Fig. 663Fig. 663.
Fig. 663.
Fig. 664Fig. 664.
Fig. 664.
Fig. 665Fig. 665.
Fig. 665.
Fig. 663is a side elevation,Fig. 664an end elevation, andFig. 665a plan view of this rest, and similar letters of reference indicate like parts in each of the three figures.ais the base, held to the lathe bed by the boltb, whose operation is the same as that already described for the head and tailstocks.
In one piece withais the armc, carrying at its head three gauge tongues or piecesd e f, which are adjustable by means of the screwsd e f, which move the gauge tongues horizontally. Through a suitable guideiis a standard or head; pivoted toaatj j, and carrying at its top three gauge tonguesk l m.
Midway between pivotsj jand the ends of the gauge tongues, is the centre or tool carrying spindleo. If a piece of work, as a jewel, be placed between the tonguesfandm,Fig. 664[swingingm, and with iti(which is pivoted atj), laterally], then the point of the centrenwill be thrown out of line with the lathe live spindle half the diameter of the jewel, because fromjto the centren, ofo, is exactly one half of the vertical distance fromjto the jewel. If then a tool be placed in the dead centre and its cutting edge is in line with the axis of spindleo, 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 betweend kthe hole may be bored to fit the piece easily, with it placed betweene lthe fit may be made barely movable, while with it placed betweenf mthe 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 screwse f g.
The tool is fed by moving spindleoby hand, the screwpbeing adjusted so that its end abuts against stopq, when the hole is bored to the requisite depth;ris simply a guide for the pieces, which being attached too, 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 mouthhofa,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 theouter spindlehas the end motion necessary to open and close the chuck, the chuck having no end motion.
Largeimage(141 kB).Fig. 666Fig. 666.
Largeimage(141 kB).
Fig. 666.
The construction of this sliding-spindle head is shown inFig. 666, in which a wire chuck is shown in position in the spindles;lis the live spindle passing through parallel bearings, so that it may have end motion when the nutmis operated. The inner spindlento which the chucks are screwed is prevented from having end motion by means of the collarpand nutqat the rear bearing. When nutmis rotated andnis held stationary by means of the pulleyp,lslides endways, and the chuck opens or closes according to the direction in which the nut moves the spindlel.
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 spindlenbeing detained in its adjusted position by the set screws.
The construction whereby the nut is permitted to revolve with spindlel, and be operated by hand to move spindlelwhen the lathe is at rest, is as follows.
The cylindrical rimtof the nut is provided with a series of notches arranged around its circumference.ris a lever whose hub envelops nutm, but has journal bearing onv.rreceives the pins, which rests upon a spiral springt. When, therefore,sis pushed down it depresses the springtand its endwenters some one of the notches in the rimt, and operates the nut after the manner of a ratchet. But so soon as the end pressure onris released, the spiral spring lifts it andmis free to revolve withlas before. The inner spindle is driven by means of the featherg.
Pulleyphas two stepsyfor the belt, and a friction stepz, 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 ofmis 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 spindlelendwise. Per contra, however, in stopping the lathe suddenly by means of the brake, there is a tendency of nutmto stop less quickly than spindlel, and this operates to unscrew nutnand release the work. To assist thisris sometimes in lathes for watch manufactories provided with a hand wheel whose weight is made sufficient for the purpose.
Largeimage(78 kB).Fig. 667Fig. 667.
Largeimage(78 kB).
Fig. 667.
Fig. 668Fig. 668.
Fig. 668.
Figs. 667and668represent 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 rightangle 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 spindlesa,b, andc, inFig. 667;acorresponds to spindleainFig. 651, driving the chuckdwhich screws onaas shown;bsimply holds the work against the facedofd, andcholds the work true by means of the centree, which enters the hole or centre in the work and is withdrawn when the work is secured by spindleb.
The chuckdis open on two sides as shown ate einFig. 668, which is an end face view of the chuck, and through these openings the work is admitted to the chuck. The rod or spindlecis then pushed, by hand, endwise, its centreeentering the hole or centre in the work (so as to hold the same axially true) and forcing the work against the inside facesd, spindlebis then operated, the facepforcing the work against faced, and between these two facesd pthe work is held and driven by friction. The spindlecand its centreeis then withdrawn by hand, leaving the hole in the work free to be operated upon.
The journal bearings for spindleaare constructed as described forainFig. 666; spindlebis operated endways withinaas follows.ais threaded atgto receive the hubhof wheeli, at the end ofbis a collar which is held to and prevented from end motion within the hubh: hence when wheeliis rotated andais held stationary (by means of the band pulley),htraverses ongand carriesbwith it. Operatingiin one direction, therefore movespagainst the work, while operating it in the other direction releases facepfrom contact with the work.
It is obviously of the first importance that the spindlecbe held and maintained axially true, notwithstanding any wear, and that it be a close fit withinbso 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 spindleaand spindleb, 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. Betweenbandcare two inverted conical steel bushes, which can also be replaced by new ones, to take up any wear that may have taken place.
Fig. 669Fig. 669.
Fig. 669.
Fig. 669represents 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 ate) screwing on in the ordinary manner. When, however, this face plate is removed a nutdscrews on in its stead, to protect the thread on the live spindle.
The chuck for driving work in the absence of face platee(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 piecesj jare to support the hand tool shown inFigs. 1313and1314, in connection with hand turning, the tool stock or handle being shown atkon the floor. The lever for securing the tailstock to or releasing it from the shears is shown att. The tail spindle is operated by a lever pivoted atgso 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 collarr, 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 to1⁄2inch 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. 670Fig. 670.
Fig. 670.
Fig. 670represents 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 sawe, 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 leveraacts similarly to a pair of pliers, one jaw of which is provided on handlea, while the other is upon the piece to whichais pivoted. The screw to be slotted is placed between the jaws ofabeneathe; handleais then moved to the left, gripping the screw stem; by depressinga, the screw head is brought up to the cuttereand the slot is cut to a depth depending upon the amount to whichais depressed, which is regulated by a screw atb; hence afterbis properly adjusted, all screw heads will be slotted to the same depth.
The frame carrying the piece to whichais pivoted may be raised or lowered to suit screws having different thicknesses of head by means of a screw, whose hand nut is shown atd.
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.
Fig. 671Fig. 671.
Fig. 671.
Fig. 672Fig. 672.
Fig. 672.
Fig. 673Fig. 673.
Fig. 673.
Figs. 671,672, and673, 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 inFig. 672as applied to ball turning.lis a belt cone upon the counter-shaft driven from the line shaft.ldrivesh, which may be termed the lathe countershaft, and from the stepped conekthe belt is connected to the lathe in the usual manner.pis a shipper bar to move the beltnupon and along the belt cones, and thus vary the speed.ris a vertical shaft extending up at the end of the lathe and carrying a segment. This segment is connected to the belt shipper barpby two cords, one passing fromr1around half the segment tor2, and the other passing fromr3tor4, 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 beltnalong the cones respectively left or right.
At the back of the lathe is a horizontal shafts, similar to an ordinary feed spindle, and connected to the segment shaft by a pair of bevel gearss2. Between the two earse e, at the rear of the lathe carriage, is a piniont, which drives the splined shafts, which works in a rackt′. The tool rest is pivoted directly beneath the ball, to be turned after the usual manner of spherical slide rests, and carries a geara2, which, as the rest turns, rotates a geara3. Upon the face of the latter is a pina4working in a slota5at the end of the rackt′; hence as the tool rest feeds, motionis transmitted froma2througha3,a4,a,t′,t, ands s2tor, which operates the belt shipperp. 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. 674Fig. 674.
Fig. 674.
Fig. 673illustrates the action of the mechanism when used for surfaces, like a lathe face plate. In this case the two gears and the rackt′simply traverse with the cross-feed slider, and the mechanism is actuated as before. InFig. 674a 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 atd, the belt guiding forks, asp2, being carried on a nut actuated by the screwd.
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. 675Fig. 675.
Fig. 675.
Fig. 675represents Hyde’s cutting-off machine, which consists of a hollow live spindle through which the bar of iron is passed and gripped by the chucksc c. Atgis a gauge rod whose distance from the tool restrdetermines the length of the work.fis a feed cone driven by a corresponding cone on the live spindle and driving the wormw, 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 standsis 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 independentdrum or wide pulley, is employed, the direction of rotation of the emery wheel being opposite to that of the work.
Fig. 676Fig. 676.
Fig. 676.
Fig. 676represents 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 inFigs. 508and509.
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 infigure. 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 raisedVs.
Fig. 677Fig. 677.
Fig. 677.
It is essential that the work revolve in a direction opposite to that of the emery wheel, for the following reasons. InFig. 677letarepresent a reamer andba segment of an emery wheel. Now supposeaandbto 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 reamerato 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 ofawould show waves and would be neither round nor smooth.
Fig. 678Fig. 678.
Fig. 678.
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 inFig. 678, in whichais the emery wheel,cthe pulley for driving the arbor, andbthe frame,dbeing 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 edgecof the reamer, thenacould be rotated untilcstood in the required position with relation tob, 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 casearemains stationary during each cut traverse; after each successive traverseamay be rotated sufficiently to give a cut for the next traverse.
Fig. 679Fig. 679.
Fig. 679.
Fig. 679represents 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 headstockbcapable of lateral swing, so as to enable the grinding of taper holes.
The manner in which these results are accomplished is as follows:
The headstockband the tailstock are attached to the bed or tablea, which is pivoted at its centre to a table beneath it, this latter table being denoted byc. This permits tableato swing laterally uponcand stand at any required angle. To enable a delicate adjustment of this angle, a screwahaving journal bearing in a lug oncis threaded through a piece carried in projection on the end ofa.
The tablectraverses 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 whichcslides. The carriage supporting the emery frame and tabledremains stationary in its adjusted position, whilec(carryingawith it) traverses back and forth.
Now, ifabe adjusted so that the line of centres is parallel with the line of motion ofc, then the work will be ground parallel, but ifabe operated to moveaupon its pivoted centre and draw the tailstock end ofatowards the operator, then the work will be ground of larger diameter at the tailblock end. Conversely, by operating screwain the opposite direction, it will be of smaller diameter at that end.
But whatever the degree of angle ofatoc, 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 headstockbis pivoted at its centre upon a piece held by bolts to the tablea, 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 pulleydbeing removed to enable the grinding of cones, chamfers, or tapers of too great an angle to permit ofasetting 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 tabled, which is capable of being operated (in a direction across the tablea) upon a carriage beneatha. 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 sincedoperates 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,dmay be operated from the feed screw disc and handlef.
The lever handle at the left hand is for operating or rather traversingcby hand;bis a pan to catch the grit and water, the water being led to the back of machine into a pail;cis a back rest to steady the work when it is slight and liable to deflection.
The slot and stops shown upon the edge ofcare to regulate the points of termination of the traverse (in the respective directions) ofc. 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. 680Fig. 680.
Fig. 680.
Fig. 680(fromThe 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 nutc, which is threaded internally to receive the threaded wings of the bushb. A collar behindcholds it in place upon the hub. To admit piecebthe front of the chuck is bored out, and afterbis inserted and its threaded wings are engaged in the ring nutca collar is fitted over it and into the counter-bore to preventbfrom having end motion unlesscis revolved.dis a split bushing that fits intob, its stem fitting the bore of the disc, or cutter to be ground: the enlarged end ofdis countersunk to receive the head of the screwe, whose stem passes throughdand threads at its end intob, so that wheneis screwed up its head expandsdand causes it to grip the bore of the disc or cutter to be ground. Aftereis screwed up the ring nutcis revolved, drawingbwithin 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 bushingd. 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 inb, then screw up the screweand then revolve the ring nutcuntil the work is brought to bear evenly and fair against the face of the chuck, and to insure this it is best not to screwevery tightly up until after the ring nutchas been operated and brought the work up fair against the chuck face.
Fig. 681Fig. 681.
Fig. 681.
Fig. 681represents 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 rollitself is employed as a corrective agent to eliminate whatever errors may exist in the guide. The rolls come to this machine turned (in the latheFig. 730), and with their journals ground true (on dead centres).
Fig. 682Fig. 682.
Fig. 682.
Fig. 681represents a perspective view of the machine, as a whole. It consists of a driving head, answering to the headstock of an ordinary lathe.b bare bearings in which the rolls are revolved to be ground.cis a carriage answering to the carriage of an ordinary lathe, but seated in sunkenV-guideways, corresponding to those on an ordinary iron planing machine. Referring toFig. 682,fis a swing-frame suspended by four links atg,h,i,j, which are upon shafts having at their ends knife edges resting in smallV-grooves on the surface of standardss, which are fixed to carriagec. The framefbeing thus suspended and being in no way fixed toc, it may be swung back and forth crosswise of the latter, the links atg,h,i,j, swinging as pendulums. At the top offare two slide restsa a, one on each end, carrying emery or corundum wheelsw, and the rollr, which rests in the bearingsb, rotates between these emery wheels. The carriagecis 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.
Fig. 683Fig. 683.
Fig. 683.
Supposing now that we have a rollr(Fig. 683), placed in position and slowly revolved, and that the carriagecis fed along by feed screwe, then the line of motion of the emery wheels will be parallel to the axis of the roll, provided, of course, that the bearingsb(Figs. 681and687) are set parallel to theV-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 theVs except in so far as concerns their height from thoseVs, 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 inFig. 683, when the high side meets the left-hand wheel it will push against it, causing the carriagecto swing to the left and to slightly raise. The pressure thus induced between the emery wheel and the rollcauses 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 restsa 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 framefis slightly resisted, as by a plate between it andc, with a spring to set up the plate againstf, 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, andfmay 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 theV-guideways of the bed, there being a slight error in the adjustment of the roll journals in the bearings onb, 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.