Practical Mechanics for Boys Part 7

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 ill.u.s.trates 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a result the split sleeve opens or spreads out and binds against the wall sufficiently to prevent the bolt from being withdrawn.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.

[Ill.u.s.tration: _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.

[Ill.u.s.tration: _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.

[Ill.u.s.tration: _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 ill.u.s.tration 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 4 : 50 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 pa.s.ses 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.