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Robot Welding

The essential guide

Fixtures for robot welding
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Welding fixtures for robotic welding

There are two types of fixtures. Assembly fixtures hold all components as loose items, which are then separately clamped. When a product is too large or is produced in low volumes it is best to pre-tack the assembly and then offer it to the robot for final welding.
In this case the robot fixture can be very simple indeed locating on some key features of the product. When designing an assembly fixture you should consider the following points:

Locate the joint. This is the only area that needs to be accurate for the robot.

Support the component on knife edges or narrow locating surfaces. This will ensure that the components are always located accurately and small amounts of weld spatter will not affect the joint positions.

If there any holes in the components you can use these as a location.

Harden locating surfaces to eliminate premature wear.

Use good quality clamps. The utilisation of a robot fixture is much higher than a manual welding fixture.

Ensure that the robot has access to the joint. Although the robot is quite flexible, it is not as flexible as a human arm and also consider the hose bundle.

If there are certain features within the product that have critical tolerances, shim the location surfaces to compensate for any distortion due to heat input. Sometimes the components have to be set away from nominal to get an accurate part. Incorporate local shielding if weld spatter is an issue in certain areas.

Manual fixtures


Customers in general industry tend to have welding fixtures with clamps that are manually applied. These represent the lowest cost and often work the best. In this example the fixture is for welding gates. Because the product features welding on both sides, it is necessary to rotate the assembly using a positioner. The fixture is mounted from a strong back, that spans across the fixture discs of the positioner. The strong back has a series of locations to accept a range of other fixtures. The strong back needs to be very rigid so that the joints will be accurately positioned.

Automatic fixtures


Customer is in the automotive industry tend to have either automatic fixtures or semi automatic fixtures (manually applied clamps, pneumatic backup and pneumatic release). There will be sensors that monitor part presence and clamp open/close condition. This information is mapped through to the robot control system or a PLC. If any of the sensors is not made an error will be flagged up. These requirements are usually the result of the automotive manufacturers insistence on quality control and traceability. Automatic fixtures are also faster to operate and will show an improvement in productivity when a large number of components and clamps need to be handled. The use of sensors in fixtures greatly increases the electrical engineering cost associated with mapping the sensor information and additional hardware such as a Human Machine Interface (HMI), PLC and bus system. Whilst customers in the automotive sector often do not have a choice, general industry customers should carefully consider the cost implications. In addition, this type of fixturing tends to make a cell more dedicated and any subsequent fixtures will need to be electrically integrated at a high on-cost.

 

Fixtures for pre-tacked assemblies

When welding very large assemblies it is generally too expensive to use assembly fixtures and in this case the assembly is pre-tacked prior to robot welding. The tacked assembly is then presented to the robot cell and held in strategic locations in a simple fixture. An advantage is that this type of fixture represents a low investment level and the robot has very good access to the joints. A disadvantage is that the manual tacking operations, which will have a bearing on the financial justification of the system.

 
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