constant 8 and change gears running from 24 to 100 teeth, increasing by 4, proceed as below:

(2 X 36) X (4 X 24) 251X2 (1X36) X (2324)

Change Gears for Metric Pitches.

72 X 96

36 X 64

When screws are cut in accordance with the metric system, it is the usual practice to give the lead of the thread in millimeters, instead of the number of threads per unit of measurement. To find the change gears for cutting metric threads, when using a lathe having an English lead-screw, first determine the number of threads per inch corresponding to the given lead in millimeters. Suppose a thread of 3 millimeters lead is to be cut in a lathe having an English lead-screw and a screw constant of 6. As there are 25.4 millimeters per inch, the number of threads per inch will equal 25.4 3. Place the screw constant as the numerator, and the number of threads per inch to be cut as the denominator:

The numerator and denominator of this fractional expression of the change gear ratio is next multiplied by some trial number to determine the size of the gears. The first whole number by which 25.4 can be multiplied so as to get a whole number as the result is 5. Thus, 25.4 X 5 = 127. Hence, one gear having 127 teeth is always used when cutting metric threads with an English lead-screw. The other gear required in this case has 90 teeth. Thus:

Therefore, the following rule can be used to find the change gears for cutting metric pitches with an English lead-screw:

Rule: Place the lathe screw constant multiplied by the lead of the required thread in millimeters multiplied by 5, as the numerator of the fraction, and 127 as the denominator. The product of the numbers in the numerator equals the number of teeth for the spindle-stud gear, and 127 is the number of teeth for the lead-screw gear.

If the lathe has a metric pitch lead-screw, and a screw having a given number of threads per inch is to be cut, first find the "metric screw constant" of the lathe or the lead of thread in millimeters that would be cut with change gears of equal size on the lead-screw and spindle stud; then the method of determining the change gears is simply the reverse of the one already explained for cutting a metric thread with an English lead-screw.

Rule: To find the change gears for cutting English threads with a metric leadscrew, place 127 in the numerator and the threads per inch to be cut, multiplied by the metric screw constant multiplied by 5, in the denominator; 127 is the number of teeth on the spindle-stud gear and the product of the numbers in the denominator equals the number of teeth in the lead-screw gear.

Planing Clearance on Threading Tools. In the following are given formulas for finding the angle to which the planer or shaper head should be set when planing threading tools with both side and front clearance. The expression "leading" side indicates the side of the tool which first enters the work when a thread is cut; the "following" side is that which enters the work last. In the formulas, a = depth of thread; b= width of flat on offset tool; c = actual width of flat; d= outside

diameter of screw; v = front clearance angle; w= one-half angle of thread; y = angle of thread helix; and x = angle to which to set the planer head when planing the tool on the side. Then, for tools with side clearance:

Use+ for leading side and for following side.

For Acme (29-degree) thread and 15 degrees clearance angle, the formula can, for all practical purposes, be written:

The width of flat on the offset tool is figured from the formula:

Example: Find the angles to which to set the planer heads when planing the sides of an Acme thread tool for a screw 2 inches in diameter having 2 threads per inch.

Lubricants for Machining Operations

Lubricants used for Different Operations. A good grade of lard oil is an excellent lubricant for use when turning or tapping steel or wrought iron and is extensively used on automatic screw machines, especially those which operate on comparatively small work. For some classes of work, especially when high-cutting

speeds are used, lard oil is not as satisfactory as soda water or some of the commercial lubricants, because the oil is more sluggish and does not penetrate to the cutting point with sufficient rapidity. Many lubricants which are cheaper than oil are extensively used on "automatics" for general machining operations. These usually consist of a mixture of sal-soda (carbonate of soda) and water, to which is added some ingredient such as lard oil or soft soap to thicken or give body to the lubricant. A cheap lubricant for turning, milling, etc., and one that has been extensively used, is made in the following proportions: 1 pound of sal-soda (carbonate of soda), 1 quart of lard oil, 1 quart of soft soap, and enough water to make 10 or 12 gallons. This mixture is boiled for one-half hour, preferably by passing a steam coil through it. If the solution should have an objectionable smell, this can be eliminated by adding about 2 pounds of unslaked lime. The soap and soda in this solution improve the lubricating quality and also prevent the surfaces from rusting. For turning and threading operations, plain milling, deep-hole drilling, etc., a mixture of equal parts of lard oil and paraffin oil will be found very satisfactory, the paraffin being added to lessen the expense. For fluting operations, paraffin oil, not mixed, has proved satisfactory. For automatic screw machine work, a good lubricant is composed of equal parts of so-called "electric cutting oil" and paraffin oil. For gear cutting, the following mixture has been extensively used: 32 gallons of mineral lard oil, 24 pounds of sal-soda, and one barrel of soft water.

For thread cutting and all other work on nickel steel or other hard stock, with machines running at high speed, the following compound has proved satisfactory: To 8 gallons of warm water add 25 to 30 ounces of borax. When fully dissolved, add two gallons of lard oil and stir thoroughly. When cold, add the contents of a No. 10 jar of "Aquadag" (condensed); mix thoroughly. An excess of borax will be indicated by the formation of more than two or three bubbles on the surface of the mixture after thorough stirring. For turning, use one No. 10 can of "oildag" and 10 gallons of oil (lard or paraffin). For general work in milling, cutting, tapping and drilling steel, the following formula has been successfully employed: Mix 96 pounds of "Cataract" compound and 21 gallons of pure water; take 12 gallons of this stock mixture, add 48 gallons of water and two No. 10 jars of Aquadag; mix thoroughly. A cheap drilling composition can be made by adding to thirty gallons of water, 5 gallons of lard oil and 20 pounds of washing soda. Put the material in a lard oil barrel, invert a steam hose into the bung and boil thoroughly. Do not use mineral oil or a barrel that has contained it.

For grinding with hard or soft wheels, use a No. 10 jar of Aquadag mixed with ten gallons of water; add one-half pound of borax or sal-soda to prevent rusting. Cast iron, except when tapping, is usually machined dry. Experiments made to determine the effect of applying a heavy stream of cooling water to a tool turning cast iron showed the following results: Cutting speed without water, 47 feet per minute; cutting speed with a heavy stream of water, nearly 54 feet per minute. Increase in speed 15 per cent. The dirt caused by mixing the fine cast-iron turnings with a cutting lubricant is an objectionable feature which, in the opinion of many, more than offsets the increase in cutting speed that might be obtained. Brass or bronze is usually machined dry, although lard oil is sometimes used for automatic screw machine work. Babbitt metal is also worked dry, ordinarily, although kerosene or turpentine is sometimes used when boring or reaming. If babbitt is bored dry, balls of metal tend to form on the tool and score the work. Milk is generally considered the best lubricant for machining copper. A mixture of lard oil and turpentine is also used for copper. For aluminum, the following lubricants can be used: Kerosene; a mixture of kerosene and gasoline; soap-water; or "aqualine" one part, water twenty parts. (See "Machining Aluminum.") When

drilling glass, use a mixture of turpentine and camphor. For drilling very hard steel, use kerosene or turpentine. When drilling rawhide, apply ordinary laundry soap to the drill at frequent intervals. For tapping iron or steel, a good grade of animal lard oil or a mixture of 10 per cent graphite and 90 per cent tallow will give good results. If a lubricant is used for tapping cast iron, it should be thin. A small amount of kerosene a few drops only will facilitate the tapping of long holes in cast iron. (See also "Lubricants for Tapping.")

Lard Oil as a Cutting Lubricant. After being used for a considerable time, lard oil seems to lose some of its good qualities as a cooling compound. There are several reasons for this: Some manufacturers use the same oil over and over again on different materials, such as brass, steel, etc. This is objectionable, for when lard oil has been used on brass it is practically impossible to get the fine dust separated from it in a centrifugal separator. When this impure oil is used on steel, especially where high-speed steels are employed, it does not give satisfactory results, owing to the fact that when the cutting tool becomes dull, the small brass particles "freeze" to the cutting tool and thus produce rough work. The best results are obtained from lard oil by keeping it thin, and by using it on the same materials- that is, not transferring the oil from a machine in which brass is being cut to one where it would be employed on steel. If the oil is always used on the same class of material, it will not lose any of its good qualities.

Prime lard oil is nearly colorless, having a pale yellow or greenish tinge. The solidifying point and other characteristics of the oil depend upon the temperature at which it was expressed, winter-pressed lard oil containing less solid constituents of the lard than that expressed in warm weather. The specific gravity should not exceed 0.916; it is sometimes increased by adulterants, such as cotton-seed and maize oils.

Navy Department Specifications for Lard Oil. - Lard oil must be of a good commercial quality, and must be purchased and inspected by weight; the number of pounds per gallon is to be determined by the specific gravity of the oil at 60 degrees F. multiplied by 8.33 pounds (the weight of a gallon of distilled water at the same temperature). Oil will not be accepted which contains a mixture of any mineral oil (10 per cent vegetable or fish oil is allowed); nor must the oil contain more acidity than the equivalent of 5 per cent of oleic acid, or show a cold test above 55 degrees F. The specific gravity must not be above 0.92, nor below 0.90.

Cutting Lubricants for Broaching Operations. For broaching steel, cutting compounds similar to those used for other machining operations, such as turning and milling, are commonly used. The J. N. Lapointe Co. recommends a lubricant for broaching steel containing 21⁄2 pounds of soda ash and 3 gallons of mineral lard oil to 50 gallons of water. The soda ash and lard oil is mixed with 10 gallons of water, and then the remaining 40 gallons of water added. When holes to be broached are of exceptional length, a good grade of oil is better than soda water or similar cutting lubricants, as the oil will cling to the cutting edges of the broach for a longer time.

Machining Aluminum. - Tools for turning, drilling, or planing aluminum should have acute cutting angles. After rough-grinding the tool, it is advisable to finish-sharpening the cutting edge on a grindstone or with an oilstone for fine work, as a keen edge is very essential. High speeds and comparatively light cuts are recommended. For drilling, a straight-fluted drill gives good results. If a twist drill is used, the cutting edges should be ground without front rake. The principal difficulty in the machining of aluminum and aluminum alloys is caused by the clogging of the chips, which become so firmly wedged between the teeth of milling

cutters, counterbores and similar tools, that they cannot be removed with a brush. This difficulty can be largely avoided by the use of the right kind of cutting lubricant. Soap-water and kerosene are commonly employed. The latter enables a fine finish to be obtained, provided the cutting tool is properly ground. For milling flat surfaces, it is preferable to use end mills rather than cylindrical cutters. The cutting edges or corners of the cutter should be sharp instead of rounded. The mill will cut better if a high cutting speed and moderate feed is employed. The depth and width of the cut are of minor importance. A cutting speed of 325 feet per minute is practicable, and from 22 to 4 cubic inches of aluminum can be removed per minute.

The following information on this subject was obtained from the Brown-Lipe Gear Co., where aluminum parts are machined in large quantities: For finishing bored holes, a bar equipped with cutters has been found more practicable than reamers. The cutters used for machining 4-inch holes have a clearance of from 20 to 22 degrees and no rake or slope on the front faces against which the chips bear. The roughing cutters for this work have a rather sharp nose, being ground on the point to a radius of about 2 inch, but for securing a smooth surface, the finishing tools are rounded to a radius of about 4 inch. When machining aluminum on the profiler, a milling cutter about 3 or 4 inches in diameter is used, having spiral teeth, which are sharp-cornered on the end of the mill. The sharp-cornered tooth has given much better service at high speed than a mill having round-cornered teeth. When aluminum is machined with an inserted-tooth milling cutter, the teeth are inclined at a slight angle, not exceeding two degrees. The cutting speed, as well as the feed, for machining aluminum, is from 50 to 60 per cent faster than the speeds and feeds for cast iron. The lubricant used by this company is composed of one part "aqualine," and 20 parts water. This lubricant not only gives a smooth finish but preserves a keen cutting edge and enables tools to be used much longer without grinding. Formerly, a lubricant composed of one part of high-grade lard oil and one part of kerosene was used. This mixture costs approximately 30 cents per gallon, whereas the aqualine and water mixture now being used costs less than 4 cents per gallon, and has proved more effective than the lubricant formerly employed.

RUNNING, PRESSED AND SHRINKAGE FITS

Classes of Fits. In ordinary machine construction, five classes of fits are commonly used: running fit; push fit; driving fit; forced fit; and shrinkage fit. The running fit, as the name implies, is employed when the parts must rotate; a push fit is not sufficiently free to rotate; the other classes referred to are used in assembling parts which must be held in fixed positions. The limits recommended by the Engineering Standards Committee of Great Britain for running fits are given in the table "Allowances and Tolerances for Running Fits. When the allowance

is smaller than for a running fit, and a moderate pressure is required in assembling the parts, the term "push fit" is sometimes used. The tables "Allowances for Different Classes of Fits" are intended to cover average machine work. (See also "Grinding Limits for Cylindrical Parts" and "Forced Fit Allowances.") As the factors which determine the proper allowances vary considerably, the dimensions given in these tables may sometimes have to be increased or decreased. For example, the allowances for forced fits usually increase with the diameter to secure greater pressure, but in some shops the allowance is made practically the same for all diameters, the increased surface area of the larger sizes giving sufficient increase in pressure. For running fits, the allowances are also increased with the diameter, but may be varied according to the length of the bearing surface.