The Cold Saw."-For sawing any section of iron while cold the cold saw is sometimes used. This consists simply of a plain soft steel or iron disk without teeth, about 42 inches diameter and 3/16 inch thick. The velocity of the circumference is about 15,000 feet per minute. One of these saws will saw through an ordinary steel rail cold in about one minute. In this saw the steel or iron is ground off by the friction of the disk, and is not cut as with the teeth of an ordinary saw. It has generally been found more profitable, however, to saw iron with disks or band-saws fitted with cuttingteeth, which run at moderate speeds, and cut the metal as do the teeth of a milling-cutter.

Reese's Fusing-disk.-Reese's fusing-disk is an application of the cold saw to cutting iron or steel in the form of bars, tubes, cylinders, etc., in which the piece to be cut is made to revolve at a slower rate of speed than the saw. By this means only a small surface of the bar to be cut is presented at a time to the circumference of the saw. The saw is about the same size as the cold saw above described, and is rotated at a velocity of about 25,000 feet per minute. The heat generated by the friction of this saw against the small surface of the bar rotated against it is so great that the particles of iron or steel in the bar are actually fused, and the "sawdust" welds as it falls into a solid mass. This disk will cut either cast iron, wrought tron, or steel. It will cut a bar of steel 13% inch diameter in one minute, including the time of setting it in the machine, the bar being rotated about 200 turns per minute.

Cutting Stone with Wire.-A plan of cutting stone by means of a wire cord has been tried in Europe. While retaining sand as the cutting agent, M. Paulin Gay, of Marseilles, has succeeded in applying it by mechanical means, and as continuously as formerly the sand-blast and band-saw, with both of which appliances his system--that of the "helicoidal wire cord "-has considerable analogy. An engine puts in motion a continuous wire cord (varying from five to seven thirty-seconds of an inch in diameter, according to the work), composed of three mild-steel wires twisted at a certain pitch, that is found to give the best results in practice, at a speed of from 15 to 17 feet per second.

The Sand-blast. In the sand-blast, invented by B. F. Tilghman, of Philadelphia, and first exhibited at the American Institute Fair, New York, in 1871, common sand, powdered quartz, emery, or any sharp cutting material is blown by a jet of air or steam on glass, metal, or other comparatively brittle substance, by which means the latter is cut, drilled, or engraved To protect those portions of the surface which it is desired shall not be abraded it is only necessary to cover them with a soft or tough material, such as lead, rubber, leather, paper, wax, or rubber-paint. (See description in App. Cyc. Mech.; also U. S. report of Vienna Exhibition, 1873, vol. iii. 316.) Ajet of sand " impelled by steam of moderate pressure, or even by the blast of an ordinary fan, depolishes glass in a few seconds; wood is cut quite rapidly; and metals are given the so-called "frosted" surface with great rapidity. With a jet issuing from under 300 pounds pressure, a hole was cut through a piece of corundrum 131⁄2 inches thick in 25 minutes.

The sand-blast has been applied to the cleaning of metal castings and sheet metal, the graining of zinc plates for lithographic purposes, the frosting of silverware, the cutting of figures on stone and glass, and the cutting of devices on monuments or tombstones, the recutting of files, etc. The time required to sharpen a worn-out 14-inch bastard file is about four minutes. About one pint of sand, passed through a No. 120 sieve, and four horse-power of 60-lb. steam are required for the operation. For cleaning castings compressed air at from 8 to 10 pounds pressure per square inch is employed. Chilled-iron globules instead of quartz or flint-sand are used with good results, both as to speed of working and cost of material, when the operation can be carried on under proper conditions. With the expenditure of 2 horse-power in compressing air, 2 square feet of ordinary scale on the surface of steel and iron plates can be removed per minute. The surface thus prepared is ready for tinning, galvanizing, plating, bronzing, painting, etc. By continuing the operation the hard skin on the surface of castings, which is so destructive to the cutting edges of milling and other tools, can be removed. Small castings are placed in a sort of slowly rotating barrel, open at one or both ends, through which the blast is directed downward against them as they tumble over and over. No portion of the surface escapes the action of the sand. Plain cored work, such as valve-bodies, can be cleaned perfectly both inside and out. 100 lbs. of castings can be cleaned in from 10 to 15 minutes with a blast created by 2 horse

power. The same weight of small forgings and stampings can be scaled in from 20 to 30 minutes.-Iron Age, March 8, 1894.

EMERY-WHEELS AND GRINDSTONES.

The Selection of Emery-wheels.-A pamphlet entitled " Emery. wheels, their Selection and Use," published by the Brown & Sharpe Mfg. Co., after calling attention to the fact that too much should not be expected of one wheel, and commenting upon the importance of selecting the proper wheel for the work to be done, says:

Wheels are numbered from coarse to fine; that is, a wheel made of No. 60 emery is coarser than one made of No. 100. Within certain limits, and other things being equal, a coarse wheel is less liable to change the temperature of the work and less liable to glaze than a fine wheel. As a rule, the harder the stock the coarser the wheel required to produce a given finish. For example, coarser wheels are required to produce a given surface upon hardened steel than upon soft steel, while finer wheels are required to produce this surface upon brass or copper than upon either hardened or soft steel.

Wheels are graded from soft to hard, and the grade is denoted by the letters of the alphabet, A denoting the softest grade. A wheel is soft or hard chiefly on account of the amount and character of the material combined in its manufacture with emery or corundum. But other characteristics being equal, a wheel that is composed of fine emery is more compact and harder than one made of coarser emery. For instance, a wheel of No. 100 emery, grade B, will be harder than one of No. 60 emery, same grade. The softness of a wheel is generally its most important characteristic. A soft wheel is less apt to cause a change of temperature in the work, or to become glazed, than a harder one. It is best for grinding hardened steel, cast-iron, brass, copper, and rubber, while a harder or more compact wheel is better for grinding soft steel and wrought iron. As a rule, other things being equal, the harder the stock the softer the wheel required to produce a given finish.

Generally speaking, a wheel should be softer as the surface in contact with the work is increased. For example, a wheel 1/16-inch face should be harder than one 1⁄2-inch face. If a wheel is hard and heats or chatters, it can often be made somewhat more effective by turning off a part of its cutting surface; but it should be clearly understood that while this will sometimes prevent a hard wheel from heating or chattering the work, such a wheel will not prove as economical as one of the full width and proper grade, for it should be borne in mind that the grade should always bear the proper relation to the width. (See the pamphlet referred to for other information. See also lecture by T. Dunkin Paret, Pres't of The Tanite Co., on Emery-wheels, Jour. Frank. Inst., March, 1890.) Speed of Emery-wheels.--The following speeds are recommended different makers:

by

"We advise the regular speed of 5500 feet per minute." (Detroit Emery. wheel Co.)

"Experience has demonstrated that there is no advantage in running

solid emery-wheels at a higher rate than 5500 feet per minute peripheral speed." (Springfield E. W. Mfg. Co.)

Although there is no exactly defined limit at which a wheel must be run to render it effective, experience has demonstrated that, taking into account safety, durability, and liability to heat, 5500 feet per minute at the periphery gives the best results. All first-class wheels have the number of revolutions necessary to give this rate marked on their labels, and a column of figures in the price-list gives a corresponding rate. Above this speed all wheels are unsafe. If run much below it they wear away rapidly in proportion to what they accomplish." (Northampton E. W. Co.)

Grades of Emery.-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.

66 a coarse rough file.

an ordinary rough file.
a bastard file.

a second-cut file.

66

66

66

70

66

66

66 a dead-smooth file.

Speed of Polishing-wheels.

...

7000 ft. per minuto 2500 revs. for largest "smallest 8000 ft. per minute 7000"

66

66

11⁄2" to 8" diam., hair 1" to 14" long, ab. 4500 Walrus-hide wheels, about..... Rag-wheels, 4 to 8 in. diameter, about... Safe Speeds for Grindstones and Emery-wheels.-G. D. Hiscox (Iron Age, April 7, 1892), by an application of the formula for centrifugal force in fly-wheels (see Fly-wheels), obtains the figures for strains in grindstones and emery-wheels which are given in the tables below. His formulæ are:

Stress per sq. in. of section of a grindstone =(.7071D X N)2 x .0000795 an emery-wheel (.7071D X N)2 X .00010226 D= diameter in feet, N = revolutions per minute.

He takes the weight of sandstone at .078 lb. per cubic inch, and that of an emery-wheel at 0.1 lb. per cubic inch; Ohio stone weighs about .081 lb. and Huron stone about .089 lb. per cubic inch. The Ohio stone will bear a speed at the periphery of 2500 to 3000 ft. per min., which latter should never be exceeded. The Huron stone can be trusted up to 4000 ft., when properly clamped between flanges and not excessively wedged in setting. Apart from the speed of grindstones as a cause of bursting, probably the majority of accidents have really been caused by wedging them on the shaft and overwedging to true them. The holes being square, the excessive driving of wedges to true the stones starts cracks in the corners that eventually run out until the centrifugal strain becomes greater than the tenacity of the remaining solid stone. Hence the necessity of great caution in the use of wedges, as well as the holding of large quick-running stones between large flanges and leather washers.

Strains in Grindstones.

LIMIT OF VELOCITY AND APPROXIMATE ACTUAL STRAIN PER SQUARE INCH OF SECTIONAL AREA FOR GRINDSTONES OF MEDIUM TENSILE STRENGTH.

The figures at the bottom of columns designate the limit cf velocity (in revolutions per minute), at the head of the columns for stones of the diameter in the first column opposite the designating figure.

A general rule of safety for any size grindstone that has a compact and strong grain is to limit the peripheral velocity to 47 feet per second.

There is a large variation in the listed speeds of emery-wheels by different makers-4000 as a minimum and 5600 maximum feet per minute, while others claim a maximum speed of 10,000 feet per minute as the safe speed of their best emery-wheels. Rim wheels and iron centre wheels are specialties that require the maker's guarantee and assignment of speed.

Strains in Emery-wheels.

ACTUAL STRAIN PER SQUARE INCH OF SECTION IN EMERY-WHEELS AT THE VELOCITIES AT HEAD OF COLUMNS FOR SIZES IN FIRST COLUMN.

Revolutions per minute.

22.67 32.65 44.45 58.05 73.47 90.71 109.76 130.62 153.30 35.47 51.08 69.51 90.81 114.94 141.90 171.71 18.40 32.72 51.12 73.62 100.21 130.88 165.65

61.81 109.41 171.23

73.62 130.88

26

86.36 152.85

30 115.04

36

165.64

Joshua Rose (Modern Machine-shop Practice) says: The average speed of grindstones in workshops may be given as follows:

Circumferential Speed of Stone.
900 feet per minute.
600

66

The speeds of stones for file-grinding, and other similar rapid grinding is thus given in the "Grinders' List.

Diam. ft... 8 71% 7 612 6 51% 5 412 4 312 3 Revs. per min. 135 144 154 166 180 196 216 240 270 308 360 The following table, from the Mechanical World, is for the diameter of stones and the number of revolutions they should run per minute (not to be exceeded), with the diameter of change of shift-pulleys required, varying each shift or change 21⁄2 inches, 24 inches, or 2 inches in diameter for each reduction of 6 inches in the diameter of the stone.

Columns 3, 4, and 5 are given to show that if we start an 8-foot stone with, say, a countershaft pulley driving a 40-inch pulley on the grindstone spindle, and the stone makes the right number (135) of revolutions per minute, the reduction in the diameter of the pulley on the grinding-stone spindle, when the stone has been reduced 6 inches in diameter, will require to be also reduced 21⁄2 inches in diameter, or to shift from 40 inches to 371⁄2 inches, and so on similarly for columns 4 and 5. Any other suitable dimensions of pulley may be used for the stone when eight feet in diameter, but the number of inches in each shift named, in order to be correct, will have to be proportional to the numbers of revolutions the stone should run, as given in column 2 of the table.

FOR GRINDING BROAD SURFACES, AS SAWS OR IRON PLATES.

Taps for Machine-screws. (The Pratt & Whitney Co.)

of an inch.

of an inch.

The Morse Twist Drill and Machine Co. gives the following table showing the different sizes of drills that should be used when a suitable thread is to be tapped in a hole. The sizes given are practically correct.