Welding robot

A welding apparatus and method for forming arc welds of thin sheet material. The apparatus forms a first pool of molten material and then discontinues the welding to permit a portion of that pool to solidify and then reestablishes a new arc at point along the periphery of the solidified previous pool and continuing this operation to form the finished weld bead. Thus, lower welding heats may be employed and greater tolerance in the welding method and apparatus is possible.

BACKGROUND OF THE INVENTION 
This invention relates to a welding robot and more particularly to an 
improved apparatus and method for arc welding thin sheets of metal. 
A variety of automatic welding devices have been proposed for arc welding, 
a most common type being the MAG welders that provide automatic gas 
shielded arc welding and which have welding torch which is moved along the 
seam to be welded by a robot with the speed of motion, diameter of weld 
rod, welding current and other factors being controlled in accordance with 
the thickness of the materials being welded. When butt welding, the two 
materials to be joined have their edges positioned in abutting 
relationship and the weld rod or wire is fed along the direction of the 
butted joint. 
However, this type of apparatus presents certain problems in connection 
with welding thin sheets of material. If the edges to be jointed are not 
completely straight and in abutting relationship without any gaps, then 
the amount of welding heat can be too great for the thickness of the 
material and voids in the weld can occur. This is because the molten metal 
can fall through the gap between the edges of the material being welded. 
Although this can be avoided by using very sensitive welding techniques 
and also by assuring extreme accuracy in the edges being joined, such 
complicated systems and the provision of such high accuracies in forming 
the edges to be welded significantly increase the cost of the apparatus 
and reduces the efficiency of the welding technique. 
It is, therefor, a principal object of this invention to provide an 
improved apparatus and method for welding thin sheets of material. 
It is a further object of this invention to provide an improved method and 
apparatus for arc welding joining edges of thin sheets of material. 
SUMMARY OF THE INVENTION 
This invention is adapted to be embodied in a method and apparatus for arc 
welding thin pieces of material using an arc welder that is supported for 
movement along the seam to be welded. In accordance with both the method 
and apparatus an arc is struck in the area to be welded and a pool of 
molten material is formed. The welding is then suspended for a time period 
so as to permit the pool to partially solidify. A further arc is then 
establish at a point spaced from the first arc and containing the 
periphery of the first pool to form a second pool and this procedure is 
continued along the seam until the weld is completed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
Referring first to FIG. 1, a welding apparatus constructed and operated in 
accordance with an embodiment of the invention is identified generally by 
the reference numeral 11. As will be described, a major portion of the 
welding apparatus 11 is conventional and those conventional components 
will be described first. The apparatus 11 is designed and construction so 
as to perform welds along a work piece indicated by the reference 
character W and shown in phantom in this Figure. 
The welding apparatus 11 includes a robot 12 that carries a welding head 13 
and which is supported for controlled movement to move along the 
pre-determined path which will determine the weld bead established on the 
work piece W. The welding head 13 is of a conventional MAG automatic 
welder that receives a weld rod in the form of a wire from a wire feeder 
14. This wire is also supplied with electrical current from a power source 
15 through a conductor 16. In addition, a ground conductor 17 is connected 
to the work piece W so as to complete the normal circuit. 
The wire from the feeder 14 is delivered to the head 13 through a conduit 
18. The conduit 18 also provides a path for shield gas from a remote 
shield gas supply, shown partially in phantom and identified by the 
reference numeral 19. A conduit 21 extends from a regulator 22 on the 
supply 19 to the conduit 18 so as to provide the gas shield in a well 
known manner. 
A main power supply, shown in phantom, 23 supplies electrical power to the 
welding transformer and welding control 15 in a well known manner. 
In a similar manner, the robot 12 is controlled by a controller 24 that 
controls the time of movement of the welding head 13 and also the path of 
movement of the robot 12. As has been noted, this construction is well 
known in the prior art and, for that reason, further detailed description 
of it is not believe to be necessary. 
A problem with this type of prior art construction may be understood by 
reference to FIG. 7 which shows a weld joint between thin pieces of sheet 
metal when an attempt is made to weld them the prior art apparatus and 
method. The shaded areas in this Figure show areas where the weld will be 
incomplete due to the generation of excess heat which causes the molten 
material to fall through gaps which are either existent or will occur 
between the edges of the material being welded. The unshaded areas show 
the areas wherein bonds will be formed without voids. This irregular bead 
occurs due to the inability to control the heat and speed of the welding 
head at any reasonable cost and still permit high production efficiencies. 
Therefor, in accordance with the invention there is provided a switch pulse 
controller, indicated generally by the reference numeral 25 that has a 
control 26 that goes to the robot 12 so as to cause it to move not a 
uniform speed but to move at step function as will be described. Of 
course, during the actual movement the speed may be uniform. In addition, 
a conductor 27 is connected to the welding controller 15 so as to switch 
the welding current on and off or increase and decrease it to make and 
break the arc, also in a manner to be described. Thus the weld is made in 
a step function rather than continuously. Although the pulse controller 25 
is depicted as being a separate element, it is to be understood that it 
can be incorporated either completely in the welding controller 15 or in 
the robot controller 24 or portions of it can be provided in each of these 
units with the units being interrelated. 
Basically the function of the switch pulse controller 25 is to first 
establish a weld arc and form a pool of molten material. The continued 
melting is then stopped to permit a portion of the pool to solidify and 
the welding head 13 is advanced to an area at the periphery of the 
solidified pool to again strike an arc and begin forming a new puddle of 
molten material. In this way, rather than continuing to form a molten pool 
as the weld head 13 moves along and requiring continuous supply of heat 
and more difficult control, the unit operates in an on off manner so as to 
reduce the total heat supplied to the base material W and thus avoid the 
defects of the prior art type of construction. 
The effect of this will now be described by particular reference to FIGS. 
2-6 with the method being described in conjunction with the control 
routine of FIG. 8. It should be noted that FIG. 8 shows both the operation 
of the switch pulse controller 25 and the related operations of both the 
welding head 13 and the robot 12. At the beginning and at the first step 
P-1 the switch pulse controller 25 initiates welding and this signal is 
transmitted to the controller 15 and also at this time the robot 
controller 24 is held in its position. To initiate welding, the welding 
current as supplied is shown at the top view of FIG. 2 and the welding 
current initially started is greater than a pre-determined value I so as 
to strike the arc. Once the arc is struck, the welding current is dropped 
to the predetermined value I so as to control the amount of heat supplied. 
The establishment of the arc is determined at the step P-2 in FIG. 8. This 
time period is the point in time "a" as seen in FIG. 2 and is established 
by determining when the arc has been struck. As may be seen in the lower 
view of FIG. 2, the welding head 13 has a lower tip portion 28 from which 
the weld rod or wire, indicated at 29 protrudes. The arc is established 
between the end of the wire 29 and the base material W and the arc is 
indicated at "a" in this Figure. Throughout the welding technique, the 
inert gas is also supplied as shown at G to shield the area being welded. 
The amount of inert gas supplied during the operation can be varied as 
desired and in accordance with any control routine. Normally it can be at 
a constant flow but it may be at different flow rates during the welding 
or may be stopped at any time on times during the operation. 
The molten weld puddle that is formed is indicated at 31 in. This welding 
operation is maintain during a pre-determined time period, as will be 
described at portion of this time period is indicated by the block b in 
FIG. 2 and will be now described by reference to FIGS. 3 and 4. As may be 
seen in FIG. 3, the actual welding current that flows through the wire 29 
to the work piece W varies during the welding cycle under the operation of 
the control 15 with the welding current being shown in the curve in FIG. 3 
and the actual condition between the weld rod 39 and the work piece W 
being shown in FIG. 4 with the corresponding points in both Figures being 
identified by the letters a, b, c and d. 
Once the arc has been stuck the tip of the wire 28 will melt during the 
time period a b and will come in contact with the material of the work 
pieces being welded W as shown in these two FIGS. (4a and 4b). Once the 
welded material comes into contact with the base piece there is a short 
circuit and the welding current rises abruptly as shown in FIG. 3. This 
change in the current while the short circuit is formed is indicated by 
the hatched area in FIG. 3 between the points a and b. As this continues, 
the molten material extending the weld rod 28 and the work piece W will 
constrict as shown in FIG. b and eventually it will drop off and form a 
void. This causes the molten material 31 to be formed as shown in FIG. 3 
and specifically the portion a thereof. 
When this occurs, the arc will continue at the point c and eventually the 
weld rod will begin to melt and form a molten segment again as shown in 
FIG. 4d with the welding current all falling off during this time period. 
The amount of weld material 31 which is built up will depends upon the time 
period T1 set. During this operation, the welding head 13 is held in a 
fixed position by the robot 12. The time T1 may be determined by 
determining the amount of molten material that is to be built up before 
the arc is moved to the new location, as will now be described. 
FIG. 8 shows how the controller determines if the arc producing time T1 has 
elapsed this being determined at the step P3. If the time T1 has not 
elapsed, the program repeats back to step P2 and the welding will continue 
until the time T1 has elapsed. When this is determined at the step P3, the 
welding robot 13 is switched off with the arc off time being shown at P4. 
This condition also appears in view b of FIG. 2 wherein it will be seen 
that the arc is now turned off and the molten pool of material 31 will 
begin to solidify. During this time, the shielding gas G may continue to 
flow. The shut off time of the arc at the point T1 is shown at the area c 
of FIG. 2 wherein a small amount of current may be supplied but not 
sufficient to establish an arc. 
The arc off will be determined by the amount of solidification which 
desired and this can be determined by running a test weld to insure that 
there is adequate solidification to avoid too much heat generation when 
the arc is again reestablished. The arc welding time T1 has been described 
as a time duration but it is understood that the amount of welding 
accomplished can also be controlled by controlling the amount of arc 
energy supplied by detecting welding current and voltage so that the time 
function can also be related to the actual amount of heat applied by the 
voltage and current as mentioned. 
After a pre-determined arc off time T2 then the welding robot 12 is 
energized so as to advance the head 13 and weld rod 29 to a new position 
indicated by the dimension L in FIG. 2c wherein the now solidified weld 
material is indicated at 31a. During this time the gas supply may be shut 
off or may be continued depending upon the desired result. 
The advancing step of the welding head 13 is initiated at the point P6 as 
shown in FIG. 8 and this is continued at a predetermined speed or variable 
speed for a time period T3. Hence, at the step P7 the controller 25 
determines if the advance time after when the arc has been switched off 
(T3) has been established. If it has not, the program moves back to the 
step P6. 
If, however, the advance time T3 has been met then the program moves to the 
step P8 to determine if the weld rod is clear of the weld material. That 
is, it is determined at the step P8 if the weld rod is stuck to the weld 
and this is determined by passing a current through the wire and detecting 
the amount of that current. If current is excessive, it is determined that 
the weld is defective and the weld rod has stuck to the weld and the 
program moves to the step SP9 so as to stop the welding operation. 
If, however, no weld defect is determined at step P8, then the program 
moves on to the program step P10 to complete the welding by again 
reestablishing an arc as shown in FIG. 2d and begin establishing a further 
pool of molten material. As may be seen, this is done in the overlap of 
the previous solidified material 31a so as to provide a continuous weld 
bead as may be seen in FIGS. 5 and 6. 
FIG. 5 is a top plan view showing the weld feed while the FIG. 6 is a 
bottom plan view showing the other side of the weld at the welded joint. 
Because the weld beads are overlapped weld strength is maximized and if 
the overlapping area is not great then the weld bead may become 
discontinuous. On the other hand, if two large overlapping occurs and the 
weld bead tends to become large and mound shaped. Hence, the amount of 
advancement L will determine the overlap and this must be determined by 
running a pass experimentally. 
As may be seen in FIG. 5, the weld beads are quite regularly produced and 
have a somewhat scale or ripple pattern. This rippling is less obvious 
when the arc time off is set shorter. The hatched portion shows slugs of 
impurities which are formed on the beads and these are formed at the 
center of the puddling area. These occur mainly when steel or stainless 
steel is being welded. Because of the intermittent welding operation these 
slug masses are deposited in a row at approximately constant intervals 
because the welding operations are intermittent and there is cooling 
between each welding operation. 
On the under side of the weld (FIG. 6) there will be some slight craters 
formed and these are shown by the shaded areas. These result from the 
cooling and pulling of the material from the center during solidification. 
These also occur at constant intervals along the weld. 
Thus, comparing this method and the weld bead with that shown in FIG. 7 of 
the prior art, it is clear that no voids in the weld will occur and a 
continuous and strong weld is formed. 
FIG. 10 Table 1 indicates the comparison of weld between thin sheets of 
metal, such as steel between the prior art continuous type welds and 
welding method and the present switch pulse welding method with various 
types of butt and lap joints being shown. With conventional welding 
techniques for butt joints and any form of gap whatsoever ends up melted 
holes formed through the joint. With lap joint conventional welding was 
possible with small gaps up to one millimeter but under cuts were observed 
in many locations. 
On the other hand, with the switch pulse welding technique there is 
tolerance for gaps and good welds can be accomplished in butt joints even 
with gaps up to 0.5 millimeters. With lap joints, good welds can be 
accomplished with gaps even up to one millimeter with no under cutting. 
Also, this Figure shows the tolerance possible in variations in millimeters 
from the aimed or desired center line C-L position of the weld. As shown 
in this Figure, the tolerance is much greater in accordance with this 
method then with the prior art type of methods. 
The welding conditions for the information shown in Table 1 using this 
invention are a welding current of 50-60 amps and a welding voltage of 
17.6-19.2 volts with the arc on time T1 arc off time T2 and advancing time 
T3 set so that the welding speed becomes 20-25 centimeters per minute and 
using a wire of 0.6 millimeters. The prior art weld in the table shows the 
results of the same condition for welding material welded by a known MAG 
welding process with a welding voltage of 37-53 amps, a welding voltage of 
16.5-17.6 volts and 50-60 centimeters per minute welding speed. 
In the method as thus far described, a time period T2 is permitted to 
elapse before the welding torch is moved. Of course, the welding torch can 
be moved immediately after turning off the arc but there may be some 
advantages in delaying the time of advancement so that more solidification 
can occur before the weld head 13 is actually advanced. Such a method is 
shown in the control routine of FIG. 9 wherein the steps which are the 
same as those of the program of FIG. 8 are identified by the same 
reference numerals. However, in this program the P5 is deleted and 
immediately after turning the arc off at the step P4 the program moves to 
the step P6 to advance the welding head. At a next step S1 it is 
determined if the arc off time T2 has been sufficient for the desired 
solidification to occur. When this has occurred the program moves on to 
the step P8 to run the abnormality check and complete the welding seam. 
The described method has been practiced with steel base thin sheets of 
material but it should be readily obvious that other materials may be 
welded using this technique such as cast iron or carbon steel which may 
require pre-heating and using the MAG welding technique or MIG welding 
aluminum or aluminum alloy sheets is also possible. The welding of metal 
having high thermal conductivity such as copper, brass, aluminum and 
magnesium is also possible as is the welding of stainless steel, 
ferrite-based alloys, alloys of the martensitic family or alloys of 
precipitation hardening. In addition, welding of titanium by TIG welding 
can also be performed. 
Thus, it should be readily apparent that the described welding apparatus 
and technique is highly useful in welding of very thin sheets of various 
materials and permits less accurate control over the edges of the material 
being welded and also of the welding control without adversely effecting 
the quality of the weld and in fact improving it. Of course, the preceding 
description is of preferred embodiments of the invention and various 
changes and modifications may be made without departing from the spirit 
and scope of the invention, as defined by the appended claims.