A new broom sweeps lean
Rapid prototyping and design for assembly techniques light the way to a vacuum cleaner headlamp that is easier to assemble.
Each year, engineers of appliances, automobiles and other products have to please two key constituencies: consumers and corporate management. Consumers want products with new and improved features and styling. Management wants products that can be assembled faster and’ cheaper.
Such was the dilemma faced by engineers at The Kirby Co., who were asked to update the company’s G4 vacuum cleaner to a new model, the G5.
One of the first features the engineers focused on was the cleaner’s headlight. The engineers needed to reduce the overall cost of the headlight without decreasing quality or safety. Aside from the lightbulb and socket assembly, the G4 headlight consisted of three main parts: a polished die-cast aluminum cap, a plastic frame bracket and a plastic lens.
The G4 headlight was assembled in several steps. The lightbulb and socket assembly was snapped into the frame bracket, and its wires were wrapped into a channel in the frame. This assembly was snapped into the polished headlight cap, and the lens was inserted. Two screws were then driven through the cap and frame bracket and into the lens. These components were assembled in a staging area and then handed over to assemblers at the unit assembly stations.
In the past, assembling the headlight raised a number of manufacturing quality issues. Inserting the frame bracket into the cap casting could be tricky. The frame bracket had sharp corners, and the size of the wire channel was different on the export models. To reduce upward glare, the lens needed to be painted. The new design would have to solve these problems.
On the other hand, the G4 headlight design couldn’t be completely scrapped. Several design elements had to be kept. For example, the headlight cap on the G4 pivoted about the unit to allow placement and removal of attachments. The pivot point and attachment method were to remain the same. The way the headlight fit into other components was predetermined, and the cap was to remain polished aluminum.
The new vacuum cleaner also needed to meet the electrical safety requirements of Underwriters Laboratories and various world testing organizations. For example, an articulated device modeled after a child’s finger must not be able to enter the appliance and contact any electrical component. Similarly, if a pin is inserted into the appliance, it cannot contact any electrical component. And, if a weight is dropped from a specified height onto the lens, the plastic must maintain its structural integrity and must not allow access to electrical components.
The new model also had to meet Kirby’s internal quality tests, which check use, abuse, excess vibration, structural integrity and failure modes.
Emphasizing Snap Fits
Using design for manufacture and assembly software from Boothroyd Dewhurst (Wakefield, RI), the Kirby engineers realized they could simplify assembly of the new headlight and reduce its cost by eliminating as many screws as possible. Not only do screws add part cost, they also add labor cost. In the end, the engineers decided that the entire headlight assembly could be snapped together, rather than fastened. The headlight socket would snap to the frame bracket, and the frame bracket and lens would snap to the cap casting.
Simply removing the two screws that held the bracket and lens to the cap decreased total assembly time by 46 percent, from 38.1 to 20.2 seconds. Incorporating snap fits into the headlight assembly also simplified repairs. The new frame bracket can be replaced on the repair line in less than 30 seconds without removing any parts. The headlight on the old model normally took a couple of minutes to replace. Screws and parts had to be removed, which could potentially damage a polished unit and incur additional rework expense.
Critics of design for assembly techniques argue that eliminating a few fasteners to save money is meaningless if product quality is compromised. But, engineers can meet both objectives. Replacing two screws with a series of snap fits doesn’t necessarily reduce the quality of the product. The quality of the assembly depends on the design of the snap fits (ASSEMBLY, August 1997, p. 32). To be safe, Kirby engineers molded two bosses in the casting and two holes in the frame bracket so that the parts can be secured with screws should the snap fits ever fail in the field.
Beside emphasizing snap fits, the Kirby engineers changed other aspects to the headlight assembly. The headlight frame bracket was modified to make installing the wires easier and to eliminate the sharp corners.