Beyond the Bowl
Vibratory bowls have long been the method of choice for feeding parts in high-speed automated assembly systems. But, they're not appropriate for every application. Screws, caps, clips and other durable parts can bounce around in vibratory bowls with little damage. Fragile components, such as painted parts, clear plastic lenses and connectors with delicate leads, may not be so lucky.
"Delicate parts should not be vibrated," says John Van De Vegt, manager of the standard products division of ATS Automation Tooling Systems (Cambridge, ON, Canada). "A good example is the plastic gears for the windshield wiper motor of a car. You don't want the gears to be nicked and dented, because that's going to affect the performance of the motor."
Another drawback of bowl feeders is more subtle. The raison d'?tre of bowl feeders is to singulate and orient individual parts from a random mass. But, asks Nicky Borcea, applications engineer with IPR Automation Sohner Plastics (Weston, CT), wouldn't it be better if the parts arrived at the line already singulated and oriented? "As soon as you lose orientation on a part, it costs you money to reorient it," he points out.
Feeding parts from custom trays solves both problems. Moreover, trays can be integrated into both sides of the assembler's supply chain.
"Trays are usually part of a system, starting from the moment that a part is made," says Borcea. At the parts supplier, a robot loads freshly machined or molded parts directly into trays. At the assembly line, a robot unloads the trays. The empty trays are then returned to the supplier, or brought to the end of the line, where another robot loads them with the finished product or subassembly.
"The pockets in the tray can be designed to accommodate intermediate steps in the machining or assembly process," explains Borcea. "So if nuts, bolts or other parts are added to the assembly, it will still fit in the tray."
Similarly, tray pockets can be designed to accept two to four different parts or products, as long as they are similar in size and shape.
In addition, trays are typically sized to fit on standard 45-by-48-inch shipping pallets. For example, a standard pallet can carry four trays sized 22.5 by 24 inches, six trays sized 15 by 24 inches or eight trays sized 12 by 22.5 inches.
When the product is no longer manufactured, the trays can be recycled.
Trays can be made from aluminum or plastic, such as high-density polyethylene and ABS. Plastic trays can be made through injection-molding or thermoforming. Which material or process to use depends on the application. There are conductive plastics for static-sensitive components; sterile plastics for medical devices; and temperature-resistant plastics for heat-treating applications.
For precision automation applications, Borcea prefers thermoformed ABS, because it holds its shape despite temperature fluctuations and aging. Trays made from virgin ABS will be more accurate than trays made from utility-grade ABS, but virgin material is more expensive. "With ABS, we can maintain a tolerance of ±0.005 inch from pocket to pocket," says Borcea. "Trays produced to a tolerance of ±0.125 inch are fine for shipping and manual loading, but they're not good for automation."
The accuracy of the trays is an important consideration, says Jim Story, president of Spectra Technologies (Euless, TX). "The robot will not put anything down more accurately than it picks it up," he explains. "If the part is 0.01 inch out of registration when the robot picks it up, it's going to be 0.01 inch out of registration when the robot puts it down. So the accuracy requirements for placement will determine how accurately you need to make your tray."
If assemblers don't want to foot the bill for highly accurate trays, there are two ways around the problem, says Story. One is to equip the robot with vision guidance. The other is to have the robot place the part in an intermediate, box-like fixture that is more accurate than the tray. Two walls of the box are 90 degrees apart and fixed. The opposing walls are moveable. When a loose part is placed in the fixture, the moveable walls close in, pushing the part against the fixed walls. "Now the robot knows exactly where the part is," says Story. "However, this only works if you've got the cycle time to make that intermediate move."
In addition to specifying the accuracy of the tray, engineers should ensure that there's enough space between tray pockets to provide clearance for the robot's gripper. Trays can be loaded or unloaded by a Cartesian, SCARA or six-axis robot.
Many assemblers use plastic trays in different colors to distinguish one batch of parts from another. This can be particularly useful if differences between parts are not immediately apparent. For example, parts that have been heat-treated can be put in a different color tray than parts that have not. Automotive suppliers with multiple OEM customers often use this strategy.
Tray handlers typically store several trays. How many trays a handler should store depends on the cycle time of the assembly system and how long engineers want it to run without human intervention, says Story.
With the SpectraFlex tray handler from Spectra Technologies, trays are manually loaded into slots on a pair of parallel vertical belts. A single tray is lowered onto a conveyor and transferred to the load position. After loading or unloading, the tray is transferred to the stack position, where a second set of vertical belts lifts it off the conveyor. Each tray tower on the standard model holds 35 trays that are 14 inches wide, 20 inches long and 1 inch tall.
A gantry-style robot is integrated into the tray handler from ATS. Trays are stored on a pair of wheeled carts that hold approximately 24 inches of stacked trays, depending on the height and configuration of the conveyor. "The robot is set up to cover both sets of trays, so you don't lose production time while you're transferring trays from one stack to the other," says Van De Vegt.
Like bowl feeders, tray feeding systems are not for every application and should be used only when necessary, notes Van De Vegt. A tray feeding system, including trays, tray handler and robot, is considerably more expensive than a bowl feeder. It also requires more floor space.