casting would be soft. High-silicon irons used in this way are called “soft (For other analyses, see pages 371 to 373.) Ferro-silicons contain a low percentage of total carbon and a high percentage of combined carbon. Carbon is the most important constituent of cast iron, and there should be about 3.4% total carbon present. By adding ferro-silicon which contains only 2% of carbon the amount of carbon in the resulting mixture is lessened. Mr. Keep found that more silicon is lost during the remelting of pig of over 10% silicon than in remelting pig iron of lower percentages of silicon. He also points out the possible disadvantage of using ferro-silicons containing as high a percentage of combined carbon as 0.70% to overcome the bad effects of combined carbon in other irons. The Scotch irons generally contain much more phosphorus than is desired in irons to be employed in making the strongest castings. It is a mistake to mix with strong low-phosphorus irons an iron that would increase the amount of phosphorus for the sake of adding softening qualities, when softness can be produced by mixing irons of the same low phosphorus. (For further discussion of the influence of silicon see page 365.) Shrinkage of Castings.-The allowance necessary for shrinkage varies for different kinds of metal, and the different conditions under which they are cast. For castings where the thickness runs about one inch, cast under ordinary conditions, the following allowance can be made: Thicker castings, under the same conditions, will shrink less, and thinner ones more, than this standard. The quality of the material and the manner of moulding and cooling will also make a difference. Numerous experiments by W. J. Keep (see Trans. A. S. M. E., vol. xvi.) showed that the shrinkage of cast iron of a given section decreases as the percentage of silicon increases, while for a given percentage of silicon the shrinkage decreases as the section is increased. Mr. Keep gives the following table showing the approximate relation of shrinkage to size and percentage of silicon: Shrinkage in Decimals of an inch per foot of Length. Mr. Keep also gives the following "approximate key for regulating fourdry mixtures" so as to produce a shrinkage of 1⁄2 in. per ft. in castings of different sections: Weight of Castings determined from Weight of Pattern. (Rose's Pattern-maker's Assistant.) Moulding Sand. (From a paper on "The Mechanical Treatment of Moulding Sand," by Walter Bagshaw, Proc. Inst. M. E. 1891.)-The chemical composition of sand will affect the nature of the casting, no matter what treatment it undergoes. Stated generally, good sand is composed of 94 parts silica, 5 parts alumina, and traces of magnesia and oxide of iron. Sand containing much of the metallic oxides, and especially lime, is to be avoided. Geographical position is the chief factor governing the selection of sand; and whether weak or strong, its deficiencies are made up for by the skill of the moulder. For this reason the same sand is often used for both heavy and light castings, the proportion of coal varying according to the nature of the casting. A common mixture of facing-sand consists of six parts by weight of old sand, four of new sand, and one of coal-dust. Floor-sand requires only half the above proportions of new sand and coal-dust to renew it. German founders adopt one part by measure of new sand to two of old sand; to which is added coal-dust in the proportion of one tenth of the bulk for large castings, and one twentieth for small castings. A few founders mix street-sweepings with the coal in order to get porosity when the metal in the mould is likely to be a long time before setting. Plumbago is effective in preventing destruction of the sand; but owing to its refractory nature, it must not be dusted on in such quantities as to close the pores and prevent free exit of the gases. Powdered French chalk, soapstone, and other substances are sometimes used for facing the mould; but next to plumbago, oak charcoal takes the best place, notwithstanding its liability to float occasionally and give a rough casting. For the treatment of sand in the moulding-shop the most primitive method is that of hand-riddling and treading. Here the materials are roughly proportioned by volume, and riddled over an iron plate in a flat heap, where the mixture is trodden into a cake by stamping with the feet; it is turned over with the shovel, and the process repeated. Tough sand can be obtained in this manner, its toughness being usually tested by squeezing a handful into a ball and then breaking it; but the process is slow and tedious. Other things being equal, the chief characteristics of a good moulding-sand are toughness and porosity, qualities that depend on the manner of mixing as well as on uniform ramming. Toughness of Sand.-In order to test the relative toughness, sand mixed in various ways was pressed under a uniform load into bars 1 in. sq. and about 12 in. long, and each bar was made to project further and further over the edge of a table until its end broke off by its own weight. Old sand from the shop floor had very irregular cohesion, breaking at all lengths of projections from 1⁄2 in. to 11⁄2 in. New sand in its natural state held together until an overhang of 234 in. was reached. A mixture of old sand, new sand, and coal-dust Mixed under rollers.. broke at 2 to 214 in. of overhang. 66 66 2 "214 134 66 66 66 66 Showing as a mean of the tests only slight differences between the last three methods, but in favor of machine-work. In many instances the fractures were so uneven that minute measurements were not taken. Dimensions of Foundry Ladles.-The following table gives the dimensions. inside the lining, of ladles from 25 lbs. to 16 tons capacity. All the ladles are supposed to have straight sides. (Am. Mach., Aug. 4, 1892.) SPEED OF CUTTING-TOOLS IN LATHES, MILLING MACHINES, ETC. Relation of diameter of rotating tool or piece, number of revolutions, and cutting-speed: Let d = diam. of rotating piece in inches, n = No. of revs. per min.; Approximate rule: No. of revs. per min. = 4 x speed in ft. per min. + diam. in inches. Speed of Cut-for Lathes and Planers. (Prof. Coleman Sellers, Stevens' Indicator, April, 1892.)-Brass may be turned at high speed like wood. Bronze.-A speed of 18 feet per minute can be used with the soft alloyssay 8 to 1, while for hard mixtures a slow speed is required-say 6 feet per minute. Wrought Iron can be turned at 40 feet per minute, but planing-machines that are used for both cast and forged iron are operated at 18 feet per minute. Machinery Steel.-Ordinary, 14 feet per minute; car-axles, etc., 9 feet per minute. Wheel Tires.-6 feet per minute; the tool stands well, but many prefer to run faster, say 8 to 10 feet, and grind the tool more frequently. In Lathes.--The speeds obtainable by means of the cone-pulley and the back gearing are in geometrical progression from the slowest to the fastest. a well-proportioned machine the speeds hold the same relation through all the steps. Many lathes have the same speed on the slowest of the cone and the fastest of the back-gear speeds. The Speed of Counter-shaft of the lathe is determined by an assumption of a slow speed with the back gear, say 6 feet per minute, on the largest diameter that the lathe will swing. EXAMPLE.-A 30-inch lathe will swing 30 inches, say, 90 inches circumfer ence 7'6"; the lowest triple gear should give a speed of 5 or 6 per minute. In turning or planing, if the cutting-speed exceed 30 ft. per minute, so much heat will be produced that the temper will be drawn from the tool. The speed of cutting is also governed by the thickness of the shaving, and by the hardness and tenacity of the metal which is being cut; for instance, in cutting mild steel, with a traverse of 3% in. per revolution or stroke, and with a shaving about 5 in. thick, the speed of cutting must be reduced to about 8 ft. per minute. A good average cutting-speed for wrought or cast Iron is 20 ft. per minute, whether for the lathe, planing, shaping, or slotting machine. (Proc. Inst. M. E., April, 1883, p. 248.) 76.4 152.8 229.2 305.6 382.0 458.4 534.8 611.2 687.6 764.9 50.9 101.9 152.8 203.7 254.6 305.6 356.5 407.4 458.3 509.3 38.2 76.4 114.6 152.8 191.0 229.2 267.4 305.6 343.8 382.0 30.6 61.1 91.7 122.2 152.8 183.4 213.9 244.5 275.0 305.6 25.5 50.9 76.4 101.8 127.3 152.8 178.2 203.7 229.1 254.6 21.8 43.7 65.5 87.3 109.1 130.9 152.8 174.6 196.4] 218.3 Speed of Cutting with Turret Lathes.-Jones & Lamson Machine Co. give the following cutting-speeds for use with their flat turret lathe on diameters not exceeding two inches: Tool steel and taper on tubing......... Threading Machinery. Ft. per minute. 20 20 Turning Forms of Metal-cutting Tools.-"Hutte," the German Engi. neers' Pocket-book, gives the following cutting-angles for using least power: Angle of Cutting-edge. Top Rake. 4o 4o 51° 51° 66° The American Machinist comments on these figures as follows: We are not able to give the best nor even the generally used angles for tools, because these vary so much to suit different circumstances, such as degree of hardness of the metal being cut, quality of steel of which the tool is made, depth of cut, kind of finish desired, etc. The angles that cut with the least expenditure of power are easily determined by a few experiments, but the best angles must be determined by good judgment, guided by expe rience. In nearly all cases, however, we think the best practical angles are greater than those given. For illustrations and descriptions of various forms of cutting-tools, see articles on Lathe Tools in App. Cyc. App. Mech., vol. ii., and in Modern Mechanism. Cold Chisels.-Angle of cutting-faces (Joshua Rose): For cast steel, about 65 degrees; for gun-metal or brass, about 50 degrees; for copper and soft metals, about 30 to 35 degrees. Rule for Gearing Lathes for Screw-cutting. (Garvin Machine Co.)-Read from the lathe index the number of threads per inch cut by equal gears, and multiply it by any number that will give for a product a gear on the index; put this gear upon the stud, then multiply the number of threads per inch to be cut by the same number, and put the resulting gear upon the screw. EXAMPLE.-TO cut 11 threads per inch. We find on the index that 48 into 48 cuts 6 threads per inch, then 6 × 4 = 24, gear on stud, and 11 X 4 = 46, gear on screw. Any multiplier may be used so long as the products include gears that belong with the lathe. For instance, instead of 4 as a multiplier we may use 6. Thus, 6 × 6 = 36, gear upon stud, and 111⁄2 × 6 = 69, gear upon screw. Rules for Calculating Simple and Compound Gearing where there is no Index. (Am Mach.)-If the lathe is simplegeared, and the stud runs at the same speed as the spindle, select some gear for the screw, and multiply its number of teeth by the number of threads per inch in the lead-screw, and divide this result by the number of threads per inch to be cut. This will give the number of teeth in the gear for the stud. If this result is a fractional number, or a number which is not among the gears on hand, then try some other gear for the screw. Or, select the gear for the stud first, then multiply its number of teeth by the number of threads per inch to be cut, and divide by the number of threads per inch on the lead-screw. This will give the number of teeth for the gear on the screw. If the lathe is compound, select at random all the driving-gears, multiply the numbers of their teeth together, and this product by the num ber of threads to be cut. Then select at random all the driven gears except one; multiply the numbers of their teeth together, and this product by the number of threads per inch in the lead-screw. Now divide the first result by the second, to obtain the number of teeth in the remaining driven gear. Or, select at random all the driven gears. Multiply the numbers of their teeth together, and this product by the number of threads per inch in the leadscrew. Then select at random all the driving-gears except one. Multiply the numbers of their teeth together, and this result by the number of threads per inch of the screw to be cut. Divide the first result by the last, to obtain the number of teeth in the remaining driver. When the gears on the compounding stud are fast together, and cannot be changed, then the driven one has usually twice as many teeth as the other, or driver, in which case in the calculations consider the lead-screw to have twice as many threads per inch as it actually has, and then ignore the compounding entirely. Some lathes are so constructed that the stud on which the first driver is placed revolves only half as fast as the spindle. This can be ignored in the calculations by doubling the number of threads of the lead-screw. If both the last condi tions are present ignore them in the calculations by multiplying the number of threads per inch in the lead-screw by four. If the thread to be cut is a fractional one, or if the pitch of the lead-screw is fractional, or if both are fractional, then reduce the fractions to a common denominator, and use the numerators of these fractions as if they equalled the pitch of the screw |