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 passage 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 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, 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
abutting 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 abutting ends heat, the current is forced into the adjacent cooler
parts, until there is a uniform heat throughout 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 volume (amperage), is allowed to pass 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 quantity or volume
passing. Amperes measure the rate at which the current is passing through
the circuit and consequently give a measure of the quantity which passes 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, passing
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 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 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 furnishing 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.
[Illustration: Figure 43--Operating Parts of a Toledo Spot Welder]
[Illustration: Figure 43a.--Method of Testing Electric Welder]
[Illustration: Figure 44.--Detail of Water-Cooled Spot Welding Head]
The transformer is carried in the base of the machine and on the left-hand
side is a regulator for controlling the voltage for various kinds of work.
The clamps are applied by treadles convenient to the foot of the operator.
A treadle is provided which instantly releases both jaws upon the
completion of the weld. One or both of the copper dies may be cooled by a
stream of water circulating through it from the city water mains
(Figure 44). The regulator and switch give the operator control of the
heat, anything from a dull red to the melting point being easily obtained
by movement of the lever (figure 45).
[Illustration: Figure 45.--Welding Head of a Water-Cooled Welder]
_Welding._--It is not necessary to give the metal to be welded any
special preparation, although when very rusty or covered with scale, the
rust and scale should be removed sufficiently to allow good contact of
clean metal on the copper dies. The cleaner and better the stock, the less
current it takes, and there is less wear on the dies. The dies should be
kept firm and tight in their holders to make a good contact. All bolts and
nuts fastening the electrical contacts should be clean and tight at all
times.
The scale may be removed from forgings by immersing them in a pickling
solution in a wood, stone or lead-lined tank.
The solution is made with five gallons of commercial sulphuric acid in
150 gallons of water. To get the quickest and best results from this
method, the solution should be kept as near the boiling point as possible
by having a coil of extra heavy lead pipe running inside the tank and
carrying live steam. A very few minutes in this bath will remove the scale
and the parts should then be washed in running water. After this washing
they should be dipped into a bath of 50 pounds of unslaked lime in 150
gallons of water to neutralize any trace of acid.
Cast iron cannot be commercially welded, as it is high in carbon and
silicon, and passes suddenly from a crystalline to a fluid state when
brought to the welding temperature. With steel or wrought iron the
temperature must be kept below the melting point to avoid injury to the
metal. The metal must be heated quickly and pressed together with
sufficient force to push all burnt metal out of the joint.
High carbon steel can be welded, but must be annealed after welding to
overcome the strains set up by the heat being applied at one place. Good
results are hard to obtain when the carbon runs as high as 75 points, and
steel of this class can only be handled by an experienced operator. If the
steel is below 25 points in carbon content, good welds will always be the
result. To weld high carbon to low carbon steel, the stock should be
clamped in the dies with the low carbon stock sticking considerably further
out from the die than the high carbon stock. Nickel steel welds readily,
the nickel increasing the strength of the weld.
Iron and copper may be welded together by reducing the size of the copper
end where it comes in contact with the iron. When welding copper and brass
the pressure must be less than when welding iron. The metal is allowed to
actually fuse or melt at the juncture and the pressure must be sufficient
to force the burned metal out. The current is cut off the instant the metal
ends begin to soften, this being done by means of an automatic switch which
opens when the softening of the metal allows the ends to come together. The
pressure is applied to the weld by having the sliding jaw moved by a weight
on the end of an arm.
Copper and brass require a larger volume of current at a lower voltage than
for steel and iron. The die faces are set apart three times the diameter of
the stock for brass and four times the diameter for copper.
Light gauges of sheet steel can be welded to heavy gauges or to solid bars
of steel by "spot" welding, which will be described later. Galvanized iron
can be welded, but the zinc coating will be burned off. Sheet steel can be
welded to cast iron, but will pull apart, tearing out particles of the
iron.
Sheet copper and sheet brass may be welded, although this work requires
more experience than with iron and steel. Some grades of sheet aluminum can
be spot-welded if the slight roughness left on the surface under the die
is not objectionable.
_Butt Welding._--This is the process which joins the ends of two
pieces of metal as described in the foregoing part of this chapter. The
ends are in plain sight of the operator at all times and it can easily be
seen when the metal reaches the welding heat and begins to soften (Figure
46). It is at this point that the pressure must be applied with the lever
and the ends forced together in the weld.
[Illustration: Figure 46.--Butt Welder]
The parts are placed in the clamping jaws (Figure 47) with 1/8 to 1/2 inch
of metal extending beyond the jaw. The ends of the metal touch each other
and the current is turned on by means of a switch. To raise the ends to the
proper heat requires from 3 seconds for 1/4-inch rods to 35 seconds for a
1-1/2-inch bar.
This method is applicable to metals having practically the same area of
metal to be brought into contact on each end. When such parts are forced
together a slight projection will be left in the form of a fin or an
enlarged portion called an upset. The degree of heat required for any work
is found by moving the handle of the regulator one way or the other while
testing several parts. When this setting is right the work can continue as
long as the same sizes are being handled.
[Illustration: Figure 47.--Clamping Dies of a Butt Welder]
Copper, brass, tool steel and all other metals that are harmed by high
temperatures must be heated quickly and pressed together with sufficient
force to force all burned metal from the weld.
In case it is desired to make a weld in the form of a capital letter T, it
is necessary to heat the part corresponding to the top bar of the T to a
bright red, then bring the lower bar to the pre-heated one and again turn
on the current, when a weld can be quickly made.
_Spot Welding._--This is a method of joining metal sheets together at
any desired point by a welded spot about the size of a rivet. It is done on
a spot welder by fusing the metal at the point desired and at the same
instant applying sufficient pressure to force the particles of molten metal
together. The dies are usually placed one above the other so that the work
may rest on the lower one while the upper one is brought down on top of the
upper sheet to be welded.
One of the dies is usually pointed slightly, the opposing one being left
flat. The pointed die leaves a slight indentation on one side of the metal,
while the other side is left smooth. The dies may be reversed so that the
outside surface of any work may be left smooth. The current is allowed to
flow through the dies by a switch which is closed after pressure is applied
to the work.
There is a limit to the thickness of sheet metal that can be welded by this
process because of the fact that the copper rods can only carry a certain
quantity of current without becoming unduly heated themselves. Another
reason is that it is difficult to make heavy sections of metal touch at the
welding point without excessive pressure.
_Lap welding_ is the process used when two pieces of metal are caused
to overlap and when brought to a welding heat are forced together by
passing through rollers, or under a press, thus leaving the welded joint
practically the same thickness as the balance of the work.
Where it is desirable to make a continuous seam, a special machine is
required, or an attachment for one of the other types. In this form of work
the stock must be thoroughly cleaned and is then passed between copper
rollers which act in the same capacity as the copper dies.
_Other Applications._--Hardening and tempering can be done by clamping
the work in the welding dies and setting the control and time to bring the
metal to the proper color, when it is cooled in the usual manner.
Brazing is done by clamping the work in the jaws and heating until the
flux, then the spelter has melted and run into the joint. Riveting and
heading of rivets can be done by bringing the dies down on opposite ends of
the rivet after it has been inserted in the hole, the dies being shaped to
form the heads properly.
Hardened steel may be softened and annealed so that it can be machined by
connecting the dies of the welder to each side of the point to be softened.
The current is then applied until the work has reached a point at which it
will soften when cooled.
_Troubles and Remedies._--The following methods have been furnished by
the Toledo Electric Welder Company and are recommended for this class of
work whenever necessary.
To locate grounds in the primary or high voltage side of the circuit,
connect incandescent lamps in series by means of a long piece of lamp cord,
as shown, in Figure 43a. For 110 volts use one lamp, for 220 volts use two
lamps and for 440 volts use four lamps. Attach one end of the lamp cord to
one side of the switch, and close the switch. Take the other end of the
cord in the hand and press it against some part of the welder frame where
the metal is clean and bright. Paint, grease and dirt act as insulators and
prevent electrical contact. If the lamp lights, the circuit is in
electrical contact with the frame; in other words, grounded. If the lamps
do not light, connect the wire to a terminal block, die or slide. If the
lamps then light, the circuit, coils or leads are in electrical contact
with the large coil in the transformer or its connections.
If, however, the lamps do not light in either case, the lamp cord should be
disconnected from the switch and connected to the other side, and the
operations of connecting to welder frame, dies, terminal blocks, etc., as
explained above, should be repeated. If the lamps light at any of these
connections, a "ground" is indicated. "Grounds" can usually be found by
carefully tracing the primary circuit until a place is found where the
insulation is defective. Reinsulate and make the above tests again to make
sure everything is clear. If the ground can not be located by observation,
the various parts of the primary circuit should be disconnected, and the
transformer, switch, regulator, etc., tested separately.
To locate a ground in the regulator or other part, disconnect the lines
running to the welder from the switch. The test lamps used in the previous
tests are connected, one end of lamp cord to the switch, the other end to a
binding post of the regulator. Connect the other side of the switch to some
part of the regulator housing. (This must be a clean connection to a bolt
head or the paint should be scraped off.) Close the switch. If the lamps
light, the regulator winding or some part of the switch is "grounded" to
the iron base or core of the regulator. If the lamps do not light, this
part of the apparatus is clear.
This test can be easily applied to any part of the welder outfit by
connecting to the current carrying part of the apparatus, and to the iron
base or frame that should not carry current. If the lamps light, it
indicates that the insulation is broken down or is defective.
An A.C. voltmeter can, of course, be substituted for the lamps, or a D.C.
voltmeter with D.C. current can be used in making the tests.
A short circuit in the primary is caused by the insulation of the coils
becoming defective and allowing the bare copper wires to touch each other.
This may result in a "burn out" of one or more of the transformer coils, if
the trouble is in the transformer, or in the continued blowing of fuses in
the line. Feel of each coil separately. If a short circuit exists in a coil
it will heat excessively. Examine all the wires; the insulation may have
worn through and two of them may cross, or be in contact with the frame or
other part of the welder. A short circuit in the regulator winding is
indicated by failure of the apparatus to regulate properly, and sometimes,
though not always, by the heating of the regulator coils.
The remedy for a short circuit is to reinsulate the defective parts. It is
a good plan to prevent trouble by examining the wiring occasionally and see
that the insulation is perfect.
_To Locate Grounds and Short Circuits in the Secondary, or Low Voltage
Side._--Trouble of this kind is indicated by the machine acting sluggish
or, perhaps, refusing to operate. To make a test, it will be necessary to
first ascertain the exciting current of your particular transformer. This
is the current the transformer draws on "open circuit," or when supplied
with current from the line with no stock in the welder dies. The following
table will give this information close enough for all practical purposes:
K.W. ----------------- Amperes at ----------------
Rating 110 Volts 220 Volts 440 Volts 550 Volts
3 1.5 .75 .38 .3
5 2.5 1.25 .63 .5
8 3.6 1.8 .9 .72
10 4.25 2.13 1.07 .85
15 6. 3. 1.5 1.2
20 7. 3.5 1.75 1.4
30 9. 4.5 2.25 1.8
35 9.6 4.8 2.4 1.92
50 10. 5. 2.5 2
Remove the fuses from the wall switch and substitute fuses just large
enough to carry the "exciting" current. If no suitable fuses are at hand,
fine strands of copper from an ordinary lamp cord may be used. These
strands are usually No. 30 gauge wire and will fuse at about 10 amperes.
One or more strands should be used, depending on the amount of exciting
current, and are connected across the fuse clips in place of fuse wire.
Place a piece of wood or fibre between the welding dies in the welder as
though you were going to weld them. See that the regulator is on the
highest point and close the welder switch. If the secondary circuit is
badly grounded, current will flow through the ground, and the small fuses
or small strands of wire will burn out. This is an indication that both
sides of the secondary circuit are grounded or that a short circuit exists
in a primary coil. In either case the welder should not be operated until
the trouble is found and removed. If, however, the small fuses do not
"blow," remove same and replace the large fuses, then disconnect wires
running from the wall switch to the welder and substitute two pieces of
No. 8 or No. 6 insulated copper wire, after scraping off the insulation for
an inch or two at each end. Connect one wire from the switch to the frame
of welder; this will leave one loose end. Hold this a foot or so away from
the place where the insulation is cut off; then turn on the current and
strike the free end of this wire lightly against one of the copper dies,
drawing it away quickly. If no sparking is produced, the secondary circuit
is free from ground, and you will then look for a broken connection in the
circuit. Some caution must be used in making the above test, as in case one
terminal is heavily grounded the testing wire may be fused if allowed to
stay in contact with the die.
_The Remedy._--Clean the slides, dies and terminal blocks thoroughly
and dry out the fibre insulation if it is damp. See that no scale or metal
has worked under the sliding parts, and that the secondary leads do not
touch the frame. If the ground is very heavy it may be necessary to remove
the slides in order to facilitate the examination and removal of the
ground. Insulation, where torn or worn through, must be carefully replaced
or taped. If the transformer coils are grounded to the iron core of the
transformer or to the secondary, it may be necessary to remove the coils
and reinsulate them at the points of contact. A short circuited coil will
heat excessively and eventually burn out. This may mean a new coil if you
are unable to repair the old one. In all cases the transformer windings
should be protected from mechanical injury or dampness. Unless excessively
overloaded, transformers will last for years without giving a moment's
trouble, if they are not exposed to moisture or are not injured
mechanically.
The most common trouble arises from poor electrical contacts, and they are
the cause of endless trouble and annoyance. See that all connections are
clean and bright. Take out the dies every day or two and see that there is
no scale, grease or dirt between them and the holders. Clean them
thoroughly before replacing. Tighten the bolts running from the transformer
leads to the work jaws.
ELECTRIC ARC WELDING
This method bears no relation to the one just considered, except that the
source of heat is the same in both cases. Arc welding makes use of the
flame produced by the voltaic arc in practically the same way that
oxy-acetylene welding uses the flame from the gases.
If the ends of two pieces of carbon through which a current of electricity
is flowing while they are in contact are separated from each other quite
slowly, a brilliant arc of flame is formed between them which consists
mainly of carbon vapor. The carbons are consumed by combination with the
oxygen in the air and through being turned to a gas under the intense heat.
The most intense action takes place at the center of the carbon which
carries the positive current and this is the point of greatest heat. The
temperature at this point in the arc is greater than can be produced by any
other means under human control.
An arc may be formed between pieces of metal, called electrodes, in the
same way as between carbon. The metallic arc is called a flaming arc and as
the metal of the electrode burns with the heat, it gives the flame a color
characteristic of the material being used. The metallic arc may be drawn
out to a much greater length than one formed between carbon electrodes.
Arc Welding is carried out by drawing a piece of carbon which is of
negative polarity away from the pieces of metal to be welded while the
metal is made positive in polarity. The negative wire is fastened to the
carbon electrode and the work is laid on a table made of cast or wrought
iron to which the positive wire is made fast. The direction of the flame is
then from the metal being welded to the carbon and the work is thus
prevented from being saturated with carbon, which would prove very
detrimental to its strength. A secondary advantage is found in the fact
that the greatest heat is at the metal being welded because of its being
the positive electrode.
The carbon electrode is usually made from one quarter to one and a half
inches in diameter and from six to twelve inches in length. The length of
the arc may be anywhere from one inch to four inches, depending on the size
of the work being handled.
While the parts are carefully insulated to avoid danger of shock, it is
necessary for the operator to wear rubber gloves as a further protection,
and to wear some form of hood over the head to shield him against the
extreme heat liberated. This hood may be made from metal, although some
material that does not conduct electricity is to be preferred. The work is
watched through pieces of glass formed with one sheet, which is either blue
or green, placed over another which is red. Screens of glass are sometimes
used without the head protector. Some protection for the eyes is absolutely
necessary because of the intense white light.
It is seldom necessary to preheat the work as with the gas processes,
because the heat is localized at the point of welding and the action is so
rapid that the expansion is not so great. The necessity of preheating,
however, depends entirely on the material, form and size of the work being
handled. The same advice applies to arc welding as to the gas flame method
but in a lesser degree. Filling rods are used in the same way as with any
other flame process.
It is the purpose of this explanation to state the fundamental principles
of the application of the electric arc to welding metals, and by applying
the principles the following questions will be answered:
What metals can be welded by the electric arc?
What difficulties are to be encountered in applying the electric arc to
welding?
What is the strength of the weld in comparison with the original piece?
What is the function of the arc welding machine itself?
What is the comparative application of the electric arc and the
oxy-acetylene method and others of a similar nature?
The answers to these questions will make it possible to understand the
application of this process to any work. In a great many places the use of
the arc is cutting the cost of welding to a very small fraction of what it
would be by any other method, so that the importance of this method may be
well understood.
Any two metals which are brought to the melting temperature and applied to
each other will adhere so that they are no more apt to break at the weld
than at any other point outside of the weld. It is the property of all
metals to stick together under these conditions. The electric arc is used
in this connection merely as a heating agent. This is its only function in
the process.
It has advantages in its ease of application and the cheapness with which
heat can be liberated at any given point by its use. There is nothing in
connection with arc welding that the above principles will not answer; that
is, that metals at the melting point will weld and that the electric arc
will furnish the heat to bring them to this point. As to the first
question, what metals can be welded, all metals can be welded.
The difficulties which are encountered are as follows:
In the case of brass or zinc, the metals will be covered with a coat of
zinc oxide before they reach a welding heat. This zinc oxide makes it
impossible for two clean surfaces to come together and some method has to
be used for eliminating this possibility and allowing the two surfaces to
join without the possibility of the oxide intervening. The same is true of
aluminum, in which the oxide, alumina, will be formed, and with several
other alloys comprising elements of different melting points.
In order to eliminate these oxides, it is necessary in practical work, to
puddle the weld; this is, to have a sufficient quantity of molten metal at
the weld so that the oxide is floated away. When this is done, the two
surfaces which are to be joined are covered with a coat of melted metal on
which floats the oxide and other impurities. The two pieces are thus
allowed to join while their surfaces are protected. This precaution is not
necessary in working with steel except in extreme cases.
Another difficulty which is met with in the welding of a great many metals
is their expansion under heat, which results in so great a contraction when
the weld cools that the metal is left with a considerable strain on it. In
extreme cases this will result in cracking at the weld or near it. To
eliminate this danger it is necessary to apply heat either all over the
piece to be welded or at certain points. In the case of cast iron and
sometimes with copper it is necessary to anneal after welding, since
otherwise the welded pieces will be very brittle on account of the
chilling. This is also true of malleable iron.
Very thin metals which are welded together and are not backed up by
something to carry away the excess heat, are very apt to burn through,
leaving a hole where the weld should be. This difficulty can be eliminated
by backing up the weld with a metal face or by decreasing the intensity of
the arc so that this melting through will not occur. However, the practical
limit for arc welding without backing up the work with a metal face or
decreasing the intensity of the arc is approximately 22 gauge, although
thinner metal can be welded by a very skillful and careful operator.
One difficulty with arc welding is the lack of skillful operators. This
method is often looked upon as being something out of the ordinary and
governed by laws entirely different from other welding. As a matter of
fact, it does not take as much skill to make a good arc weld as it does to
make a good weld in a forge fire as the blacksmith does it. There are few
jobs which cannot be handled successfully by an operator of average
intelligence with one week's instructions, although his work will become
better and better in quality as he continues to use the arc.
Now comes the question of the strength of the weld after it has been made.
This strength is equally as great as that of the metal that is used to make
the weld. It should be remembered, however, that the metal which goes into
the weld is put in there as a casting and has not been rolled. This would
make the strength of the weld as great as the same metal that is used for
filling if in the cast form.
Two pieces of steel could be welded together having a tensile strength at
the weld of 50,000 pounds. Higher strengths than this can be obtained by
the use of special alloys for the filling material or by rolling. Welds
with a tensile strength as great as mentioned will give a result which is
perfectly satisfactory in almost all cases.
There are a great many jobs where it is possible to fill up the weld, that
is, make the section at the point of the weld a little larger than the
section through the rest of the piece. By doing this, the disadvantages
of the weld being in the form of a casting in comparison with the rest of
the piece being in the form of rolled steel can be overcome, and make the
weld itself even stronger than the original piece.
The next question is the adaptability of the electric arc in comparison
with forge fire, oxy-acetylene or other method. The answer is somewhat
difficult if made general. There are no doubt some cases where the use of a
drop hammer and forge fire or the use of the oxy-acetylene torch will make,
all things being considered, a better job than the use of the electric arc,
although a case where this is absolutely proved is rare.
The electric arc will melt metal in a weld for less than the same metal can
be melted by the use of the oxy-acetylene torch, and, on account of the
fact that the heat can be applied exactly where it is required and in the
amount required, the arc can in almost all cases supply welding heat for
less cost than a forge fire or heating furnace.
The one great advantage of the oxy-acetylene method in comparison with
other methods of welding is the fact that in some cases of very thin sheet,
the weld can be made somewhat sooner than is possible otherwise. With metal
of 18 gauge or thicker, this advantage is eliminated. In cutting steel, the
oxy-acetylene torch is superior to almost any other possible method.
_Arc Welding Machines._--A consideration of the function and purpose
of the various types of arc welding machines shows that the only reason for
the use of any machine is either for conversion of the current from
alternating to direct, or, if the current is already direct, then the
saving in the application of this current in the arc.
It is practically out of the question to apply an alternating current arc
to welding for the reason that in any arc practically all the heat is
liberated at the positive electrode, which means that, in alternating
current, half the heat is liberated at each electrode as the current
changes its direction of flow or alternates. Another disadvantage of the
alternating arc is that it is difficult of control and application.
In all arc welding by the use of the carbon arc, the positive electrode is
made the piece to be welded, while in welding with metallic electrodes this
may be either the piece to be welded of the rod that is used as a filler.
The voltage across the arc is a variable quantity, depending on the length
of the flame, its temperature and the gases liberated in the arc. With a
carbon electrode the voltage will vary from zero to forty-five volts. With
the metallic electrode the voltage will vary from zero to thirty volts. It
is, therefore, necessary for the welding machine to be able to furnish to
the arc the requisite amount of current, this amount being varied, and
furnish it at all times at the voltage required.
The simplest welding apparatus is a resistance in series with the arc. This
is entirely satisfactory in every way except in cost of current. By the use
of resistance in series with the arc and using 220 volts as the supply,
from eighty to ninety per cent of the current is lost in heat at the
resistance. Another disadvantage is the fact that most materials change
their resistance as their temperature changes, thus making the amount of
current for the arc a variable quantity, depending on the temperature of
the resistance.
There have been various methods originated for saving the power mentioned
and a good many machines have been put on the market for this purpose. All
of them save some power over what a plain resistance would use. Practically
all arc welding machines at the present time are motor generator sets, the
motor of which is arranged for the supply voltage and current, this motor
being direct connected to a compound wound generator delivering
approximately seventy-five volts direct current. Then by the use of a
resistance, this seventy-five volt supply is applied to the arc. Since the
voltage across the arc will vary from zero to fifty volts, this machine
will save from zero up to seventy per cent of the power that the machine
delivers. The rest of the power, of course, has to be dissipated in the
resistance used in series with the arc.
A motor generator set which can be purchased from any electrical company,
with a long piece of fence wire wound around a piece of asbestos, gives
results equally as good and at a very small part of the first cost.
It is possible to construct a machine which will eliminate all losses in
the resistance; in other words, eliminate all resistance in series with the
arc. A machine of this kind will save its cost within a very short time,
providing the welder is used to any extent.
Putting it in figures, the results are as follows for average conditions.
Current at 2c per kilowatt hour, metallic electrode arc of 150 amperes,
carbon arc 500 amperes; voltage across the metallic electrode arc 20,
voltage across the carbon arc 35. Supply current 220 volts, direct. In the
case of the metallic electrode, if resistance is used, the cost of running
this arc is sixty-six cents per hour. With the carbon electrode, $2.20 per
hour. If a motor generator set with a seventy volt constant potential
machine is used for a welder, the cost will be as follows:
Metallic electrode 25.2c. Carbon electrode 84c per hour. With a machine
which will deliver the required voltage at the arc and eliminate all the
resistance in series with the arc, the cost will be as follows: Metallic
electrode 7.2c per hour; carbon electrode 42c per hour. This is with the
understanding that the arc is held constant and continuously at its full
value. This, however, is practically impossible and the actual load factor
is approximately fifty per cent, which would mean that operating a welder
as it is usually operated, this result will be reduced to one-half of that
stated in all cases.