Principles the same for large, small weldments
September 10, 2012
The concepts of positioning are the same for all weldments, large or small. A properly positioned weldment, regardless of the size, reduces welder fatigue, increases safety, improves weld quality, and saves on production floor space.
It is a well-known fact that positioning equipment can assist welders in maneuvering and welding large assemblies. What might not be so well-known is that many small assemblies also can be mechanically positioned to provide the welder with the same benefits.
The principles of positioning are the same for all weldments, large or small. The base product is affixed to the positioning equipment and then maneuvered by mechanical means into a position that allows the most effective welding and assembly. As parts or subassemblies are added, the entire weldment is moved to allow easy access to weld joints.
A properly positioned weldment, regardless of the size, reduces welder fatigue, increases safety, improves weld quality, and saves on production floor space. By moving the weldment using mechanical means and positioning the welding area into a comfortable range, welders are not forced to weld out of position or in an uncomfortable position.
Safety is improved when the weldment is anchored to a suitable positioning device. Cranes, chains, slings, and other nonrigid methods of moving a part might create uncontrolled motion, which can be dangerous. With the help of a positioner, welders don’t have to maneuver themselves underneath a possibly heavy weldment, reducing the risk of injury from falling sparks, slag, or parts.
While many welders are qualified to do overhead and vertical welding, downhand welds often require less training, allowing new welders to produce quality welds. Gravity helps the welder in a downhill weld, resulting in equal legs on fillet welds, smoother bead surface, and reduced cleanup and rework times.
By combining a positioner with a welding power source and a torch stand, a welder can perform semiautomatic welding that is productive and ergonomically friendly. The positioner holds the part and maneuvers it under a stationary torch. This torch can be fitted with a weaving device to allow oscillation to fill large gaps or V-grooves. Consistent speed and torch position improve the quality of the weld with greater repeatability. By using a communication cable between the integrated positioner and a welding power supply, the operator only needs to signal a start through a foot pedal or a start button, and the welding cycle will continue until the signal is automatically sent that it has completed. This method, typically used on a circumferential weld, can incorporate dwell times to create a puddle and fill the crater. The completed part is removed and another is started.
Regardless of the size of the weldment, welders should keep these five suggestions in mind when selecting, operating, and maintaining a positioner.
Selecting the right positioning device for the job involves accounting not only for the weight and size of the weldment, but also for the center of gravity (COG) and how far it is from the positioning device. COG is the point at which the weldment balances on all axes equally. As the distance increases from the device, more torque is applied to the positioner. The COG changes as the welder adds material and parts to the positioner, so these changes must be taken into account.
The table rotates by a variable-speed electric motor and can be hand- or foot-controlled. The positioner motor and control should be selected based on the size and speed that are required to perform the desired welding operation.
The positioner manufacturer’s specifications will guide the motor and control choice. It’s important for welders to check the specifications for both horizontal and vertical loading to make sure capacity is adequate to handle the weldment. The positioner should be able to withstand the largest possible load.
How a weldment is attached to a positioning device is as important as the positioner itself, because this is the point where separation would naturally occur. Production fixtures are designed for a specific application performed repeatedly. This type of fixture is mounted permanently to the positioner, and its specific shape allows for easy part alignment.
Round parts often are attached by a three-jaw chuck. The part must not pull away from the jaws when the part extends from the table. Also, the part may expand or contract from the heating and cooling that occur during and after the welding process, which can change the grip that the chuck has on the workpiece.
Fixtures and chucks add weight and distance from the faceplate, which needs to be considered when sizing the positioning device. While the weight of a weldment applies torque to the device, the distance that weight is applied multiplies the torque by the increase in distance. For example, a 50-lb. weldment that is 3 inches away from the face of the positioning table creates 12.5 ft.-lbs. of torque in the vertical position. If the distance is increased to 6 in., the torque increases to 25 ft.-lbs. at the mounting surface. The increased torque may require a larger-capacity machine.
Many positioners have slots that allow the workpiece to be bolted to the face. Welding the part to the positioner is often a good way to prevent shear forces from sliding the part as it is maneuvered. Any of these methods, whether stand-alone or combined, will work if applied properly.
If a weldment is cylindrical, it is eligible to be rolled. Small turning rolls—powered or idler type—can rotate a pipe or vessel to enable downhand welds. The power rolls provide steady rotation, producing an even circumferential weld. Idler rolls are not powered but can be added in series to support longer pipes and vessels. Often these are used for adding flanges to pipe ends and connecting pipes and ends to vessels.
The combination of a roller-type pipe stand and a vertical-faced table positioner provides stability and safety when a round part is extended outward. When the rollers provide two points of contact, the weight is distributed evenly, and the COG can be supported.
Even with small positioning equipment it is important that the unit be mounted to a flat, even surface to prevent it from tipping. If mounting holes are provided, they should be used to secure the positioner to a stable surface to prevent tipping when or if it encounters an unexpected force. A positioner mounted to a workbench or stand must be secured as well.
During welding, a ground current should be connected to the positioner itself. The ground current transfers from the table and into the chassis, which eliminates having to remove and replace a welding clamp continuously. Without proper grounding, electrical parts can be damaged and substandard weld deposits made.
The positioner should be compatible with the electric current produced by the welding process. Also, all ground cables must be secured tightly by removing any paint before bolting to the carriage.