Modern Machine-Shop Practice Part 139

The emery wheel should be of a grade of not less than 60 or more than 70. If it is too coa.r.s.e it leaves a rough edge, which may, however, be smoothed with an oilstone slip. If the wheel is too fine it is apt to _burn_ the cutter, or in other words, to soften the cutting edge, which may be known by a fine blue burr that may be seen on the front face of the tooth, the metal along this line being softened.

The diameter of the wheel may be larger for small cutters than for large ones, since the teeth of small cutters clear the wheel better. The larger the wheel the less the curvature on the top of the tooth.

[Ill.u.s.tration: Fig. 2012.]

[Ill.u.s.tration: Fig. 2013.]

For general work a diameter of 2-1/2 inches will serve well, the thickness being about 5/16 inch or 3/8 inch. The speed of a wheel of this diameter varies in practice from 3,000 to 4,500 revolutions per minute, but either too fast or too slow a speed will cause the wheel to burn the cutter, and the same thing will occur if the cutter is fed too fast to the wheel, or if too deep a cut is taken. The finishing cut should obviously be very small in amount, especially in cutters of large diameter, for otherwise the wear in the diameter of the wheel will sensibly affect the teeth height, those last ground being the highest.

CHAPTER XXIII.--EMERY WHEELS AND GRINDING MACHINERY.

EMERY WHEELS AND GRINDING.--Emery grinding operations may be divided into four cla.s.ses as follow:--

1st. Tool or cutter grinding, in which the emery wheel is used to sharpen tools which, from their shape, were formerly softened and sharpened by the file, already largely treated in the preceding chapter.

2nd. Cylindrical grinding, in which both the work and the emery wheel are revolved, as has been explained with reference to grinding-lathes.

3rd. Flat surface grinding, in which the emery wheel takes the place of the ordinary steel cutting tool; and

4th. Surface grinding, in which the object is to remove metal or to smoothen surfaces.

The distinctive feature of the various makes of solid emery wheels lies in the material used to cement the emery together, and much thought and experiment are now directed to the end of discovering some cementing substance which will completely meet all the requisite qualifications.

Such a material must bind the emery together with sufficient strength to withstand the centrifugal force due to the high speeds at which these wheels must be run to work economically; and it must neither soften by heat nor become brittle by cold. It must not be so hard as to project above the surface of the wheel; or in other words, it should wear away about as fast as does the emery. It must be capable of being mixed uniformly throughout the emery, so that the wheel may be uniform in strength, texture, and density. It must be of a nature that will not spread over the surface of the emery, or combine with the cuttings and form a glaze on the wheel, which will prevent it from cutting. This glazing is, in fact, one of the most serious difficulties to be encountered in the use of emery wheels for grinding purposes, while it is a requisite for polishing uses, as will be explained farther on. Many of the experiments to prevent glazing have been in the direction of discovering a cement which would wear away under about the same amount of duty as is necessary to wear away the cutting angles of the grains of emery, thus allowing the emery to become detached from the wheel, rather than to remain upon it in a glazed condition.

With the same grade of emery the wheel will cut more freely and glaze less in proportion as the cementing material leaves the wheel softer, but the softer the wheel the more rapidly it will wear away; indeed it is the dislodgement of the emery points as soon as they have become dulled that produces freedom from glazing. It may be remarked, however, that the nature of the material operated upon has a good deal to do with the glazing; thus wrought iron will glaze a wheel more quickly than hardened steel, and bra.s.s more quickly than wrought iron, while on the other hand soft cast iron has less tendency than either of them to glaze. Glazing occurs more readily in all cases upon fine than upon coa.r.s.e wheels. Glazing is more apt to occur as the work is pressed more firmly to the wheel, and with broad and flat surfaces rather than with cylindrical ones. An excellent material for removing the glaze from an emery wheel is a piece of ordinary pumice stone.

The princ.i.p.al cements used in the manufacture of emery wheels are as follows, each representing the cement for one make of wheel:--

1. Hard rubber. 2. Chemical charcoal (leather cut down by acid and used to prevent shrinkage), and glue. 3. Oxychloride of zinc. 4. Sh.e.l.lac. 5.

Linseed oil and litharge. 6. Silicate of soda and chloride of calcium.

7. Celluloid. 8. Oxychloride of magnesium. 9. Infusoria. 10. Ordinary glue.

The vitrified emery wheel is made with a cement which contracts slightly while cooling, leaving small pores or cells through which water, introduced at the centre, is thrown (by centrifugal force) to the surface. This causes, when the wheel is rotating, a constant flow of water from the centre to the surface, carrying off the cuttings and the detached emery.

In order that an emery wheel shall run true with its bore it must fit the driving spindle, and in order that it may do this closely the wheel bore is sometimes filled with lead, the latter being bored out to fit the spindle. If the bore of the emery wheel itself were made a tight fit to the spindle it would abrade the spindle in being put on, and the pressure of the fit if any would tend to split the wheel. A common method of securing emery wheels to their spindles is to fill the bore of the wheel with lead, and bore it out to fit the spindle of the emery grinding machine. The f.l.a.n.g.es between which the wheel is held are recessed so as to grip the wheel at and near their perimeters only.

Between the f.l.a.n.g.e and the wheel a thin disk of sheet-rubber is sometimes used to afford a good bedding for the f.l.a.n.g.e.

The forms of the perimeters of emery wheels are conformed to suit the form of the work to be ground, and it is found that from the great strength of the emery wheel it can be used to a degree of thinness that cannot be approached in any kind of grinding stone. For instance, vulcanite emery wheels 18 inches in diameter and having 3/16 inch thickness, or face as it is commonly termed, are not unfrequently used at a speed of some 5,000 feet of circ.u.mferential feet per minute, whereas it would be altogether impracticable to use a grindstone of such size and shape, because the side pressure would break it, no matter at what speed it were run. Indeed, in the superior strength of the emery wheels of the smaller sizes lies their main advantage, because they can be made to suit narrow curvatures, sweeps, recesses, &c., and run at any requisite speed under 5,000 feet per minute, and with considerable pressure upon either their circ.u.mferential or radial faces.

GRADES OF COa.r.s.eNESS OR FINENESS OF EMERY WHEELS.--Emery is found in the form of rock, and is crushed into the various grades of fineness. The crushing is done either between rollers or by means of stamps, the latter, however, leaves the corners of the grains the sharpest, and hence the best for cutting, though not for polishing purposes. The grades of emery are determined by pa.s.sing the crushed rock through sieves or wire cloths having from eight to ninety wires to the inch; thus, emery that will pa.s.s through a sieve of sixty wires to the inch is called No. 60 grade.

The finest grade obtained from the manufactory is that which floats in the atmosphere of the stamping room, and is deposited on the beams and shelves, from where it is occasionally collected. Washed emery is used by plate-gla.s.s workers, opticians, and others that require a greater degree of fineness than can be obtained by the sieve.

The numbers representing the grades of emery run from 8 to 120, and the degree of smoothness of surface they leave may be compared to that left by files as follows:

8 and 10 represent the cut of a wood rasp 16 " 20 " " coa.r.s.e rough file 24 " 30 " " ordinary rough file 36 " 40 " " b.a.s.t.a.r.d "

46 " 60 " " second cut "

70 " 80 " " smooth "

90 " 100 " " superfine "

120, F & FF " " dead smooth "

The F and FF emery is flour emery which has been washed to purify it.

The following are the kinds of wheel suitable for the respective purposes named:--

Kind of work. Kind of wheel.

For rough grinding, such as on the edges of iron } or steel plates, for removing the protuberances on } Coa.r.s.e grain and castings or on narrow surfaces where rough grinding } hard texture.

is sufficient. }

For narrow surfaces, such as moulding knives, lathe } Medium grain and tools, saw gumming, &c. } hard texture.

For free cutting without gumming on broad surfaces } Medium grain and on iron, steel, or bra.s.s. } soft texture.

For grinding fine tools, such as milling machine } cutters, or for work in which the duty is not great } Fine grain and while the wheel requires to keep its shape and keep } soft texture.

true. }

For smooth grinding on soft metals, as cast iron } Fine grain and and bra.s.s. } hard texture.

[Ill.u.s.tration: Fig. 2014.]

When the work is presented to the wheel unguided, the wheel wears out of true, because the work can follow the wheel, hence it becomes necessary to true the wheel occasionally. This can be done by a tool such as in Fig. 2014, which is applied by hand on the hand rest, and corresponds to the tool shown in Fig. 2061 for grindstones, or by the use of a diamond set in a tool to be held by hand or in a slide rest. The diamond produces the most true and smooth work, but the cut of the wheel is at first impaired by the action of the diamond, which is not the case with the tool in Fig. 2014.

Corundum is a mineral similar to emery, and corundum wheels are made and used in the same manner as emery wheels. Their cutting qualifications are, however, superior to those of the emery wheel, both cutting more freely and being more durable with less liability to glaze.

SPEEDS FOR EMERY WHEELS.--The speed at which an emery wheel may be run without danger of bursting varies according to the thickness or breadth of face of the wheel, as well as according to the quality of the cementing material and excellence of manufacture. Hence, although a majority of manufacturers recommend a speed of about 5,000 circ.u.mferential feet per minute, that speed may be largely exceeded in some cases, while it would be positively dangerous in others. It is, in fact, impracticable in the operations of the workshop to maintain a stated circ.u.mferential speed, because that would entail a constant increase of revolutions to compensate for the wear in the diameter of the wheel. Suppose, for example, that a wheel when new is a foot in diameter: a speed of about 1,600 revolutions per minute would equal about 5,000 circ.u.mferential feet; whereas, when worn down to 2 inches in diameter, the revolutions would require, to maintain the same circ.u.mferential speed, to be about 9,500 per minute, entailing so many changes of pulleys and counter-shafting as to be impracticable. In practice, therefore, a uniform circ.u.mferential speed does not exist, the usual plan adopted being to run the large-sized wheels, when new, at about the speed recommended by the manufacturer of the kind of wheel used, and to make such changes in the speed of the wheel during wear as can be accomplished by changing the belt upon a three-stepped cone pulley, and perhaps one, or at most two, changes of pulley upon the counter-shaft. It is sometimes practicable to use wheels of a certain diameter upon machines speeded to suit that diameter, and to transfer them to faster speeded machines as they diminish in diameter. Even by this plan, however, only an approximation to a uniform speed can in most cases be obtained, because as a rule certain machines are adapted to certain work, and the breadth of face and form of the edge of the emery wheel are very often made to suit that particular work. Furthermore, a new wheel is generally purchased of such a size, form, and grade of emery as are demanded by the work it is intended at first to perform.

Neither is it, as a rule, practicable to transfer the work with the diametrically reduced wheel to the lighter and faster-speeded grinding machine. So that, while it is desirable to run all emery wheels as fast as their composition will with safety admit, yet there are practical objections to running small wheels at a rate of speed sufficient to make their circ.u.mferential velocities equal to those of large wheels. The speeds recommended for the various kinds of wheels now in use vary from about 2,700 to 5,600 circ.u.mferential feet per minute; but the speeds obtaining in workshops average between 2,000 and 4,000 feet for wheels 3 inches and less in diameter, and from about 3,000 to 5,600 feet for wheels above 12 inches in diameter. Wheels above 15 inches in diameter, and of ample breadth of face, are not unfrequently run at much greater velocities.

On account of the high velocity at which emery wheels operate, it is necessary that they be very accurately balanced, otherwise the unequal centrifugal motion causes them to vibrate very rapidly, every vibration leaving its mark upon the work.

The method of balancing adopted by one firm is as follows: The arbors are of cast iron, and are cast standing vertical so as to induce equal density in the metal, it having been found that if the arbors were cast horizontally the lower part of the metal would from the weight of the molten metal be more dense than that at the top of the casting. In casting the arbors upright, the difference in the density of metal simply causes one end of the arbor to be more dense than the other, and the difference being at a right angle to the plane of revolution has no tendency to cause vibration. The driving pulleys are cast horizontal to obtain equal density, and after being turned are carefully balanced. The driving pulleys are held to the arbors by being bored a driving fit, and are driven on so as to avoid the use of keys, which would throw the wheels out of balance.

The centrepiece and f.l.a.n.g.e to hold the wheel to the arbor are turned and balanced. The nut to hold the wheel is a round one, which is easier to balance than a hexagon nut. After the centrepiece is put on the arbor, the whole is tried for balance, and corrected if necessary. The pulley is then put on and the whole is again balanced, and so on, the arbor being balanced after each piece is added, so that while each piece is balanced of itself the whole is balanced after the addition of each separate piece.

[Ill.u.s.tration: Fig. 2015.]

[Ill.u.s.tration: Fig. 2016.]

The emery or corundum wheel is then put on the arbor and tried for being in balance. The method of correcting the balance of the wheel is as follows: The arbor with the wheel on is placed in the lathe, the wheel turned true with a diamond tool (the wheel revolving at a comparatively slow speed). The arbor is then revolved at its proper speed (5,000 circ.u.mferential feet per minute), and a point applied to just meet the circ.u.mference will touch the wheel where it is heaviest, leaving a line as shown in Fig. 2015 at A. The centre of the arbor is then moved over towards this line as shown in Fig. 2016, in which W is the wheel, the location of the line A (marked as above) being as denoted by the arc A, and C represents the arbor whose centre is moved over towards the arc A.

When therefore the arbor is again put in the lathe, it will run out of true by reason of the centre at one end having been altered. A cut is taken down that radial face of the wheel which faces the end of the arbor that has had its centre moved so that the wheel is turned thinner where the mark (A, Fig. 2016) is. The amount of cut to be taken off is a matter of judgment and trial, since it must be just sufficient to compensate for the greater density of the wheel on that side. This greater density, be it noted, occurs from the difficulty in mixing the corundum or other abrasive grains with the cementing material with entire uniformity throughout the ma.s.s.

By this method of balancing, the wheel will remain in true balance notwithstanding its wear, because the balancing proceeds equally from the perimeter towards the centre of the wheel.

[Ill.u.s.tration: Fig. 2017.]