Error-Proofing Your Assembly: Part 2
When assemblers talk about the Japanese concept of poka-yoke (pronounced POH-kah YOH-kay), or mistake-proofing, the first thing that comes to mind is often manual assembly. Error-proofing lore is replete with simple solutions to vexing quality control problems, like adding a simple feature to a part so it becomes obvious if it hasn't been placed in a fixture the right way.
However, there is also a high-tech approach to error proofing. Electric fastening tools, servo-driven presses, sensors and process-monitoring software not only make it possible to control process parameters-they also serve to wave a red flag when an assembly isn't done right.
In the area of threaded fastener installation, for example, a screw presenter can be fitted with a photoelectric sensor that monitors each time a screw has been picked up. If no screw has been picked up, the tool will not cycle.
In more complicated assemblies where the operator must install more than one type of screw, multiple screw presenters can work in concert to ensure that the right screws are installed in the right holes. If, for example, the operator picks up a fastener from "Screw Presenter B" without installing all the required fasteners from "Screw Presenter A," the controller can sound an alarm or even deactivate the tool.
Another way to prevent errors during screwdriving is to build intelligence into the tool, something that can be done with both electric and pneumatic systems. For example, the Minimat-F handheld screwdriver from Deprag Inc. (Lewisville, TX) is a pneumatic system that has a variety of functions to prevent mistakes. It counts screwdriving cycles, monitors assembly times, recognizes part changes and shuts off automatically when a set torque is reached.
"If you know it takes between 0.8 and 1.1 seconds to drive a screw, you can set those times as your operating window," says Deprag general manager Jan Aijkens. "If the assembly time is shorter than 0.8 second or longer than 1.1 seconds, you know there's a problem."
Taking its error-proofing capabilities to an even higher level, the tool can be linked to a switch-equipped fixture that keeps track of the number of fasteners installed in each part. Each time the fixture is loaded, the counter and timer reset, and a new cycle begins. The switch can be a sensor inside the fixture itself, or it can be a switch that is activated when a hinged template closes on the part. Note that in addition to activating the switch, the template provides visual cues to the operator, helps the operator guide the bit into each hole, and protects the finish on the part from being scratched by errant bits.
Of course, using today's "smart" electric tools, assemblers can monitor a whole host of parameters, to ensure that fasteners are correctly installed. For example, DC nutrunners and controllers from AIMCO (Portland, OR) include digital torque-value displays and green "pass" light signals. They can also detect cross-threading or stripped fasteners.
Electric tools like these can even be integrated with an encoder-equipped tool arm, so that the controller not only monitors a fastener's installation parameters, but the point at which it was installed in 3-D space and the sequence in which the different fasteners were installed. This system, although pricey, can provide the ultimate in quality control in a workcell environment where precision is critical.
Of course, threaded fastener installation is not only area where electronics and process monitoring technologies can help assemblers quickly catch mistakes. Pressing, riveting and radial and orbital forming systems can all be equipped with sensors and controllers that provide real-time feedback and warnings when an assembly isn't performed correctly. For example, an orbital forming machine can be equipped with a strain gauge to measure forming pressure and a linear potentiometer to measure forming distance. Together, these sensors detect rivets or bosses that are too long or too short, too hard or too soft, explains Werner R. Stutz, vice president of Taumel Assembly Systems (Patterson, NY).
Along these same lines, using process monitoring systems like an Electro-Mechanical Assembly Press or a Pro monitor and test system from Promess Inc. (Brighton, MI) assemblers cannot only install press fittings to extremely tight tolerances, but monitor the relationships between multiple parameters like force and position. In the areas of ultrasonic welding of plastics and metals, and heat welding and staking of plastics, sensor-equipped machines can track collapse distances, time, energy, force and temperature to ensure precise attachments-and let an operator know immediately when something goes wrong.
Poka-Yoke and Automation
Automation is often lauded as the best way to eliminate mistakes during assembly. But, in the real world mistakes are inevitable, so a typical high-speed automated assembly system is equipped with numerous devices to verify that was supposed to happen at a particular station, actually did happen.
"You must have checks in the line," says Paul Beduze, business development manager at Mikron Corp. Denver (Aurora, CO). "Anything can happen: A part may not load correctly. A gripper may drop a part. You don't want to do 10 operations on an assembly only to discover at the end of the line that a part was out of line or it didn't feed. You don't want to add value to a defective product."
Verification technologies range from simple sensors that merely detect the presence of a part, to complex machine vision systems that take highly accurate dimensional measurements at production speed. One of the simplest verification technologies is a touch probe. When a pallet arrives at the station, the probe extends to touch the part. If the probe stops too low, the machine knows the fixture is empty. If the probe stops too high, the machine knows the part is not fully seated in its nest.
Similarly, machine vision systems can be used to detect the presence or absence of an assembly's various parts, or a leak detector can be used to check the integrity of an air- or watertight seal. Whatever the case, the tester will signal the system controller to stop any further work being done on the problem assembly or separate it for rejection or rework.
Ultimately, a battery of checks are an integral part of an assembly system. For example, feeder bowls are a type of error-proofing device. The bowls ensure that parts enter the system in the correct orientation, and they often weed out defective parts. A sensor at the end of the feed track can perform one last verification, such as detecting a part that hangs too low on the track. Another sensor can tell the system that a part is present and ready for pick up.
Verification devices may increase the size of an assembly system, but they should not affect cycle time. "I've never had a customer say there were too many checks," says Beduze.