to be cut, and of the lead-screw, respectively. Then use that part of the rule given above which applies to the lathe in question. For instance, suppose It is desired to cut a thread of 25/32-inch pitch, and the lead-screw has 4 threads per inch. Then the pitch of the lead-screw will be 14 inch, which is equal to 8/32 inch. We now have two fraction, 25/32 and 8/32, and the two screws will be in the proportion of 25 to 8, and the gears can be figured by the above rule, assuming the number of threads to be cut to be 8 per inch, and those on the lead-screw to be 25 per inch. But this latter number may be further modified by conditions named above, such as a reduced speed of the stud, or fixed compound gears. In the instance given, if the lead-screw had been 211⁄2 threads per inch, then its pitch being 4/10 inch, we have the fractions 4/10 and 25/32, which, reduced to a common denominator, are 64/160 and 125/160, and the gears will be the same as if the lead-screw had 125 threads per inch, and the screw to be cut 64 threads per inch.

On this subject consult also "Formulas in Gearing," published by Brown & Sharpe Mfg. Co., and Jamieson's Applied Mechanics.

Change-gears for Screw-cutting Lathes.-There is a lack of uniformity among lathe-builders as to the change-gears provided for screwcutting. W. R. Macdonald, in Am. Mach., April 7, 1892, proposes the following series, by which 33 whole threads (not fractional) may be cut by changes of only nine gears:

Ten gears are sufficient to cut all the usual threads, with the exception of perhaps 112, the standard pipe-thread; in ordinary practice any fractional thread between 11 and 12 will be near enough for the customary short pipethread; if not, the addition of a single gear will give it.

In this table the pitch of the lead-screw is 12, and it may be objected to as too fine for the purpose. This may be rectified by making the real pitch 6 or any other desirable pitch, and establishing the proper ratio between the lathe spindle and the gear-stud.

Metric Screw-threads may be cut on lathes with inch-divided leading-screws, by the use of change wheels with 50 and 127 teeth; for 127 centimetres = 50 inches (127 X 0.393749.9999 in.).

Rule for Setting the Taper in a Lathe. (Am. Mach.)-No rule can be given which will produce exact results, owing to the fact that the centres enter the work an indefinite distance. If it were not for this circumstance the following would be an exact rule, and it is an approximation as it is. To find the distance to set the centre over: Divide the difference in the diameters of the large and small end of the taper by 2, and multiply this quotient by the ratio which the total length of the shaft bears to the length of the tapered portion. Example: Suppose a shaft three feet long is to have a taper turned on the end one foot long, the large end of the taper being two 2-1 3 inches and the small end one inch diameter. 2

X=1%1⁄2 inches.

Electric Drilling-machines-Speed of Drilling Holes in Steel Plates. (Proc. Inst. M. E., Aug. 1887, p. 329.)-In drilling holes in the shell of the S.S. "Albania," after a very small amount of practice the men working the machines drilled the g-inch holes in the shell with great rapidity, doing the work at the rate of one hole every 69 seconds, inclusive of the time occupied in altering the position of the machines by means of differential pulley-blocks, which were not conveniently arranged as slings for this purpose. Repeated trials of these drilling-machines have also shown that, when using electrical energy in both holding-on magnets and motor

amounting to about 34 H.P., they have drilled holes of 1 inch diameter through 14inch thickness of solid wrought iron, or through 15% inch of mild steel in two plates of 13/16 inch each, taking exactly 134 min. for each hole. Speed of Drills. (Morse Twist-drill and Machine Company.)-The following table gives the revolutions per minute for drills from 1/16 in. to 2 in. diameter, as usually applied:

One inch to be drilled in soft cast iron will usually require: for 4-in. drill, 160 revolutions; for 2-in. drill, 140 revolutions; for 34-in. drill, 100 revolutions; for 1-in. drill, 95 revolutions. These speeds should seldom be exceeded. Feed per revolution for 4-in. drill, .005 inch; for 1⁄2-in. drill, .007 inch; for 34-in. drill .010 inch.

The rates of feed for twist drills are thus given by the same company: Diameter of drill.................. 14 3% 泊

...........

1/16

Revs. per inch depth of hole. 125 125 120 to 140

MILLING-CUTTERS.

1 12

3/4 1 inch feed per min.

George Addy, (Proc. Inst. M. E., Oct. 1890, p. 537), gives the following: Analyses of Steel.-The following are analyses of milling-cutter blanks, made from best quality crucible cast steel and from self-hardening "Ivanhoe" steel:

The first analysis is of a cutter 14 in. diam., 1 in. wide, which gave very good service at a cutting-speed of 60 ft. per min. Large milling-cutters are sometimes built up, the cutting-edges only being of tool steel. A cutter 22 in. diam. by 51⁄2 in. wide has been made in this way, the teeth being clamped between two cast-iron flanges. Mr. Addy recommends for this form of tooth one with a cutting-angle of 70°, the face of the tooth being set 10° back of a radial line on the cutter, the clearance-angle being thus 10°. At the Clarence Iron-works, Leeds, the face of the tooth is set 10° back of the radial line for cutting wrought iron and 20° for steel.

Pitch of Teeth.-For obtaining a suitable pitch of teeth for millingcutters of various diameters there exists no standard rule, the pitch being usually decided in an arbitrary manner, according to indivídual taste.

For estimating the pitch of teeth in a cutter of any diameter from 4 in. to 15 in., Mr. Addy has worked out the following rule, which he has found capable of giving good results in practice:

Pitch in inches = √(diam. in inches x 8) × 0.0625 = .177 √/diam.

J. M. Gray gives a rule for pitch as follows: The number of teeth in a milling-cutter ought to be 100 times the pitch in inches; that is, if there were 27 teeth, the pitch ought to be 0.27 in. The rules are practically the same, for if d = diam., n = No. of teeth, p = pitch, c = circumference, c = pn 100p2 pn; d= = 31.83p2; p = √.0314d = .177 √d; No. of teeth, n, = 3.14d ÷ p.

П

=

π

Number of Teeth in Mills or Cutters. (Joshua Rose.)-The teeth of cutters must obviously be spaced wide enough apart to admit of the emerywheel grinding one tooth without touching the next one, and the front faces of the teeth are always made in the plane of a line radiating from the axis of the cutter. In cutters up to 3 in. in diam. it is good practice to provide 8 teeth per in. of diam., while in cutters above that diameter the spacing may be coarser, as follows:

Diameter of cutter, 6 in.; number of teeth in cutter, 40

Speed of Cutters.-The cutting speed for milling was originally fixed very low; but experience has shown that with the improvements now in use it may with advantage be considerably increased, especially with cutters of large diameter. The following are recommended as safe speeds for cutters of 6 in. and upwards, provided there is not any great depth of material to cut away:

Feet per minute..
Feed, inch per min...

Steel.
36

1/2

Wrought iron. Cast iron.
48
60
1
12%

Brass. 120

22%

Should it be desired to remove any large quantity of material, the same cutting-speeds are still recommended, but with a finer feed. A simple rule for cutting-speed is: Number of revolutions per minute which the cutter spindle should make when working on cast iron 240, divided by the diameter of the cutter in inches.

Speed of Milling-cutters. (Proc. Inst. M. E., April, 1883, p. 248.)The cutting-speed which can be employed in milling is much greater than that which can be used in any of the ordinary operations of turning in the lathe, or of planing, shaping, or slotting. A milling-cutter with a plentiful supply of oil, or soap and water, can be run at from 80 to 100 ft. per min., when cutting wrought iron. The same metal can only be turned in a lathe, with a tool-holder having a good cutter, at the rate of 30 ft. per min., or at about one third the speed of milling. Á milling-cutter will cut cast steel at the rate of 25 to 30 ft. per min.

The following extracts are taken from an article on speed and feed of milling-cutters in Eng'g, Oct. 22, 1891: Milling-cutters are successfully employed on cast iron at a speed of 250 ft. per min.; on wrought iron at from 80 ft. to 100 ft. per min. The latter materials need a copious supply of good lubricant, such as oil or soapy water. These rates of speed are not approached by other tools. The usual cutting-speeds on the lathe, planing, shaping, and slotting machines rarely exceed about one third of those given above, and frequently average about a fifth, the time lost in back strokes not being reckoned.

The feed in the direction of cutting is said by one writer tovary, in ordinary work, from 40 to 70 revs. of a 4-in. cutter per in. of feed. It must always to an extent depend on the character of the work done, but the above gives shavings of extreme thinness. For example, the circumference of a 4-in. cutter being, say, 121⁄2 in., and having, say, 60 teeth, the advance corresponding to the passage of one cutting-tooth over the surface, in the coarser of the above-named feed-motions, is 1/40 X 1/60 1/2400 in.; the finer feed gives an advance for each tooth of only 1/70 × 1/60 = 1/4200 in. Such fine feeds as these are used only for light finishing cuts, and the same authority recommends, also for finishing, a cutter about 9 in. in circumference, or nearly 3 in. in diameter, which should be run at about 60 revs. per min. to cut tough wrought steel, 120 for ordinary cast iron, about 80 for wrought

Iron, and from 140 to 160 for the various qualtities of gun-metal and brass. With cutters smaller or larger the rates of revolution are increased or diminished to accord with the following table, which gives these rates of cutting-speeds and shows the lineal speed of the cutting-edge:

Feet per minute...

Steel. Wrought Iron. Cast Iron. Gun-metal. Brass. 45

60

90

105

120

These speeds are intended for very light finishing cuts, and they must be reduced to about one half for heavy cutting.

The following results have been found to be the highest that could be attained in ordinary workshop routine, having due consideration to economy and the time taken to change and grind the cutters when they become dull: Wrought iron-36 ft. to 40 ft. per min.; depth of cut, 1 in.; feed, 5% in. per min. Soft mild steel-About 30 ft. per min.; depth of cut, 14 in.; feed, 34 in. per min. Tough gun-metal-80 ft. per min.; depth of cut, 1⁄2 in.; feed, 34 in. per min. Cast-iron gear-wheels-26% ft. per min.; depth of cut, 1⁄2 in.; feed, 34 in. per min. Hard, close-grained cast iron-30 ft. per min.; depth of cut, 2% in.: feed, 5/16 in. per min. Gun-metal joints, 53 ft. per min.; depth of cut, 13% in.; feed, 5% in. per min. Steel-bars-21 ft. per min.; depth of cut, 1/32 in.; feed, 34 in. per min.

A stepped milling-cutter, 4 in. in diam. and 12 in. wide, tested under two conditions of speed in the same machine, gave the following results: The cutter in both instances was worked up to its maximum speed before it gave way, the object being to ascertain definitely the relative amount of work done by a high speed and a light feed, as compared with a low speed and a heavy cut. The machine was used single-geared and double-geared, and in both cases the width of cut was 101⁄2 in.

Single-gear, 42 ft. per min.; 5/16 in. depth of cut; feed, 1.3 in. per min. = 4.16 cu. in. per min. Double-gear, 19 ft. per min.; 3 in. depth of cut; feed, 5% in. per min. = 2.40 cu. in. per min.

Extreme Results with Milling-machines. - Horace L. Arnold (Am. Mach., Dec. 28, 1893) gives the following results in flat-surface milling, obtained in a Pratt & Whitney milling-machine: The mills for the flat cut were 5'' diam., 12 teeth, 40 to 50 revs. and 4%" feed per min. One single cut was run over this piece at a feed of 9 per min., but the mills showed plainly at the end that this rate was greater than they could endure. At 50 revs. for these mills the figures are as follows, with 47" feed: Surface speed, 64 ft., nearly; feed per tooth, 0.00812": cuts per inch, 123. And with 9'' feed per min.: Surface speed, 64 ft. per min.; feed per tooth, 0.015"; cuts per inch, 662%.

At a feed of 4%" per min. the mills stood up well in this job of cast-iron surfacing, while with a 9" feed they required grinding after surfacing one piece; in other words, it did not damage the mill-teeth to do this job with 123 cuts per in. of surface finished, but they would not endure 66% cuts per inch. In this cast-iron milling the surface speed of the mills does not seem to be the factor of mill destruction: it is the increase of feed per tooth that prohibits increased production of finished surface. This is precisely the reverse of the action of single-pointed lathe and planer tools in general: with such tools there is a surface-speed limit which cannot be economically exceeded for dry cuts, and so long as this surface-speed limit is not reached, the cut per tooth or feed can be made anything up to the limit of the driving power of the lathe or planer, or to the safe strain on the work itself, which can in many cases be easily broken by a too great feed.

In wrought metal extreme figures were obtained in one experiment made in cutting keyways 5/16" wide by " deep in a bank of 8 shafts 14" diam. at once, on a Pratt & Whitney No. 3 column milling-machine. The 8 mills were successfully operated with 45 ft. surface speed and 191⁄2 in. per min. feed; the cutters were 5" diam., with 28 teeth, giving the following figures, in steel: Surface speed, 45 ft. per min.; feed per tooth. 0.02024"; cuts per inch, 50, nearly. Fed with the revolution of mill. Flooded with oil, that is, a large stream of oil running constantly over each mill. Face of tooth radial. The resulting keyway was described as having a heavy wave or cutter-mark in the bottom, and it was said to have shown no signs of being heavy work on the cutters or on the machine. As a result of the experiment it was decided for economical steady work to run at 17 revs., with a feed of 4" per min., flooded cut, work fed with mill revolution, giving the following figures: Surface speed, 224 ft. per min.; feed per tooth, 0.0084"; cuts per inch, 119.

An experiment in milling a wrought-iron connecting-rod of a locomotive on a Pratt & Whitney double-head milling-machine is described in the Iron Age, Aug. 27, 1891. The amount of metal removed at one cut measured 3% in. wide by 13/16 in. deep in the groove, and across the top 1⁄2 in. deep by 434 in. wide. This represented a section of nearly 4% sq. in. This was done at the rate of 134 in. per min. Nearly 8 cu. in. of metal were cut up into chips every minute. The surface left by the cutter was very perfect. The cutter moved in a direction contrary to that of ordinary practice; that is, it cut down from the upper surface instead of up from the bottom.

66

Milling" with " or against" the Feed.-Tests made with the Brown & Sharpe No. 5 milling-machine (described by H. L. Arnold, in Am. Mach., Oct. 18, 1894) to determine the relative advantage of running the milling-cutter with or against the feed-" with the feed " meaning that the teeth of the cutter strike on the top surface or "scale" of cast-iron work in process of being milled, and "against the feed " meaning that the teeth begin to cut in the clean, newly cut surface of the work and cut upwards toward the scale-showed a decided advantage in favor of running the cutter against the feed. The result is directly opposite to that obtained in tests of a Pratt & Whitney machine, by experts of the P. & W. Co.

In the tests with the Brown & Sharpe machine the cutters used were 6 inches face by 4% and 3 inches diameter respectively, 15 teeth in each mill, 42 revolutions per minute in each case, or nearly 50 feet per minute surface speed for the 42-inch and 33 feet per minute for the 3-inch mill. The revolution marks were 6 to the inch, giving a feed of 7 inches per minute, and a cut per tooth of .011". When the machine was forced to the limit of its driving the depth of cut was 11/32 inch when the cutter ran in the "old " way, or against the feed, and only 4 inch when it ran in the "new way, or with the feed. The endurance of the milling-cutters was much greater when they were run in the "old" way.

Spiral Milling-cutters.-There is no rule for finding the angle of the spiral; from 10 to 15° is usually considered sufficient; if much greater the end thrust on the spindle will be increased to an extent not desirable for some machines.

Milling-cutters with Inserted Teeth.-When it is required to use milling-cutters of a greater diameter than about 8 in., it is preferable to insert the teeth in a disk or head, so as to avoid the expense of making solid cutters and the difficulty of hardening them, not merely because of the risk of breakage in hardening them, but also on account of the difficulty in obtaining a uniform degree of hardness or temper.

He

Milling machine versus Planer. For comparative data of work done by each see paper by J. J. Grant, Trans. A. S. M. E., ix. 259. says: The advantages of the milling machine over the planer are many, among which are the following: Exact duplication of work; rapidity of production the cutting being continuous; cost of production, as several machines can be operated by one workman, and he not a skilled mechanic; and cost of tools for producing a given amount of work.

POWER REQUIRED FOR MACHINE TOOLS.

Resistance Overcome in Cutting Metal. (Trans. A. S. M. E., viii. 308.)-Some experiments made at the works of William Sellers & Co. showed that the resistance in cutting steel in a lathe would vary from 180,000 to 700,000 pounds per square inch of section removed, while for cast iron the resistance is about one third as much. The power required to remove a given amount of metal depends on the shape of the cut and on the shape and the sharpness of the tool used. If the cut is nearly square in section, the power required is a minimum; if wide and thin, a maximum. The dulness of a tool affects but little the power required for a heavy cut. Heavy Work on a Planer.-Wm. Sellers & Co. write as follows to the American Machinist: The 120 planer table is geared to run 18 ft. per minute under cut, and 72 feet per minute on the return, which is equivalent, without allowance for time lost in reversing, to continuous cut of 14.4 feet per minute. Assuming the work to be 28 feet long, we may take 14 feet as the continuous cutting speed per minute, the .8 of a foot being much more than sufficient to cover time loss in reversing and feeding. The machine carries four tools. At " feed per tool, the surface planed per hour would be 35 square feet. The section of metal cut at 34" depth would be .75" X .125" X 4 = .375 square inch, which would require approximately 30,000 lbs.