THE AUTOMOTIVE ROBOT TIG APPLICATION:

This weld report deals with the robot TIG welding issues on one of the big three cars. The parts required 10 to 15 precise small tack welds. The tacked parts were later brazed, The welds were made with a Fanuc Arc Mate 100 robot, and a Lincoln 350 amp "pulsed" square wave power source. The welding issues at this tier one part supplier were extensive.For more than a year they had struggled to attain 40 % weld efficiency from the robot. The tack welds were frequently missing, arc starts issues were extensive, and the tack welds would leak. After rectified the problem, I wrote the following report. The issues were reported under the following topics.

[1] The Fundamental Requirements of a Robot "TIG" Weld.
[2] The Robot and it's program.
[3] The Power Source.
[4] Controlling Weld Quality / Weld Productivity.
[5] The Fixture and Positioning Table.
[6] Robot Personnel Requirements.

THE FUNDAMENTAL ROBOT / TIG WELD REQUIREMENTS.

The six-decade-old TIG process is a sensitive weld process with many weld variables that can influence both weld quality and productivity. Adding a robot to the TIG process greatly increases the opportunity for additional issues. Using the following process control information and weld recommendations will provide benefits for this application.

Small robot TIG tack welds typically will require more consideration than longer or larger welds. The following are reasons for the Robot /TIG weld concerns.

[a] The primary issue with the TIG process is there is no way to control the tungsten tip life. For any individual considering TIG with a robot if possible select the Plasma welding process. This improved version of the TIG process developed "25" years ago provides improvements in this area. Also the plasma configeration can be altered to suit the specific welds.

[b] Small TIG tack welds combined with a robot high arc/on arc/off weld duty cycle will negatively impact the tungsten life.

[c] Your tack weld cycle times are typically less than a second. In this short time frame the robot interface and power source have to communicate four sets of weld data.

[1] High frequency on, and arc established.
[2] Provide start weld data.
[3] Provide weld data.
[4] Provide end weld data.

Tack weld issues mean short weld cycle times, you have to ensure the weld equipment and interface purchased for the robot provides the ability to deliver the data in micro-seconds.

[d] Due to the small amount of robot "TIG" installations and a lack of focus on weld process expertise, few of the major robot companies or robot integrators have TIG / Robot experience. On the subject of weld process and application expertise, this is derived through knowledge experience and time. Few robot companies provide this fundamental commodity.

[e] The robot weld data presented in your robot teach pendant is designed for MIG welding rather than TIG.

[f] Pulsed is a beneficial weld process for TIG tacking without the use of a filler metal. However the Lincoln pulsed power source and robot you purchased does not have the capability to provide stable pulsed parameters in the "short" weld cycle times used. This is just one example of one of the issues that needs careful consideration before you purchase a robot/power source for a demanding application.

[g] The smaller the weld the more precision is required by the robot. Longer or larger welds are typically more forgiving. Your Fanuc robot tool center point (TCP) is rudimentary, and needs to be checked at the start of each shift. Also with the tungsten placement variations noted, either the robot or positioning table are not accurate or consistent.

Unfortunately as is common with most robot installations the weld process requirements and variables appear to have been given minimum consideration by the vendors involved.

TIG WELD CURRENT CONTROL: With the TIG welding process it's especially important to control the current during the TIG "arc start" and at the "arc end." In a manual TIG welding application, the welder may use a variable amp control which he or she regulates through a foot current control. During the weld, the manual welder may ramp up the weld current at that arc start from a low to a high current. The weld current ramp up assists the welder in;

[a] establishing a controlled arc start,

[b] creating a specific amount of molten weld pool at the arc start.

The "ramp up of weld current" can prolong the tungsten tip life and also provide a less forceful arc start. In reducing arc force at the arc start, less molten metal expulsion is produced; this can reduce the potential to contaminate the tungsten with weld during the arc initiation. Also a controlled weld current "ramp up" can provide improved control of the weld fluidity and attain the desired "weld puddle size", (note; this is an important feature when producing TIG welds "without the addition of filler weld metal"

IT'S IMPORTANT FOR YOUR OPERATION TO PROGRAM THE ROBOT TO RAMP UP AND RAMP DOWN THE CURRENT DURING THE WELD CYCLE

POOR WELD PROCEDURE CHOICE: Your application was set initially to tack weld without robot torch movement". It's fine to use this stationary TIG tack weld method if;

[a] The parts are perfect.
[b] The parts are the same thickness and the weld gaps controlled and consistent
[c] No fixture issues.
[d] The robot and positioner accuracy is always +/- 0.005.
[e] The TCP is accurate and maintained daily.
[f] The tungsten shape and length does not change.

Of course we live in the real world where we rarely will see manufacturing or process perfection. It's also important to note that many robots and part positioner's are not as accurate as they should be. Controlling a robot TCP is difficult on a robot with a rudimentary TCP control, and of course your TIG tungsten will always have wear issues.

 

 

The bottom line, an experienced weld process engineer (a missing link on this application) would have known that to compensate for the known TIG / Robot / Part Weld issues, you have to provide a forgiving weld rather than a stationary tack.

For this Mid west robot installation, RSI was the Detroit integrator, Fanuc supplied robot and Lincoln Electric provided the power source.

 

 

 

With this unique application, the TIG torch has to first establish a weld puddle between two parts of different gage. The weld puddle is established on the thicker of the two parts. The robot is then programmed with a push or lead angle to move the weld puddle between the two parts. This fundamental tack weld approach is necessary for you to attain consistent quality weld tacks on parts with variable thickness and variable gaps.

THE CONFUSION OF THE WELD DATA PRESENTED
BY THE FANUC ROBOT.

For every 100 arc welding robots sold in the USA, 99 may end up as MIG robots and the one remaining may be used for TIG. Robot arc welding programs presented in the teach pendant are typically designed for MIG welding which will use very different weld data. Few robot manufactures provide a custom TIG program designed and dedicated to meet the needs of a TIG or plasma weld. Also when you examine how ineffective the Lincoln power source bells and whistles are along with the poor pulsed tack weld performance, you will know how much consideration Lincoln/Fanuc have given to pulsed TIG tacking applications.

As mentioned your programmer was provided with a robot unit which provided a Lincoln "pulsed" TIG power source, however no control of the pulsed weld parameters was provided in the Fanuc teach pendant. Even if this application did not use pulsed, surely its logical when you purchase a pulsed power source to be able to control the pulsed parameters from the robot teach pendant.

Pulsed will provide future weld application benefits. However on this specific low arc on time application there would be a concern of the stability of the pulsed arc parameters when communicating between a robot and power source interface in a weld cycle time of less than a second.

Your Lincoln power source also provided a TIG weld "start option". This weld start option provided a variable percentage of the weld current and a time. However if we used the minimum time settings available on this option, the weld arc would "stay on all the time".

Your power source provided "end weld data" in the form of "crater fill current and time" This feature also did not function. The weld reality is you purchased a power source with many bells and whistles none of which you use.

WHEN I CALLED FANUC TO ADDRESS SOME OF THE FUNDAMENTAL ROBOT WELD ISSUES, THEIR RESPONSE WAS THEY WOULD HAVE TO ASK LINCOLN. SURELY LINCOLN COULD USE ITS TRAINING FACILITIES TO PROVIDE FANUC EMPLOYEES WITH WELD PROCESS TRAINING.?

WELDING POWER SOURCE OVERKILL

Few welding manufacturers are aware that even for the most sophisticated complex welds, an intelligent robot needs a "basic" power source with one important feature, interface capability with the ability to instantly communicate and respond to the robot pendant instructions.

 

 

YOUR ROBOT PROGRAMMER WAS PROVIDED A ROBOT PENDANT WHICH PROVIDED WELD DATA THAT HAD LITTLE TO DO WITH THE ACTUAL WELD REALITY:

The robot TIG weld schedule provided a weld data window showing both Amps and IPM. The amps in this window were not the actual weld current, nor was the amp number we set a percentage of the real weld current as indicated on an amp meter. The weld current indicated on the Lincoln power source amp meter also had no relationship to the real weld current as read on a DC amp meter.

HOW CAN ISO REQUIREMENTS BE APPLIED TO A WELD UNIT IN WHICH ACTUAL WELD DATA HAS NO REALITY WITH THE WELD EQUIPMENT UTILIZED?

What about the IPM was this the weld speed? The IPM was likely the wire feed in./min used for setting a "MIG" weld. The robot weld travel speed was where it should be in the arc data lines, however to add confusion for the programmer one weld data line was in English measurements and the next line would be in metric. Remember to an experienced Fanuc programmer, this would not be an issue, however your programmer is in learning stage and none of the above was pointed out to him.

 

The robot training provided by the robot companies involved trained the programmer on a "MIG" welding robot. There was minimal info on setting an effective TIG welding program. It's also a key point that the Fanuc programming book has almost no data on the subject of TIG welding.

The repeatability and accuracy of the Fanuc robot raises questions. Right after a TCP check, the robot was found to be 0.040 to 0.060 farther from the joint for which it was programmed. At this time we do not know if the positioning table is the issue or the robot. I requested that the table be checked, and

The robot intergrator has the responsibility to ensure that the data in the pendant, and the data on the power source meters is "actual: and calibrated before the cell is installed. In this installion neither of these functions was performed.

The robot manufacture has a responsibility to ensure that his training program and literature provided completely covers the welding process utilized.

To control your TIG tack welds requires the following.
1. Determine the weld current range for the job. Our weld tests revealed 60 to 90 amps. Note this is the weld schedule amps. At the time we set the welds we did not have an amp meter. Once your equipment is calibrated, with an amp meter find out the actual weld current range utilized for your future weld procedures.

2. At the arc start data ramp the current up 10 to 20 amps higher than the selected weld current. Put in a "wait time" of 0.1 to 0.3 sec. The extra current will assist in getting the arc started, consider the "wait time" at the arc start as a control for the desired amount of weld required before the robot moves.

3. In the next arc weld program line place another arc start, this time with no wait time. In this line we have the actual weld current. Note the tack weld travel range will typically fall between 10 and 20 in./min.

 

 

PORES IN THE TACK WELD CENTER RESULTED IN LEAKS:


To eliminate pores or micro cracks in the tacks, both can result in leaks. In the weld end data lower the weld current so that 5 to 20 amps is indicated. Hold this low end current from ½ to 1 second.

POOR CABLE MANAGEMENT LED TO EXTENSIVE ROBOT ARC START ISSUES.

To avoid touching the tungsten with the work it takes high frequency (HF) to help initiate a TIG arc. In this installation during numerous arc starts the HF was not going the tungsten tip. In trying to figure out where the HF was I used a small fluorescent tube which revealed that the HF was jumping to the other cables which were all grouped together touching each other. From the other cables the HF then jumped to a metal post (supported the torch), the post is anchored to the floor. Once we separated the cables and insulated the post from the cables the HF went back to where it belongs. Again HF issues in TIG have been well documented for decades however neither the Fanuc or Lincoln literature dealt with the HF issues that are unique to a robot cell installation.

You can anticipate the occasional arc start issue with any TIG application. In the event of an arc ignition failure a robot is typically programmed to provide more than one arc start. In this application the arc restart option was not enabled, and was still none functional when I left. Again the responsibility lies with the integrator to ensure the process options required are switched on. Look into this situation.

 

THE TIG WELD GAS WAS SET AT 150% HIGHER THAN IT SHOULD HAVE BEEN.

The argon gas flow rates for this application were set at 40 cuft/hr, this is a typical setting for MIG rather than TIG. TIG gas flow rates are typically 10 to 15 cuft/hr. It's important to keep this flow on during the total weld cycle. At the weld completion keep the post flow flowing to protect the tungsten from the atmosphere during its cool down cycle.. We have marked the flow meter; I would recommend a Smith flow control. It can be locked and also reduces the high gas surge which comes through each time the gas is switched on High gas flow rates or gas surges are not only a wasteful they can add to weld turbulence agitating the weld pool causing large pore porosity or weld contamination of the tip.

TUNGSTEN CONSIDERATIONS:
To establish the TIG arc, the TIG power source provides high frequency (HF). The HF ionizes the argon gas which improves the weld gas conductivity before the TIG weld current is applied. With TIG it's important that the tungsten does not make contact with either the part or weld as tungsten contamination can occur. Tungsten contamination will "lower" the melting temperature of the tungsten causing the tungsten end to ball or oxide, this reduces the stability of the weld current transfer. For these reasons;

[a] All persons who handle the tungsten should use clean gloves.
[b] Only grind the tungsten on a dedicated grind wheel.
[c] Do not use a tungsten if oxide evident on it's surface, break off the contaminated part.
[d] Program the robot so the tungsten is a minimum of "0.060" at the weld start, and "0.070" at the weld end.
[e] At this time you are changing the tungsten every 50 parts. If you contaminate a new tungsten on the first part you will have a high probability of extensive weld rework. Cut a window in the robot door cell door, program the robot home position so the TIG gun nozzle and tungsten is always visible to the robot operator.

 

 

 

 

 

 

 

[f] Provide the robot operator with a tungsten gage. At any time the operator can stop the robot when it's at the home position, and without entering the robot cell, reach through the access window and replace and reset the tungsten. The tungsten stick out from the nozzle should be 6 mm.

PROVIDE A TUNGSTEN STICKOUT GAGE:

 

 

 

 

The long-term solution to the tungsten tacking issues is to change the welding process to "plasma welding"

 

 

TRAINING TRAINING TRAINING

Your programmer needs more fundamental robot program training. In a plant such as yours you also should have a second individual that can step up to the programming plate if Nick is busy or absent.

 

Each robot weld program should be clearly identified in the program, and each program should be saved on a disc. The manufacturing manger should keep a copy of this disc in a fireproof box.

WELD PROCESS CONTROLS

Once the program you have is calibrated and fine-tuned, a large weld process control board should be mounted outside the robot cell. The board should identify the parts, each weld and the weld data used. As the key weld parameter is weld current you could use a large amp-meter attached to this board so your customers can see that the current you set for weld number 5 is the current attained on the amp meter. Personnel responsible for programming should realize that this process control data should be maintained. At the start of each shift and after lunch part of the weld process control instructions should be, operators;

[a] Check the gas cylinder contents and flow. Consider a bank of two to four cylinders so you will not run out of gas.

[b] Install new tungsten; use the tungsten stick out gage.

[c] Perform a TCP check.

[d] Weld three parts, have the programmer or supervisor provide a signature stating all welds are OK or adjustment required.

WELD PROCEDURES?

At this time I believe you have "no pre-qualified weld procedures". From a product liability perspective this could be a serious issue. Also from a customer or ISO perspective what happens when one of your customers asks to see your weld procedures?

CONCLUSION: As this report indicates extensive issues were generated. To get to the root cause of the production problems each issue had to be identified then rectified. As the issues were numerous and my visit short, I missed an opportunity to really optimize your process to its peak capability. However with the recommendations of this report you will have the opportunity for dramatic improvement

Contractual and Vendor responsibility. As you are aware you cannot depend on vendors of equipment for specific process or application expertise. However when purchasing automated equipment it's beneficial if you ask the vendor the "right questions". It's also beneficial on future automated equipment purchases to stipulate in the purchase contract that a production run of four hours should be provided to prove the equipment and process. Also ensure that the product literature, programs, training and equipment provided are really applicable to your needs.

Regards E.F.Craig.

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