case shows the wheel engaged in grinding a flat surface, and the fourth is a wheel grinding internally. In each case practice demands that the wheel shall be progressively softer in bond or grade and is some proof of a consistency in the action of grinding wheels. The Contact Area of a Wheel THE most probable explanation of this may be that as the contact area increases more work is required from each individual kernel of grit and it the sooner becomes dulled; this requires that the bond must be more friable both to allow it to escape easily and to minimize the pressure required to make the wheel cut as the cutting area becomes greater. Following on this reasoning we are able to choose a list of wheels which would be suitable for almost all purposes, and which would be as follows if of Norton grade: This collection of wheels would be suitable for almost any type of grinding machines, though when the wheels are exceptionally narrow a grade or one-half grade higher might be possible; it would, of course, be a matter for a little trial and experiment. The wheels for external cylindrical work may preferably be combination wheels, but for plane surface and internal work they are better made of single grit, about 36 or 46. The great contact area of wheel in these two classes of work is liable to generate much heat so that an open and porous wheel is preferable. Wheel Pressure and Wear As the wheel is a disk built up from a numerous assortment of minute cutting tools which are held in position by a more or less friable bond, in using it we must bring it to bear on the work with a pressure that shall not be so great as to tear these minute tools from their setting until their cutting efficiency is exhausted, for if we do so we are wasting the wheel. To gage the exact amount of the pressure required is a matter of judgment and experience, though where automatic feeds are provided on a machine the right amount of pressure or feed is soon determined. It will also be readily understood that a regular automatic feed is more reliable for the purpose than a possibly erratic hand one. The automatic feed may be set to give a certain depth of cut at each pass of the wheel, and its amount of wear noted; if this wear be found excessive the depth of cut may be reduced. It must not be here forgotten that work speed also enters into this consideration and that a high work speed will tend to wear the wheel excessively; inversely a reduced work speed will reduce the amount of wear. Having these points in mind the right combination of depth of cut and work speed is soon arrived at, and an approximate judgment attained for the future. Grinding Allowances THE amount of stock left for removal by the grinding wheel and the method of preparing the work have both much bearing on the economic use of grinding wheels, and heavy and unnoticed losses often occur through want of a few precautionary measures. The necessary amount of stock to leave on a piece of work as a grinding allowance depends firstly on the type of machine employed, the class of labor engaged in preparing it, and whether it has to be hardened or otherwise. In powerful machines, which will remove stock rapidly, the grinding allowance may be anything from 2 to 4 inch. There are many cases of an especial character when the grinding allowance stated may be exceeded to advantage so long as discretion is used. Straight shafts may often be ground direct from the black bar of raw material inch above finished size, or when shafts of this character must have large reduction on the ends they can be roughly reduced in the turret lathe while in their black state and finished outright more economically in the grinding machine. Very hard qualities of steels or chilled rolls are other cases where it is often more economical to use the grinding machine without any previous machining process, and though there may be sometimes an alarming waste of abrasive material its cost is as nothing compared with other savings that are made. Grinding allowances for hardened work are usually larger than for soft work, to allow for possible distortion; so that individual experience alone can determine the amount to be left. It is sufficient to say that the allowances on case-hardened or carbonized work should not be excessive; otherwise the hardened surface may be ground away. Grinding Hardened Work As far as the actual grinding of hardened work goes, it is indispensable that the whole portion of a piece that is to be ground should be roughed over previous to the final finishing; if it is at all possible to allow some little time to elapse between the two operations so much the better, more especially if it has bent in hardening and been afterward straightened; this will allow of the development of any strain that may be present. Both for special and standard work in a factory a table of grinding allowances can be compiled as a result of experience and posted in a conspicuous position. If this be done and trouble taken to see that it is adhered to, it will save much trouble and be a means of avoiding much unnecessary expense. It is necessary to slightly undercut the corners of shoulders so as to preserve the corner of the grinder's wheel intact. A piece of work should never be prepared in such a manner as to form a radius on the corner of the wheel, for to get the wheel face flat again means much waste of wheel and wear of diamond. Where fillets or radii are necessary they are better got out with a tool, for even if they are to be ground they must be turned good to allow the wheel to conform to their shape. The only excusable reason for grinding a round corner is when the work is hardened or in some special case where the expense incurred is warranted. Use of Water WATER should be applied at the right spot. This spot must be right at the grinding point, whether it be internal, external, or planesurface work, and must be delivered with sufficient force to keep the wheel face clean. If this is not done there is a kind of mud accumulated at the grinding point, which causes glazing. Water is, or should be, used in grinding processs not as a means of quenching heat but rather to prevent its creation and radiation, and so the actual grinding point is the best place to apply it. It is a necessary means of keeping the work at an equable temperature so as to obviate distortion and to make the matter of taking dimensions an actuality rather than a guessing matter. applies equally to all kinds of grinding. This HERE it is perhaps well to give the question of diamonds some little consideration as they are sometimes a very expensive item. A diamond is a very essential part of a grinding machine's equipment, for in its absence a good and highly finished grade of work is an impossibility. It is perhaps unnecessary to state that they should be the hardest rough stones procurable, and that the larger they are the cheaper they are in the end. With regard to their size: This is a known proportionate element in their price per carat, but a large stone allows of a more secure hold in its setting and so the danger of losing it is reduced. As a further precaution against this danger the diamond tool should always be held by mechanical means when using it except in cases which are unavoidable; this may be in cases where profile shapes have to be turned on the wheel face. An attempt to turn by hand a perfectly flat face on a wheel, which is necessary for finishing, must of a necessity end in failure. As a means of preservation of the diamond a full stream of water should be run on it when in use and many light chips are preferable to a few heavy ones. The main thing is to watch that it does not get unduly heated, for this is disastrous to it. Where large quantities of material have to be removed from a wheel the ordinary wheel dresser may be employed to reduce the bulk of the stock, and the diamond only used for finishing to shape. Setting the Diamonds DIAMONDS may be obtained ready fixed in suitable holders or the rough stones may be bought and set by any competent toolmaker. The illustrations show various methods by which they may be held securely and require but little explanation. First, Fig. 5 is the method most commonly used, the diamond being either peened or brazed in position. One disadvantage of this method is that the diamond is apt to break with a chance blow of the peening chisel, or the heat from brazing will sometimes cause fractures; neither is it so easily reset when its point becomes dulled as are the other methods shown. Fig. 6 requires no explanation except that it is advisable to pack the diamond with shredded asbestos fiber to act as a cushion; this method allows of quick resetting. Fig. 7 consists of a small steel cap tapped out to fit the stock as shown. Enough shredded asbestos fiber is inserted between the diamond and stock to hold it firmly in position. This method also allows of quick and safe resetting. The fourth method, Fig. 8, is covered by patent rights and its advantage can be seen at a glance; as the diamond wears, the small peg containing it can be revolved in the stock to present a new cutting edge and be so clamped in position. SPEED FOR GRINDING WHEELS The table below shows the number of revolutions per minute for various diameters of emery wheels, to cause them to run at the peripheral rates of 4000, 5000 and 6000 feet per minute. Ordinarily a speed of 5000 feet is employed, though sometimes the speed is somewhat lower or higher for certain cases. GRADING ABRASIVE WHEELS THE Norton Company uses 26 grade marks, the Carborundum Company 19, while the Safety Emery Wheel Company uses 40. The following table is a comparison between the grade designations of the Norton Company and the Carborundum Company. Intermediate letters between the grade designations indicate relative degrees of hardness between them; the Norton Company manufacturing four degrees of each designation, while the Carborundum Company manufactures three. The Safety Emery Wheel Company's grade list is an arbitrary one with the following designations: C. Extra Soft A. Soft P. Medium O. Hard E. Extra Hard H. Very Soft M. Medium Soft I. Medium Hard |