Design for Manufacturability or Design for Assembly (DFM or DFA) means taking stock of the tiniest component and assembly steps in a product development cycle—from the simplest fastener to the overall material.
The Caradon Ideal Group, a boiler manufacturer in the U.K., was having trouble keeping up with their competitors. For starters, the company had problems with its existing fastening system. During transportation of the boilers, the screws assembled into spring clips would often loosen and fall into the bottom of the unit or get lost, says Mike Goldthrop, senior production engineer at Caradon. Technicians were constantly removing panels, finding or replacing clips, and reassembling the outer metal jackets.
"We had to do something to reduce production costs, eliminate field repairs, and remain competitive," says Goldthrop. After employing DFM techniques for a new boiler design, the company decided to implement a different type of fastener...the High Torque Fastening System.
This fastener, a combination of screw manufacturing and stamping, involves a screw and a helical form punched into the assembly base material, says John Emmett, managing director of High Torque Fastener Systems Ltd. (Swansea, Wales, U.K.). The thread form has a large crest and a root dimension that matches the mating material of the assembly. The fastener sports a buttress type thread that is designed to capture the outside edge of the material. This match allows zero run-down, similar to a machine screw, while achieving joint strengths double that of sheet metal screws or inserts. The helix is stamped directly into the base material during metal fabrication, and is punched in either pre-plated or post plated materials, automating the formation of the thread.
"Thanks to High Torque, we have no more loose clips," says Goldthrop, "And we've seen extensive savings in installation."
In addition, the company eliminated nuts, inserts, spring clips, and thread-forming screws with one High Torque screw, reducing assembly time by about 15 seconds per part. At 10 parts per unit, that is a total of 2.5 minutes per unit, and Caradon saves approximately $10,000 (U.S.) in inventory-handling costs annually, an overall cost reduction of 25-30%.
With these kinds of results, Caradon engineers are designing the High Torque System into all new products and utilizing DFM in the design phase.
In another case, Household Articles in Hollywell, North Wales, used DFM techniques and High Torque to redesign a coffee pot, saving 37% in manufacturing costs.
DFM examines three areas of a product: part reduction, flipping and turning during assembly, and fasteners, says John Souza, application manager for QSN Industries.
Changing multiple metal parts into one plastic molded part, or using pre-assembled components, eliminates parts and can cut overhead and inventory costs.
By reducing the flipping and turning of a part during assembly, companies can increase productivity, reducing fixturing and tooling costs, and improve space requirements.
Replacing fasteners such as inserts, weld nuts, lock washers, and nuts, drilling and tapping with specially designed screws also increases volume production, reduces inventory costs, and yields a more reliable product.
Companies can put this design technique into practice on their own, or they can rely on specially designed software.
Engineers at Compaq (Marlborough, MA) used DFM software and injection molding to save money on grommets used in a family of high-end servers. The grommets fit into sheet metal cutouts in the server rack and prevent wire chafe on the many power and signal cables that run from rack to rack. Forty or more grommets may be used in a single server.
David Meeker, principal engineer at Compaq, says that engineers originally considered using an existing part, a rubber elastomer grommet that had been used in a previous design. But, he says, "Given the large volumes required, the design team didn't like the cost of the grommet nor its assembly time. We thought we could do better with a different material and a completely new part."
After brainstorming, the design team came up with a concept sketch for an injection-molded grommet that would snap into place in the sheet metal. But before committing resources to design the part, they assessed the potential savings, including tooling costs. Compaq engineers used DFM analysis software for injection molded parts from Boothroyd Dewhurst (Wakefield, RI) for a preliminary cost estimate and Design for Assembly software, also from Boothroyd Dewhurst, to evaluate and reduce assembly times for the new design.
With a DFM estimated part cost of 17% less than the cost of the rubber elastomer grommet, they went with the new design, and cut 33% off assembly time.
Compaq designers then used the costing data from the DFM software as a basis for soliciting quotes from mold makers. The manufacturer produces the grommet for Compaq in a single-cavity tool that runs multiple batches. Set-up and change-out time are both minimal, and ROI was instantaneous. "We quickly recovered the costs of the design team and the tooling," says Meeker.
Compaq also uses DFM software to estimate costs on bezels, light pipes, frame retainers, module retainers, and custom card guides, all with substantial savings.
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