Page 33 - MetalForming June 2009

Had to be Mistake-Proof, Economical
Upon being awarded the CRH con- tract in 2007, Aaron Volz, manufactur- ing engineering manager at Orchid Mount Juliet, guided his team toward a goal of creating an assembly process with multiple lines of defense to ensure mistake-proof production.
“We began by considering what could go wrong throughout every stage of the process, and then set out to build a staking assembly cell with the neces- sary features to prevent these potential errors,” Volz says.
The team also sought to build a sys- tem as economically as possible, result- ing in cost savings for Orchid and CRH.
“Our staking cell combines various time-proven technological tools,” Volz emphasizes. “Many components were already inhouse and were brought together to build a very robust system on a tight budget.”
Components used to construct the assembly cell include a dial table, stak- ing guns, multi-camera vision system, electropneumatic regulators, pressure switches, various sensors, conveyor sys- tem, safety cage and a 10-year-old Motoman robot resurrected from stor- age. The only significant purchase was the dial table, from Tri City Automation Inc., Guelph, Ontario, Canada.
Risk of Human Error Eliminated
As the team brainstormed common failure modes that could result in a bad part reaching the customer, human fac- tors inherent to repetitive assembly received a great deal of attention. The key to success lay in tying all quality- control features back to each individual part number, according to Volz. Once the cell operator enters a part number into the computer, the staking cell’s vision system checks to verify that the correct parts are loaded, including prop- er orientation and presence of all nec- essary components. Loading a wrong part, leaving off a part or component, mixing parts from two products, or placing the part or component on the staking fixture incorrectly will result in
Once the cell operator enters a part number into the computer, the staking cell’s vision system checks to verify that the correct parts are loaded, including proper orientation and presence of all necessary components. Any condition not met results in an error and the dial table will not rotate until the operator corrects that error.
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an error, and the dial table will not rotate until the operator corrects that error. This, says Volz, eliminates approx- imately 90 percent of the fallout on the backside.
As the parts rotate along the circuit, electropneumatic regulators set, and PLC-controlled pressure switches con- firm pressure settings on every staking cycle, ensuring that the proper pres- sure is applied each time, as specified by the product number. Variables that can’t be verified electronically are verified by the vision system, preventing the intro- duction of human error.
Robot Always Focused
The next step in preventing errors is off-loading via the robot. The PLC ana-
lyzes feedback from every measured variable and provides the robot with good/bad part information. The robot then discards any bad parts first, virtu- ally eliminating mishap potential as a bad part is never carried over the con- veyor leading to the finished-parts con- tainer. Should a sensor fail, multiple layers of programming back-checks stop production and automatically sig- nal the robot to discard any suspect product.
Cell designers included the robot as an additional means to eliminate human error.
“A person might make errors due to the repetitive nature of the off-loading task,” says Volz. “The robot doesn’t lose focus or become fatigued.” MF