The Fabricator - August 2009

CFIGURE 3 The swing diameter of this headstock and tailstock positioner (end view shown) allows room for the entire workpiece and fixture components.

CFIGURE 4 Shifting the unit downward compensates for center of gravity issues. However, the shift downward also causes the edges to move slightly outside the swing diameter.

CFIGURE 5 This setup satisfies both center-of-gravity and swing-diameter requirements.

Visit www.thefabricator.com and type these article titles in the search box:

A “Fixturing on display: POP display fabricator integrates robotic workcell efficient for small runs”

A “Moving to modular welding fixturing”

A “Getting the needed fixtures”

Want more information?

line of the headstock/tailstock positioner, the more torque (or horsepower) will be required to roll the fixture and the part. This usually means the fixture and part need to shift downward to maintain the CG near the positioner centerline. Looking at Figure 3, you can see that although there is no problem with the swing diameter, the CG is too high. Figure 4 shows how the unit can be shifted downward to improve the CG, but now the fixture is outside the swing diameter. In this case, you must adjust the fixtures to maintain the proper CG while still working within the envelope of the swing diameter, as shown in Figure 5.

Turntable positioners—also called indexing or lazy Susan positioners— generally have two or more stations (see Figure 6). One is for part loading/un-loading, while the other station is for robotic welding. When welding is complete, the table indexes, and the process repeats. The diameter of the table will affect the size and number of parts that can be fixtured simultaneously. And like the headstock/tailstock positioner, turntable systems must also handle the weight of both the part and the fixturing.

Stationary positioners do not move the parts being held, but rather allow the robot to move (sometimes down a track) and perform welding while the parts are held in place (see Figure 7). For the most efficient setup, ensure

that the robot can reach all of the weld joints without excessive handling and part repositioning.

Modular Fixturing

Modular fixturing allows you to fixture parts in various configurations consistently—important for robotic welding systems, which expect parts to be in the same position every time. Using a modular system allows for flexibility: welding one assembly, changing to another, changing back to the original, and so on. Such fixtures have repeatability within ±0.005 inch.

your environment and process. By taking an objective look at your assemblies, you should be able to define or refine your process to maximize efficiency, and answer critical questions about your fixturing before you buy your system:

• Can I load loose parts into the robot, or will I need tack fixtures to optimize the process?

• Can the positioner support the weight of the parts and tooling?

•Is the swing diameter of the positioner adequate to support the dimensions of my parts and tooling?

• Do changes in the center of gravity need to be accounted for in the fixturing?

•Is the span of the positioner adequate to support the dimensions of my parts and tooling?

Although critical to the success of the entire system, fixturing is sometimes overlooked. Ideally, it should be considered during the first stages of specifying the system requirements. To

CFIGURE 6 This turntable positioner allows one part to be loaded while another is being welded.

The Goal of Effective Fixturing

The goal should be to ensure that you end up with a system optimized for

ensure a successful robotic welding process integration, it’s essential to consider how fixturing will affect the specifications of the overall system. Simply put, the more consistently fixtured the part, the better a robot can weld it. ■