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THE ACME OR 29° THREAD.

The Acme or 29° thread is neither a square thread nor a V thread. The sides of the thread are inclined 14° from the perpendicular, making an angle of 29°. It is used in many instances where a coarse pitch screw is desired that has but a small friction on its sides. It has a much greater shearing strength than the square thread and is used quite extensively in many places in preference to the square thread.

EXAMPLES.

standard thread

1. What is the double depth of a United States having a pitch of inch? Answer, .10825 inch. 2. A shaft 1 inch in diameter having a V thread of -inch pitch, and a nut is desired. What size must the hole be drilled in the stock for the nut before cutting the thread? Give answer to three decimal places. Answer, .6536 inch.

3. What size tap drill will be used for drilling a hole to be tapped for a g-inch bolt having 11 United States standard threads per inch on it? Answer to 3 decimal places. Answer, .507 inch.

4. What size hole should be bored in a piece of work that is to be threaded to receive a shaft 14 inches in diameter having a V thread of + pitch? Answer to 3 decimal places. Answer, 1.002 inches.

5. To find the size of drill to use in drilling a hole that is to be tapped for a thread corresponding to a thread on a piece of work, such as a stud, or in making a nut for a piece of work, such as a bolt, etc., find the double depth of the thread and subtract it from the outside diameter of the bolt or stud, etc. The remainder will be the diameter of the hole to be bored. Find the single depth of a thread (V thread) having a pitch of inch. Answer, .10825 inch.

6. Find the double depth of the same thread. Answer, .2165 inch. 7. Find the double depth of a V thread having a pitch of 16 inch. Answer, .1732 inch.

8. Find the single depth of a United States standard thread having a pitch of inch. Answer, .1299 inch.

9. The diameter of a valve is 4 inches. What will be the area of the valve? Answer, 12.5664 inches.

10. How far off the seat must the valve be lifted to allow a stream of liquid, the area of whose cross section is equal to that of the valve, to pass through in a full stream? Answer, 1 inch.

THE PITCH OF A THREAD.

tooth or

The distance from the center of one screw thread or gear serration of any kind to the center of the next is called the pitch. In screws with a single thread th' pitch is the same as the lead, but not otherwise.

THE LEAD OF A THREAD.

The advance made by a screw in one revolution is called the lead of the screw. The term is often confused with the pitch of a thread, but has not the same meaning unless in the case of a single thread. With a double thread the lead is twice as much as the pitch.

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TAPERS.

The term taper, when applied to a piece of work, means that the piece of work or a part of the piece of work gradually diminishes in diameter.

AMOUNT OF TAPER.

The amount of taper a piece of work has is the difference between the two dimensions which are taken at the beginning and at the ending of the taper.

Thus: If a pin 12 inches long measures 4 inches in diameter at one end and is 3 inches in diameter at the other end, the difference between 4 inches and 3 inches, or 1 inch, is the amount of taper.

1. If a wedge is made that is 23 inches at one end and tapers to an inch at the other end, what amount of taper has the wedge? Answer, 24 inches.

LENGTH OF TAPER.

When the length of taper is spoken of it means the distance the taper runs on the work measured along the axis of the work. Thus: If the length of a piece of stock is 10 inches and a taper the length of which is 6 inches is cut on it, the work begins to taper or decrease in diameter 4 inches from the end and continues to taper until it reaches the other end.

TAPER PER INCH.

The taper per inch of a piece of work is the amount the work tapers in one inch measured along the axis of the work.

Thus: If a piece of work had a taper whose length was 6 inches and the amount of taper was an inch, the taper per inch would be ÷6 or inch.

1. What is the taper per inch of a piece of work that has a taper 9 inches long and the amount of taper is inch? Answer, inch. 2. What is the taper per inch of a piece of work with a 4-inch taper in 12 inches? Answer, inch.

3. If the centers of a lathe are in central position with each other and a piece of work is turned to a cylindrical shape, the work will caliper the same diameter on each end. If the tailstock or dead center is the slightest amount out of line with the live center, the work will be tapered when a cut is taken with the tool. It must be understood in turning up a piece of work in a lathe if a cut of an inch in depth is taken, the diameter of the work will be reduced inch. If a cut of of an inch is taken, the work will be reduced of an inch in diameter, and so on. In turning a taper that is to run a certain. distance along the center line of a piece of work, the length of the work or the distance between the lathe centers must be taken into consideration when estimating the amount of set-over to be given the tailstock. If we have a piece of work 24 inches long and a taper 10 inches long is to be cut on it, the amount of taper to be inch, the amount the tailstock would be set off center would be of an inch, while if the taper ran 14 inches along the work, the amount of set-over would be of an inch. The rule in common use by machinists in estimating the amount of set-over to give the tailstock for cutting a taper is as follows: Multiply the length of the work over all or the distance

between centers by one-half the taper per inch, the product will be the amount necessary to set the tailstock over.

In very accurate work the depth of the centers of the work are to be taken into consideration. These are added together and their sum subtracted from the length of the work over all.

Example: Find the amount of set over necessary to give the tail stock in order to cut a taper, the amount of which is 3 inch, the taper to run 8 inches along a piece of work 17 inches long.

64

64

Solution. The work tapers of an inch in 8 inches; in 1 inch it will taper, or 4 inch. Therefore, the taper per inch is 4 inch. Half the taper per inch is X4 inch, or inch. Half the taper per 138 inch equals inch. Multiplied by the length of the work, 17×125 equals 1 inch, the amount necessary to set the tail stock over.

64

The smallest graduations on a machinist's scale are 64ths. So it will be necessary to reduce the fraction to 64ths. In the above case it may be done by reducing both numerator and denominator, thus

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It may also be reduced to 64ths by simple proportion in the following

manner:

X

128=64, or 51 : 128 :: X : 64
x=25

1. What amount will it be necessary to set the tail stock over to cut a taper of inch, the taper to run 9 inches on a piece of stock 15 inches in length? Answer, inch.

2. A piece of work is 27 inches long and it is desired to cut a 1-inch taper 8 inches long. How far over should the tail stock be set? Answer, 11 inches.

MILLING MACHINE AND FITTINGS.

Milling may be defined as the process of removing by a cutting tool that is rotated about its own axis and has one or more cutting edges that are successively brought against the work. Milling machines are of the vertical and horizontal types.

A milling machine consists of certain essential parts, which in some form or other must exist in any of its various modifications. These parts are the frame, the spindle, the table, the feed mechanism, and the cutting tool. The function of the table is to serve as a support to the work, which may be directly secured to it. The feed mechanism serves to move the work past the cutting tool. It may operate directly upon the table or upon the dividing head, or upon both, as desired.

The cutters are made in various shapes and sizes and are classed as follows: Plain milling cutters with either straight or helical cutting edges, hollow milling cutters, slotting cutters, angular cutters, fly cutters and gear-tooth cutters, gang-mill cutters. All milling cutters

are so made that in grinding the shape or form is not changed. Fittings used on milling machine are arbors which fit spindle and to which cutters are secured; dividing head, or index head, which rotates the work, tailstock, chuck, vice, collets, and index plates.

The index head is made so as to rotate the work, and is so constructed that a given number of turns of the crank handle rotates the

shaft to which the center is attached. It is so constructed that the spindle can be swung in a vertical plane through an arc of about 110° to 200°; the head is graduated in degrees, and a zero mark is placed on the frame and shows how many degrees the axis of the spindle deviates from its horizontal position; universal index heads are constructed so that the feed screw of the table and the worm shaft may be connected by suitable gearing, so that the index spindle will revolve as the table moves in a straight line. A combination of these two movements allow helixes and spirals to be cut.

INDEXING ON A MILLING MACHINE.

RULE I: To obtain the number of turns the index crank must make, divide the number of turns required for one revolution of the index head spindle by the number of divisions into which the periphery of the

work is divided.

Example: 40 revolutions of index crank-1 revolution of index head. How many turns must the crank make to divide a periphery of the given work into 12 parts? Answer, 40÷12=311⁄2•

RULE II: To measure the fractional parts of a turn of the index crank, select an index circle having a number of holes that is divisible by the denominator of the fraction when it is reduced to its lowest terms. Multiply the number of holes in the index circle thus selected by the fraction to obtain the number of holes that the latch pin must be moved for the fractional part of a turn.

Example: It requires 40 turns of the index crank to make 1 revolution of the index head spindle. How much must the crank be moved in order to divide the circumference of a piece of work in 28 equal divisions?

Solution.-40 turns=1 revolution.

28 divisions

11 to lowest terms = 13. Index plates available 37, 39, 41, 43, 49. So 49 X 21 holes.

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