Optimising the service life of welding robots

In the two previous blogs on how to reduce downtime with welding robots, the focus was on the robot torch and its wear parts.

1. Selection of the robot welding torch according to the application ✓
2. Selection of wear parts such as contact tip, gas nozzle, gas distributor and liner ✓
3. Cable assembly and cable assembly guide
4. Robot torch mount and switch-off protection
5. Torch cleaning station

In this blog article, we will go over all the remaining parts of a robotic welding system where downtime can occur: The cable assembly and its guide, and the torch mount. But we will also take a closer look at an optional equipment – the important and practical torch cleaning station. It should not be missing from any welding robot system if you want to optimise the service life of the welding robot in your company.

3. Cable assembly and cable assembly guide

Cable assemblies in robot welding are under constant load. They are continuously in motion, have to reach all parts of sometimes complex components, and are quickly moved from one point to be welded to the next. Depending on the intensity of the movement, the load on individual components is very high and so is the wear.

There are two variants of cable assembly guide for welding robots:

• OA – over-arm = cable assembly guide outside along the robot arm
• TA – through-arm = cable assembly guide through the robot arm

Each variant has its advantages and disadvantages and offers approaches for optimising the service life of welding robots.

Over-arm welding robot

An external cable assembly is subject to constant swerving or skidding movements due to the movements of the robot arm. This means that there is a constant danger of it getting caught on the component or on fixtures and tearing off. The robot itself does not notice this incident in the process unless the switch-off protection in the torch head mount registers this and stops the robot. In such cases, there is a risk of a shorter service life.

The high dynamics of the movement results in a more or less rapid fatigue process of the cable assembly components. Hoses, gas lines, power cables or control cables can thus fail.

With an over-arm welding robot, offline programming – i. e. without manual teaching at the welding torch – is only possible to a limited extent, as the skidding movements are too complex or unpredictable and not repeatable for the software. This is a major reason why more through-arm welding robots are used nowadays.

Advantages of OA welding robots

• Good accessibility to the cable assembly when replacing wearing parts such as the liner

Disadvantages of OA welding robots

• High susceptibility due to high dynamics of the cable assembly movement
• Danger of getting stuck
• Only limited offline programming possible
• Unsafe TCP stability
• Heavy weight load on the switch-off protection
• Faster product ageing due to flying spatter and UV radiation of the arc

Tips

 Cable assemblies guided outside the robot arm must be fastened or suspended in such a way that pendulum movements and bending moments are kept to a minimum. This avoids the so-called "florist's wire effect", i. e. breaking at particularly stressed points.

 Cable assemblies should be guided in the largest possible radii with the help of balancers and the suspension positions to ensure good wire feeding.

 Never use self-made solutions for mounting (e. g. cable ties)! Instead, make sure that the individual components in the cable assembly – such as the power cable, control cable, coolant hose, gas hose – can move freely against each other in the special dimensionally stable outer hose.

Through-arm welding robot

An internal cable assembly always lies in a defined area, can only move a few centimetres all around and is not exposed to any skidding or swerving movements. This means that offline programming is no problem at all and ensures a stable TCP and thus a long service life.

A TA welding robot system – also called a hollow shaft robot – on the other hand is naturally exposed to strong torsional forces. After the third robot axis from the wire feed, everything is in motion in the direction of the welding process. A cable assembly must be able to cope with these rotary movements. ABICOR BINZEL has developed special BIKOX^® cables for this purpose, which withstand these movements. Some manufacturers offer switch-off protection and built-in sliding contacts for TA welding robots. With these, low torsional forces occur within the hollow shaft robot arm. We have explicitly decided against the development of such cable assemblies, since high current transfer, abrasion, wear and susceptible sliding contacts are on the one hand expensive and on the other hand represent too many disturbance variables for failures.

Advantages of TA welding robots

• Protected cable assembly, no swerving, no getting caught
• Good service life
• Offline programming possible
• Assured TCP stability
• Switch-off protection is not overloaded

 Disadvantages of TA welding robots

• High torsional forces that have to be compensated by special cable assemblies.
• Depending on the mobility of the wire feeder, a TA cable assembly can be more cumbersome to install and dismantle.

The torch technologies for TA welding robot systems from ABICOR BINZEL are very sophisticated, so that they can be used for an above-average period of time, depending on the application and process.

Cable assemblies guided inside the robot arm are more suitable if you want to reduce the downtimes of welding robots due to their considerably longer service life and lower susceptibility to faults during the process.

4. Robot torch mount and switch-off protection

Each robot welding torch neck is attached to a mount which, in most cases, also has an integrated switch-off protection function. This torch mount or switch-off protection also offers potential for optimising the service life of welding robots. In the case of hollow shaft robots, the structure is predefined and the switch-off protection corresponds to the mount. For welding robots with an external cable assembly, the torch mount must be designed according to the application and with the correct torch geometry – and in such a way that there are very few tight radii in the cable assembly during the process.

A possible source of faults with torch mounts on an externally routed cable assembly can be the use of two mounting elements instead of a one-piece mount with a defined TCP. It is a challenge to position these two mounting elements exactly. To prevent the TCP from slipping on the Y-axis, all parts must be additionally pinned. Even if the dowel pins supplied have the correct diameter, there is a risk that the user will set the boreholes incorrectly. If a normal drill with a diameter of 6 mm is used instead of a drill with a diameter of 6H7 mm, this extra tenth can move the holder plates minimally despite the screw connection. For the TCP, this displacement, however small, can amount to 2-3 mm under certain circumstances. This can be compared with the leverage effect of an articulated bus: a slight steering movement by the bus driver causes the passengers in the last row of seats to swing to the side.

As a result, the robot welding torch no longer welds where it should. In addition, the torch neck may be damaged when it enters the cleaning station.

Tips

• When connecting the robot flange to the torch neck, ensure that there are as few mechanical interfaces as possible.
• Ensure precise pinning.
• One-piece mounts are recommended.

5. Torch cleaning station

Depending on the welding process, power source and parameter settings, welding spatter occurs during MIG/MAG robot welding. These spatters adhere to the front area of the robot welding torch and thus also to the gas nozzle. If the weld spatter is not removed, turbulences or reductions of the shielding gas occur. As a result, the required shielding gas coverage deteriorates and there is a risk of pore formation. Regular cleaning of the gas nozzle is therefore a standard part of robot welding. Automated cleaning stations are used for this purpose.

There are different types of cleaning stations offering the cleaning methods by: CO₂, magnet, sandblasting and reamer. For automated cleaning in MIG/MAG robot welding, reamer cleaning is established to approx. 90 %, which always achieves satisfactory cleaning results. The decisive factor in reamer cleaning is the ratio of the reamer to the torch head, which should always be ideally suited to each other. In order to achieve the best possible cleaning results, suitable reamers are offered for the different torch head geometries. During the cleaning process, the inner contour of the gas nozzle as well as the outer contour of the contact tip and contact tip holder must be cleaned as well as possible to keep the outlet holes of the shielding gas free.

The cleaning device, which in most cases is equipped with an air motor, must have sufficient power. The cleaning station itself must also be very stable to withstand a crash without having to readjust and realign everything.

Recommended: Maintenance units

The cleaning station itself is ideally a maintenance unit that performs reaming, blowing out, wire cutting and spraying the front end with anti-spatter agent in one process. With such a maintenance unit, it is important to ensure that the blade for wire cutting has an appropriate hardness – a hardness between 62 and 64 HRC (Hardness Rockwell C) is recommended, which stands for a hard cutting edge and high cutting resistance and falls under the upscale hardness of blade steels – and does not wear out prematurely. The cleaning station itself is ideally a maintenance unit that performs reaming, blowing out, wire cutting and spraying the front end with anti-spatter agent in one process. With such a maintenance unit, it is important to ensure that the blade for wire cutting has an appropriate hardness – a hardness between 62 and 64 HRC (Hardness Rockwell C) is recommended, which stands for a hard cutting edge and high cutting resistance and falls under the upscale hardness of blade steels – and does not wear out prematurely. When spraying in, it is important not to spray on too much anti-adhesive, which then drips down on its way to the workpiece – but also not too little, otherwise no effect is achieved and weld spatter adheres more strongly. Good cleaning stations as maintenance units spray in very efficiently and catch dripping liquid. This can be filtered and reused accordingly.

Frequency of cleaning intervals

How often a welding robot should stop at the cleaning station must be considered on an individual basis. If the cleaning intervals are programmed too short one after the other, the gas nozzle, contact tip and contact tip holder remain nice and clean, but this also means more wear on these wearing parts and possibly longer cycle times. If the cleaning intervals are too far apart, more weld spatter will accumulate and cleaning will become more difficult.

When a reamer is used in a maintenance unit, the loosened weld spatter is pushed upwards by the movements of the reamer, where of course it must not remain. For this reason, all robot welding torches from ABICOR BINZEL have an integrated blow-out hose. During the cleaning process, compressed air is fed through the gas boreholes via a valve in the wire feed, whereby the loosened spatter is blown out downwards. The subsequent spraying with anti-spatter agent after cutting the wire reduces spatter adhesion and makes subsequent reaming easier.

Tips

• Maintenance units with integrated reamer, wire cutting device and injection unit with anti-spatter protection extend the service life of the wear parts close to the process.
• Only use reamers suitable for the torch geometry.
• Look for stable and high-quality cleaning stations.
• Adjust reaming frequency and reaming time individually.

Robot programming

For any company, the biggest source of error in robot programming is the human being. A minimum requirement for this job is to have welding skills. There is a saying in this regard: "You can teach a welder to programme, but you can't teach a programmer to weld”. Here, as everywhere, exceptions prove the rule.

Precautionary measures during programming

Possible precautionary measures during programming to prevent downtimes and optimise the service life of welding robots are:

• When teaching, programme as few movements as possible
• Avoid jerky movements as far as possible
• Keep skidding and swerving movements to a minimum
• Observe limits for the axis movements
• Do not provoke collisions, e. g. by driving too close to devices
• Programme inputs and outputs with external devices such as cleaning station to the correct control intervals, duration of reaming time, etc.
• Proper process and power source settings for the robot movements

But programming is not always the only reason for a collision, for example. Components can be faulty, a clamping element can fail or was not closed properly and parts are no longer in their original position. These are, however, mechanically related causes, but they can occur. In these cases, the switch-off protection ensures that the welding robot stops.

Basically, although you may have successfully completed a two-week training course on programming for industrial robots, this does not mean that you will be able to programme absolutely error-free in the future. Robot programming is a lifelong process. You always learn something new – and above all, you never stop learning.

Human and machine – two that get along well

We can no longer close our eyes to automation. Human and machine will move even closer together in automated welding in the future. There are other industries where it's more "human versus machine", in other words, humans are needed less and less. The handling of an automatic machine that supports human workers in their activities, relieves them and thus makes time available for other welding tasks, is an essential aspect to be accepted by the user.

We need to open the doors for automated welding. Then we will be more efficient in production, remain competitive and be able to use the workforce and the knowledge of the employees in a more targeted way.

Here you will find future-oriented automation solutions for:

• cobots
• welding tractors
• orbital welding
• collaborative pipe welding

A visit to the website www.binzel-abicor.com or also to the ABICOR BINZEL YouTube channel is worthwhile!

Questions about programming your robot or about the application? The ABICOR BINZEL team of programmers and application engineers will be happy to support you. Contact us and find the right solution for you with our automation experts – also for your start into automated welding.

Happy welding!