Modern Machine-Shop Practice Part 141

Fig. 2039 represents 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.

[Ill.u.s.tration: Fig. 2038.]

It is obvious that if a piece of work, as A in Fig. 2040, be held steadily upon the rest R, its end will be ground to the curvature of the emery wheel W, 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.

Furthermore, if the piece of work be of a less width than the thickness of the wheel, as in the top view in Fig. 2041, the work A will wear a groove on the wheel, and its side edges will therefore become rounded off unless it be given sufficient motion in the direction of D and E to 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 to Fig. 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.

Suppose, for example, that in Fig. 2042 the duty has been confined to that part of the wheel face from A to the perimeter and the wheel would wear as shown, the result being the same whether the width or distance from the shoulder A to 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.

This difficulty may be overcome by recessing the wheel face, as in Fig.

2043, in which the wheel is shown in section.

In some cases, as for grinding the knives for wood-working machines, hollow cylindrical wheels, such as in Fig. 2044. are used, the duty being performed on the end face B B of the wheel, and the work being traversed in the direction of the arrows. The wheel is here gripped between the f.l.a.n.g.e F and the collar C, which fits accurately to the end of the driving spindle S, so as to be held true, and secured by screws pa.s.sing through C and into F, or the end of S may be threaded to receive a nut to screw against C.

The circ.u.mferential 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 in Fig. 2045. In this case, however, the cut must be taken while the work P is travelling in the direction denoted by the arrow J, and no cutting must be done while the work is travelling back in the direction of K. After the work has traversed back in the direction of K, 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 pa.s.ses 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 from wear, which is not found sufficient to be of practical importance. If the whole surface of the work does not pa.s.s across or beneath the wheel thickness the wheel face may wear taper. Suppose, for example, that in Fig. 2047, P is a piece of work requiring to have produced in it a groove whose bottom is to be parallel to the lower surface F. Then the upper work surface being taper the thick side A would wear away the side B of the wheel, and the groove ground would not be parallel to F.

[Ill.u.s.tration: Fig. 2039.]

[Ill.u.s.tration: Fig. 2040.]

[Ill.u.s.tration: Fig. 2041.]

[Ill.u.s.tration: Fig. 2042.]

[Ill.u.s.tration: Fig. 2043.]

[Ill.u.s.tration: Fig. 2044.]

Another method of grinding flat surfaces is to mount the emery wheel beneath a table T in Fig. 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.

[Ill.u.s.tration: Fig. 2045.]

[Ill.u.s.tration: Fig. 2046.]

[Ill.u.s.tration: Fig. 2047.]

In Fig. 2049 T represents a table and B a guide bar thereon. The depth of cut taken will be equal to the height the emery wheel projects above the surface A of the table, hence when the bar has been moved nearly half-way across the table its surface will be as in Fig. 2050, the bar occupying the position shown in Fig. 2051. Now the part of the bar that has pa.s.sed over the table will not rest upon it as is shown in Fig.

2051. When the bar has pa.s.sed over the emery wheel more than half of the bar length, its end F, Fig. 2052, will fall to meet the half D of the table, and end E will lift from the half C of 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 ground practically 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.

[Ill.u.s.tration: Fig. 2048.]

[Ill.u.s.tration: Fig. 2049.]

[Ill.u.s.tration: Fig. 2050.]

POLISHING WHEELS.--For polishing purposes as distinguished from that of grinding, various forms of polishing wheels are employed. For the rougher cla.s.s 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.

[Ill.u.s.tration: Fig. 2051.]

[Ill.u.s.tration: Fig. 2052.]

[Ill.u.s.tration: Fig. 2053.]

[Ill.u.s.tration: Fig. 2054.]

[Ill.u.s.tration: Fig. 2055.]

The joints of the sections or segments are broken--that is to say, suppose in Fig. 2053 that 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 lines A, B, C, &c., up to F. 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 about 1/4 inch 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 circ.u.mferential 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 circ.u.mference of the wheel sole leather is used, its thickness being about 1/4 inch; 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 in Fig.

2054 at A, and the joint made diagonal, as shown in Fig. 2055 at A.

If the leather were put on with a square b.u.t.t 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 in the 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, leaving 1/10 inch 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, about 3/16 inch in diameter, driven through the leather into the wood and cut off slightly below the surface of the leather.

[Ill.u.s.tration: Fig. 2056.]

[Ill.u.s.tration: 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 in Fig. 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 coa.r.s.er 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 circ.u.mference of the wheel, and cover it as quickly as possible with the emery. Then take a piece of board about 3/4 inch 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 circ.u.mferential 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 circ.u.mference of the wheel, as ill.u.s.trated in Fig. 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 circ.u.mference 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 a.s.sistant 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 circ.u.mference. These wheels are excellent appliances, whether employed upon iron, steel, or bra.s.s. Their sizes run from 1-1/2 inch to about 8 inches in diameter, and the hair of the brush should not exceed from 1 to 1-1/4 inches 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 bra.s.s 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 bra.s.s polishing, the first stages are performed with powdered pumice-stone mixed with sufficient oil to hold it together. This material has considerable cutting qualifications. The next process is with rouge and crocus mixed, and for very fine finishing rotten-stone.

Solid leather wheels are much used by bra.s.s-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 a.s.sume 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 bra.s.s-work, an excellent polishing composition is composed of crocus 2 parts, wax 1 part, rouge 1/8 part, 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 bra.s.s, 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.

[Ill.u.s.tration: Fig. 2058.]

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 coa.r.s.e 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 coa.r.s.eness 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 cla.s.s of work named:--

FOR GRINDING MACHINISTS' TOOLS.

----------------+-------------+-----------------+---------------- Name of stone. |Kind of grit.|Texture of stone.|Color of stone.

----------------+-------------+-----------------+---------------- Nova Scotia. |All kinds, |All kinds, from |Blue or |from finest |hardest to |yellowish grey.

|to coa.r.s.est. |softest. | | | | Bay Chaleur |Medium to |Soft and sharp. |Uniformly light (New Brunswick).|finest. | |blue.

| | | Liverpool or |Medium to |Soft, with sharp |Reddish.

Melling. |fine. |grit. | ----------------+-------------+-----------------+----------------