TABLE OF DIVISIONS CORRESPONDING TO GIVEN CIRCUMFERENTIAL DISTANCES This table gives approximate number of divisions and distances apart on circumference, corresponding to a known diameter of work. It is useful in milling-machine work in cutting mills, saws, ratchets, 25 12 31 16 6 8 9 38 19 44 22 II 50 25 13 6789 75 38 19 13 15 II 879 6699 100 50 25 17 13 10 7 789 080 1649 126 63 31 21 16 13 10 200 100 50 34 25 20 17 14 12 II ΙΟ 226 113 56 38 28 23 8 13 II IO 19 16 14 13 II IO 18 16 14 12 I I ΙΟ 20 17 15 14 13 12 IO 30 25 22 19 17 15 14 13 II 402 201 100 67 50 40 34 29 25 22 20 For example: A straddle mill, say, 5 inches in diameter, is to be cut with teeth apart. Without a table of this kind the workman will have to go to the trouble of multiplying the diameter by 3.1416 and then divide by to find the number of teeth to set up for. In the table, under and opposite 5, he can find at once the number of divisions, as 36. Where the table shows an odd number of teeth, one more or less can, of course, be taken if it is important to have even number of teeth. GRINDING AND LAPPING GRINDING WHEELS AND GRINDING The Commercial Abrasives EMERY, corundum, carborundum, and alundum are the ordinary commercial abrasive materials. They vary in hardness, though it does not follow that the hardest grit is the best for cutting purposes; the shape and form of fracture of the particles must also be taken into consideration. We may imagine a wheel made up from diamonds, the hardest substance in nature, and whose individual kernels were of spherical form; it is quite obvious that it would be of little service as a cutting agent; on the other hand, if these kernels were crystalline or conchoidal in form it would probably be the ideal grinding wheel. Emery is a form of corundum found with a variable percentage of impurity; it is of a tough consistency and breaks with a conchoidal fracture. Corundum is an oxide of aluminum of a somewhat variable purity according to the neighborhood in which it is mined; its fracture is conchoidal and generally crystalline. Carborundum is a silicide of carbon and is a product of the electric furnace; it breaks with a sharp crystalline fracture. Alundum is an artificial product, being a fused oxide of aluminum. It is of uniform quality with about 98 per cent. of purity. It breaks with a sharp, conchoidal crystalline fracture and has all the toughness of emery. Grading of Wheels Of the many firms engaged in the manufacture of grinding wheels there are probably no two which have a similar method of grading or designating the hardness of their wheels. The Norton Company, which is probably the oldest in the field, uses the letter method, which may be said to be the simplest. That is, they take M for their medium-hard wheel and the letters before M denote in regular alphabetical progression the progressively softer wheels. Moreover they use a+mark for denoting wheels which vary in temper from the standards. Thus a wheel may be harder than the standard K, and still be not so hard as the standard L; in this case it is known as K+. The Carborundum Company adopts a somewhat similar method of grading, the difference being that although M denotes its mediumhard wheel the letters before M denote the progressively harder grades. Various other American companies use the letter method of grading to some extent, but all have individual ideas as to what degree of hardness should constitute an M or medium-grade wheel. Then there are firms both in America and on the continent of Europe which discard the letter method of grading or else use it in conjunction with numbers or fractions of numbers such as 2H, 1M and so on. The selection of suitable wheels for machine grinding may be said to be governed by the following points, namely, the texture of the material to be ground, the arc of wheel contact with work and the quality of finish required. The first and last of these points can for convenience' sake be taken in conjunction. The quality of surface finish is dependent on the condition of the wheel face and depth of cut rather than on the fineness of the grit in the wheel. A wheel of so fine a grit as 100 will give an indifferent finish if it is not turned true and smooth. It may be assumed that for all general purposes the aim in view is to procure a wheel which will fulfil two conditions, that is, that it shall first remove stock rapidly and at the same time give a decent finish. Wheels made from a combination of grit of different sizes are the best for this purpose, as may be seen from the following explanation. Coarse wheels of an even number of grit will remove stock faster than will fine wheels of an even number, because their depth of cut or penetration is greater. They, however, fail in giving a high surface finish except in grinding very hard material, because they are not compact enough. The Combination Grit Wheel WITH the combination wheel the conditions are different and it seems better at removing stock than does the coarse, even grit wheel. It may be safe to assume from this that something of a grindstone action takes place, that is, that the finer particles of grit become detached from the bond and both roll and cut in their imprisoned condition between the larger particles. For finishing purposes this wheel has all the compactness and smooth face of a wheel which was made solely from its finest number of grit; and for roughing, it enables a depth of cut to be got which is within the capacity of its largest kernels. With regard to the texture or hardness of material ground it may be taken as a general rule that the harder the material is, the softer the bond of wheel should be, and that cast iron and hardened steel bear some relation to each other as far as grinding wheels are concerned, for the same wheel is usually suitable for both materials. Too large an assortment of wheels is likely to lead to confusion and we may take the Norton plain cylindrical grinding machine as being a case in point of a limited assortment of wheels; at the sams time it will be a starting point to illustrate choice of wheels under various grinding conditions. In this machine four different grade wheels, all of 24 combination grit, are found sufficient for all classes of material that it is ordinarily required to grind. These include high- and low-carbon steels, cast iron, chilled iron, and bronze or composition metals. These wheels are graded J, K, L, and M. Hard Wheels ONE of the greatest advantages accruing from grinding is that it ignores the non-homogeneity of material and that it machines work with the lightest known method of tool pressure, thus avoiding all deflections and distortions of material which are a natural result of the more severe machining processes. Yet these objects are too often defeated by the desire for hard and long-lived wheels. A wheel that is too hard or whose bond will not crumble sufficiently under the pressure of cut will displace the work and give rise to many unforeseen troubles. It is also a prolific cause of vibration which is antagonistic to good and accurate work. The advantage claimed for it, that it gives a better surface finish, is a deceptive one, for it mostly obtains this finish at the expense of accuracy. Quality of finish, that is, accurate finish, is merely a question of arranging of work speed, condition of wheel face and depth of cut. In the machine mentioned the suitability of wheels to materials and conditions is found to be as follows, the wheels being in each case of a combination of alundum grit: For hard chilled iron and large diameters of cast iron and hardened steel For medium chilled iron and medium diameters of cast iron 24 J 24 K 24 L 24 M The table given may, speaking generally, be what would be chosen in the way of wheels for the materials given, and in actual practice they soon give evidence as to whether they are suitable. It may be gathered from the table that diameter of work is a factor in the choice of a wheel. This refers to area of wheel contact and is governed by what is shown in the table when broad differences of diameter occur; for instance, it might be necessary to use the K wheel for a large diameter of high carbon steel if the L wheel was evidently too hard. Speed and Efficient Cutting The efficient cutting of a wheel depends very much on the speed of the work, and an absence of knowledge in this respect may often lead to a suitable wheel's rejection. Revolving the wheel at the speed recommended by the maker is the first necessity, and if it is found unsuitable after experimenting with various speeds it should be changed for a softer or harder one as the conditions indicate. Starting from the point that a wheel is desired that shall remove the maximum amount of stock with the minimum amount of wear on the wheel, the indications and method of procedure may be as follows; only it must be understood that this refers to cases where an ample supply of water is being delivered at the grinding point. If, after trying all reasonable work speeds, a wheel should burn the work, or refuse to cut without excessive pressure, or persistently glaze the surface of the work, it is too hard for that particular work and material and may be safely rejected. If, after trying all reasonably reduced work speeds, a wheel should lose its size quickly and show all signs of rapid wear, it is too soft for that particular work and material and may be rejected. These indications refer to all ordinary cases and it may be gathered that the most economical wheel is that which acts in such a manner as to be a medium between the two cases. There is still another point to bear in mind with regard to the size of the grit in the wheel, but which refers more especially to very hard materials such as chilled iron. Either a coarse or combination wheel may go on cutting efficiently in roughing cuts because pressure is exerted, but may begin to glaze when this pressure is much relieved as in finishing cuts. A careful microscopic scrutiny of a wheel that displays this tendency would seem to lead to the following assumption: When a Wheel is Sharp THE wheel face when newly trued with the diamond tool, which is necessary to obtain an accurate finish, shows a promiscuous arrangement of particles, some of which present points and others present a broader face with a rough and granular surface. When the wheel is presented to the hard surface of the work the high points of this granular face and the sharp contour of the kernels will go on cutting until they are dulled and worn down, after which their face area is too great to enter the surface without undue pressure. When the wheel has reached this condition the microscope shows these broader-faced kernels polished to a metallic luster, which bears out the explanation tendered and also makes the remedy quite apparThis is to use a wheel of very fine grit for finishing purposes in these cases or else keep the coarser wheel in condition by repeated dressings with the diamond tool. ent. Wheel Contact REFERENCE to Figs. 1 to 4 will show what actual practice requires in the choice of a wheel so far as the question of wheel contact is concerned. A wheel is shown in contact with four different varieties of work, all of which we will suppose to be of the same material, the depth of cut, much exaggerated, being the same in each case. In the first case it is a shaft of small diameter, and the wheel contact being the smallest the harder grade of wheel would be suitable, comparatively speaking. Assuming that this wheel was found to be suitable it would probably require a softer wheel for the next case, which is a shaft of larger diameter, and the wheel contact proportionately greater. To continue the comparison still further, the third |