Fig. 2026 bottomSection.Fig. 2026.
Section.Fig. 2026.
InFig. 2025a cylinderris shown lying in a conical recess, and end views of the cylinder are shown atvandw. Now suppose the line of contact of the roll or cylinder upon the recess represents the cutting edge of the drill, and that we consider the clearance at the outer end, and at that part that in revolving would describe the circleq, and on referring to circlevand the outer circle of the recess, and also to circleswandq, it is seen that there is more clearance forvthan there is forw, and that the clearance of the latter would be still less ifqwere of smaller diameter, and it follows that the clearance is less in proportion as the point of the drill is approached. In determining the amount of clearance, therefore, we are compelled to make it sufficient for the point of the drill, and this under this system of grinding is excessive for the outer diameter of the drill, causing it to dull quickly, it being borne in mind that as the outer corner of the cutting edge of a drill describes the largest circle of any point of the cutting edge it obviously performs the most cutting duty in removing metal, and furthermore revolves at the highest rate of cutting speed, both of which cause it to dull the most rapidly. InFig. 2026we have a conerlying in the coned recess, an end view of the cone being shown atvandw, and if we again consider the line of contact of the cone on the recess to represent the cutting edge and the circumferential surface of the cone as the end surface of the drill, we observe in the end viewsvandwthat the clearance is equal for the two positions, or by varying the degree of taper of the cone we may regulate the amount of clearance at will. It is found preferable, however, to give more clearance as the point of the drill is approached so as to increase the cutting capacity; hence, in this case, the outer corner of the drill has the least clearance, which greatly increases its endurance for the reasons already mentioned, and which were further pointed out in the remarks upon drilling in the lathe. There remains, however, an additional advantage in this method of grinding which may be pointed out, inasmuch as that the clearance produced by the method shown inFig. 2019, while capable of being governed from end to end of the cutting edge, yet increases as the heel of thelandis approached, making the central cutting edge (c,Fig. 2028) more curved in its length so that it approachesthe form of cutting edge of the fiddle drill and this enhances its cutting capability.
Fig. 2027Fig. 2027.
Fig. 2027.
Fig. 2028Fig. 2028.
Fig. 2028.
Referring again to the general view of the machine inFig. 2019, the chuck is supported or carried by the shaft having the ball leverf, which is clearly seen in the rear view,Fig. 2027, and the rod carrying the sleeveb(which holds the centre for supporting the shank end of the drill) is secured to the back of the chuck, as seen in the same figure. When, therefore, leverfis moved over, the drill is moved through an arc of a circle of which the axis of the shaft offis the centre, and this it is that gives clearance to the cutting edge of the drill.
The drill being chucked, the emery wheel is brought up to it by means of the hand wheele, which moves the frameclaterally, the grinding being done by the side face of the emery wheel. On the same shaft aseis a lever which may be used in connection with the stop or pin (against which it is shown lying) to enable an adjustment of the depth of cut taken by the wheel separately when grinding each lip, and yet to permit both cutting edges of the drill to be gauged to the same length.
Suppose, for example, that the point of a drill has been broken so that it requires several cuts or traverses of the emery wheel to bring it up to a point again; then when this has been done on one cutting edge the lever may be set to the stop, so that when the grinding of the second cutting edge has proceeded until the lever meets the stop both edges will be known to be ground of the same length, and will, therefore, perform equal cutting duty when at work.
The depth of cut being adjusted, the leverdis operated to pass the side face of the emery wheel back and forth along the cutting edge of the drill, this lever rocking the framecon which the emery wheel is mounted back and forth in a line parallel to the cutting edge of the drill. Different angles of one cutting edge of the drill to the other are obtained by swivelling the frame carrying the shaft of leverf. The emery wheel is cased in except at a small opening where it operates upon the drill, and may, therefore, be liberally supplied with water without the latter splashing over. Water is continuously supplied to the emery wheel by an endless belt pump, which also delivers water on the end of the drill, enabling heavy grinding cuts to be taken without danger of softening the drill at the cutting edge, which is otherwise apt to occur. The following is the method of operating the machine: Open the jaws of the chuck by means of the hand wheela, insert the drill from the back of the chuck towards the face of the stone, letting the end of the drill rest on the lower jaw, with the cutting edge just touching the end stop; close the jaws temporarily, while the back centrebis run up and clamped; then release the jaws, hold the drill back against the back centrebwith the left hand, at the same time rotating hard against the two side stops on the jaws; then tightly closing the jaws, clamp the drill by means of the hand wheela, using the right hand for this purpose. Throw ball-handlefpart way back, and by means of hand wheelefeed up the stone until it just touches the drill. Bring ball-handlefforward and give additional feed; pass the stone over the face of the drill, back and forth, by leverd, moving ball-handlefback a little between each two cuts. This slices off the stock to be removed; then when entirely over the face of the lip being ground, hold leverdstationary, and rotate the drill against the stone by means of ball-handlef. By this means a heavy slicing cut can be taken and a final smooth finish obtained without any risk of drawing the temper of the drill.
When one lip has been thus formed, slack up the jaws of the chuck, turn the drill half around, pressing its lips as before against the side stops on jaws, and at the same time be sure to hold the drill firmly back against the back centreb(pay no attention to the end stop, which is only used in locating the drill endways in the first setting), tighten chuck, and grind the second lip without any readjustment of the stone. The lips will then be of equal length. During all these manipulations the stop that is arranged in connection with hand wheelecan be slack, and may rest against the pin in the bed made to receive it.
Fig. 2027represents a rear view of the machine, at which there is an attachment for thinning the point of the drill, which is advantageous for the following reasons. InFig. 2028we have a side and an end view of a twist drill, and it can be shown that the angular piece of cutting edgecthat connects the two edgesaandbcannot be given sufficient angle to make it efficient as a cutting edge without giving clearance and angle excessive to the edgesaandb.
Fig. 2029Fig. 2029.
Fig. 2029.
InFig. 2029we may consider the angle of the cutting edge at the cornerhand at the pointsfandg. First, then, it is obvious that the front face for the pointhis represented by the linehh, that forfby lineff, and that forgbygg, and it appears thaton account of the spiral of the flute the front face has less angle to the drill axis as the point of the drill is approached.
Fig. 2030Fig. 2030.
Fig. 2030.
Considering the end of the drill, therefore, as a cutting wedge, and considering the cutting edge at the two pointscande, inFig. 2030, the end face being at the same angle, we see that the pointchas the angleaand pointethe angleb; at the drill point there will be still less cutting angle, and it has, therefore, the least cutting capacity. To remedy this the attachment shown in thefigureis employed, consisting of a frame or head carrying a thin emery wheel, and capable of adjustment to any angle to suit the degree of spiral of the drill flute.
Fig. 2031Fig. 2031.
Fig. 2031.
By means of this emery wheel a groove is cut in the flute at the point of the drill, as shown inFig. 2031, ataandb, thus reducing the length ofc, and therefore increasing the cutting capacity and correspondingly facilitating the feed of the drill. It is found, indeed, that by this means the drill will perform 15 per cent. more duty.
It is obvious, however, that as the thickness of drills at the point increases in proportion to the diameter of the drill, this improvement is of greater advantage with large than with small drills. The reason for augmenting the thickness at the centre with the drill diameter is that the pressure of the cut acts to unwind the spiral of the drill, and if the drill were sufficiently weak at its axis this unwinding would occur, sensibly enlarging the diameter of hole drilled, more especially when the drill became partly dulled and the resistance of the cut increased. By means of the small groovesaandb, however, the point is thinned while the strength of the drill is left unimpaired.
Fig. 2032represents Brown & Sharpe’s surfacing grinder, designed to produce true and smooth surfaces by grinding instead of by filing. In truing surfaces with a file a great part of the operator’s time is occupied in testing the work for parallelism, and applying it to the surface plate to test its flatness or truth, whereas in a machine of this kind both the parallelism and the truth of the work are effected by the accurate guiding of the machine table in its guideways. Furthermore, a high order of skill is essential to the production of work by filing that shall equal for parallelism and truth work that is much more easily operated upon in the machine. The machine is provided with two feed motions, the first of which is in a line parallel with the axis of the emery wheel driving spindle, and is communicated (by means of the small hand wheel on the right) to the lower table, which moves inV-guides provided upon the base plate of the machine. Upon this lower, and what may be termed cross-feed table slides, in suitable guideways, the work-holding or upper table, which is operated (by the large hand wheel) to traverse the work back and forth beneath the grinding wheel. Both these feed motions are operated by hand, automatic feed motions being unnecessary for work of the size intended to be operated upon in this machine. The grinding wheel spindle is carried in a bearing carried in a vertical slide, and is fed to its depth of cut by means of the vertical feed screw and hand wheel shown. The spindle passes through the bearing and carries a pulley at the back of the machine, which pulley is driven by a belt passing over idler pulleys at the back of the machine, by means of which the tension of the driving belt may be regulated.
Fig. 2033represents The Tanite Co.’s machine for surface grinding such work as locomotive guide bars. The emery wheelnis mounted beneath a tablet, whose upper surface is planed true, and which has two cylindrical stemsc dfitting into the bored guidese. The stems are threaded at their lower ends to receive a screw, on the lower end of which is a bevel-gearfmeshing into a similar geargon the shaft actuated by the hand wheelw, hence by operatingwthe height of the table face may be adjusted to suit the diameter of the wheel.
The surface to be ground is laid upon the face of the table, and the operator moves it by hand, slowly passing it over the emery wheel, which projects slightly through the opening shown through the centre of the table. The operator stands at the end of the machine so as to be within reach of the wheel, and the direction of rotation is towards him, so that the work requires to be pushed to the cut and is not liable to be pulled too quickly across the table by the emery wheel.
Fig. 2034Fig. 2034.
Fig. 2034.
Fig. 2034represents an emery grinding machine for grinding the bores of railroad car axle-boxes. The circumference of the emery wheel is dressed to the curvature of the box bore by a diamond toolawhich swings on a centre in its frame, and can be adjusted to any arc. Once set, it can only turn the prescribed arc with accuracy. In order to avoid the necessity of the foreman having to set the tool, a gauge is also furnished. This consists of a spindle adjustable with a nut in such a way that its two points rest in the centres on which the diamond tool revolves. It is only necessary for a diskbturned accurately to the diameter of the bearing, to be prepared, and this the apprentice can place on the spindle, adjust the latter, and screw down the diamond tool until it touches the periphery of the disk. A nut is then fastened on the diamond tool, and the frame is lifted on the ways beneath the wheel, when the moving of the handle turns the face of the wheel to the exact circle desired.
To adjust the brass in the chuckc, it is first set on the axled. The chuck is then placed on framee, in such a way that theVs fit. Handlefthen moves a cam that clamps the brass between the jawsg, one set of which swings on a pivot ath. The brass is thus adjusted in such a manner that, despite the imperfections in moulding, it is ground accurately with the least removal of metal. The chuckcfits into planed guides on the tablei, and is thus brought in exact line with the motion of the wheel. The crankjserves to move the table to and fro on the rodsk, and the table also rises and falls on planed ways, being pressed up by springs. The hand wheel gives vertical adjustment to the whole bed by means of a chain beneath it. There is a pulley by which a suction fan, to remove dust, &c., may be driven. The machine is capable of fitting from 150 to 500 car brasses per day.
Fig. 2035Fig. 2035.
Fig. 2035.
Fig. 2035represents an emery planing machine. The emery wheel, which takes the place of the cutting tool of an ordinary shaping machine, is upon a spindle driven by the pulleyaupon the spindleb, which is traversed endways by means of the connecting rod which is actuated by a crankedriven by the cone pulleyc. The work-holding tablegis traversed by the handlekor automatically through wheelh, which through suitable gearing drives the spindlei. The blower or fan is to draw off the cuttings and emery. It is obvious that any of the usual forms of work-holding devices may be employed.
Fig. 2036Fig. 2036.
Fig. 2036.
Fig. 2036represents an ordinary form of emery grinding machine for general purposes.arepresents the frame affording journal bearing for the driving spindle driven by the cone pulleyp, having the fast flangesfand collarsc, which are screwed up to hold the emery wheel by the nutn, the direction of spindle rotation being denoted by the arrows. The thread at the endkof the spindle must be a right-hand one, and that at the other endlmust be a left-hand, so that the resistance against the nut shall in both cases be in a direction to screw the nuts up and cause them to bind or grip the wheels more firmly, and not unscrew and release the wheels. Upon the frameaare the lugsdto carry the hand restsrands, which are adjustable, and are secured in their adjusted position by the handle nutse. The restsis of the same form and construction as a lathe hand rest, while that atris angular, to support the tool while applying it to the side as well as to the circumference of the wheel.
Fig. 2037Fig. 2037.
Fig. 2037.
Fig. 2037represents a machine for grinding the knives for wood-planing machines, and having a hand feed only. It consists of an emery wheel mounted upon a spindle and with a slide rest in front of it. Mounted on the slide rest is a frame for holding the knife, and a set-screw for adjusting the angle of the knife to the wheel. The slide rest is traversed by means of the hand wheel operating a pinion in the rack shown.
Fig. 2038Fig. 2038.
Fig. 2038.
Fig. 2038represents a swing frame for carrying and driving an emery wheel to be used on the surfaces of castings, its construction permitting it to be moved about the casting to dress its surface. The overhead countershaft carries the grooved driving wheela. Atbis a vertical shaft pivoted atiby the forked bearing which swings upon the countershaft. The forklat the lower end of shaftbcarries a shaft on which is the forkc′,chaving journal bearing on it, and the driving pulleyj. Forkdhas journal bearing on the same shaft as pulleyj, and is fast upon the rod or arme, which affords journal bearing to the emery wheelkon a shaft having handlesh h. Motion to the emery wheel is conveyed through the beltsfandg. To counterbalance the frame the weightwis employed, permitting the frame to be readily swung. The upper fork carryingb, being pivoted to the shaft ofa, permitsbto swing to any required position. The pivot atipermitsbto rotate in a vertical plane; the pivot ofc′ catdaffords vertical movement toe; the pivot atdallowseto rotate about its ownaxis, hence the wheelkcan be moved about laterally, raised, lowered, or have its plane of revolution varied at will by simply swinging the handlesh hto the required plane. The emery-wheel shaft is pivoted upon the fork carrying it, so that the emery wheel can be turned to stand in a horizontal plane if desired.
Fig. 2039Fig. 2039.
Fig. 2039.
Fig. 2039represents an emery belt machine, in which the belt runs vertically and its tension is adjusted by the idler pulley shown at the top of the frame.
Fig. 2040Fig. 2040.
Fig. 2040.
It is obvious that if a piece of work, asainFig. 2040, be held steadily upon the restr, its end will be ground to the curvature of the emery wheelw, and that if it be required to grind the surface flat the piece must be raised and lowered as denoted by the dotted lines, the amount of this motion being determined by the thickness of the piece.
Fig. 2041Fig. 2041.
Fig. 2041.
Furthermore, if the piece of work be of a less width than the thickness of the wheel, as in the top view inFig. 2041, the workawill wear a groove on the wheel, and its side edges will therefore become rounded off unless it be given sufficient motion in the direction ofdandeto cause it to traverse across the full width of the wheel face, and as this motion would require to be simultaneous with the vertical motion explained with reference toFig. 2040, it is not practicable to grind true level surfaces upon the perimeter of the wheel. As the sides of the wheel are flat, however, it is self-suggestive to apply the work to the side faces. But in this case, also, that part of the wheel surface that performs grinding duty will gradually wear away, leaving a shoulder or projecting surface upon the wheel.
Fig. 2042Fig. 2042.
Fig. 2042.
Suppose, for example, that inFig. 2042the duty has been confined to that part of the wheel face fromato the perimeter and the wheel would wear as shown, the result being the same whether the width or distance from the shoulderato the perimeter of the wheel represents the width of the work held steadily against the wheel or the traverse of a narrower piece of work.
Fig. 2043Fig. 2043.
Fig. 2043.
This difficulty may be overcome by recessing the wheel face, as inFig. 2043, in which the wheel is shown in section.
Fig. 2044Fig. 2044.
Fig. 2044.
In some cases, as for grinding the knives for wood-working machines, hollow cylindrical wheels, such as inFig. 2044. are used, the duty being performed on the end faceb bof the wheel, and the work being traversed in the direction of the arrows. The wheel is here gripped between the flangefand the collarc, which fits accurately to the end of the driving spindles, so as to be held true, and secured by screws passing throughcand intof, or the end ofsmay be threaded to receive a nut to screw againstc.
Fig. 2045Fig. 2045.
Fig. 2045.
Fig. 2046Fig. 2046.
Fig. 2046.
Fig. 2047Fig. 2047.
Fig. 2047.
The circumferential surface of a wheel may be employed to grind a flat surface, providing that the work be traversed to the wheel, as in the side view inFig. 2045. In this case, however, the cut must be taken while the workpis travelling in the direction denoted by the arrowj, and no cutting must be done while the work is travelling back in the direction ofk. After the work has traversed back in the direction ofk, and is clear of the wheel, the cut is carried farther across the work by moving or feeding the work in the direction of the arrow in the front view,Fig. 2046. In this case the whole surface of the work passes beneath the wheel thickness, and the wheel face wears parallel to the wheel axis, producing a true plane (supposing the work to be moved in straight lines), save in so far as it may have been affected by the reduction of the diameter of the emery wheel fromwear, which is not found sufficient to be of practical importance. If the whole surface of the work does not pass across or beneath the wheel thickness the wheel face may wear taper. Suppose, for example, that inFig. 2047,pis a piece of work requiring to have produced in it a groove whose bottom is to be parallel to the lower surfacef. Then the upper work surface being taper the thick sideawould wear away the sidebof the wheel, and the groove ground would not be parallel tof.
Fig. 2048Fig. 2048.
Fig. 2048.
Another method of grinding flat surfaces is to mount the emery wheel beneath a tabletinFig. 2048, letting the top of the wheel emerge through an opening in the table, and sliding the work upon the trued upper surface of the table. The surface of the table thus becomes a guide for the work. To obtain true work in this way, however, it is necessary that the cut taken by the emery wheel be a very light one, as will be perceived from the following considerations.
Fig. 2049Fig. 2049.
Fig. 2049.
Fig. 2050Fig. 2050.
Fig. 2050.
Fig. 2051Fig. 2051.
Fig. 2051.
Fig. 2052Fig. 2052.
Fig. 2052.
InFig. 2049trepresents a table andba guide bar thereon. The depth of cut taken will be equal to the height the emery wheel projects above the surfaceaof the table, hence when the bar has been moved nearly half-way across the table its surface will be as inFig. 2050, the bar occupying the position shown inFig. 2051. Now the part of the bar that has passed over the table will not rest upon it as is shown inFig. 2051. When the bar has passed over the emery wheel more than half of the bar length, its endf,Fig. 2052, will fall to meet the halfdof the table, and endewill lift from the halfcof the table, causing the bar surface to be ground rounding in its length. If, however, the cut taken be a very light one the surface may be groundpractically true, because the bar will bend of its own weight and lap down to fit the table at both ends. Furthermore it will be noted that in the case of a large surface in which the wheel might sensibly wear in diameter before it had operated over the whole of the work surface, the table may be lowered or the wheel may be raised (according to the construction of the machine), to offset the wear of the wheel, or rather to take it up as it were.
Polishing Wheels.—For polishing purposes as distinguished from that of grinding, various forms of polishing wheels are employed. For the rougher class of polishing, wooden wheels covered with leather coated with fine emery that is allowed to glaze are employed. For a finer degree of polish the wheels are covered with lead to which various polishing materials are occasionally applied, while for the finest polishing rag or buff wheels are the best. Wooden polishing wheels are built up of sections of soft wood fastened together by gluing, and with wooden pegs in place of nails or screws.
Fig. 2053Fig. 2053.
Fig. 2053.
The joints of the sections or segments are broken—that is to say, suppose inFig. 2053that 1, 2, 3, &c., up to 6, represent the joints of the six sections of wood forming one layer of the wheel, the next six sections would have their joints come at the dotted linesa,b,c, &c., up tof. To prevent them from warping after being made into a wheel it is advisable to cut out the sections somewhere near the size in the rough and allow them to lie a day or two before planing them up and fitting them together; the object being to allow any warping that may take place to do so before the pieces are worked up into the wheel, because if the warping takes place afterwards it will be apt to throw the wheel out of true, whereas it is necessary that these wheels be very true, not only so that they may not prove destructive to their shaft bearings, but that they may run steady, and not shake or terrible, and because the work can be made much more true and smooth with a true than with an untrue wheel. Only one layer of segments should be put on in one day, and they should be put on as quickly as possible after the glue is applied, so that the latter shall not get cold. So soon as each segment is put into its place it should be clamped firmly to its seat and driven firmly up to the joint of the next one, and when the layer is completed it should be left clamped all night to dry. In the morning one clamp should be removed, and that section fastened by boring small holes and driving therein round and slightly tapered soft-wood pegs of about1⁄4inch diameter. The whole of the sections being pegged the next layer of segments may be added, and so on until the required width of wheel is attained. The whole wheel should then be kept two days before it is turned, and as little as possible should be taken off in the turning process. The circumferential surface should be turned slightly rounding across its width, and as smoothly as possible. It is practicable to proceed with the construction of the wheel without waiting between the various operations so long as here advised, but the wheel will in that case be more apt to get, in time, out of true. To cover the circumference of the wheel sole leather is used, its thickness being about1⁄4inch; it should be put on soft and not hardened by hammering at all, and with the flesh side to the wood. The joint of the leather should not be made straight but diagonal with the wheel face, the leather at the edge of the joint being chamfered off, as shown inFig. 2054ata, and the joint made diagonal, as shown inFig. 2055ata.
Fig. 2054Fig. 2054.
Fig. 2054.
Fig. 2055Fig. 2055.
Fig. 2055.
If the leather were put on with a square butt joint there would likely be a crease in the joint, and the emery or other polishing material would then strike the work with a blow, as well as presenting a keener cutting edge, which would make marks inthe work no matter what pains might be taken to prevent it. This, indeed, is found to occur to a slight extent upon very fine polishing, even when the joint of the leather is made as above; and the means taken to obviate it is to not put any polishing material on the immediate joint and to wipe off any that may get there, leaving1⁄10inch clear of polishing material. It is obvious that in fastening the wheel to its shaft it should be put on so that it will run in the direction of the arrow, providing the operator works with the wheel running from him, as is usually the case with large wheels, that is to say, wheels over 18 inches in diameter. In any event, however, the wheel should be put on so that the action of the work is to smooth the edge of the leather joint down upon the wheel, and not catch against the edge of the joint, which would tend to rough it up and tear it apart. The leather should be glued to the wheel, which may be slightly soaked first in hot water. The glue should be applied very hot, and the leather applied quickly and bound tightly to the wheel with a band. One end of the leather may be first glued to the wheel and fastened with a few tacks to hold it while it is stretched tightly round the wheel; the leather itself should be softened by an application of hot water, but not too much should be applied. After the leather is glued to the wheel it is fastened with soft wooden pegs, about3⁄16inch in diameter, driven through the leather into the wood and cut off slightly below the surface of the leather.
Fig. 2056Fig. 2056.
Fig. 2056.
Fig. 2057Fig. 2057.
Fig. 2057.
Wheels of this kind are sometimes made as large as 5 or 6 feet in diameter, in which case the truth of the wheel may be preserved by letting in a wrought-iron ring, as shown inFig. 2056, fastening the rings with wood screws. The wheels thus constructed are covered with emery of grades varying from No. 60 to 120, and flour emery. The coarser grades perform considerable cutting duty as well as polishing. The manner of putting the emery, and fastening it, upon the wheel is as follows:—The face of the wheel is well supplied with hot glue of the best quality, and some roll the wheel in the emery, in which case the emery does not adhere so well to the leather as it does when the operation is performed as follows:—Let the wheel either remain in its place upon the shaft, or else rest it upon a round mandrel, so that the wheel can revolve upon the same. Then apply the hot glue to about a foot of the circumference of the wheel, and cover it as quickly as possible with the emery. Then take a piece of board about3⁄4inch thick and 28 inches long, the width being somewhat greater than that of the polishing wheel, and placing the flat face of the board upon the circumferential surface of the wheel, work it by hand, and under as much pressure as possible, back and forth, so that each end will alternately approach the circumference of the wheel, as illustrated inFig. 2057, the movement being indicated by the dotted lines. By adopting this method the whole pressure placed upon the board is brought to bear upon a small area of the emery and leather, and the two hold much more firmly together than would be the case if the circumference of the wheel were glued and then rolled in a trough of emery, because the time occupied in spreading the glue evenly and properly over the whole wheel surface would permit it to cool before receiving the emery, whereas it is essential that the glue be hot so that it may conform itself to the shape of the grains of emery and hold them firmly.
The speed at which such wheels are used is about 7,000 feet per minute. The finest of emery applied upon such wheels is used for cast iron, wrought iron, and steel, to give to the work a good ordinary machine finish; but if a high polish or glaze is required, the wheels are coated with flour emery, and the wheel is made into a glaze-wheel by wearing the emery down until it gets glazed, applying occasionally a little grease to the surface of the wheel. Another kind of glaze-wheel is made by covering the wooden wheel with a band of lead instead of a band of leather, and then applying to the lead surface a mixture of rouge, crocus and wax, worn smooth by applying to it a piece of sheet steel or a piece of flint-stone before applying the work. Others add to this composition a little Vienna lime. For flat surfaces, or those requiring to have the corners or edges kept sharp, it is imperative that such wheels as above described—that is to say, those having an unyielding surface—be used; but where such a consideration does not exist, brush and rag wheels may be used. In Europe comparatively large flat surfaces requiring a high polish are finished upon wooden wheels made of soft wood and not emeried, the polishing material employed being Vienna lime. The lime for ordinary use is mixed with water, and is applied by an assistant on the opposite side of the wheel to the operator. For superfine surfaces the Vienna lime is mixed with alcohol, which increases its efficiency; and here it may be as well to note that Vienna lime rapidly deteriorates from exposure to the air, so that it should be kept as little exposed as possible.
Brush-Wheels.—These are polishing wheels of wood with a hair brush provided around the circumference. These wheels are excellent appliances, whether employed upon iron, steel, or brass. Their sizes run from 11⁄2inch to about 8 inches in diameter, and the hair of the brush should not exceed from 1 to 11⁄4inches in length. The speed at which they should be run is about 2,500 for the largest, and up to 4,500 revolutions per minute for the smaller sizes. In ordinary grinding and polishing practice in the United States, brush wheels are used with Vienna lime in all cases in which the lime is used by itself—that is to say, unmixed with wax, crocus, or rouge, or a mixture of the same. In watchmaking, however, and for other purposes in which the truth of the work is an important element, Vienna lime is applied to wooden or even metal, such as steel, polishing wheels, which are in this latter case always of small diameter. An excellent polishing composition is formed of water 1 gill, sperm oil 3 drops, and sufficient Vienna lime to well whiten the mixture. The brush may be let run dry during the final finishing. For polishing articles of intricate shape, brush wheels are superior to all others. If the articles to be polished are of iron, or steel, the first stage of the process is performed with a mixture of oil and emery, Vienna lime being used for final finishing only. The wheels to which Vienna lime is applied should not be used with any other polishing material, and should be kept covered when not in use, so as to keep them free from dust.
For brass work, brush wheels are used with crocus, with rouge, or with a mixture of the two, with sufficient water, and sometimes with oil, to cause the material to hold to the brush and not fly off from the centrifugal force. For very fine brass polishing, the first stages are performed with powdered pumice-stone mixed with sufficient oil to hold it together. This material has considerablecutting qualifications. The next process is with rouge and crocus mixed, and for very fine finishing rotten-stone.
Solid leather wheels are much used by brass-finishers. The wheels are made of walrus hide glued together in disks, so as to obtain the necessary thickness of wheel. The disks are clamped between pieces of board so soon as the glue is applied, so as to make a good joint, and also keep the wheel flat and prevent it from warping during the drying process. Such wheels may be run at a velocity of 8,000 feet per minute, and with any of the polishing materials already referred to. After the wheel is made and placed upon its spindle or mandrel it may be turned true with ordinary wood-turning tools—and it may here be remarked that rag wheels may be trued in the same way. The spongy nature of these wheels renders them very efficient for polishing purposes, for the following reasons: The polishing materials become imbedded in the leather and are retained, and become mixed and glazed with a fine film of the material being polished, which film possesses the very highest polishing qualifications. These walrus wheels may be used with pumice, crocus, rouge, or Vienna lime, according to the requirements of the case, or even with a mixture of flour emery and oil; and they possess the advantage of being less harsh than leather or lead-covered wheels, while they are more effectual than the latter, and will answer very well for flat surfaces.
Rag polishing wheels are formed of disks of rags, either woollen or strong cotton, placed loosely side by side, and clamped together upon the mandrel at the centre only. Their sizes range usually from 4 to 8 inches in diameter, and they are run at a speed of about 7,000 feet per minute. They assume a disk form when in motion from the centrifugal force generated from the great speed of rotation. They are used for the fine polishing only, and not upon work requiring the surfaces to be kept very flat or the corners very sharp. For use upon steel or iron, they are supplied with a polishing material composed of Vienna lime 3 parts, crocus 3 parts, beeswax 3 parts, boiled up together, allowed to cool off, and then cut into cakes. These cakes are dipped in oil at the end, which is then applied to the rag wheel occasionally during the polishing process. For brass-work, an excellent polishing composition is composed of crocus 2 parts, wax 1 part, rouge1⁄8part, the wax being melted, and the ingredients thoroughly mixed. This mixture gives to the metal a rich color. It is dipped in oil and then applied to the rag wheel. It may be used to polish fine nickel-plating, for which purpose it is an excellent material. Nickel-plated articles having sharp corners should be polished with fine rouge mixed with clear water and a drop of oil, the mixture being applied to the rag wheel with the finger of the operator. Any of the compositions of rouge, crocus, and rotten-stone may be used for brass, copper, or nickel-plated work upon rag wheels, while for iron or steel work the same materials, separate or in combination, may be used, though they are greatly improved by the addition of Vienna lime. When, however, either of these materials is used singly, it should be applied to the rag wheels with a brush; and if it is used dry, it must be at a greatly reduced speed for the wheel, which is sometimes resorted to for very fine polishing.