How to Choose the Right Positioner for Your Robotic Welding Application

Excellence in welding automation is conditional upon the capabilities of the components used. This is especially true for positioner technology, which can significantly vary in style, size and capacity. No matter the specs, however, a keen understanding of positioner types and features will go a long way to facilitate the decision-making process when looking for the most robust and reliable positioner for demanding application needs.

POSITIONER TYPES
A welding positioner is a device that controls the rotation of a part with different types of rotators that allow for 360 deg of rotation and access to the part. The use of such equipment during the robotic welding process has the ability to accelerate production and improve weld quality, especially for large weldments. Just as ergonomics is important to enhance the comfort and productivity for skilled welders, proper positioning matters for robots as well. In order for a robot to reach specific weldments and minimize cycle time, parts must be easily accessible. Regardless of part size, however, each welding positioner used is uniquely designed for specific application requirements.

Single Axis
The most affordable and basic type of positioner is designed with a single axis. Whereas most positioners are simply multiple headstocks and tailstocks in different connected configurations, single axis positioners can be mounted solely as a headstock on a floor or table top. This accommodates simple and/or small part welding, such as small metal frames, cylinders or components that make up a larger part. Frequently, fabricators combine a headstock with a tailstock to support long part spans or peripheral weld tools, as tailstocks spin freely to help support the load of the part and tooling. This is ideal for parts up to 5 m long, such as tower trussing, trailers or agricultural parts. For weldments with increased payload, two headstocks can be used in conjunction with a single tailstock.

• Traditional hard-mount tooling vs. newer patented technology. A growing number of manufacturers are moving from traditional hard-mount systems to a more scalable approach to meet a wide variety of application requirements. Traditionally, alignment of a headstock and tailstock has been done on a common base with the guidance of laser-accurate leveling procedures, because misaligned headstock/tailstocks or headstock/headstocks can cause significant damage to the part and tooling. A substantial reduction of positioner life can also occur. To avoid this scenario, manufacturers are implementing the use of an innovative fixture mounting system called MotoMount™ that provides two degrees of freedom to simplify tooling and to reduce stress on positioner bearings up to 70 percent on three-meter spans or larger.

Unlike traditional hard-mount tooling, this newer fixture mounting system has compliant bearings on both the head and tailstock. Not only does this allow for up to two degrees misalignment in any plane – reducing stress on the positioner and subsequent tooling – but also, use of this technology improves overall repeatability of the fixture by up to 75 percent, allowing the fixture to relax in its natural state.

Ferris Wheel
Ideal for medium-to large-sized parts that are 3 to 5 m long, the two-station design of the Ferris Wheel positioner offers the best floor space efficiency. Like most two-sided positioners, these popular positioners are generally rated by capacity (i.e., 1,255 kg), which is the total tool and part payload per side of the unit. Due to the size capacities, small footprint and excellent cycle time, they are extremely popular in the automotive parts and similar industries with demanding environments. This positioner type provides two trunnion axes on each side of a major “sweep” axis that turns the positioner in an over-under fashion – much like its namesake. The axes are servo-driven, saving cycle time by allowing parts to turn into position while the trunnion stations rotate over or under the sweep axis. Typically, a central arc screen divides the two sides (or workstations), keeping the operator safe from spatter and arc flash exposure. These two stations allow the operator to be unloading and loading one side, while the part on the other side is being welded.

Turntables and H-Frames
Popular with pre-engineered welding workcells, turntable positioners utilize a base with axis mounted to the floor and a table top where stationary tooling can be added. Likewise, these positioners use an arc screen to divide two sides (or workstations). Unlike Ferris Wheel positioners, turntables rotate in a “lazy Susan” motion to move parts from the operator’s loading/unloading station to the inside of the workcell. Simple small-to-medium sized parts, such as office equipment or automotive suspensions, are ideal for this positioner.

Although parts cannot be moved while welding with a turntable, an H-Frame positioner is the cousin to a traditional turntable where an additional headstock and tailstock on each side of the positioner is used in lieu of a table. One benefit of this positioner style is that another axis can be added where the head and tailstock attach to the floor-mounted base. This allows a part to be turned end-over-end, as well as spun with the headstock while being welded. Medium parts with complex, multi-sided weldments work well with this setup, especially when utilized with coordinated robot motion during welding.

Multi-Axis
Just as the H-Frame can add capability to a regular turntable positioner, other servo axes, headstocks and bases can be combined to make a variety of specialty positioners, such as “tilt-rotate” or “skyhook” positioners. These multi-axis positioners provide the added flexibility to accommodate parts and tooling of different sizes and complexities. Larger, boxy or tubular parts are usually well-suited for these types of positioners, as they enable the welding of a part in the optimum position. For increased access or agility, it is not uncommon to find these positioners mounted to walls, tracks or other surfaces. For ultimate flexibility, a six-axis robot arm can be used as a “jigless” solution to a positioner.

POSITIONER RECOMMENDATIONS
Customer standards require welders to use the best technology possible to achieve high-quality welds. To meet these demands via the robotic welding process, automation experts suggest welding operators use the following:

• Standard coordinated motion. A highly advanced software function ensures consistent coordinated motion between all positioner axes, as well as the welding robot. Not only does this simplify programming, but also, it enables fast and consistent travel speeds, while maintaining the highest weld standards.
• Integrated tooling power, air and communications. Certain hardware accessories give robot users the option to manage multiple controller integrations, reducing redundant wiring and I/O modules. This prevents damage to cables and provides easy troubleshooting.
• Specifications for use. It is important for companies to be sure that the positioners in place are uniquely designed for welding and satisfy current specs for withstanding tough conditions –

1. a) Positioners should be life tested. It is of the utmost importance for a positioner to have the ability to operate at maximum speed for the number of shifts and hours designated.
2. b) Integrated carbon brushes may be needed on positioners for certain applications. This helps provide the long life and high amperage capacity required for multiple robot utilization and pulsed welding.
3. c) The positioner’s width and payload capacity should meet current and future needs. Whether a positioner is to be used for a single job or multiple jobs, the equipment purchased should maintain the best possible sweep and cycle time while in use.

• Optional Functional Safety Unit. A Functional Safety Unit (FSU) for a positioner works with the FSU of a robot controller to define the permissible range of motion. Operator safety is also ensured in the event a worker crosses into a denied area that is protected by light curtains or area scanners. Similarly, the FSU can prevent damage to the positioner or robot if either attempt to leave its defined work space (even by human programming error).

Choosing the right positioner and components to enable a unique welding setup is critical for manufacturers looking to gain the flexibility needed to improve weld quality and decrease cycle times. To do this, decision makers should have a clear understanding of the positioner options and features available. Adapting innovative technologies, such as a positioner equipped with an innovative fixture mounting system, could go a long way to saving time and money, and satisfying stringent customer demands.