The essential guide

ROBOT WELDING

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Key issues for robotic welding

Component fit up

The robot will consistently move to its programmed positions to a very high accuracy (± 0.04 mm) and it is therefore important that the components are accurately positioned. These are usually loaded in a welding fixture, but large assemblies may be pre-tacked. If the joint is not located in the right position, the robot will still move to its programmed point and as a result the weld will be in the wrong place. As a guide, the accepted tolerance for joint positioning is ± half a welding wire thickness. For example if you use a 1.0 mm wire the total tolerance can be ± 0.5 mm, so the total tolerance is 1.0 mm. This tolerance will also depend on the material thickness and type of joint. If these conditions cannot be maintained, it is possible to locate the joint by means of a tactile sensor and this is commonly used in welding large fabrication where it is difficult to maintain the required cutting and bending tolerances. It is also possible to use a vision sensor, which is faster, but has the disadvantage of being a bolt-on item and may restrict access. If the joint is expected to deviate from its programmed path, due to for example large amounts of heat input on large fabrications, it will be possible to track the seam using a through the arc joint sensor.

Gap conditions

The tolerance on a gaps should not exceed a total tolerance of 0.5 mm to 1.0 mm depending on material thickness. Larger gaps can be bridged by using the weaving facilities of the robot, but it is important that the gap condition is consistent The welding parameters are tuned to provide optimum results for a particular condition and if that condition changes, it will have an effect on the quality of the weld. If gap conditions vary then this can be measured using a vision system, which will make adaptive fill corrections to the welding process. It is almost always preferable to ensure that the components are consistent rather than relying on sensing technology. The cost of these sensors is relatively high and requires quite a high level of expertise from the robot programmer.

Joint design

It is important to have the correct joint design and if this could be changed to suit robotic welding, it will be much easier to achieve success. The ideal joint is a fillet joint, but an overlap joint is also easy to weld. For thin gauge materials joint design and gap conditions become even more critical and it essential that the components are cut and bent with CNC equipment.

By modifying the joint design of an outside corner, it will be much easier for the robot to weld. Even if the integrity of an outside corner has to be maintained it would be helpful if the joint could be designed so there is a lap condition.

Butt joints on thin gauge materials represent a serious challenge for robot welding. By changing the design to an overlap joint the success rate can be dramatically improved.

Quantities

If there are too many variants that are only produced in low volumes, there will be an additional investment requirement for fixtures. Companies should consider the minimum batch volumes and if these are too low, it will affect the uptime on the robot system due to set up times. It is possible to integrate robots in a flexible manufacturing system (FMS) where single products can be welded without set up times. These systems require high capital investments. The alternative can by a system that comprises more than two work station, in which case any fixture change over does not affect the robot's productivity.

Ouside corner welding

Critical outside corner joint

Outside corner joint Filler weld joint Butt joint position Overlap joint position

Change outside corner joint to fillet joint

Change butt joint to overlap joint