To err is human; to prevent errors is the engineer's job.

"The best laid schemes o' mice and men gang aft a-gley," wrote Robert Burns in To a Mouse. Assemblers who aren't familiar with the works of the 18th century Scottish poet surely know a more contemporary American truism: Whatever can go wrong, will go wrong.

Indeed, on the assembly line, old Murphy has a host of able-bodied assistants: fatigue, inattentiveness, time pressure, and, sadly, even carelessness. What's more, Murphy costs manufacturers money. According to ASSEMBLY's eighth annual capital equipment spending survey, 41 percent of assembly plants will buy equipment this year to reduce the cost of scrap and rework. Another 22 percent will buy equipment to cut material costs.

To circumvent Murphy, U.S. manufacturers are embracing the Japanese concept of poka-yoke (pronounced POH-kah YOH-kay), or mistake-proofing. Poka-yoke devices are mechanisms that prevent mistakes from being made or that make mistakes obvious at a glance. Besides reducing the cost of scrap, rework and warranty claims, mistake-proofing provides a less obvious benefit: It gives workers time to think about how to improve the assembly process. Instead of acting as mere extensions of their machines, operators are free to manage their environment.

The most common sources of product defects are errors in setting up workpieces and equipment, omitted processing steps, processing errors, missing parts, and wrong parts. More often than not, the devices for preventing these errors are simple and inexpensive. In some cases, errors can be prevented by changing the shape of the part or the fixture. In others, electronics come into play. A sensor can be installed to ensure that an assembly process won't begin until the correct part has been installed in the correct position. Or, a limit switch can be used to make certain that the correct number of assembly steps has been performed.

A classic example of a poka-yoke device is a parts kit. All the parts required for an assembly are loaded into the kit, which is typically a thermoformed plastic tray. Cavities in the tray are designed to match the shape or number of parts in the assembly. Thus, before assembly, the operator knows if any parts are missing. And, if any parts are left over in the tray after assembly, the operator instantly knows that a mistake has been made.

In the book Poka-Yoke: Improving Product Quality by Preventing Mistakes (Productivity Press, 1988), editor Nikkan Kogyo Shimbun offers an example of how product design can be modified to prevent mistakes. An electrical products company made a timer switch that was equipped with a frequency selection mechanism so it could be used in areas with either 50- or 60-hertz power. The 50- and 60-hertz gears were installed next to each other on the same shaft. The only difference between the two gears was three teeth, making it difficult to tell the gears apart with the naked eye. And because the gears fit interchangeably on the shaft, assembly errors were common.

To error-proof the assembly, engineers modified the holes in the gears and the shape of the shaft to that the gears could only fit on the shaft one way. In addition, the 50-hertz gear was molded in white plastic, and the 60-hertz gear was molded in blue plastic, so they could be identified at a glance. Installation errors were eliminated at no increase in product cost.

Sometimes, mistakes can be prevented not by looking at specific assembly designs or procedures, but by re-examining the entire line, says Robert J. Simmons, vice president of Pro-Line (Haverhill, MA). "With modular workstations, you can set up your line for progressive assembly," he explains. "Each operator is responsible for adding one component of the assembly. That helps with error-proofing, because each operator has only one task to do and he can be trained to do it very well."

Simply becoming better organized can also be a big help in reducing errors, says Simmons. For example, a workstation's uprights can be equipped with clearly marked, color-coded part bins. Document holders can display assembly drawings or instructions. "Some manufacturers mount a flat-screen computer monitor onto the uprights of a workstation," Simmons adds. "Each step in the assembly process comes up on the screen. Workers access assembly information through a touch-screen menu. It's all right there in front of them."


According to ASSEMBLY's capital spending survey, 91 percent of plants that use threaded fasteners in their products install those fasteners manually. To Jan Aijkens, general manager at Deprag Inc., that's an open invitation for mistakes. Operators who feel pressure to meet output can easily forget to install a fastener. Worse, they might intentionally skip over fasteners, misadjust their screwdrivers, or tamper with quality control measures. "The only way to be 100 percent foolproof is to automate," says Aijkens.

Still, there are ways to prevent mistakes during manual screwdriving operations. To begin with, engineers should analyze their product designs to minimize the number of different screws used in the assemblies. It's also a good idea to set up the line so that one operator is responsible for installing one type of screw. This will reduce the possibility that an operator could install the wrong screw.

Electronics can also help. For example, a screw presenter can be fitted with a photoelectric sensor that will monitor each time a screw has been picked up. If no screw has been picked up, the tool will not cycle. If the operator must install more than one type of screw, the 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. For example, Deprag's Minimat-F handheld screwdriver 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," explains Aijkens. "If the assembly time is shorter than 0.8 second or longer than 1.1 seconds, you know there's a problem."

Similarly, if the assembly requires four screws, and only three are installed before the product is removed from the fixture, the tool will deactivate and an alarm will sound.

The tool works in conjunction with a switch inside the part fixture. 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 parts. Besides activating the switch, the template serves several other functions. It provides visual cues to the operator, and its beveled openings help the operator guide the bit into each hole. The template also protects the finish on the part from being scratched by errant bits.

Another error-proofing option for screwdriving is a tool support arm that provides positional feedback, such as the Intelli-Arm from Automation Tool Co. (Cookeville, TN). When coupled with an electronic control system, this feedback allows engineers to control actuation of the screwdriver based on its position in space. The device ensures that a fastener is installed at each position in an assembly and that the fasteners are installed in the correct sequence.

Electronics also play a role in error-proofing other fastening operations, such as pressing, riveting, and radial and orbital forming. 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).

"If the pressure and dimensional criteria are not met, the part can be checked, or, in an automated system, the part can be rejected," he says.

Poka-Yoke and Automation

Automation is often lauded as the best way to eliminate mistakes during assembly. Nevertheless, examine any multistation automated assembly system, and you'll see evidence of the Cold War adage: "Trust, but verify." Indeed, error-proofing techniques are not limited to manual assembly operations. 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. In either case, the probe signals the system controller to ignore the problem pallet. If the part is present, all downstream operations will be performed on the pallet.

Many checkpoints are an integral part of an assembly system. For example, feeder bowls are a type of error-proofing device, Beduze points out. 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.

Wire Processing

Assembling a wire harness for even the simplest electronic device presents myriad opportunities for errors. Assemblers could enter incorrect information into automatic wire cutting and stripping equipment. They could choose the wrong terminal for an assembly, or they could route a wire to the wrong pin on a connector. Or, they might "adjust" a machine's controls in a misguided attempt to increase its output.

"Anyone who is paid by the piece will try to boost the output of that machine as much as possible," says Peter Waas, engineering manager at Komax Corp. (Buffalo Grove, IL).

Fortunately, manufacturers of wire processing equipment have devised many ways to prevent errors. For example, to prevent data entry errors, automatic wire processing equipment can be networked. "With networked equipment, an engineer or supervisor only has to enter the numbers once from his computer, and that information would then be transmitted to all the machines on the shop floor," says Waas. "Networking also prevents mistakes when there are engineering changes, which do not always get communicated to all the operators."

Bar codes can prevent operators from choosing the wrong wire or terminal for an assembly. Before starting an operation, the worker would scan codes on the bill of materials, wire spools, terminal reels and applicators. "If the parts don't match what's on the bill of materials, the machine won't start," says Waas.

Similarly, color codes can be used to match terminals with their applicators. For example, a green dot can be placed on an applicator and all the reels of terminals associated with it. "But," warns Waas, "that only works if you work with a limited number of terminals."

Crimp height and pull force are critical measures of the quality of crimped connections, and such test equipment can be integrated into many fully automatic wire processing systems. Measurements of crimp height, for example, can be automatically transmitted to the crimping press via a serial interface, and the data can be stored in a log. "If the crimp height does not match the predefined specification, the machine will automatically adjust itself," says Waas. "The machine will not start full production until it achieves the correct height."

Wire color and patterns have long been used to help workers route wires during harness assembly, but mistake-proofing doesn't end there. For example, HellermannTyton (Milwaukee) supplies equipment to print custom wire labels on demand. The labels can be printed with alphanumeric characters as well as 1D and 2D codes. Alternatively, an ink-jet printer can be used to mark termination information directly on the wire, says Peter Doyon, vice president of product management at Schleuniger Inc. (Manchester, NH).

The Easy-Wire system from Cirris Systems Corp. (Salt Lake City) guides operators through the process of assembling and testing wire harnesses and backplanes. The system consists of a tester, a grid board and modular, snap-in components for holding wires and connectors. A PC displays a schematic of the harness and assembly instructions. Visual and audible signals guide the operator to the correct positions on the board. Once the operator terminates one wire, the system automatically indexes to the next one.

"The nice thing about that type of system is that assembly and testing take place at the same time," Doyon points out.

Do You Poka-Yoke?

How have you error-proofed your process? Share your ideas with your colleagues! Send us your poka-yoke success stories, and we'll publish the best ones in a subsequent issue of ASSEMBLY. Send your stories to Thanks!