Part 12 (1/2)
(Per Cubic Inch) Pounds Pounds Lead............ .410 Wrought Iron..... .278 Copper.......... .320 Tin.............. .263 Bronze.......... .313 Cast Iron........ .260 Bra.s.s........... .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 Bra.s.s............ .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 forms of electric welding apparatus are in use, one producing heat by the resistance of the metal being treated to the pa.s.sage of electric current, the other using the heat of the electric arc.
The resistance process is of the greatest use in manufacturing lines where there is a large quant.i.ty 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, becomes heated in attempting to carry the large volume of current. The degree of heat depends on the amount of current and the resistance of the conductor.
In an electric circuit the ends of two pieces of metal brought together form the point of greatest resistance in the electric circuit, and the ab.u.t.ting ends instantly begin to heat. The hotter this metal becomes, the greater the resistance to the flow of current; consequently, as the edges of the ab.u.t.ting ends heat, the current is forced into the adjacent cooler parts, until there is a uniform heat throughout the entire ma.s.s. The heat is first developed in the interior of the metal so that it is welded there as perfectly as at the surface.
[Ill.u.s.tration: 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 volume (amperage), is allowed to pa.s.s across these dies, and in going through the metal to be welded, heats the edges to a welding temperature. It may be explained that the voltage of an electric current measures the pressure or force with which it is being sent through the circuit and has nothing to do with the quant.i.ty or volume pa.s.sing. Amperes measure the rate at which the current is pa.s.sing through the circuit and consequently give a measure of the quant.i.ty which pa.s.ses in any given time. Volts correspond to water pressure measured by pounds to the square inch; amperes represent the flow in gallons per minute. The low voltage used avoids all danger 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 machine at a higher voltage and lower amperage than is actually used between the dies, the low voltage and high amperage being produced by a transformer incorporated in the machine itself. By means of windings of suitable size wire, the outside current may be received at voltages ranging from 110 to 550 and converted to the low pressure needed.
The source of current for the resistance welder must be alternating, that is, the current must first be negative in value and then positive, pa.s.sing from one extreme to the other at rates varying from 25 to 133 times a second. This form is known as alternating current, as opposed to direct current, in which there is no changing of positive and negative.
The current must also be what is known as single phase, that is, a current which rises from zero in value to the highest 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 impulses during this time.
As long as the current is single phase alternating, the voltage and cycles (number of alternations per second) may be anything 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 power without producing useful work. Direct current may be changed to alternating by having a direct current motor running an alternating current dynamo, or the change may be made by a rotary converter, although this last method is not so satisfactory as the first.
The voltage 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 ample capacity for the welder being used, it will be very hard to avoid a fall of voltage when the current is forced to pa.s.s 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 simple test for fall of voltage is made by connecting an incandescent electric lamp across the supply lines at some point near the welder. The lamp should burn with the same brilliancy when the weld is being made as at any other time. If the lamp burns dim at any time, it indicates a drop in voltage, and this condition should be corrected.
The dynamo furnis.h.i.+ng the alternating current may be in the same building with the welder and operated from a direct current motor, as mentioned above, or operated from any convenient shafting or source of power. When the dynamo is a part of the welding plant it should be placed as close to the welding machine as possible, because the length 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 dynamo so that the voltage may be increased by cutting 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 in making a weld, that portion of the rheostat resistance between two arms determining the voltage is short circuited. This lowers the resistance and the field magnets of the dynamo are made stronger so that additional voltage is provided to care for the resistance in the metal being heated.
A typical machine is shown in the accompanying cut (Figure 43). On top of the welder are two jaws for holding the ends of the pieces to be welded.
The lower part of the jaws is rigid while the top is brought down on top of the work, acting as a clamp. These jaws carry the copper dies through which the current enters the work being handled. After the work is clamped between the jaws, the upper set is forced closer to the lower set by a long compression lever. The current being turned on with the surfaces of the work in contact, they immediately heat to the welding point when added pressure on the lever forces them together and completes the weld.
[Ill.u.s.tration: Figure 43--Operating Parts of a Toledo Spot Welder]
[Ill.u.s.tration: Figure 43a.--Method of Testing Electric Welder]
[Ill.u.s.tration: Figure 44.--Detail of Water-Cooled Spot Welding Head]