Preformed Line Products (PLP) is a worldwide designer, manufacturer and supplier in the overhead power lines industry for the Australian and Southeast Asia markets. A clear challenge the company often faces is the utilization of products manufactured by specialist manufacturers in both markets that use their own methods, standards and specifications. The end result is difficulty combining hardware parts from two different suppliers.
When product manufacturers combine several different components, they perform ‘tolerance stacking’, which analyzes the accumulated variation allowed by specified dimensions and tolerances. Every component may have a flaw or deviation from the specifications that can be tolerated by the overall design to form a functional whole.
Phil Timbrell, Manager of Engineering Services at PLP Australia, faced this problem with the process for manufacturing spreader rods, which are used to hold low-voltage lines in windy conditions. The two primary parts of this component are a pultruded rod, and a spring that clamps the rod to a conductor.
Previously, the industry put what Timbrell calls ‘dumb tolerances’ on products. “We tell our vendors that we can’t afford for the interference to be more than a half a millimeter,” he says. “And then we split the difference of that variation between the two part manufacturers, and ask them to comply with two equal sets of tolerances.”
However, this arbitrary ‘down-the-middle’ approach is potentially inefficient. Timbrell decided to use existing statistical process control (SPC) data from the external suppliers to model a probabilistic approach to tolerance stacking with @RISK. “This would result in tolerances that were both within the scope of the external supplier and also could be quantified into the correct tolerance stacking,” he says.
This new approach has been very successful for manufacturing the spreader rod at PLP Australia. Now, rather than applying a one-size-fits all approach when ensuring both vendors meet the tolerance-stacking requirements, Timbrell is able to give them specific guidance that works for them.
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