Fig. 1: When a robot is mounted on a transporter, mounting the cleaner to the moving base saves cycle time during operation.

Fig. 2 An automatic nozzle cleaning station should be mounted within easy reach of the robot.

Regardless of industry, all robotic arc welding systems have two things 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 often is overlooked, or these accessories are treated as nonessential options during the purchase of a robot system. However, considering that $5,000 to $10,000 of peripheral equipment would provide several more minutes of production per shift from your $100,000+ robotic welding cell, the relatively minor additional investment in peripheral equipment can yield significant benefits. The return on these peripherals certainly will be less than one year, in part because of the 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 can 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 for a large weldment on a construction machine. Often, a sprayer is used to deliver antispatter compound inside the nozzle to make it easier to remove the spatter on the next cleaning cycle (see figures1 and 2).

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 does not 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 that drip from the nozzle and onto the parts.
Inspect the torch when tips are changed to verify that gas ports are not plugged and that 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 to 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 repeatable, but the key to keeping them operating correctly is to make sure that the end of the weld wire, or tool center 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 work cell and within reach of the robot to create a reference point for the tool.

If there is some reason to suspect the robot is not repeating, the technician can call up a program to move the wire to that reference point. The technician then can see how far the tool center point is off and in which direction it has deviated.

This process normally indicates the cause of the problem and leads the technician to the proper corrective action. If the technician can get the wire back to its reference position, it is likely that the problem has been fixed. This routine works, but requires the operator to detect a problem, alert maintenance staff, then wait for the technician to fix it. The cell typically is 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 devices normally involve some programmed robot motion to a fixed gage to measure the position of the weld wire, and that information then is used to calculate the tool center point.

Fig. 3 On-line alignment gage performs automatic tool center point check. Alignment pin provides visual reference for manual inspection.

These gages may be a contact type that has the wire physically touching the gage or they could be a noncontact type that detects wire position when the wire 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 tool center point is within tolerance, then production continues (see figure 3).

Checking the tool center point 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 tool center point gage, then maintenance personnel know they need to focus on a problem with tooling or part variation. Checking the tool center point 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 tool center point 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 then can be used to measure the offset of a deviated tool center point. The robot does 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 inspected visually and checked for false torch condition readings that could be caused by a bent weld wire or more serious torch problems. Often, torch errors can be remedied by changing the contact tip.

The robot also can offset the programs to compensate for the tool deviation detected during the gage routine. Prior to updating the tool center point 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 tool center point can be off and still shift. Compensating programs for gross tool center point 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 tool center point 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 tool center point 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 tool center point mechanically. Almost all torch manufacturers provide removable torch barrels and have alignment jigs for checking and adjusting the tool center point. However, few users take advantage of these jigs and use them as intended.

A tool center point variation might be the result of a bent torch barrel. This problem is easily remedied by exchanging a barrel with one that has been verified in the jig. Depending on the torch design, changing 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. Users should have an alignment jig on site to allow them to verify that new torch barrels have the same tool center point as barrels they replace. Some users have operators try to fix a suspected problem by replacing torch barrels and contact tips, prior to alerting maintenance staff. A simple alignment jig allows the operator to check a barrel visually and easily.

An automotive suspension component manufacturer was plagued by “wandering” program points. The manufacturer was using torch alignment jigs to verify the tool center point position prior to exchanging torch barrels, and was using reference points inside the cell to verify proper wire position.

The manufacturer’s technicians were editing weld positions often, and the technicians suspected torch barrels were being distorted by the heat from welding.

An on-line alignment gage was implemented, the robot programs were changed to automatically update the tool center point every few parts, and the problem immediately got worse. The robot began placing welds off seam randomly after a tool center point update.

This led the maintenance staff to concentrate on wire positioning, and they identified a problem with inconsistent wire cast. That random wire flipping plagued them for weeks and caused constant editing, and it was the on-line gage that 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 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.

Fig. 4 –A tip change window provides easy and safe access for servicing torch.

This improves uptime and reduces cost. It also improves safety by allowing the 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 (see figure 4).

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 tool center point 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 rely on placing the end of the wire into the weld joint accurately and 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 usually requires an automatic wire cutter to cut the wire to a consistent length for searching. For thinner-gage applications in which repeatable results are required, a wire brake option in the torch is helpful. A wire brake is a small air cylinder that 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 touchsensing 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. Through-the-arc seam-tracking requires the robot to weave in the joint. It tracks the joint by keeping the wire stick out the same on each leg of the weave.

Laser sensors also are available for seam-finding and seam-tracking. They are more expensive, but they are faster and can find thin lap joints. They also can provide additional information on joint gap that can be used to create programs that adapt to changing joint geometry.

However, the camera used in laser sensing sometimes can interfere with joint access, and that limits the use of lasers in some applications.

Here’s an example: A manufacture of large storage racks had variation in fit-up and had to slow welding speeds to ensure that the robot always made a quality weld. This slower speed compromised the required throughput.

A laser sensor was installed to detect the joint condition ahead of the weld. The robot slowed on joints that had gap, but traveled faster on joints that had good fitup. 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
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 they do so at all. Often, the tools that could improve productivity on a daily basis are laying unused in a maintenance crib.

Chris Anderson is a degreed Welding Engineer from Ohio State (1983) with 24 years of experience in robotic welding applications. Currently, he is Motoman’s Technology Leader for Welding with responsibilities to specify and develop new products. He served on the RIA R15.06 Robot Safety Committee and AWS D16 Robotic and Automated Welding Committee. He also served as Chairman of the Dayton Section of AWS from 2003-2007.