so that it may be swivelled to set in either the vertical or horizontal position.

Fig. 662 represents a jewelers' rest for this lathe. It fits on the bed in the place of the tailstock, and is used for cutting out the seats for jewels, in plates, or settings. It is especially constructed so as to receive the jewel at the top and bore the seating to the proper diameter, without requiring any measurements or fitting by trial, and the manner in which this is accomplished is as follows:--

Fig. 663 is a side elevation, Fig. 664 an end elevation, and Fig. 665 a plan view of this rest, and similar letters of reference indicate like parts in each of the three figures. A is the base, held to the lathe bed by the bolt B, whose operation is the same as that already described for the head and tailstocks.

In one piece with A is the arm C, carrying at its head three gauge tongues or pieces D E F, which are adjustable by means of the screws _d_ _e_ _f_, which move the gauge tongues horizontally. Through a suitable guide I is a standard or head; pivoted to A at J J, and carrying at its top three gauge tongues K L M.

Midway between pivots J J and the ends of the gauge tongues, is the centre or tool carrying spindle O. If a piece of work, as a jewel, be placed between the tongues F and M, Fig. 664 [swinging M, and with it I (which is pivoted at J), laterally], then the point of the centre N will be thrown out of line with the lathe live spindle half the diameter of the jewel, because from J to the centre N, of O, is exactly one half of the vertical distance from J to the jewel. If then a tool be placed in the dead centre and its cutting edge is in line with the axis of spindle O, it will bore a hole that will just fit the jewel. Hence placing the jewel between the two tongues sets the diameter to which the tool will bore and determines that it shall equal the diameter of the jewel.

The object of having three pair of gauge tongues is to enable the obtaining of three degrees of fit; thus with a piece placed between D K the hole may be bored to fit the piece easily, with it placed between E L the fit may be made barely movable, while with it placed between F M the fit may be too tight to be a movable one save by pressure or driving, each degree of fit being adjusted by means of the screws _e_ _f_ _g_.

The tool is fed by moving spindle O by hand, the screw P being adjusted so that its end abuts against stop Q, when the hole is bored to the requisite depth; R is simply a guide for the piece S, which being attached to O, prevents it from rotating.

In watch manufactories special chucks and appliances are necessary to meet their particular requirements. There is found to exist, for example, in different rods of wire of the same nominal diameter, a slight variation in the actual diameter, and it is obvious that with the smaller diameters of wire the split chucks will pass farther within the mouth _h_ of A, Fig. 651, because the splits of the chucks will close to a greater extent, and the cones on the chucks therefore become reduced in diameter.

If then it be required to turn a number of pieces of work to an exact end measurement, or a number of flanges or wheels to equal thicknesses, without adjusting the depth of cut for each it becomes necessary to insure that the successive pieces of work shall enter the chucks to an equal distance, notwithstanding any slight variation in the work diameter at the place or part where it is gripped by the chuck.

To accomplish this end what is termed a sliding-spindle head is employed. In this the _outer spindle_ has the end motion necessary to open and close the chuck, the chuck having no end motion.

The construction of this sliding-spindle head is shown in Fig. 666, in which a wire chuck is shown in position in the spindles; L is the live spindle passing through parallel bearings, so that it may have end motion when the nut M is operated. The inner spindle N to which the chucks are screwed is prevented from having end motion by means of the collar _p_ and nut _q_ at the rear bearing. When nut M is rotated and N is held stationary by means of the pulley P, L slides endways, and the chuck opens or closes according to the direction in which the nut moves the spindle L.

To regulate the exact distance to which the work shall be placed within the chuck, a piece of wire rod may be placed within the hollow spindle N being detained in its adjusted position by the set screw S.

The construction whereby the nut is permitted to revolve with spindle L, and be operated by hand to move spindle L when the lathe is at rest, is as follows.

The cylindrical rim _t_ of the nut is provided with a series of notches arranged around its circumference. R is a lever whose hub envelops nut M, but has journal bearing on V. R receives the pin S, which rests upon a spiral spring T. When, therefore, S is pushed down it depresses the spring T and its end W enters some one of the notches in the rim _t_, and operates the nut after the manner of a ratchet. But so soon as the end pressure on R is released, the spiral spring lifts it and M is free to revolve with L as before. The inner spindle is driven by means of the feather G.

Pulley P has two steps Y for the belt, and a friction step _z_, around which passes a friction band operated by the operator's foot to stop the lathe quickly. This performs two functions, as follows. The thread of M is a left-hand one so that the inertia of the nut will not, when the lathe is started, operate to screw the nut back, and release the chuck jaws from the work, by moving spindle L endwise. Per contra, however, in stopping the lathe suddenly by means of the brake, there is a tendency of nut M to stop less quickly than spindle L, and this operates to unscrew nut N and release the work. To assist this R is sometimes in lathes for watch manufactories provided with a hand wheel whose weight is made sufficient for the purpose.

Figs. 667 and 668 represent a pump centre head for watch manufactories, being a device for so chucking a piece of work that a hole may be chucked true and enlarged or otherwise operated upon, with the assurance that the work will be chucked true with the hole. Suppose two discs be secured together at their edges, their centres being a certain distance apart, as, for example, a top and bottom plate of a watch movement, and that the holes of one plate require to be transferred to the other, then by means of this head they may be transferred with the assurance that they shall be axially in line one with the other, and at a right angle to the faces of the plates, as is necessary in setting jewels in a watch movement.

In holes of such small diameters as are used in watch work, it is manifestly very difficult to set them true by the ordinary methods of chucking and it is tedious to test if they are true, and it is to obviate these difficulties that the pump centre head is designed. Its operation is as follows.

There are in this case three spindles A, B, and C, in Fig. 667; A corresponds to spindle A in Fig. 651, driving the chuck D which screws on A as shown; B simply holds the work against the face _d_ of D, and C holds the work true by means of the centre _e_, which enters the hole or centre in the work and is withdrawn when the work is secured by spindle B.

The chuck D is open on two sides as shown at E E in Fig. 668, which is an end face view of the chuck, and through these openings the work is admitted to the chuck. The rod or spindle C is then pushed, by hand, endwise, its centre _e_ entering the hole or centre in the work (so as to hold the same axially true) and forcing the work against the inside faces _d_, spindle B is then operated, the face _p_ forcing the work against face _d_, and between these two faces _d_ _p_ the work is held and driven by friction. The spindle C and its centre _e_ is then withdrawn by hand, leaving the hole in the work free to be operated upon.

The journal bearings for spindle A are constructed as described for A in Fig. 666; spindle B is operated endways within A as follows. A is threaded at G to receive the hub H of wheel I, at the end of B is a collar which is held to and prevented from end motion within the hub H: hence when wheel I is rotated and A is held stationary (by means of the band pulley), H traverses on G and carries B with it. Operating I in one direction, therefore moves _p_ against the work, while operating it in the other direction releases face _p_ from contact with the work.

It is obviously of the first importance that the spindle C be held and maintained axially true, notwithstanding any wear, and that it be a close fit within B so as to remain in any position when the lathe is running, and thus obviate requiring to remove it. To maintain this closeness of fit the following construction is designed. Between spindle A and spindle B, at the chuck end of the two, is a steel bush which can be replaced by a new one when any appreciable wear has taken place. Between B and C are two inverted conical steel bushes, which can also be replaced by new ones, to take up any wear that may have taken place.

Fig. 669 represents an improved hand lathe by the Brown and Sharpe Manufacturing Company, of Providence, R. I. It is specially designed for the rapid production of such cylindrical work as may be held in a chuck, or cut from a rod of metal passing through the live spindle, which is hollow, so that the rod may pass through it. Short pieces may be driven by the chuck or between the centres of a face plate (shown on the floor at _e_) screwing on in the ordinary manner. When, however, this face plate is removed a nut _d_ screws on in its stead, to protect the thread on the live spindle.

The chuck for driving work in the absence of face plate _e_ (as when the rod from which the work is to be made is passed through the live spindle) may be actuated to grip or release the work without stopping the lathe. The pieces _j_ _j_ are to support the hand tool shown in Figs. 1313 and 1314, in connection with hand turning, the tool stock or handle being shown at _k_ on the floor. The lever for securing the tailstock to or releasing it from the shears is shown at _t_. The tail spindle is operated by a lever pivoted at _g_ so that it may be operated quickly and easily, while the force with which the tail spindle is fed may be more sensitively felt than would be the case with the ordinary wheel and screw, this being a great advantage in small work. The tail spindle is also provided with a collar _r_, that may be set at any desired location on the spindle to act as a stop, determining how far the tail spindle can be fed forward, thus enabling it to drill holes, &c., of a uniform depth, in successive pieces of work.

The live spindle is of steel and will receive rods up to 1/2 inch in diameter. Its journals are hardened and ground cylindrically true after the hardening. It runs in bearings which are split and are coned externally, fitting into correspondingly coned holes in the headstock. These bearings are provided with a nut by means of which they may be drawn through the headstock to take up such wear in the journal and bearing fit, as may from time to time occur.

It is obvious that the lathe may be removed from the lower legs and frame and bolted to a bench, forming in that case a bench lathe.

Fig. 670 represents a special lathe or screw slotting machine, as it is termed, for cutting the slots in the heads of machine or other screws. The live spindle drives a cutter or saw _e_, beneath which is the device for holding the screws to be slotted, this device also being shown detached and upon the floor.

The screw-holding end of the lever _a_ acts similarly to a pair of pliers, one jaw of which is provided on handle _a_, while the other is upon the piece to which _a_ is pivoted. The screw to be slotted is placed between the jaws of _a_ beneath _e_; handle _a_ is then moved to the left, gripping the screw stem; by depressing _a_, the screw head is brought up to the cutter _e_ and the slot is cut to a depth depending upon the amount to which _a_ is depressed, which is regulated by a screw at _b_; hence after _b_ is properly adjusted, all screw heads will be slotted to the same depth.

The frame carrying the piece to which _a_ is pivoted may be raised or lowered to suit screws having different thicknesses of head by means of a screw, whose hand nut is shown at _d_.

The frame for the head of the machine is hollow, and is divided into compartments as shown, in which are placed the bushings used in connection with the screw-gripping device, to capacitate it for different diameters of screws, and also for the wrenches, cutters, &c.

Figs. 671, 672, and 673, represent a lathe having a special feed motion designed and patented by Mr. Horace Lord, of Hartford, Connecticut. Its object is to give to a cutting tool a uniform rate of cutting speed (when used upon either flat or spherical surfaces), by causing the rotations of the work to be retarded as the cutting tool traverses from the centre to the perimeter of the work, or to increase as the tool traverses from a larger to a smaller diameter. If work of small diameter be turned at too slow a rate of cutting speed, it is difficult to obtain a true and smooth surface; hence, as the tool approaches the centre, it is necessary to increase the speed of rotation. As lathes are at present constructed, it is necessary to pass the belt from one step to another of the driving cone, to increase the speed. In this two disadvantages are met with. First, that the increase of speed occurs suddenly and does not meet the requirements with uniformity. Second, that the strain upon the cutting tool varies with the alteration of cutting speed. As a result, the spring of the parts of the lathe, as well as of the cutting tool, varies, so that the cut shows plainly where the sudden increase or decrease (as the case may be) of cutting speed has occurred. The greatest attainable degree of trueness is secured when the cutting speed and the strain due to the cut are maintained constant, notwithstanding variations of the diameter.

This, Mr. Lord accomplishes by the following mechanism: Instead of driving the lathe from an ordinary countershaft, he introduces a pair of cones which will vary the speed of the lathe as shown in Fig. 672 as applied to ball turning. L is a belt cone upon the counter-shaft driven from the line shaft. L drives H, which may be termed the lathe countershaft, and from the stepped cone K the belt is connected to the lathe in the usual manner. P is a shipper bar to move the belt N upon and along the belt cones, and thus vary the speed. R is a vertical shaft extending up at the end of the lathe and carrying a segment. This segment is connected to the belt shipper bar P by two cords, one passing from _r_^{1} around half the segment to _r_^{2}, and the other passing from _r_^{3} to _r_^{4}, so that if the segment be rotated, say to the right, it and the bar will move as denoted by the dotted lines, or if moved in an opposite direction, the bar motion will correspond and move the belt N along the cones respectively left or right.

At the back of the lathe is a horizontal shaft S, similar to an ordinary feed spindle, and connected to the segment shaft by a pair of bevel gears S^{2}. Between the two ears _e_ _e_, at the rear of the lathe carriage, is a pinion _t_, which drives the splined shaft S, which works in a rack T´. The tool rest is pivoted directly beneath the ball, to be turned after the usual manner of spherical slide rests, and carries a gear _a_^{2}, which, as the rest turns, rotates a gear _a_^{3}. Upon the face of the latter is a pin _a_^{4} working in a slot _a_^{5} at the end of the rack T´; hence as the tool rest feeds, motion is transmitted from _a_^{2} through _a_^{3}, _a_^{4}, _a_, T´, T, and _s_ _s_^{2} to R, which operates the belt shipper P. As it is the rate of tool feed that governs the speed of these motions, the effect is not influenced by irregularity in feeding; hence the speed of the work will be equalized with the tool feed under all conditions. The direction of motion of all the parts will correspond to that of the tool feed from which their motion is directed, and therefore the work speed will augment or diminish automatically to meet the requirements.

Fig. 673 illustrates the action of the mechanism when used for surfaces, like a lathe face plate. In this case the two gears and the rack T´ simply traverse with the cross-feed slider, and the mechanism is actuated as before. In Fig. 674 a different method of actuating the belt shipper is illustrated. A pulley is attached to the intermediate stud of the change gears, being connected by belt to the shipper, which is threaded as shown at _d_, the belt guiding forks, as _p_^{2}, being carried on a nut actuated by the screw _d_.

CUTTING-OFF MACHINE.--The cutting-off machine is employed to cut up into the requisite lengths pieces of iron from the bar. As the cutting is done by a tool, the end of the work is left true and square and a great saving of time is effected over the process of heating and cutting off the pieces in the blacksmith's forge, in which case the pieces must be cut off too long and the ends left rough.

Fig. 675 represents Hyde's cutting-off machine, which consists of a hollow live spindle through which the bar of iron is passed and gripped by the chucks C C. At G is a gauge rod whose distance from the tool rest R determines the length of the work. F is a feed cone driven by a corresponding cone on the live spindle and driving the worm W, which actuates the self-acting tool feed, which is provided with an automatic motion, which throws the feed out of action when the work is cut off from the bar. The stand S is movable and is employed to support the ends of long or heavy bars.

To finish work smooth and more true than can be done with steel cutting tools in a lathe, what are known as grinding lathes are employed. These lathes are not intended to remove a mass of metal, but simply to reduce the surfaces to cylindrical truth, to true outline and to standard diameter, hence the work is usually first turned up in the common lathe to the required form and very nearly to the required diameter, and then passed to the grinding lathe to be finished. The grinding lathe affords the best means we have of producing true and smooth cylindrical parallel work, and in the case of hardened work the only means. In place of steel cutting tools an emery wheel, revolved at high speed from an independent drum or wide pulley, is employed, the direction of rotation of the emery wheel being opposite to that of the work.

Fig. 676 represents Pratt and Whitney's weighted grinding lathe. The headstock and tailstock are attached to the bed in the usual manner, the frame carrying the emery wheel is bolted to the slide rest as shown, the rest traversing by a feed spindle motion. The carriage traverse is self-acting and has three changes of feed, by means of the feed cones shown.

To enable the lathe to grind taper work (whether internal or external) the lathe is fitted with the Slate taper attachment shown in Figs. 508 and 509.

It is obvious that in a lathe of this kind, there must be an extra overhead shaft, driving a drum of a length equal to the full traverse of the lathe carriage, or of the plate carrying the head and tailstocks, and the arrangement of this drum with its belt connection to the pulley on the emery wheel arbor, is sufficiently shown in figure. To protect the ways of the bed from the abrasion that would be caused by the emery and water falling upon them, guards are attached to the carriage extending for some distance over the raised [V]s.

It is essential that the work revolve in a direction opposite to that of the emery wheel, for the following reasons. In Fig. 677 let A represent a reamer and B a segment of an emery wheel. Now suppose A and B to revolve in the direction that would exist if one drove the other from frictional contact of the circumferential surfaces, then the pressure of the cut would cause the reamer A to spring vertically and a wedging action between the reamer and wheel would take place, the reamer vibrating back and forth under varying degrees of this wedging; as a result the surface of A would show waves and would be neither round nor smooth.

In the absence of a proper grinding lathe, an ordinary lathe is sometimes improvised for grinding purposes, by attaching to the slide rest a simple frame and emery wheel arbor with pulley attached as in Fig. 678, in which A is the emery wheel, C the pulley for driving the arbor, and B the frame, D being a lug for a bolt hole to hold the frame to the lathe rest.

In some cases the work may remain stationary and the emery wheel only rotate. Thus, suppose it was required to grind the necessary clearance to relieve the cutting edge C of the reamer, then A could be rotated until C stood in the required position with relation to B, and the revolving emery wheel may either be traversed along, or the work may traverse past the wheel, according to the design of the grinding lathe, but in either case A remains stationary during each cut traverse; after each successive traverse A may be rotated sufficiently to give a cut for the next traverse.

Fig. 679 represents Brown and Sharpe's universal grinding lathe.

This lathe is constructed to accomplish the following ends. First, to have the lathe centres axially true with the work when grinding tapers, so that the lathe centres shall not wear and gradually throw the work out of true from the causes explained in the remarks on turning tapers in a lathe of ordinary construction.

Second, to have the headstock B capable of lateral swing, so as to enable the grinding of taper holes.

The manner in which these results are accomplished is as follows:

The headstock B and the tailstock are attached to the bed or table A, which is pivoted at its centre to a table beneath it, this latter table being denoted by C. This permits table A to swing laterally upon C and stand at any required angle. To enable a delicate adjustment of this angle, a screw _a_ having journal bearing in a lug on C is threaded through a piece carried in projection on the end of A.

The table C traverses back and forth past the emery wheel, after the manner of an ordinary iron planing machine, the mechanical parts effecting this motion being placed within the bed upon which C slides. The carriage supporting the emery frame and table D remains stationary in its adjusted position, while C (carrying A with it) traverses back and forth.

Now, if A be adjusted so that the line of centres is parallel with the line of motion of C, then the work will be ground parallel, but if _a_ be operated to move A upon its pivoted centre and draw the tailstock end of A towards the operator, then the work will be ground of larger diameter at the tailblock end. Conversely, by operating screw _a_ in the opposite direction, it will be of smaller diameter at that end.

But whatever the degree of angle of A to C, the line of centres of the head and tailstocks will be axially true with the axial line of the work, hence the work centres are not liable to wear off true, as is the case when the tailstock only sets over (as will be fully explained in the remarks on taper turning).

To grind conical holes the headstock B is pivoted at its centre upon a piece held by bolts to the table A, so that it is capable of being swung laterally to the degree requisite for the required amount of taper in the work bore, and of being locked in that adjusted position, the work being held in a chuck screwed upon the spindle in the usual manner. The pulley _d_ being removed to enable the grinding of cones, chamfers, or tapers of too great an angle to permit of A setting over to the required degree. The line of cross-feed motion of the emery wheel may be set to the required angle as follows.

The frame carrying the emery wheel arbor is fixed to a table D, which is capable of being operated (in a direction across the table A) upon a carriage beneath A. This carriage, or saddle (as it may perhaps be more properly termed), is pivoted so as to allow of its movement and adjustment in a horizontal plane, and since D operates in the slide of the carriage, its line of motion in approaching or receding from the line of centres will be that to which the saddle is set. This enables the grinding of such short cones as the circumferences of bevelled cutters, chamfers, &c., at whatever angle the saddle may be set, however, D may be operated from the feed screw disc and handle _f_.

The lever handle at the left hand is for operating or rather traversing C by hand; _b_ is a pan to catch the grit and water, the water being led to the back of machine into a pail; _c_ is a back rest to steady the work when it is slight and liable to deflection.

The slot and stops shown upon the edge of C are to regulate the points of termination of the traverse (in the respective directions) of C. A guard is placed over the emery wheel to arrest and collect the water cuttings, &c., which would otherwise fly about.

A large amount of work which has usually been filed in a lathe, can be much more expeditiously and accurately finished by grinding in this machine.

Work to be ground may obviously be held in the same chucks or work-holding appliances as would be required to hold it to turn it with cutting tools, or where a quantity of similar work is to be done special chucks may be made.

Fig. 680 (from _The American Machinist_) shows a special chuck for grinding the faces of thin discs, such as very thin milling cutters, which could not be held true by their bores alone. The object of the device is to hold the cutter by its bore and then draw it back against the face of the chuck, which, therefore, sets it true on the faces. The construction of the chuck is as follows. The hub screws upon the lathe like an ordinary face plate, and has a slot running diametrically through it. Upon its circumference is a knurled or milled nut C, which is threaded internally to receive the threaded wings of the bush B. A collar behind C holds it in place upon the hub. To admit piece B the front of the chuck is bored out, and after B is inserted and its threaded wings are engaged in the ring nut C a collar is fitted over it and into the counter-bore to prevent B from having end motion unless C is revolved. D is a split bushing that fits into B, its stem fitting the bore of the disc, or cutter to be ground: the enlarged end of D is countersunk to receive the head of the screw E, whose stem passes through D and threads at its end into B, so that when E is screwed up its head expands D and causes it to grip the bore of the disc or cutter to be ground. After E is screwed up the ring nut C is revolved, drawing B within the chuck and therefore bringing the inside face of the disc or cutter against the face of the chuck or face plate, and truing it upon the bushing D. All that is necessary therefore in using the chuck is to employ a bushing of the necessary diameter for the bore of the cutter, insert it in B, then screw up the screw E and then revolve the ring nut C until the work is brought to bear evenly and fair against the face of the chuck, and to insure this it is best not to screw E very tightly up until after the ring nut C has been operated and brought the work up fair against the chuck face.

Fig. 681 represents the J. Morton Poole calender roll grinding lathe, which has attained pre-eminence both in Europe and the United States from the great accuracy and fine finish of the work it produces.

In all other machine tools, surfaces are made true either by guiding the tool to the work or the work to the tool, and, in either case, guide-ways and slides are employed to determine the line of motion of the tool or the work, as the case may be. These guideways and slides are usually carried by a framing really independent of the work, so that the cutting depends entirely upon the truth or straightness of the guideways, and is not determined by the truth, straightness, or parallelism of the work itself. As a result, the surface produced depends for its truth upon the truth of the tool-guiding ways. In the Poole lathe, however, while guideways are necessarily employed to guide the emery wheels in as straight a line as is possible, by means of such guides, the roll itself is employed as a corrective agent to eliminate whatever errors may exist in the guide. The rolls come to this machine turned (in the lathe Fig. 730), and with their journals ground true (on dead centres).

Fig. 681 represents a perspective view of the machine, as a whole. It consists of a driving head, answering to the headstock of an ordinary lathe. B B are bearings in which the rolls are revolved to be ground. C is a carriage answering to the carriage of an ordinary lathe, but seated in sunken [V]-guideways, corresponding to those on an ordinary iron planing machine. Referring to Fig. 682, F is a swing-frame suspended by four links at G, H, I, J, which are upon shafts having at their ends knife edges resting in small [V]-grooves on the surface of standards S, which are fixed to carriage C. The frame F being thus suspended and being in no way fixed to C, it may be swung back and forth crosswise of the latter, the links at G, H, I, J, swinging as pendulums. At the top of F are two slide rests A A, one on each end, carrying emery or corundum wheels W, and the roll R, which rests in the bearings B, rotates between these emery wheels. The carriage C is fed along the bed as an ordinary lathe carriage, and the emery wheels are revolved from an overhead countershaft. Now, it will be found that from this form of construction the surface of the roll, when ground true, serves as a guide to determine the line of motion of the emery wheels, and that the emery wheels may be compared to a pair of grinding calipers that will operate on such part of the roll length as may be of larger diameter than the distance apart of the perimeters of the emery wheels, and escape such parts in the roll length as may be of less diameter than the width apart of those perimeters; hence parallelism in the roll is inevitable, because it is governed solely by the width apart of the wheel perimeters, which remain the same, while the wheels traverse the roll, except in so far as it may be affected by wear of emery-wheel diameters in one traverse along the roll.

Supposing now that we have a roll R (Fig. 683), placed in position and slowly revolved, and that the carriage C is fed along by feed screw E, then the line of motion of the emery wheels will be parallel to the axis of the roll, provided, of course, that the bearings B (Figs. 681 and 687) are set parallel to the [V]-guideways in the bed, and that these guideways are straight and parallel. But the line of travel of the emery wheels is not guided by the [V]s except in so far as concerns their height from those [V]s, because the swing-frame is quite free to swing either to the right or to the left, as the case may be. Its natural tendency is, from its weight, to swing into its lowest position, and this it will obviously do unless some pressure is put on it in a direction tending to swing it. Suppose, then, that instead of the roll running true, it runs eccentrically, or out of true, as it is termed, as shown in Fig. 683, when the high side meets the left-hand wheel it will push against it, causing the carriage C to swing to the left and to slightly raise. The pressure thus induced between the emery wheel and the roll causes the roll surface to be ground, and the grinding will continue until the roll has permitted the swing-frame to swing back to its lowest and normal position. When the high side of the roll meets the right-hand emery wheel it will bear against it, causing the swing-frame to move to the right, and the pressure between the wheel and the roll will again cause the high side of the latter to be reduced by grinding. This action will continue so long as the roll runs out of true, but when it runs true both emery wheels will operate, grinding it to a diameter equal to the distance between the emery-wheel perimeters, which are, of course, adjusted by the slide rests A A. If the roll is out of true in the same direction and to the same amount throughout its length, the emery wheel will act on an equal area (for equal lengths of roll) throughout the roll length; but the roll may be out in one direction at one part and in another at some other part of the length; still the emery wheel will only act on the high side, no matter where that high side may be or how often it may change in location as the carriage and wheels traverse along the roll. Now, the roll does not run true until its circumference is equidistant at every point of its surface from the axis on which the roll revolves, and obviously when it does run true its circumference is parallel to the axis of revolution of the roll, because this axis is the line which determines whether the roll runs true or not, and therefore the swing-frame is actually guided by the axis of revolution of the roll, and will therefore move parallel to it.

It is obvious that if by any means the swinging of frame F is slightly resisted, as by a plate between it and C, with a spring to set up the plate against F, then the emery wheels will be capacitated to take a deeper cut than if the frame swing freely, this plan being adopted until such time as the roll is ground true, when both wheels will act continuously and simultaneously, and F may swing freely.

A screw may be used to set up the spring and plate when they are required to act.

Suppose now that the roll was not set exactly level with the [V]-guideways of the bed, there being a slight error in the adjustment of the roll journals in the bearings on B, and the emery-wheels would vary in height with relation to the height of the roll axis, and theoretically they would grind the roll of larger diameter at one end than at the other.

This, however, is a theoretical, rather than a practical point, as may be perceived from Fig. 684, in which R is a part of a section of a roll, and W a part of a section of a wheel. Now, assuming that the [V]-ways were as much as even a sixteenth out of true, so far as height is concerned, all the influence of the variation in height is shown by the second line of emery-wheel perimeter, shown in the figure, the two arcs being drawn from centres, one of which is 1/16th inch higher than the other. It is plain, then, that with the ordinary errors found in such [V]-guideways, which will not be found to exceed 1/30th of an inch, no practical effect will be produced upon the roll. Again, if one [V] is not in line with the other, no practical effect is produced, because if the carriage C were inclined at an angle, though the plane of rotation of the emery-wheel would be varied, its face would yet be parallel to the roll axis. If the [V]s were to vary in their widths apart (the angles of the [V]s being 45° apart), all the effect it would have would be to raise or lower the carriage C to one-half the amount the [V]s were in error. It will be thus perceived that correctness of the roll both for parallelism and cylindricity is obtained independent of absolute truth in the [V]-guides.

Referring now to some of the details of construction of the lathe, the slide rest A, Fig. 683, is bored to receive sockets D D, Fig. 685, and is provided with caps, so that the sockets may be firmly gripped and held axially true one with the other. The socket-bores are taper, to receive the taper ends of the arbor _x_, and are provided with oil pockets at each end. There is a driving pulley on each side of the emery-wheel, and equal belt-speed is obtained as follows: Two belt driving drums M N are employed, and each belt passes over both, as in Figs. 683 and 685, and down around the pulleys P. The diameter of the drum N is less than the diameter of the drum M by twice the thickness of the belt, thus equalizing inside and outside belt diameters, since they both pass over the pulley of the emery-arbor. The piece T is a guard to catch the water from the emery-wheels, and is hinged at the back so that the top is a lid that may be swung back out of the way when necessary.