The essential guide

ROBOT WELDING

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Positioners for robotic welding

When a joint cannot be addressed in the correct welding position, it will be necessary to rotate the assembly so that the robot can weld the joint in the horizontal-vertical position, vertical down hand position (for materials up to 3 mm thick) or gravity position. Overhead welding is perfectly OK for a manual process, but should be avoided with a robot since weld spatter can get lodged onto the tip. This will then affect the wire feed mechanism with the result that the system stops due to a weld error.

Interface into the robot controller

Positioners, sometimes called manipulators, tend to be servo controlled and are driven from the robot controller and programmed from the robot teach pendant. The movement of the positioner is integrated and programmed together with the robot, which means that the robot can move to the joint, whilst the positioner is bringing the joint into position. Alternatively if the assembly has an orbital weld (e.g. an exhaust system), the positioner can rotate the assembly whilst the robot remains stationary or moves along with the joint if the orbital weld is eccentric.

Handling capacity

Positioners will be specified to a certain handling capacity and this depends on the weight of the assembly plus the fixture and the off-set. Typically postioners are modular in construction and start at 250 kg and may go up to 10,000 kg or even larger.

Types of positioners

There are many types of positioners to suit the particular application and cell layout.

Single station v multi stations

A cell with just a single station is suitable when welding large assemblies that have a long weld cycle. In this case the load and unload time are less significant. In all other cases a twin station cell is the common approach since the operator load cycle can be fully or partly absorbed within the robot weld cycle. The is the key to improving productive compared to manual welding. In case of small batch volumes it is not unusual to see a robot system that features three work stations, allowing a tool change to be made whilst the system can still operate between two stations.

Single station positioners v twin station positioner

A limiting factor for twin station positioners is the width of the assembly or fixture. This should fall within the specified swing diameter or else there will be interference with the central anti flash screen or floor. Wider assemblies are therefore held in a single station positioner. There could also be a good reason to create two separate work stations when welding different products where logistics and material flow can be an issue.

Linear tracks

If the joints to be welded are outside the working envelope of the robot,the robot can be mounted on a linear servo track, which will extend the reach of the robot. Linear tracks are also used when two workstations are positioned in an in-line layout.







Gantries

A gantry system dramatically extends the working envelope of the robot. The robot is inverted and carried in the Y direction along the work piece, if required in the X direction across the work piece and it is also possible to have a Z direction that moves the robot up and down. The Y axis of the gantry can be long enough to create two separate workstations in order to increase the system's productivity. The additional axes are fully servo controlled and are controlled and programmed from the same robot control system. It is also possible to have additional carriers to have multiple robots.

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Head and tail stock positioner

Twin station single axis positioner with horizontal station index. The vertical station index variant is better suited to longer assemblies as the turning circle will not interfere with reach of the robot.

Small 2-axes positioner

Large 2-axes positioner

A robot mounted on linear servo track increases the working envelope or allows for stations to be positioned next to each other.