Spot welding in body shells

Automotive engineering Underbody protection

On a welding line where a large number of different car bodies are produced, new models need to be introduced with as little downtime as possible.

Spot welding in body shells

Cell description

A major automobile manufacturer operates multiple welding lines for car bodies. They use Kuka KR125 robots and Comau H4 with spot welding guns.

A special feature of the system is that a large number of different models are produced on the same lines and new models need to be introduced with as little downtime as possible for the line that is running.

Task definition

In the past it was found that when spot welding guns were replaced a significant point shift occurred. Re-teaching often resulted in further crashes as the production line is extremely restricted in terms of space and welding points in the wheel arch, for example, can only be modified to a limited extent without incurring the risk of another collision.

What made things more difficult was that the re-teach work had to be carried out for all models. These were not available, however, at the time the work was being carried out. It could be that the model would not appear until the next shift. If the operator forgot to stop the line and adjust the robot programs, another crash occurred.

The task also included ensuring that the Comau robot could be replaced. For this type of robot there is no repeatable and verifiable procedure for mastering.


An instrument called a measuring bone was developed specifically for measuring spot welding guns. This is an arm with 2 measuring balls – one at each end – and a 1:10 morse taper as an adaptation in the middle. The electrode cap is removed with a gun and the measuring bone is attached onto the E-arm. The gun is then closed. The measuring bone is now firmly positioned on the E-arm and the two measuring balls can be measured with tool:in. The TCP is then the centre of gravity of the two balls. The measuring bone is available for the commonly used cap sizes of 13, 16 and 20 millimetres.

It can be assumed that when a measurement needs to be made the line is full of car bodies and it would be an unacceptable expense to remove them from the production line. For this reason an angled pillar was developed that can be screwed into holders on the conveyor belt even though the car body is still in the station.

For the Comau robots another loop:in was carried out additionally.

Customer benefit

Maintenance requires 15 minutes for the measurement with tool:in after a welding gun has been replaced.  After this the production programs of all models are reliably in alignment.

At a later time the electrode caps were replaced by others with 2 mm more copper, to save costs. Thanks to LaserLAB and tool:in it was possible to carry out the conversion quickly and smoothly.

In the meantime several Comau robots have been replaced and measured again with loop:in without any subsequent problems arising with respect to production quality.