When welding tolerances become a trip to hell

Poorly prefabricated components, problems with correct clamping, material distortion due to heat from the process ... very often it is a major challenge to meet the required position tolerances during welding. Does this sound familiar to you?

With manual welding, irregularities can still be levelled out quite well, with good welding skills. Automated welding and robot welding are a different story. Here, the problem is solved in different ways. From “I'll just add more welding wire to the process and the joint will be sufficiently filled” to “we rely exclusively on automated optical seam tracking because we simply can't afford expensive rework or even rejects”.

In this blog, we would like to introduce you to different approaches on how you can comply with the specified welding tolerances in automated welding and welding with robots in accordance with the various DIN standards. You will be amazed: what may look like a low-cost solution turns out to be a money pit in the long run. On the other hand, initially challenging investments can save both time and money during the time they are in use.

Before welding comes finding

Before a robot or welding machine can make a seam, it must know where the joint is located. Of course, you specify this to the welding robot. In reality, however, the joint is not exactly the same each time. Therefore, the challenge is to weld good quality despite different actual joint positions, sizes, joint widths, etc.

If welding tolerances are exceeded, the weld is defective and must be reworked or even scrapped.

There are 3 approaches to compensate the above deviations:

1. Conventional process management
2. Seam finding
3. Seam tracking

Most commonly used: Conventional process management

Conventional process management involves approaches that attempt to achieve satisfactory weld quality while minimizing effort. These are:

1. Using more welding wire and ensure that the weld is oversized and overfilled ensure that the actual joint is welded
2. Try to ensure that the components come out of the previous process steps better prepared – which is difficult to control
3. Use more complex clamping fixtures
4. Possibly change the welding sequence so that the heat distortion is reduced as much as possible
5. Program the robot more precisely – which is a constantly changing battle as part positions change, requiring constant touching up of the robot program

Advantage:

There is hardly any investment in advance. Exceptions are optimized clamping fixtures, which are generally more expensive, or investments in the further training of robot programmers.

Disadvantages:

They cost time,

• if too much filler material is applied and energy is introduced – and may require later rework
• in the case of reprogramming, because welding path positions have to be adjusted frequently to match the changing parts. This also requires more skilled staff
• because defective components have already been produced, as reprogramming is reactive

They harbour hidden costs,

• if poorly produced components have to be reworked
• because more material used for the welding wire also means a higher consumption of shielding gas at a slower welding speed

They make quality assurance more difficult,

• because too many poorly produced components have to be reworked and reinspected

Time-consuming: seam search

There are two ways of searching for seams to compensate for welding tolerances: tactile – i. e. by touching the wire or gas nozzle, for example – or optical.

With tactile seam finding, the robot must first detect where the weld is to start. To do this, it approaches the component until it comes into contact with the welding wire or gas nozzle.

Advantage:

• Everything you need is usually already included in the system
• No additional costs

Disadvantages:

• The joint to be welded must be large – at least 2–3 mm of clear feature
• Only suitable for short welding paths and not for online seam tracking
• Does not measure seam geometry such as gaps or offsets
• Requires reprogramming of the welding robot

Optical seam search in turn has 3 different approaches:

A laser spot travels at right angles over the surface of the component and scans it without touching it. The robot receives the information where the welding process must begin.

Advantages:

• Faster than tactile seam tracking, but still quite slow
• Generally cost-effective alternative compared to triangulation
• Suitable for short and long seams

Disadvantages:

• Requires a clear weld feature to recognize the joint
• Investment costs for sensor system and its installation

This technology also makes a non-contact distance measurement by means of a laser triangulation. For this purpose, a semiconductor laser diode projects a laser line onto the workpiece directly below the sensor head. The sensor head is equipped with a camera that records the shape reflected by the workpiece and can use this to measure the position of the joint.

Advantages:

• Very fast system, side and height measurements are possible in less than 1 second
• Very flexible system that can be used for many seam types – particularly good for short seams such as tack welds
• Touchless detection

Disadvantage:

• Higher capital costs than other finding techniques described above for sensor system and its installation

A camera uses an algorithm to identify the position of the joint based on differences in brightness between the joint and the surrounding material. This principle works before arc ignition but is easily upset by changes in illumination or to the surface finish or the materials. This approach is not suitable for seam tracking during welding itself, as the welding light is a major disruptive factor. There is a camera system from Novarc on the market, which is used in the spool welding robot SWR.

Advantages:

• Measures sides of a joint very quickly
• Suitable for short and long seams

Disadvantages:

• Requires contrast to recognize the joint and features
• Very sensitive to surface finish of components
• Very sensitive to optical interference (daylight, indoor lighting, etc.)
• High investment costs for sensor system and its installation

Efficient: seam tracking

The most efficient way to maintain welding tolerances is seam tracking. It is reliable and rework costs can be saved. There is tactile and optical seam tracking.

Tactile seam tracking has 2 approaches:

Advantages:

• The system already includes everything you need for this type of seam tracking. No additional investment is therefore necessary
• Suitable for long weld seams and also for tracking

Disadvantages:

• Slow seam search, therefore, longer cycle time
• Sheets must be thicker
• You must weave to be able to track, so welding speeds are inherently lower

Advantages:

• Relatively cheap procedure
• Easy handling

Disadvantages:

• Inaccurate measurement
• No adaptive welding possible
• Machine welding only – not for robotic welding

Optical seam tracking also has 2 practical approaches:

Advantages:

• Fast seam search and welding at full process speed
• Measurement of seam geometry can allow for adaptive control and an optimized weld
• Suitable for many seam types, long weld seams and also for tracking

Disadvantage:

• High capital costs
• Experience with connection to robot controller or PLC required

Advantages:

• Fast procedure
• Good for welding long seams, also for tracking

Disadvantages:

• Expensive technology that is only suitable for a few applications
• Great challenge to deal with correctly interpreting the weld image to measure the seam position
• Very application specific

Conclusion

Only one technology is flexible enough to be used in various applications: optical seam tracking using laser triangulation. It is the only method that is able to recognize dynamic deformations of a component, such as distortion caused by process heat, and to compensate for welding tolerances in real time.

Optical seam tracking with the iST ARC sensors from ABICOR BINZEL correct the weld path in real time. They can recognize and track workpiece joints from a gap width of 0.1 mm.

Of course, there is no solution for ‘the one’ seam tracking. Every application is individual. If you are looking for an approach that can offer the best combination of speed, robustness and flexibility, you do best with optical seam tracking with laser triangulation.

Have you already had experience with the systems mentioned here? Tell us here!

Source Image 4: https://www.carpano.it/tactile-seam-tracker-ig-3/