Fig. 2058 represents a polishing device used to polish the surface of engravers’ plates. It consists of a spindle d, carried in bearings b, and, having no collars, it is capable of end motion through those bearings. The spindle is pressed downward by a spring a, carrying at its end a piece c, which is capped to receive the end of the spindle d and the piece e which threads into the spindle, thus making a sort of universal joint. The spindle d is run by the pulley p, and carries a piece of stone s, the work w resting upon the plate or table t. The stone being set to one side of the centre of the spindle, each part of its surface describes a circle, the centre of which is outside of the stone, thus making the effectiveness of the centre of the stone greater by increase of motion. To raise the stone from the work the spindle is raised by means of the chord f, or the table t may have a simple lever motion. The work is moved about and around and beneath the revolving stone. Water, oil, benzine or alcohol is used to keep the stone clear and wash away the cuttings. The device saves a good deal of hand work in the preparatory stages of grinding, although it can be used only with soft stones.

Grindstones and Tool Grinding.—The general characteristics of grindstones are as follow:—

For rapid grinding a coarse and an open grit is the most effective. The harder the grit the more durable the stone, but the liability of the stone to become coated or glazed with particles of the metal ground from the work is increased. With a given degree of coarseness a soft grit stone will grind a smoother surface than a hard grit one.

The finer the grit the smoother the surface it will grind. In all stones, however, it is of prime importance that the texture be even throughout the stone, because the soft or open-grained part will wear more rapidly than the close or hard grained. All grindstones are softer when water-soaked than when dry, and will cut more freely, because the water washes away the particles of metal ground from the work, and prevents them from glazing the stone. It follows from this, however, that a stone should not be allowed to rest overnight with its lower part resting in water, as the wear of the stone will be unequal until such time as it has become equally saturated. Furthermore the balance of the stone is destroyed, and if run at a maximum speed, as in the case of stones used to grind up large edge tools, the unbalanced centrifugal force generated on the water-soaked side may cause the stone to burst. The following stones are suitable for the class of work named:—

The flanges for grindstones should be trued on both faces, and should pass easily over the grindstone shaft, and there should be between these collars and the stone an elastic disk, as of wood or felt, which will bed fully against the surface of the stone. It is preferable also if the under faces of these collars be recessed to within an inch of their perimeters so as to confine the grip to the outer edges of the faces.

The process of grinding large surfaces is entirely distinct from that of small ones, because of the difficulty in the former of getting rid of the cuttings. As an illustration of this point it may be remarked that a stone that has become dulled and glazed from operating upon a broad area of surface, as say a large plate, may be both cleaned of the cuttings and sharpened by grinding upon it a roller of, say, 1 or 1¹⁄₄ inches in diameter. This roller is laid across the “horn” or rut of the stone, and pressed firmly against it, the bar being allowed to slowly rotate. What is commonly termed grinding is the class of grinding that is followed as a trade, such as file grinding, saw grinding, plate grinding, edge tool and cutlery grinding. In all this class of grinding the speeds of the stones is very much greater than for machine-shop tool grinding. For all the above, save cutlery grinding, the stones when new are of a diameter from 5 to 8 feet, and of a width of from 8 to 15 inches. The stones used by cutlers are about 15 inches in diameter, and from ¹⁄₂ inch to 3 inches thick. The average speed of grindstones in workshops may be given as follows:—

The speeds of stones for file grinding and other similar rapid grinding is thus given in the “Grinders’ List.”

These speeds are obviously obtained by reducing the diameter of the pulley on the grindstone shaft each time the stone has worn down 6 inches less in diameter, and give a uniform velocity of stone if the 8 feet stone be driven with a pulley 32 inches in diameter. Each shift (or change of pulley) giving a pulley 2 inches less in diameter.

The following table (from the Mechanical World) is for the diameter of stones and the number of revolutions they should run per minute (not to be exceeded), with the diameter of change or shift pulleys required, varying each shift or change 2¹⁄₂ inches, 2¹⁄₄ inches, or 2 inches in diameter for each reduction of 6 inches in the diameter of the stone:—

“Columns 3, 4, and 5 are given to show that if you start an 8 feet stone with, say, a countershaft pulley driving a 40 inch pulley on the grindstone spindle, and the stone makes the right number (135) of revolutions per minute, the reduction in the diameter of the pulley on the grinding-stone spindle, when the stone has been reduced 6 inches in diameter, will require to be also reduced 2¹⁄₂ inches in diameter, or to shift from 40 inches to 37¹⁄₂ inches, and so on similarly for columns 4 and 5. Any other suitable dimensions of pulley may be used for the stone when 8 feet in diameter, but the number of inches in each shift named, in order to be correct, will have to be proportional to the number of revolutions the stone should run, as given in column 2 of the table.”

In all grinding operations it is necessary that the stone should run true. This is sometimes accomplished by so mounting the stones in their frames that their perimeters touch at the back of each stone, one stone running slightly faster than the other. Or sometimes the work is placed between the two stones, as in Fig. 2059, which represents a plan frequently used to grind circular saws; c in the figure represents the grinding-stone and a the saw. Long saws are mounted vertically as in Fig. 2060, a representing a frame to which the upper end of the saw is attached and driven by a disk crank and connecting rod as shown, the two stones c e may, in this case, be of equal diameter.

Fig. 2061 represents a grindstone truing device (for tool-grinding stones) in which a series of serrated disks are employed in place on a threaded roll. The disks are fed to the stone by the hand wheel and screw, and are traversed back and forth across the stone face by means of the lever handle shown.

The fast running grindstones used for heavy and coarse grinding are trued by a process known as hacking. The high spots of the stone are marked by holding a piece of coal to the stone while it revolves slowly, and a tool similar to an adze is used to cut or chop indentations in the stone. The highest spots will be most plainly marked by the coal, and the hacking is spaced closer together in these places, the hacking marks crossing each other and varying in depth to suit, obviously being deepest where the marks are blackest. The hacking also sharpens the stone. To prevent the stone from wearing uneven across its face the file grinder mounts the stone in a very ingenious manner, causing it to traverse automatically, back and forth, while rotating.

This device is shown in Fig. 2062, in which a represents the grindstone spindle having journal bearing at b b, but as there are no collars on the journals, a can move endwise through b b. Fast to a are the collars c and c′ (sometimes the face of the pulley hub is made to serve instead of c′); s is a sleeve fitting easily to a, and containing a return groove, as shown; d is a fixed arm carrying a pin which projects down into the groove of s, as shown; p is the pulley driving a, and w w are suspended weights. The operation is self-acting, as follows: The shaft revolving causes the sleeve to revolve by friction, and the pin causes the sleeve to move endwise; its end face abutting against the face of the collar on one side, or the face of the pulley on the other side, as the case may be, causing the shaft to travel in that lateral direction. When the pin has arrived at the end of the groove, the stone ceases lateral motion (there being left a little play between the faces of the sleeve and of the collar and pulley face for this special purpose), while the cam travels in the opposite lateral direction, getting fairly in motion until it strikes the face, when it slowly crowds the face over. In travelling to the right it crowds against the face of the collar c′, and in traveling to the left, as shown in the figure, against the face of the collar c. The swing thus given to the stone is a slow and very regular one, the motion exciting surprise from its simplicity and effectiveness, especially when it is considered that the friction of the rotation of a shaft about 2¹⁄₂ inches diameter in a smooth hole about 6 inches long is all that is relied upon to swing a ponderous stone.

The following are the considerations that determine in grinding tools or pieces held by the hands to the grindstone. Upon the edge of a tool that last receives the action of the stone there is formed what is termed a feather-edge, which consists of a fine web of metal that bends as the tool is ground, and does not become detached from the tool in the grinding. The amount or length of this feather-edge increases as the work is thinner, and is greater in soft than in hardened steel. It also increases as the tool or piece is pressed more firmly to the stone.

To prevent its formation on such tools as plane blades or others having thin edges, the tool is held as at g in Fig. 2063, the top of the stone running towards the workman, and the tool is held lightly to the stone during the latter part of the grinding operation. With the tool held on the other side of the stone as at c, and pressed heavily to the stone, a feather-edge extending as long as from d to e may be formed if the tool has a moderate degree only of temper, as, say, tempered to a dark purple. The feather-edge breaks off when the tool is put to work, or when it is applied to an oil-stone, leaving a flat place instead of a sharp cutting edge. In well-hardened and massive tools, such as the majority of lathe tools, the amount of feather-edge is very small and of little moment, but in thin tapered edges, even in well-hardened tools, it is a matter of importance.

After a tool is ground it is often necessary to remove the feather-edge without having recourse to an oil stone. This may be accomplished by pressing the edge into a piece of wood lengthways with the grain of the wood, and while holding the cutting edge parallel with the line of motion, draw it towards you and along the grain of the wood, which removes the feather-edge without breaking it off low down, as would be the case if the length of the cutting edge stood at a right angle to the line of motion.

The positions in which to hold cutting tools while grinding them are as follows: The bottom faces of lathe tools and the end faces of tools such as scrapers should be ground with the tool laid upon the grindstone rest as in Fig. 2064, the stone running in the direction of the arrow. The best position for thin work as blades is at f providing the stone runs true, for otherwise the tool edge will be liable to catch in the stone. With an untrue stone the position shown in Fig. 2065 is the best, the blade being slowly reciprocated across the face of the stone.

If the facet requires to be ground rounding and not flat the position at c, Fig. 2064, is the best, the work being moved to produce the roundness of surface. If the tool is to be ground hollow or somewhat to the curvature of the stone, as in Fig. 2066, the curve being from b to c, the position at b is the best. At position d the tool cannot be held steadily; hence, that position is altogether unsuitable for tool grinding purposes.

For grinding the top faces of lathe or planer tools or other similar shaped pieces that must be held with their length at a right angle (or thereabouts) to the plane of the rotation of the stone, the tool is held in the hands, and the hands are supported by the grindstone rest as in Fig. 2067, the fingers being so placed that should the tool catch in the stone it will slip from between the fingers and not carry them down with it upon the tool rest.

Tools to be ground to a sharp point should be ground at the back of the stone, that is to say, with the top of the stone running away from the operator, and the point should be slowly moved across the width of the stone to prevent wearing grooves in its surface.

To produce a finer edge than is possible with the grindstone, the oil-stone is brought into requisition, the shape of the oil-stone being varied to suit the shape of the tool. Three kinds of oilstone are in general use, Turkey stone, Arkansas stone, and Washita stone, the latter being softer and of inferior quality to the two former. The best quality of Arkansas stone is of a milky white color, of very fine and even grain, and very hard, being impervious to a file; but there are softer grades. An oil-stone should be of even grain throughout, so that it may wear even throughout, and produce a smooth and unscored edge. Arkansas stone is rarely obtainable in lengths above 6 inches, on account of the presence of fine seams of hard quartz, which wears less than the stone, and forms a projection that scores the cutting edge of the tool, and the same applies to the Turkey stones.

For tools fully hardened and not tempered the hardest oilstones are best; but for tools that are tempered, as tools for woodwork, a softer grade of stone is preferable, since it will cut the most free.

When an oil-stone has worn out of shape it may be dressed on a grindstone; but if a flat surface is required it is best to true it by a piece of coarse sand-paper laid upon a flat true surface.

The action of an oil-stone is to smooth the surfaces; but while doing this the oil-stone itself forms what is termed a wire-edge, which resembles a feather-edge, except that it is smoother and more continuous. It is caused by the weak edge of the blade giving way under the pressure with which it is held to the stone. To reduce the wire-edge as much as possible the tool is pressed very lightly to the oil-stone during the latter part of the stoning, and is frequently turned over. If the motion of the tool upon the oil-stone is parallel with the line of cutting edge, the wire-edge will be greater than if the line of motion were at a right angle to it.

Again, the strokes performed while the cutting edge is advancing upon the oil-stone produce less wire-edge than the return strokes, hence the finishing process consists of a few light strokes upon one and then upon the other facet repeated several times. Now let it be observed that, the wire-edge will never be turned toward the facet last oil-stoned, and cannot be obviated by the most delicate use of the stone; but after the stoning proper is finished, the operator will lay one facet quite level with the face of the stone, and then give to the blade, under a very light pressure, forward diagonal motion, and then perform the same operation with the other facet upon the stone, the last facet operated upon being usually the straight and not the bevelled one. To still further reduce the wire-edge for very fine work, the operator sometimes uses a piece of leather belt, either glued to a piece of wood, as upon the lid of the oil-stone box, or some attach it at each end to projecting pieces of wood, while yet others lap the tool upon the palm of the hand. In giving an edge to a razor, the process may be carried forward in the usual way by means of straps, the first strokes being long ones made under a slight pressure, the strokes getting shorter and the pressure lighter as the process proceeds, until at last the motion and contact are scarcely perceptible.

When, as in the case of plane blades and carpenters’ chisels, the area of face is large, it is advantageous to grind the face somewhat concave, as in Fig. 2068, so that the heel and the point only of the tool has contact with the oil-stone, thus reducing the area to be stoned and steadying the tool, because, the area being small, the heel as well as the edge may be allowed to rest upon the oil-stone without unduly prolonging the stoning operation.

Chapter XXIV.—GEAR-CUTTING MACHINES.

The Brainard automatic gear cutter, Figs. 2069, 2070, 2071 and 2072 is arranged to cut spur, bevel, and worm-wheels, and is of that class where the manipulations required in gear cutting are all performed by the machine itself, thus dispensing with the care of an attendant except to place the wheels in position and set the machine for the proper depth and length of cut. The manner in which these results are accomplished will be seen from the following description, reference being had to the engravings. The wheel to be cut (a, Fig. 2070) is held upon a mandrel b fitted to the spindle c, which is mounted in firm bearings upon a column or standard d. To the face of the standard is gibbed a sliding knee e. Upon this knee is placed the cutter slide f, which is arranged to be inclined for bevel-gear cutting, and to be swung aside in cutting worm-wheels. Rotary cutters are carried on arbors fitted to the cutting spindle (g, Fig. 2071). Power for driving the cutter is applied to the pulley h, mounted upon the cutter spindle.

The cutter slide f is operated through the medium of a screw and bevel-gears from a shaft h¹, which is arranged to revolve alternately in opposite directions from a continuous motion of the driving cone pulley t, receiving, motion from the feed pulley i, through the means of a swinging arm, carrying a receiving pulley and cone as is shown in Fig. 2069.

The method of obtaining these opposite motions of the shaft h¹ will be seen in Fig. 2071. To the block h² which supports the shaft h¹ is secured a gear h³, which engages with a pinion h⁴ mounted loosely on the cone pulley i¹. Side by side with this gear is placed a second gear h⁵ also engaging with the pinion h⁴ and having one tooth less than the gear h³. This gear is mounted loosely on the shaft h¹ and is sleeved through the block h², and to it is secured a ratchet clutch j.

This arrangement produces a motion analogous to that of worm gearing; the revolution of the cone i¹ carrying the pinion h⁴, causes the gear h⁵ to be moved in the opposite direction to that of the cone i¹, and at a speed of one tooth for each revolution of the cone. The cone i¹ carries on its outer end a second clutch j¹. The shaft h¹ is made hollow, and two clutches are secured to a rod playing loosely on the hollow shaft, and arranged to be engaged alternately with the clutches j and j¹. This engagement is effected by means of a bell crank k, operated by a shipper rod k¹ on which adjustable dogs are placed, arranged to be operated by the cutter slide f.

This arrangement of feed shipping motion is very positive in its action, and allows of a very quick return of the cutter slide. The parts are so proportioned that the slide returns thirty-three times as fast as the forward motion, and yet on the very fastest speeds there is no perceptible jar of the parts. The entire mechanism can be disconnected from the feed screw, when desired, by disengaging the clutch j³ on the feed screw. The means employed for spacing the wheel blank are shown in Figs. 2070 and 2072. At the rear end of the spindle c is secured a worm-wheel l. This worm-wheel is made in two parts screwed firmly together. By this construction the wheel is made very accurately. The screw holes in the ring l¹ are slightly elliptic. After the wheel has been hobbed out the position of the ring is changed and the wheel re-hobbed, and so on until the teeth will match perfectly in any position of the ring, when the ring is pinned and screwed on permanently. This wheel is driven by a worm m in connection with change gearing m¹, m², in such a way that one turn of the shaft m³ serves for all divisions. To the shaft m³ is secured a graduated plate o, to which is secured a latch plate o¹ by means of a T-slot and bolts. The latch plate o¹ is secured in this manner in order that the plate o may be turned any desired amount of “set over” in bevel-gear cutting, without disturbing the change gearing or latch. This dividing mechanism is driven by an independent belt from the countershaft to the pulley p, which is secured to a pinion p¹, running loose on a stud. The pinion p¹ engages with a gear p² mounted loosely on the shaft m³. This gear is made to drive the latch plate o¹ at the proper time by means of friction plates, which are set to the required tension by check nuts. The latch plate o¹ is held by a spring latch v, which is secured to an arm v¹ mounted loosely on a stud. The arm v¹ is moved by a disk v² carrying a secondary latch v³. This secondary latch v³ has on one side a roll which engages with a fixed cam v⁴ which trips the latch v³ from its connection with the arm v¹, thus allowing the spring on the latch v to return it to its seat in the latch plate o¹.

The disk v² is moved by a steel ribbon (s, Fig. 2070) which is connected to a pair of plates, t t¹, held together by a T-slot and bolts, and mounted loosely upon the carriage which carries the cutter slide f. The object of the double plates is to take up the slack ribbon, in any required position of the carriage, on the knee e. To the inside plate t¹ is connected a shipper rod t², which carries a dog and is operated by the return motion of the cutter slide f. A spiral spring coiled on the stud supporting the disk v² returns the disk to its original position on the forward motion of the cutter slide f and reseats the secondary latch v³ in its seat in the arm v¹. This arrangement of dividing mechanism requiring but one turn of the shaft m³ possesses some very decided advantages over the ordinary way of simple gearing and multiplied turns. The latch v is tripped immediately after leaving its seat in the plate o¹, and is returned by its spring against the periphery of the plate, and is surely seated by means of a lip on the upper side of the plate. Should it, however, fail by reason of any accident no harm will be done as the gear will be correctly spaced whenever the latch is seated, only one or more spaces will have been missed. Another advantage is that the feed gear can be disconnected and the latch withdrawn, thus allowing the gear to be revolved for the purpose of examination without any necessity for remembering the exact number of turns. When the latch is again seated the gear will be always properly spaced.

Fig. 2073 represents the same machine made half automatic, or in other words the feed is automatic, but when the cut is through, the worm that actuates the feed is thrown out of gear by a catch which lets the box or bearing at the left hand of the worm shaft drop vertically, this catch being operated by a stop on the side of the cutter slide. The method of arranging the feed mechanism so that it shall remain undisturbed, and require no alteration or adjustment at whatever height the knee carrying the cutter slide may be, is substantially the same as that already described with reference to the universal milling machine in Fig. 1893, while the dividing mechanism and other general features are the same as in the full automatic, with the exception of the mechanism for operating the cutter during the return stroke, and operating the dividing mechanism, both of which operations are done by hand in the half-automatic machine.

Fig. 2074 represents a Whitworth machine in which the cutter is carried in a vertical spindle carried in a sliding head. a is the driving pulley, b a pair of bevel-gears, and c a pinion driving the cutter spindle d, the cutter being at e. The cutter spindle has journal bearing at each end in arms upon the sliding head f, which is operated along the slideway of h by the gear-wheel g, receiving motion from the worm at c; at k is the index wheel, the wheel to be cut being carried on its shaft at m. The head n, carrying the index-wheel shaft, may be moved along the bed on which it slides by the handle p, which operates a screw within the bed, and engaging a nut on the under side of n. The worm for the worm-wheel k is carried beneath the wheel by a bracket from n, and being on a splined shaft moves with n. p is the handle for the divisions, the latter being obtained by means of change wheels at j, which connect with the worm shaft. By employing change gears the handle p makes a complete turn for any division, and is locked in a recess, which determines when an exact turn has been made. The range of a machine of this design is very great, because of the length of the bed on which the head n slides, which may be longer than would be practical if it stood upright.

Fig. 2075 represents a gear planing machine, shown with a bevel-gear in place. The main spindle is horizontal upon a fixed head, and has its dividing mechanism at the back of the machine. A single pointed tool is used in a slide rest, operated (by crank motion) upon the horizontal slideway shown, which may be set at any required angle for bevel-wheels. The cut is carried from the point to the flank of the tooth, and is put on by a rod and ratchet motion, the rod striking against the stop seen beneath the cross slide for the slide rest, and on the side of the horizontal slideway.

Figs. 2076, 2077, 2078, 2079, 2080, 2081, and 2082 represent different views of a gear-cutting machine, which consists of a bed plate a a, Figs. 2077, 2078, and 2079, having an extension at end a², to support the hollow cylindrical column a³, which carries an overhead shaft a, at one end of which is a four-step cone a³, for driving the cutter feed motions. At the other end are the tight and loose pulleys for driving this shaft, upon which is also a series of grooved pulleys a⁵, arranged in the form of a cone. The object of this is to drive the cutter. At the base of the column a³ is a corresponding series of grooved pulleys, also arranged in the form of a cone a⁶. A round belt is employed. The shaft on which a⁶ is placed extends through the column, and on its opposite end a grooved pulley is also placed. This serves to drive a belt which, passing over a series of idle pulleys, as will be seen by reference to Figs. 2076 and 2077, drives the rotary cutter.