How to Use Arc Welding Peripherals to Improve Robot Uptime and Quality
Authored by Chris Anderson, Technology Leader, Welding - Motoman Inc.

Regardless of industry, all robotic arc welding systems have one thing in common: the robot has a torch on the end and the manufacturer is interested in increasing productivity and reducing downtime. Unfortunately, peripheral torch maintenance equipment is often overlooked or these accessories are treated as nonessential options during the purchase of a robot system. However, considering that $5,000-$10,000 of equipment will provide several more minutes of production per shift from your $100,000+ robotic welding cell, this relatively minor additional investment yields significant benefits. The ROI on these peripherals will certainly be less than one year, particularly if you factor in reduced rework and less demand on maintenance personnel.

Torch Cleaning Station
The most common peripheral accessory in a robotic welding cell is a nozzle cleaning station. Weld spatter build-up on the torch nozzle can interfere with the proper flow of shielding gas and lead to porosity in the weld, causing poor quality parts. Automated torch cleaning improves quality and reduces downtime. The frequency of nozzle cleaning varies greatly depending on the application -- from every few parts in an automotive environment, to several times on a single part on a large weldment for a construction machine. Often a sprayer is used to deliver anti-spatter compound inside the nozzle to make it easier to remove the spatter on the next cleaning cycle. The following tips will help you optimize the use of your automatic torch cleaning stations:

• Do not wait until the nozzle has collected a heavy ring of spatter before performing a cleaning cycle. This excessively wears the nozzle cleaner. Additionally, if the spatter ring is too heavy, the cleaner doesn't always remove all of the spatter

• Try different adjustments to the anti-spatter injector to find the optimum balance. You want a light coating on the inside of the nozzle -- not excess droplets dripping from the nozzle and onto the parts.

• Inspect the torch when tips are changed to verify that gas ports are not plugged and the nozzle is completely clear of spatter. Some users replace nozzles at the time of tip change and have operators clean the nozzles off-line for reuse later.

• Be sure and follow manufacturers instructions on where to program the nozzle for clamping and how to adjust the stroke of the reamer for a proper cleaning cycle. Improper clamping and reamer stroke length can lead to excessive nozzle and gas diffuser wear.

• Locate the torch cleaner conveniently within reach of the robot. Multiple robots can sometimes share a single nozzle cleaner. Robots on transporters can benefit from having the nozzle cleaner mounted on the traveling base.

Torch Alignment
Robots are very repeatable, but the key is to make sure the end of the weld wire, or Tool Control Point (TCP), is just as repeatable. Tip wear, wire cast and

torch crashes can cause the wire position to vary even though the robot is positioned consistently. If welds are placed off location, it is common to hear users complain that "the robot is not repeating." However, several issues can lead to a missed weld location and a few peripheral products can speed troubleshooting and recovery to reduce downtime.

Manufacturers typically place a punch mark or pointer on some fixed object inside the robot workcell and within reach of the robot to create a reference point for the tool. If there is some reason to suspect the robot was not repeating, the technician might call up a program that moved the wire over to this reference point. The technician can then see how far the TCP is off and which direction it has deviated. This process normally leads the technician to the cause of the problem, and the proper corrective action. If the technician can get the wire back to the reference position, the problem has probably been fixed. This routine works, but requires the operator to detect a problem, alert maintenance staff, and then wait for the technican to fix it. The cell is typically out of production during this time.

Use On-line Gage to Verify Wire Placement Accuracy
Robot vendors have devices available that can detect torch misalignment automatically. These normally involve some programmed robot motion to a fixed gage to measure the position of the weld wire, and that information is then used to calculate the TCP. These gages may be contact type with the wire physically touching the gage or non-contact type which detects wire position when it breaks a light beam. The gage provides an automatic GO/NO-GO check for proper wire positioning. The robot is programmed to detect the wire position in the gage and this can be compared to the original programmed position. If the TCP is within tolerance, then production continues.

Checking the TCP at regular intervals during production improves quality and uptime. Passing the on-line gage routine during production proves machine capability to consistently place the weld wire in proper position. This puts an end to claims that the "robot is not repeating." If welds are being made off location and the robot is passing the TCP gage, then maintenance personnel know they need to focus on a problem with tooling or part variation. Checking the TCP at frequent intervals can detect variation before it creates poor weld quality.

A trailer hitch manufacturer experienced wire position changes as the contact tip enlarged over time due to wear. Now, when the robot fails the gage routine, operators change contact tips and resume operation. The gage routine reruns automatically to verify that the wire is within tolerance with the new tip before the robot resumes production. The gage is located next to the nozzle cleaner to reduce cycle time as it moves from torch nozzle ream to TCP check. If the robot fails the gage routine after a tip change (which happens only a small percentage of the time), then the operators alert the robot technician.

Use Gage to Locate Deviation and Update Programs
If the robot fails the GO/NO-GO check, the gage can then be used to measure the offset of a deviated TCP. The robot performs a search pattern to detect wire offset in the X and Y directions, and also searches in the Z direction for the end of the torch nozzle. If the torch fails the gage check, it needs to be visually inspected and checked for false torch condition readings caused by a bent weld wire or more serious torch problems. Often, torch errors can be remedied by changing the contact tip.

The robot can also offset the programs to compensate for the tool deviation detected during the gage routine. Prior to updating the TCP with the offset that has been calculated, the robot should prompt for manual intervention. The operator must inspect the torch to make sure no major problem exists. The robot program normally has a maximum limit on how far the TCP can be off and still shift. Compensating programs for gross TCP deviation (greater than 5 mm) may cause the torch or robot arm to impact the part or tooling at program points with minimum clearance. It is important that the robot routine recheck the updated TCP in the gage to verify that it is within tolerance prior to resuming production.

It is helpful to have an "undo" feature to return to the original TCP condition in case tolerances from updating several times start to stack up. Most users will have a mechanical reference, such as a pointer, for this original aligned torch condition.

Torch Alignment Jigs
Torch barrel alignment jigs can be used to maintain the TCP mechanically. Virtually all torch manufacturers provide removable torch barrels and have alignment jigs for checking and adjusting the TCP. However, few users take advantage of these jigs and use them as intended.

A TCP variation might be the result of a bent torch barrel. This problem is easily remedied by exchanging barrels with one that has been verified in the jig. Depending on the torch design, changing out the barrel can be accomplished in 30 seconds to a few minutes. Production can resume while torch barrels are inspected and maintained off-line.

Torch manufacturers cannot bend torch barrels exactly the same due to tolerances. Having an alignment jig on site allows users to verify that new torch barrels have the same TCP as the existing barrels they are replacing. Some users have operators try to fix a suspected problem by replacing torch barrels and contact tips, prior to alerting maintenance staff. An operator can visually check the barrel using a simple jig.

An automotive suspension component manufacturer had been plagued by "wandering" program points. They were using torch alignment jigs to verify the TCP position prior to exchanging torch barrels, as well as using reference points inside the cell to verify proper wire position. The technicians were editing weld positions often, and they suspected torch barrels were being distorted by the heat from welding. An on-line alignment gage was implemented and the robot programs were changed to automatically update the TCP every few parts. The problem immediately got worse, and the robot began placing welds off seam randomly after a TCP update. This led maintenance staff to concentrate on wire positioning and they identified a problem with inconsistent wire cast. This random wire flipping had plagued them for weeks and caused constant editing, while the on-line gage led them to the source of the problem within a couple of hours. Once the wire problem was resolved, the need to constantly edit positions was eliminated.

Tip Change Window
While the advantages of having automatic torch maintenance and alignment checks have been outlined, manual intervention is necessary to inspect and maintain torches. A tip change window provides a convenient way for operators to service the torch. This option consists of a box with a safety latch that allows personnel to quickly and safely change out the welding torch tip, then restart the robot program using the control box - all without having to enter the robot cell or use the robot teach pendant to move the robot to a safe position. This not only improves uptime, it also reduces cost and improves safety by allowing tip change to be performed by less-skilled employees in a more open and less hazardous area than is typically found inside a robot cell.

Tip change takes an average of 60 seconds per robot. It can be performed at specified part counts, when a period of arc time has been met or when the torch fails the TCP gage. The operator also can activate a switch on the control box to have the robot present the torch for tip change.

Joint Sensing and Wire Cutters
The above devices involve reliably placing the end of the wire into the weld joint repeatedly. However, some manufacturing conditions prevent the weld joint from being presented in a repeatable location. Touch-sensing is a proven technology that applies a voltage to the end of the torch and uses the robot as a probe to detect the part location. The robot can measure the difference between the programmed and actual position and use this value to shift the weld positions.

Some manufacturers apply a voltage to the weld wire and use it to search for the part. This generally requires an automatic wire cutter to cut the wire to a consistent length for searching. For thinner-gage applications where repeatable results are required, a wire brake option in the torch is helpful. This is a small air cylinder which clamps the wire in the torch body to prevent cable slack from causing the wire length to grow or shrink. While the wire cutter adds some cycle time, it is possible to program the robot to perform searches in multiple weld locations prior to welding.

The other method of touch-sensing is to apply a voltage to the torch and search with the nozzle. This saves cycle time by eliminating the need to cut the wire to length. However, the outside of the nozzle to end of weld wire is not a controlled dimension and can vary from nozzle deformation, wire cast or build-up of spatter.

Touch-sensing can be used to detect start and end locations or multiple searches can be combined along a long seam. For contoured parts, it is common to use through-the-arc seam-tracking to keep the torch in the joint after touch-sensing has placed it in the proper starting position. The through-the-arc seam-tracking requires the robot to weave in the joint and it tracks the joint by keeping the wire stick out the same on each leg of the weave.

Laser sensors are also available for seam-finding and seam-tracking. They are more expensive, but are faster, can find thin lap joints, and can provide additional information on joint gap. The joint gap information can be used to create programs that adapt to the changing joint geometry. However, the camera can sometimes interfere with joint access, which limits the use of lasers in some applications.

A manufacture of large storage racks had variation in fit-up and had to slow the welding speed down to ensure that the robot always made a quality weld. This slower speed compromised their required throughput. A laser sensor was installed to detect the joint condition ahead of the weld. The robot slowed down on joints which had gap, but traveled faster on joints with good fit-up. The net increase in production justified the laser sensor. Because the storage racks have a critical function, the user also implemented laser cameras to inspect the weld joints to verify that they were cosmetically acceptable.

Summary
In summary, the key to getting payback on robotic peripherals is in how they are implemented in production. Too often, manufacturers are in a rush to get their robotic welding system into production and don't take time to implement the welding peripherals properly (if at all). Often, the tools that could be improving productivity on a daily basis are laying unused in a maintenance crib.

      For more information contact:

      Motoman, Inc.

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      www.motoman.com