Modern Machine-Shop Practice Part 52

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 circ.u.mferences 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 quant.i.ty of similar work is to be done special chucks may be made.

[Ill.u.s.tration: Fig. 680.]

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 circ.u.mference 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 bus.h.i.+ng 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 pa.s.ses 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 bus.h.i.+ng D. All that is necessary therefore in using the chuck is to employ a bus.h.i.+ng 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.

[Ill.u.s.tration: Fig. 681.]

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.

[Ill.u.s.tration: Fig. 682.]

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 circ.u.mference is equidistant at every point of its surface from the axis on which the roll revolves, and obviously when it does run true its circ.u.mference 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.

[Ill.u.s.tration: Fig. 683.]

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.

[Ill.u.s.tration: Fig. 684.]

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, a.s.suming 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 pa.s.ses 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 pa.s.s 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.

[Ill.u.s.tration: Fig. 685.]

[Ill.u.s.tration: Fig. 686.]

The method of securing the emery-wheels is shown in Fig. 686. Two f.l.a.n.g.es Z (made in halves) are let into the wheel, and clamp the wheel by means of the screws shown. The bore of these f.l.a.n.g.es Z is larger than the diameter of pulleys P, so that the emery-wheels may be changed on the arbor without removing the pulley. Fig. 687 represents an end view of the bearings B for the roll to revolve in, being provided with three pieces, the two side ones of which are adjustable by the set-screws, so as to facilitate setting the roll parallel with the bed of the lathe.

The height is adjusted by means of screws K, K, which may also be used in grinding a roll of large diameter at the middle of its length, by occasionally raising the roll as the carriage C proceeds along the roll (the principle of this action is hereafter explained with reference to turning tapers on ordinary lathe work). When the wheels have traversed half the length of the roll, the screws K are operated to lower it again, it being found that the effect of a slight operating of the screws K is so small that the workman's judgment may be relied upon to use them to give to a roll with practical accuracy any required degree of enlarged diameter at the middle of its length with sufficient accuracy for all practical purposes.

[Ill.u.s.tration: Fig. 687.]

There are, however, other advantages of this system, which may be noted as follows. When a single emery-wheel is used there is evidently twice the amount of wear to take a given amount of metal off (per traverse) that there is when two wheels are used, and furthermore the reduction of every wheel diameter per traverse is evidently twice as great with one wheel as it is with two. From some experiments made by Messrs. Morton Poole, it was found that using a pair of 10-inch emery-wheels it would take 40,000 wheel traverses along an average sized calender roll, to reduce its diameter an inch, hence the amount of error due to the reduction of the emery-wheel diameters, per traverse, may be stated as 1/40000 of an inch per traverse, for the two wheels.

[Ill.u.s.tration: Fig. 688.]

Now referring to Fig. 688, let R represent a roll and W W the two emery-wheels.

Suppose the wheels being at the end of a traverse, the roll is 1/40000 inch larger at that end on account of the wear of the emery-wheels, then each wheel will have worn 1/40000 inch diameter or 1/80000 inch radius, hence the increase of roll diameter is equal to the wear of wheel diameter.

[Ill.u.s.tration: Fig. 689.]

Now, suppose that one wheel be used as in Fig. 689, and its reduction of _diameter_ will be equal to that of the two wheels added together, or 1/20000 inch, this would be 1/40000 in the radius of the wheel, producing a difference of 1/20000 difference in the diameter of the wheel.

There is another advantage, however, in that a finer cut can be easier put on in the Poole system, because if a feed be put on of 1/100th inch, the roll is only reduced 1/100th inch in diameter, but if the same amount of feed be put on with a single wheel, it will reduce the roll 1/50th inch, hence for a given amount of feed or movement of emery-wheel towards the roll axis, the amount of cut taken is only half as much as it would be if a single wheel is used. This enables a minimum of feed to be put on the wheel, wear being obviously reduced in proportion as the feed is lighter and the duty therefore diminished.

The method of driving the roll is as follows: Shaft _t_, Fig. 681, runs in bearings in the head, and spindle _r r'_ pa.s.ses through, and is driven by shaft _t_. A driving pulley is fitted on the spindle at end _r'_, at the other end is a driving chuck _p_ for driving the roll through the medium of a _wabbler_, whose construction will be shown presently. Spindle _r_ may be adjusted endwise in _t_, so that it may be adjusted to suit different lengths of rolls without moving the bearing blocks B.

The wabbler is driven by _p_ and receives the end of the roll to be ground, as shown in Fig. 690, the end of the roll being a taper square and fitting very loosely in a square taper hole in the end of the wabbler; similarly _p_ may have a taper square hole loosely fitting the squared end of the wabbler. The looseness of fit enables the wabbler to drive the roll without putting any strain on it tending to lift or twist it in its bearings in block B, and obviates the necessity for the axis of the rolls to be dead in line with the axis of _r r'_. Various lengths of wabblers may be used to suit the lengths of roll and avoid moving blocks B, and it is obvious also that if the ends of the roll are round instead of square, two set-screws may be used to hold the roll end being set diametrically opposite, and if set screws are used in _p_ to drive the wabbler they should be two in number, set diametrically opposite, and at a right angle to the two in the wabbler, so that it may act as a universal joint.

[Ill.u.s.tration: Fig. 690.]

The method of automatically traversing the carriage C is as follows: Referring to Fig. 681, two gears _a_, _b_ are fast upon shaft _t_, gear _a_ drives _c_ which is on the same shaft as _e_, gear _b_ drives _d_ which drives a gear not seen in the cut, but which we will term _x_, it being on the same shaft as _c_ and _e_. Now if _e_ is driven through the medium of _a_ _c_, it runs in one direction, while if it is driven through the medium of _b_ _d_ _x_, it revolves _e_ in the opposite direction, and since _e_ drives _g_ and _g_ is on the end of the feed screw (E, Fig. 682) the direction of motion of carriage C is determined by which of the wheels _a_ or _b_ drives _e_. At _h_ is a stand affording journal bearing to a shaft _n_, whose end engages a clutch upon the shaft of wheels _c_, _x_ and _e_. On the outer end of shaft _n_ is ball lever _l"_, whose lower end is attached to a rod _k_, upon which are stops _l l'_ adjustable along rod _k_ by means of set-screws.

At _m_ is a bracket embracing rod _k_.

Now suppose carriage C to traverse to the left, and _m_ will meet _l_ moving rod _k_ to the left, the ball _i_ will move up to a vertical position and then fall over to the right, causing the clutch to disengage from gear _c_ and engage with the unseen gear _x_, reversing the motion of _e_ and of _g_, and therefore of carriage C, which moves to the right until _m_ meets _l'_ and pushes it to the right, causing _i_ to move back to the position it occupies in the engraving, the clutch engaging _c_, which is then the driving wheel for _e_.

SCREW MACHINE.--The screw machine is a special form of lathe in which the work is cut direct from the bar, without the intervention of forging operations, and it follows therefore that the bar must be large enough in diameter to suit the largest diameter of the work, the steps or sections of smaller diameter being turned down from the full size of the bar. The advantages of the screw machine are, that the work requires no centring since it is held in a chuck, that forging operations are dispensed with, that any number of pieces may be made of uniform dimensions without any measuring operations save those necessary when adjusting the tool for the first piece, and that it does not require skilled labor to operate the machine after the tools are once set.

The capacity of the screw machine is, therefore, many times greater than that of a lathe, while the diameters and lengths of the various parts of the work will be more uniform than can be done by caliper measurements, being in this case varied by the wear of the cutting edges of the tools only, which eliminates the errors liable to independent caliper measurement. Hollow work, as nuts and washers, may be equally operated on being driven by a mandril held in the chuck.

Fig. 691 represents Brown and Sharpe's Number 1 screw machine, which is designed for the rapid production of small work.

Three separate tool-holding devices may be employed: first, cutting tools may be placed in the holes shown to pierce (horizontally) the circular head F; second, tools may be fixed in the tool posts shown in the double slide rest, which has two slides (one in the front and one at the back of the line of centres); and third, tools may be placed in what may be termed the screw-cutting slide-rest J.

F is a head pierced horizontally with seven holes, and is capable of rotation upon L; when certain mechanism is operated L slides on D and the mechanism of these three parts is arranged to operate as follows.

The lever arms K traverse L in D. When K is operated from right to left, L advances towards the live spindle until arrested at some particular point by a suitable stop motion, this stop motion being capable of adjustment so as to allow F to approach the live spindle a distance suitable for the work in hand.