According to Murphy’s Law, whatever can go wrong, will. Here’s how engineers are getting around that and banishing mistakes from the assembly process:
During the assembly process, many factors make Murphy’s Law—which says that things that can go wrong, will go wrong—seem all the more inevitable. They range from operator fatigue to inattention to being in too much of a hurry. Most commonly, product defects result from not setting up workpieces and equipment correctly, skipping processing steps, messing up processing, missing parts, and selecting the wrong parts. And manufacturers pay dearly for such mistakes.
A recent survey by Assembly Magazine on annual capital equipment spending found that 41% of assembly plants are planning to purchase equipment this year to lower the cost of scrap and rework, while another 15% will snap up equipment to reduce material costs.
Utilizing Poka-Yoke
To outwit Murphy’s Law, U.S. manufacturers are relying on poka-yoke (pronounced POH-kah YOH-kay)—the Japanese approach to error-proofing. Poka-yoke mechanisms stop mistakes from happening or make them very conspicuous, thereby lessening the cost of scrap, rework, and warranty claims.
What’s more, they let workers survey the whole assembly process and come up with ways to improve it. And most poka-yoke devices are simple and affordable. They include electronic devices that do not activate the assembly process until the right part has been installed in the right position as well as precautions such as changing the shape of the part or the fixture so that things could only fit together in the correct manner.
Aside from making adjustments to particular assembly designs or procedures, manufacturers can also guard against mistakes by reconsidering the entire line, notes Robert J. Simmons, vice president of Pro-Line in Massachusetts. For example, manufacturers with modular workstations in place could adopt progressive assembly, in which each operator adds a single component of the assembly.
In addition, getting better organized could circumvent errors, he observes. For example, clearly labeling and color-coding part bins can prevent parts from getting misplaced, while prominently displaying assembly drawings or instructions makes important information readily available to operators.
Fastening is one area where the concept of poka-yoke can yield huge gains. Assembly Magazine’s capital spending survey found that 91% of plants using threaded fasteners in their products mount these fasteners manually, making them potential breeding grounds for operator mistakes. To error-proof manual screw driving operations, engineers can reduce the number of different screws required by assemblies as well as set up the line so that one operator takes care of installing one type of screw. Another effective error-proofing technique is affixing photoelectric sensors to screw presenters. Such sensors can track if screws are in place or if the correct screws are in the correct holes.
Applying Intelligence to Assembly
Manufacturers could also choose tools with built-in intelligence, such as Deprag’s Minimat-F, a handheld screwdriver that can keep track of cycles and assembly times, distinguish part changes and automatically power off based on torque. Also handy are tool support arms, which give feedback on position. Such electronic devices work well with other fastening operations as well, including pressing, riveting, and radial and orbital forming.
Error-proofing delivers benefits not only in manual assembly operations but in automated systems as well. In such lines, poka-yoke mechanisms can serve as verification that the correct processes have taken place at each station. Such checks could include sensors that determine if a part is present as well as sophisticated machine vision systems that can precisely measure dimensions at production speed. Also effective in preventing mistakes are touch probes, which can ensure that parts are completely seated in their nests, and feeder bowls, which can make certain that parts have the correct orientation.
In short, Murphy’s Law doesn’t have to rule the assembly line. By embracing the Japanese concept of error-proofing, manufacturing engineers can drive mistakes out of the assembly process.