Part 12 (1/2)
(Per Cubic Inch) Pounds Pounds Lead 410 Wrought Iron 278 Copper 320 Tin 263 Bronze 313 Cast Iron 260 Brass 300 Zinc 258 Steel 283 Aluminum 093
EXPANSION OF METALS
(Measured in Thousandths of an Inch per Foot of Length When Raised 1000 Degrees in Temperature) Inch Inch Lead 188 Brass 115 Zinc 168 Copper 106 Aluminum 148 Steel 083 Silver 129 Wrought Iron 078 Bronze 118 Cast Iron 068
CHAPTER VI
ELECTRIC WELDING
RESISTANCE METHOD
Two distinct for heat by the resistance of the e of electric current, the other using the heat of the electric arc
The resistance process is of the greatest use in e quantity of one kind of work to do, many thousand pieces of one kind, for instance The arc method may be applied in practically any case where any other form of weld may be made The resistance process will be described first
It is a well known fact that a poor conductor of electricity will offer so much resistance to the flow of electricity that it will heat Copper is a good conductor, and a bar of iron, a comparatively poor conductor, when placed between heavy copper conductors of a welder, becoe voluree of heat depends on the amount of current and the resistance of the conductor
In an electric circuit the ends of two pieces of reatest resistance in the electric circuit, and the abutting ends instantly begin to heat The hotter this reater the resistance to the flow of current; consequently, as the edges of the abutting ends heat, the current is forced into the adjacent cooler parts, until there is a uniforhout the entire mass The heat is first developed in the interior of the metal so that it is welded there as perfectly as at the surface
[Illustration: Figure 42--Spot Welding Machine]
The electric welder (Figure 42) is built to hold the parts to be joined between two heavy copper dies or contacts A current of three to five volts, but of very great volue), is allowed to pass across these dies, and in going through thetee of an electric currentsent through the circuit and has nothing to do with the quantity or volu A through the circuit and consequently give a iven time Volts correspond to water pressure measured by pounds to the square inch; aallons per er to the operator, this pressure not being sufficient to be felt even with the hands resting on the copper contacts
Current is supplied to the welding e than is actually used between the dies, the low voltage and high a produced by a transfors of suitable size wire, the outside currentfrom 110 to 550 and converted to the low pressure needed
The source of current for the resistance welder ative in value and then positive, passing fro fro current, as opposed to direct current, in which there is no changing of positive and negative
The current le phase, that is, a current which rises frohest point as a positive current and then recedes to zero before rising to the highest point of negative value Two-phase of three-phase currents would give two or three positive ile phase alternating, the voltage and cycles (nu convenient Various voltages and cycles are taken care of by specifying all these points when designing the transformer which is to handle the current
Direct current is not used because there is no way of reducing the voltage conveniently without placing resistance wires in the circuit and this uses poithout producing useful work Direct currenta direct current e h this last e used in welding being so low to start with, it is absolutely necessary that it be maintained at the correct point If the source of current supply is not of a used, it will be very hard to avoid a fall of voltage when the current is forced to pass through the high resistance of the weld The current voltage for various work is calculated accurately, and the efficiency of the outfit depends to a great extent on the voltage being constant
A si an incandescent electric lamp across the supply lines at some point near the welder The la made as at any other time If the lae, and this condition should be corrected
The dyna currentwith the welder and operated from a direct current motor, asor source of power When the dyna plant it should be placed as close to the welding th of the wire used affects the voltage appreciably
In order to hold the voltage constant, the Toledo Electric Welder Company has devised connections which include a rheostat to insert a variable resistance in the field windings of the dyna this resistance out at the proper time An auxiliary switch is connected to the welder switch so that both switches act together When the welder switch is closed ina weld, that portion of the rheostat resistance between two are is short circuited This lowers the resistance and the field er so that additional voltage is provided to care for the resistance in theheated
A typical ure 43) On top of the welder are ts for holding the ends of the pieces to be welded