PRACTICAL MECHANICS FOR BOYS TEXT

pins serve as the pivots for the angles formed by the two shafts. Fig. 69. TRAMMEL FOR MAKING AN ELLIPSE.--This is a tool easily made, which will be of great service in the shop. In a disc (A), preferably made of brass, are two channels (B) at right angles to each other. The grooves are undercut, so that the blocks (C) will fit and slide in the grooves and be held therein by the dove-tailed formation. Each block is longer than the width of the groove, and has an outwardly projecting pin which passes through a bar (D). One pin (E) is movable along in a slot, but is adjustable at any point so that the shape of the ellipse may be varied. The end of the bar has a series of holes (G) for a pencil, so that the size of the ellipse may also be changed. [Illustration: _Fig. 62. Bevel Gears_ _Fig. 63. Miter Gears_ _Fig. 64. Crown Wheel_ _Fig. 65. Grooved Friction Gears_ _Fig. 66. Valve_ _Fig. 67. Cone Pulleys_ _Fig. 68. Universal Joint_] Fig. 70. ESCAPEMENTS.--Various forms of escapements may be made, but the object of all is the same. The device is designed to permit a wheel to move intermittingly or in a step by step movement, by the swinging motion of a pendulum. Another thing is accomplished by it. The teeth of the escapement are cut at such an angle that, as one of the teeth of the escapement is released from one tooth of the escapement wheel, the spring, or the weight of the clock, will cause one of the teeth of the escapement wheel to engage the other tooth of the escapement, and give the pendulum an impulse in the other direction. In the figure, A is the escapement, B the escapement wheels and _a_, _b_, the pallets, which are cut at suitable angles to actuate the pendulum. Fig. 71. SIMPLE DEVICE TO PREVENT A WHEEL OR SHAFT FROM TURNING BACK.--This is a substitute for a pawl and ratchet wheel. A is a drum or a hollow wheel and B a pulley on a shaft, and this pulley turns loosely with the drum (A). Four tangential slots (C) are cut into the perimeter of the pulley (B), and in each is a hardened steel roller (D). It matters not in what position the wheel (B) may be, at least two of the rollers will always be in contact with the inside of the drum (A), and thus cause the pulley and drum to turn together. On reversing the direction of the pulley the rollers are immediately freed from binding contact. Fig. 72. RACKS AND PINIONS.--The object of this form of mechanism is to provide a reciprocating, or back-and-forth motion, from a shaft which turns continually in one direction. A is the rack and B a mutilated gear. When the gear turns it moves the rack in one direction, because the teeth of the gear engage the lower rack teeth, and when the rack has moved to the end its teeth engage the teeth of the upper rack, thus reversing the movement of the rack. Fig. 73. MUTILATED GEARS.--These are made in so many forms, and adapted for such a variety of purposes, that we merely give a few samples to show what is meant by the term. [Illustration: _Fig. 69. Trammel_ _Fig. 70. Escapement_ _Fig. 71. Device for Holding Wheel_ _Fig. 72. Rack and Pinion_ _Fig. 73. Mutilated Gears_ _Fig. 74. Shaft Coupling_] Fig. 74. SIMPLE SHAFT COUPLING.--Prepare two similarly formed discs (A, B), which are provided with hubs so they may be keyed to the ends of the respective shafts. One disc has four or more projecting pins (C), and the other disc suitable holes (D) to receive the pins. Fig. 75. CLUTCHES.--This is a piece of mechanism which is required in so many kinds of machinery, that we show several of the most approved types. Fig. 76. BALL AND SOCKET JOINTS.--The most practical form of ball and socket joints is simply a head in which is a bowl-shaped cavity the depth of one-half of the ball. A plate with a central opening small enough to hold in the ball, and still large enough at the neck to permit the arm carrying the ball to swing a limited distance, is secured by threads, or by bolts, to the head. The first figure shows this. Fig. 77 illustrates a simple manner of tightening the ball so as to hold the standard in any desired position. Fig. 78. TRIPPING DEVICES.--These are usually in the form of hooks, so arranged that a slight pull on the tripping lever will cause the suspended articles to drop. Fig. 79. ANCHOR BOLT.--These are used in brick or cement walls. The bolt itself screws into a sleeve which is split, and draws a wedge nut up to the split end of the sleeve. As a result the split sleeve opens or spreads out and binds against the wall sufficiently to prevent the bolt from being withdrawn. Fig. 80. LAZY TONGS.--One of the simplest and most effective instruments for carrying ice, boxes or heavy objects, which are bulky or inconvenient to carry. It grasps the article firmly, and the heavier the weight the tighter is its grasp. Fig. 81. DISC SHEARS.--This is a useful tool either for cutting tin or paper, pasteboard and the like. It will cut by the act of drawing the material through it, but if power is applied to one or to both of the shafts the work is much facilitated, particularly in thick or hard material. [Illustration: _Fig. 75. Clutches_ _Fig. 76. Ball and Socket Joints_ _Fig. 77. Fastening Ball_ _Fig. 78. Tripping Devices_ _Fig. 79. Anchor Bolt_ _Fig. 80. Lazy Tongs._ _Fig. 81. Disc Shears._] Fig. 82. WABBLE SAW.--This is a most simple and useful tool, as it will readily and quickly saw out a groove so that it is undercut. The saw is put on the mandrel at an angle, as will be seen, and should be run at a high rate of speed. Fig. 83. CRANK MOTION BY A SLOTTED YOKE.--This produces a straight back-and-forth movement from the circular motion of a wheel or crank. It entirely dispenses with a pitman rod, and it enables the machine, or the part of the machine operated, to be placed close to the crank. Fig. 84. CONTINUOUS FEED BY THE MOTION OF A LEVER.--The simple lever with a pawl on each side of the fulcrum is the most effective means to make a continuous feed by the simple movement of a lever. The form shown is capable of many modifications, and it can be easily adapted for any particular work desired. [Illustration: _Fig. 82. Wabble Saw_ _Fig. 83. Continuous Crank Motion_ _Fig. 84. Continuous Feed_ _Fig. 85. Crank Motion_ _Fig. 86. Ratchet Head_ _Fig. 87. Bench Clamp_] Fig. 85. CRANK MOTION.--By the structure shown, namely, a slotted lever (A), a quick return can be made with the lever. B indicates the fulcrum. Fig. 86. RATCHET HEAD.--This shows a well-known form for common ratchet. It has the advantage that the radially movable plugs (A) are tangentially disposed, and rest against walls (B) eccentrically disposed, and are, therefore, in such a position that they easily slide over the inclines. Fig. 87. BENCH CLAMP.--A pair of dogs (A, B), with the ends bent toward each other, and pivoted midway between the ends to the bench in such a position that the board (C), to be held between them, on striking the rear ends of the dogs, will force the forward ends together, and thus clamp it firmly for planing or other purposes. [Illustration: _Fig. 88. Helico-Volute Spring_ _Fig. 89. Double Helico-Volute_ _Fig. 90. Helical Spring_ _Fig. 91. Single Volute Helix-Spring_ _Fig. 92. Flat Spiral or Convolute_ _Fig. 93. Eccentric Rod and Strap_ _Fig. 94. Anti-Dead Center for Foot-Lathes_] Fig. 88. HELICO-VOLUTE SPRING.--This is a form of spring for tension purposes. The enlarged cross-section of the coil in its middle portion, with the ends tapering down to the eyes, provides a means whereby the pull is transferred from the smaller to the larger portions, without producing a great breaking strain near the ends. Fig. 89. DOUBLE HELICO-VOLUTE.--This form, so far as the outlines are considered, is the opposite of Fig. 88. A compression spring of this kind has a very wide range of movement. Fig. 90. HELICAL SPRING.--This form of coil, uniform from end to end, is usually made of metal which is square in cross-section, and used where it is required for heavy purposes. Fig. 91. SINGLE VOLUTE HELIX-SPRING.--This is also used for compression, intended where tremendous weights or resistances are to be overcome, and when the range of movement is small. Fig. 92. FLAT SPIRAL, OR CONVOLUTE.--This is for small machines. It is the familiar form used in watches owing to its delicate structure, and it is admirably adapted to yield to the rocking motion of an arbor. Fig. 93. ECCENTRIC ROD AND STRAP.--A simple and convenient form of structure, intended to furnish a reciprocating motion where a crank is not available. An illustration of its use is shown on certain types of steam engine to operate the valves. Fig. 94. ANTI-DEAD CENTER FOR FOOT-LATHES.--A flat, spiral spring (A), with its coiled end attached to firm support (B), has its other end pivotally attached to the crank-pin (C), the tension of the spring being such that when the lathe stops the crack-pin will always be at one side of the dead-center, thus enabling the operator to start the machine by merely pressing the foot downwardly on the treadle (D). CHAPTER VII PROPERTIES OF MATERIALS A workman is able to select the right metals because he knows that each has some peculiar property which is best adapted for his particular use. These with their meaning will now be explained. ELASTICITY.--This exists in metals in three distinct ways: First, in the form of _traction_. Hang a weight on a wire and it will stretch a certain amount. When the weight is removed the wire shrinks back to its original length. Second: If the weight on the wire is rotated, so as to twist it, and the hand is taken from the weight, it will untwist itself, and go back to its original position. This is called _torsion_. Third: A piece of metal may be coiled up like a watch spring, or bent like a carriage spring, and it will yield when pressure is applied. This is called _flexure_. Certain kinds of steel have these qualities in a high degree. TENACITY.--This is a term used to express the resistance which the body opposes to the separation of its parts. It is determined by forming the metal into a wire, and hanging on weights, to find how much will be required to break it. If we have two wires, the first with a transverse area only one-quarter that of the second, and the first breaks at 25 pounds, while the second breaks at 50 pounds, the tenacity of the first is twice as great as that of the second. To the boy who understands simple ratio in mathematics, the problem would be like this: 25 x 4 : 50 x 1, or as 2 : 1. THE MOST TENACIOUS METAL.--Steel has the greatest tenacity of all metals, and lead the least. In proportion to weight, however, there are many substances which have this property in a higher degree. Cotton fibers will support millions of times their own weight. There is one peculiar thing, that tenacity varies with the form of the body. A solid cylindrical body has a greater strength than a square one of the same size; and a hollow cylinder more tenacity than a solid one. This principle is well known in the bones of animals, in the feathers of birds, and in the stems of many plants. In almost every metal tenacity diminishes as the temperature increases. DUCTILITY.--This is a property whereby a metal may be drawn out to form a wire. Some metals, like cast iron, have absolutely no ductility. The metal which possesses this property to the highest degree, is platinum. Wires of this metal have been drawn out so fine that over 30,000 of them laid side by side would measure only one inch across, and a mile of such wire would weigh only a grain, or one seven-thousandth of a pound. MALLEABILITY.--This is considered a modification of ductility. Any metal which can be beaten out, as with a hammer, or flattened into sheets with rollers, is considered malleable. Gold possesses this property to the highest degree. It has been beaten into leaves one three-hundred-thousandth of an inch thick. HARDNESS.--This is the resistance which bodies offer to being scratched by others. As an example, the diamond has the capacity to scratch all, but cannot be scratched by any other. ALLOYS.--Alloys, that is a combination of two or more metals, are harder than the pure metals, and for this reason jewelry, and coins, are usually alloyed. The resistance of a body to compression does not depend upon its hardness. Strike a diamond with a hammer and it flies to pieces, but wood does not. One is brittle and the other is tough. The machinist can utilize this property by understanding that velocity enables a soft material to cut a harder one. Thus, a wrought iron disc rotating rapidly, will cut such hard substances as agate or quartz. RESISTANCE.--All metals offer more or less resistance to the flow of an electric current. Silver offers the least resistance, and German silver the greatest. Temperature also affects the flow. It passes more easily over a cold than a warm conductor. PERSISTENCE.--All metals on receiving heat, will retain it for a certain length of time, and will finally cool down to the temperature of the surrounding atmosphere. Some, like aluminum, retain it for a long time; others, as iron, will give it off quickly. CONDUCTIVITY.--All metals will conduct heat and cold, as well as electricity. If one end of a metal bar is heated, the heat creeps along to the other end until it has the same temperature throughout. This is called _equalization_. If a heated bar is placed in contact with another, the effect is to increase the temperature of the cold bar and lower that of the warm bar. This is called _reciprocity_. MOLECULAR FORCES.--_Molecular_ attraction is a force which acts in such a way as to bring all the particles of a body together. It acts in three ways, dependent on the particular conditions which exist. First: _Cohesion_. This exists between molecules which are of the same kind, as for instance, iron. Cohesion of the particles is very strong in solids, much weaker in liquids, and scarcely exists at all between the particles in gases. Second: _Adhesion_ is that property which exists between the surfaces of bodies in contact. If two flat surfaces are pressed together, as for instance, two perfectly smooth and flat pieces of lead, they will adhere. If, for instance, oil should be put on the surfaces, before putting them together, they would adhere so firmly that it would be difficult to pull them apart. Third: _Affinity_. This is another peculiarity about materials. Thus, while cohesion binds together the molecules of water, it is chemical affinity which unites two elements, like hydrogen and oxygen, of which water is composed. POROSITY.--All matter has little hollows or spaces between the molecules. You know what this is in the case of a sponge, or pumice stone. Certain metals have the pores so small that it is difficult to see them except with a very powerful glass. Under great pressure water can be forced through the pores of metals, as has been done in the case of gold. Water also is porous, but the spaces between the molecules are very small. COMPRESSIBILITY.--It follows from the foregoing statement, that if there are little interstices between the molecules, the various bodies can be compressed together. This can be done in varying degrees with all solids, but liquids, generally, have little compressibility. Gases are readily reduced in volume by compression. ELASTICITY.--This is a property by virtue of which a body resumes its original form when compressed. India rubber, ivory and glass are examples of elasticity; whereas, lead and clay do not possess this property. Air is the most elastic of all substances. INERTIA.--This is a property of matter by virtue of which it cannot of itself change its state of motion or of rest. Newton's first law of motion is, in substance, that matter at rest will eternally remain at rest, and matter in motion will forever continue in motion, unless acted on by some external force. A rider is carried over the head of a horse when the latter suddenly stops. This illustrates the inertia of movement. A stone at rest will always remain in that condition unless moved by some force. That shows the inertia of rest. MOMENTUM.--This is the term to designate the quantity of motion in a body. This quantity varies and is dependent on the mass, together with the velocity. A fly wheel is a good example. It continues to move after the impelling force ceases; and a metal wheel has greater momentum than a wooden wheel at the same speed, owing to its greater mass. If, however, the wooden wheel is speeded up sufficiently it may have the same momentum as the metal one. WEIGHT.--All substances have what is called _weight_. This means that everything is attracted toward the earth by the force of gravity. Gravity, however, is different from weight. All substances attract each other; not only in the direction of the center of the earth, but laterally, as well. Weight, therefore, has reference to the pull of an object toward the earth; and gravity to that influence which all matter has for each other independently of the direction. CENTRIPETAL FORCE.--This attraction of the earth, which gives articles the property of weight, is termed centripetal force--that is, the drawing in of a body. CENTRIFUGAL FORCE.--The direct opposite of centripetal, is centrifugal force, which tends to throw outwardly. Dirt flying from a rapidly moving wheel illustrates this. CAPILLARY ATTRACTION.--There is a peculiar property in liquids, which deserves attention, and should be understood, and that is the name given to the tendency of liquids to rise in fine tubes. It is stated that water will always find its level. While this is true, we have an instance where, owing to the presence of a solid, made in a peculiar form, causes the liquid, within, to rise up far beyond the level of the water. This may be illustrated by three tubes of different internal diameters. The liquid rises up higher in the second than in the first, and still higher in the third than in the second. The smaller the tube the greater the height of the liquid. This is called _capillary attraction_, the word capillary meaning a hair. The phenomena is best observed when seen in tubes which are as fine as hairs. The liquid has an affinity for the metal, and creeps up the inside, and the distance it will thus move depends on the size of the tube. THE SAP OF TREES.--The sap of trees goes upwardly, not because the tree is alive, but due to this property in the contact of liquids with a solid. It is exactly on the same principle that if the end of a piece of blotting paper is immersed in water, the latter will creep up and spread over the entire surface of the sheet. In like manner, oil moves upwardly in a wick, and will keep on doing so, until the lighted wick is extinguished, when the flow ceases. When it is again lighted the oil again flows, as before. If it were not for this principle of capillary attraction, it would be difficult to form a bubble of air in a spirit level. You can readily see how the liquid at each end of the air bubble rounds it off, as though it tried to surround it. SOUND.--Sound is caused by vibration, and it would be impossible to convey it without an elastic medium of some kind. _Acoustics_ is a branch of physics which treats of sounds. It is distinguished from music which has reference to the particular kinds. _Sounds_ are distinguished from _noises_. The latter are discordant and abrupt vibrations, whereas the former are regular and continuous. SOUND MEDIUMS.--Gases, vapors, liquids and solids transmit vibrations, but liquids and solids propagate with greater velocity than gases. VIBRATION.--A vibration is the moving to and fro of the molecules in a body, and the greater their movement the more intense is the sound. The intensity of the sound is affected by the density of the atmosphere, and the movement of the winds also changes its power of transmission.