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Welcome to the world of “positioneering”

To find the right welding positioner for a job, a fabricator needs to ask the right questions

A positioner makes a welder’s job easier, no matter the project’s size.

Everyone knows what a welding positioner is, but do they know about “positioneering”—the engineering science behind positioning weldments?

More than likely, most fabricators don’t give it a lot of thought. They are more focused on producing quality parts and meeting delivery deadlines. They often lean on fabricator skill and overtime to complete complex jobs rather than accessories that could result in less rework.

That’s where welding positioners can make a big difference. As the device mechanically manipulates the workpiece, the welder can access areas that previously would have been difficult to reach. Additionally, multiple assembly points can be ergonomically positioned.

These in-position welds contribute to less spatter and cleanup because the welder doesn’t have to struggle with technique in difficult-to-reach areas. The positioner has the workpiece in the proper position, and the welder can work with gravity, welding in a downward motion. Weld size is likely to be more consistent, and deposition is greater.

The elimination of manual positioning also contributes to safety and production efficiency. A positioner reduces the need for crane use and rigging, which eliminates the time and risk associated with this type of tricky material movement. The positioner holds and positions even the largest of loads without chains and straps.

A positioners can do all of these things—if it is the correct tool for the job. Unfortunately, positioners normally are not designed around an existing part. In fact, the part normally is not designed to be manufactured with a positioner.

That’s where positioneering can help. It can deliver a positioner that simplifies manufacturing of a part and possibly provide assistance with other projects down the line.

Know What Position the Positioner Needs to Be In

Before learning what influences a positioner design, a fabricator needs to know the options available for positioning a workpiece. The four basic styles are:

  • Manual Positioners. These are typically for lighter workpieces, but can handle up to 4,000 lbs. (see Figure 1). Many work under the universal balance principle, in which the rotational axis center of gravity intersects with that of the tilt axis. When that occurs, workpieces can be rotated 360 degrees around both axes. A welder then can maneuver a workpiece with little effort, perhaps without the need to break an arc.
  • Floor Turntables. These positioners are pretty self-explanatory; they rotate parallel to the shop floor (see Figure 2). These turntables can handle capacities from 250 to 40,000 lbs. Because they often are used with semiautomated welding equipment, the latest drive technology is necessary to ensure smooth acceleration and deceleration control. Expecting the rotational axis to be repeatable within ±0.005 inches per inch of radius is not unusual.
  • Head- and Tailstock Equipment. This positioning equipment—consisting of a powered rotational headstock and a nonpowered tailstock—is for large, awkward weldments (see Figure 3). The rotary table is perpendicular to the floor. It comes in fixed and adjustable heights. Some of these positioner setups can accommodate as much as 240,000 lbs. between the head- and tailstock.
  • Rotate-and-Tilt Positioners. These tables rotate to assist with welding or assembly activities and tilt for different points of access (see Figure 4). Variable-speed drives and motors deliver precise movement for smaller positioner units, and larger devices rely upon gear-driven systems. The size of these systems ranges from a benchtop unit to a heavy-weight positioner capable of moving a 1-million-lb. workpiece.

The rotate-and-tilt positioner is most frequently found in metal fabricating operations because of its ability to work with weldments of different sizes, if specified correctly. Factors such as part size, application, center-of-gravity location, and fixturing, however, apply to the selection of any type of positioner.

Know the Application

What is the process? Is it welding, assembly, or machining? This determines what type of access the fabricator needs and what orientation makes sense to present the part.

Figure 1
Manual positioners are designed for lighter workpieces to which the welder needs full access.

For example, if the process is welding, not only does the fabricator have to accommodate the welder or welding equipment, but also how the delivery of gas and wire to the workpiece is done. Also, different welding processes call for different speeds of rotation, which involves a discussion of drive technology and accuracy.

Know the Part’s Weight

Simply stated, if a fabricator has a 10,000-lb. part, it needs a 10,000-lb.-rated positioner. But that’s a short-sighted decision, because a positioner loaded to its maximum capacity 100 percent of the time will have a reduced lifespan.

The fabricator wants to consider the largest part it is going to place on the positioner but also think about future growth—possibly even larger parts. This gives the company the production answer it needs for current jobs and keeps the door open for both lighter and heavier jobs.

Part weight also influences the fixtures and mounting plates that are required to affix the workpiece to the positioner.

Know the Part’s Center of Gravity

The center of gravity is defined as the point that is exactly in the center of a symmetrical weldment. A properly defined center of gravity ensures that the positioner is able to move the part that is placed upon it because the tilt torque and rotational torque are correctly rated for the machine.

Example A is a workpiece with two ends and two sides made of 1-in. steel plate. Each end is 1 by 12 by 12 in. and weighs 40.8 lbs. Each side is 1 by 12 by 34 in. and weighs 115.6 lbs. All parts of the weldment add up to 312.8 lbs.

If one end is secured to the table of a gear-driven positioner, the center of gravity is 18 in. out from or above the table. If either side or the open top or bottom is secured to the table, the center of gravity is 6 in. above it.

On the other hand, Example B is an asymmetrical workpiece. In this instance, a 2-in.-thick bottom is added to Example A. The 2- by 12- by 36-in. box weighs 557.6 lbs. The center of gravity, however, is not 7 in., which is halfway from the top or bottom. Because nothing was added to the top to balance the weight of the bottom, a fabricator can’t quickly assume what the center of gravity is based on calculations done for Example A.

The center of gravity always moves toward any part that is added to a symmetrical weldment. Locating this center of gravity is a function of moments, which takes into account force or the tendency to produce motion. A moment is weight multiplied by length (the arm). To help determine the moment, a reference plane is needed; in this instance, the top surface of the positioner’s table acts as the reference plane. (When these calculations are being done, fabricators should always plan to mount the weldment’s heavy side to the table.)

Each separate symmetrical component has a known weight and center of gravity, and the center of gravity for the entire part is a known distance from the tabletop surface. Multiplying weight by distance (arm) gives the pound-inch moment of the part. Adding all the moments and dividing by total weight gives the location of the center of gravity of the weldment.

Referring back to Example B, a fabricator doesn’t need to focus on each side separately because the sides and ends have similar mass, weighing 312.8 lbs., with the center of gravity in the center of that mass. With the 2-in. plate between it and the table, the center of gravity for that column (the sides and the ends) is 8 in. above the table. Multiplying the 312.8 lbs. by 8 in. results in a 2,502.4 lb.-in. moment of the part.

Figure 2
Rotary tables operate just as the name suggests. They typically are used with semiautomated welding applications in which consistent part speed is necessary to ensure consistent welds.

The 244.8-lb. bottom’s center of gravity is 1 in. above the table. The gives it a 244.8 lb.-in. moment.

Adding 244.8 lb.-in. and 2,502.4 lb.-in. produces a 2,747.2 lb.-in. total moment. Dividing 2,747.2 lb.-in. total moment by 557.6 lb. (the total weldment weight) locates the center of gravity as being 4.9 in. above the tabletop surface.

This calculation is not a one-time exercise. Anytime a part or assembly is added to the base weldment, the center of gravity has to be recalculated to ensure the positioner is rated correctly for the job.

Know the Part’s Dimensions

It seems like common knowledge, but sometimes a company fails to consider leaving enough clearance to swing the work above the floor when the table is in the full-tilt position. The decision-maker doesn’t take into account the entire swing radius, but rather focuses on a limited movement range.

Positioners can be designed to have manually adjusted or powered-elevated bases to adjust the table height for complete swing radius of a part.

In the case of a fixed-based machine, a shop can alter the elevation with the use of a riser.

Knowledge Is Power

These guidelines can help a fabricator begin the search for the welding positioner best-suited for a job—and possibly other jobs. Of course, it is only the beginning.

It behooves any fabricator to consult with the welders that will be working with the positioner. Sometimes the reality gap between front-office engineers and shop floor workers is pretty significant. The welders will provide a real-world description of just how they need to interact with the part, which will result in a capital equipment investment that is guaranteed to increase production efficiency instead of shop floor frustration.

Additionally, several options for a welding positioner need to be considered. What type of controls are needed? What safety features should be included in the final system installation?

Fortunately, positioneering is not a black art. By gathering the right data and asking some simple questions, a fabricator can find the right positioning table for current and future jobs.

About the Author

Don Burgart

Welding Product Manager

635 W. Main St.

Arcade, NY 14009

585-492-2400