Get a Grip on the Basics of Robot End-of-Arm Tooling
By Trent P. Fisher

Even the best robot is only as good as its end-of-arm tooling (EOAT). EOAT that fails to do its job drives up reject rates as it drops your parts. It misses sprues, potentially damaging valuable tooling. It causes robots to wear out prematurely. In short, it generates productivity problems that nibble away at the very economic benefits that attracted you to robotics in the first place. Making matters worse, EOAT isn't cheap. At $1000 to $10,000 per set, the EOAT investment can rival that of a brand-new robot.

While molders will often spend hours deciding on a robot purchase, many give little thought to the EOAT that will go on it. What they end up with is inflexible tooling systems that weigh too much and take up too much space. Even worse, they often end up with EOAT that simply doesn't work. There's no such thing as "do-all" robot tooling.

The good news is that selecting the right EOAT for your application can be easy if you ask the right questions and keep a few simple considerations in mind.

WHICH IS THE RIGHT GRIPPER?

No one gripper style will secure every part. The more types of parts you mold, the greater your need for a wide variety of gripper styles. Make sure your EOAT vendor offers all the components you'll need in standard sizes, so you can mix and match appropriate grippers, arms, and frame elements. Also make sure your vendor is familiar with a wide variety of robot manufacturers.

This modular, standardized approach is what lets you tackle the toughest parts-removal applications without relying on expensive custom work. For instance, some auto bumpers must be "flexed" out of the tool and require a combination of gripper styles (see figure 1).

Vacuum cups are the first choice for most applications, and there is a huge variety of styles to choose from. Shapes include oval and round and sizes can range from 0.5 in. up to 4 in. diameter. They can be made of Viton fluoroelastomer, silicone, other rubbers, or polyurethane. Picking the right cup material, size, and shape is where experience—and some trial and error—comes in.

Cups typically engage in one of three ways: at the front or side of the part or at an angle. For side engagements, short-stroke pneumatic cylinders advance the cups to grip the part after the EOAT frame moves into position. To engage parts at an angle, choose a bellows cup (see figure 2).

For parts that want to fly off the mold, or that run at fast cycles, spring arms let you get the cups in place before ejection and then accept the part like a mitt accepts a baseball. Hexshaped gripper-arm profiles will keep grippers from turning when their orientation to the part must remain fixed.

Even with all the variety available in cups, they won't work in every application. Sometimes holes in the part preclude use of vacuum cups. Cups also can cause cosmetic problems on Class A surfaces. And rough surfaces may not work well with cups, either. In any of these cases, you'll need alternative ways to secure parts—which is where pliers and fingers come into play.

Pliers, while normally used to grab a sprue, can also be used to grab the part itself. Customized inserts can make the pliers' jaws match the contours of the part. Other plier options include electronic sensing to make sure the part is present before the robot moves away from the mold.

Fingers, which look like a half-pair of pliers, are used to latch onto the side of the part. They're also more secure than a vacuum cup.

For many applications, you'll need sprue cutters. Look for ones that can be mounted on whatever frame system you use. Although there are many types of cutters, not all of them can be mounted easily.

ADJUSTABILITY IS KEY

No matter how good your EOAT and no matter who built it, you will always need to adjust it. Adjustability protects your investment by allowing you to re-use tooling components on more than one part. Features for easy adjustability should be built in, and adjustments should be as easy as turning a bolt or a screw-in gripper.

What components should be adjustable? All grippers—whether cups, pliers, or fingers—should be able to attach at any location along the EOAT frame. Slotted frames are one way to ensure such mounting freedom.

Once the gripper arm is clamped to the frame, you should also be able to adjust it up and down, rotate the gripper in its clamp, and change the angle of the gripper.

SIZING TIPS FOR MAKING IT FIT

It's always surprising to see how many molders EOAT that doesn't fit well—or at all. Before buying even a dime's worth of tooling, wise molders compare the proposed EOAT with drawings of the part and mold to ensure a good fit.

When it does fit well, EOAT lines up with the mold's center line. As a rule of thumb, the tooling frame should also be as large as the part to be removed. In a multicavity tool, the frame should be big enough to reach the outer limits of all the cavities. If you build the frame too small, you may not have the range of adjustments that you might need later. So to specify the right size frame, you will need to know the mold dimensions as well as the part spacing and orientation.

You'll also need to know just how far the mold opens to make sure the EOAT won't run afoul of the mold. Since it must fit into a confined space, EOAT should be compact and as low-profile as possible. Clamps that can mount helnw nr on the side of the frame can help herc.

KEEP THE WEIGHT DOWN

The EOAT and the part weight together must not exceed robot capacity. That may sound obvious, but there will be a trail of worn-out robot parts leading to the door of any molder who forgets this fact. Most plastics molding robots are lesigned for relatively lightweight parts, but heavy EOAT causes excess wear on bearings, guide rails, EOAT and other robot components.

If you want your robot to last, choose tooling that's as light as possible. Favor open frames constructed from hollow aluminum extrusions. And look for other hollowed-out components wherever feasible. Gripper arms, for instance, can be bored out to shave a few ounces.

Keeping the weight down, however, doesn't mean scrimping on the frame size. Even if it adds some weight, the frame must still be big enough to cover the parts.

READY FOR QUICK CHANGES?

Custom molders may have quick-mold-change capabilities, but those won't help much if the robot tooling still takes a couple of hours to change. If you change jobs frequently, your EOAT systems should have quick-change provisions.

For instance, one solution is to have a quickchange chuck permanently attached to the robot. All the EOAT is affixed to a dove-tail that slides in and out of the chuck. All air connections are made directly to the chuck. With this arrangement, changing EOAT involves sliding in the plate and tightening a screw. Then plug in the quick-disconnect air connections. It takes a few minutes. 
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Reprinted from September 1997 issue of 
PT Auxiliaries, a supplement of Plastics Technology Magazine, a publication of the 
Bill Communications, Inc.