innova iovations CASTING
Converting a Bumper for Landscaping and Grounds Care
or simply performance, there are paths to serving consumers better. One manufacturer was look- ing to improve the efficiency of a rear bumper on a mower. For this machine, the previous bumper design wrapped around the mower and displaced the center of grav- ity, a problem for this zero-turn mower because it is a necessity that the mass of the rear bumper is properly balanced to provide a counterweight when the mower is operated on inclines. The bottom surface had an angle and position that made it scrape when loading onto trailers with steep inclines. The original also had a thin sec- tion in the center that could crack if it was inadvertently backed into immovable objects like trees or poles, forcing users to remove a broken (and costly) 67-lb. bumper, potentially causing lost revenue. Also, the previous bumper design required post casting third party machining including drilling and tapping of holes necessary to mount the bumper to the mower impacting the cost of the final product. In recognition of the problems and opportunities for improve- ment, the company and Waupaca worked together to make the product better. One of the first decisions was to transform the casting into two parts. Origi- nally, the bumper was cast as a one-cavity mold on a 32 x 38 in. Waupaca-built vertical molding machine, but casting the bumper on a two-piece made it possible for Waupaca to make 1 ½ bumpers per mold on a machine that was just 28 x 34 in. A challenge of designing the
C
ompanies are always looking for ways to improve their prod- ucts. Whether its efficiency, durability
casting was to position the offset parting line so it could fit three cavities into each mold. To do that, Waupaca’s designers reduced its height by 3.5 in. (89 mm), a criti- cal difference that let the foundry team fit three halves of the bumper in each mold. Waupaca also aided the company in the design of the attachment of the bumper castings to the mower. The focus was specifi- cally on the mounting holes that are now cast without the use of cores, opposed to features requiring a core and/or any secondary machining operations. As-cast slotted features that did not require additional fas- teners were designed into each half of the bumper, providing a second point of securing the casting to the mower and the adjacent other half. Following early and extensive in-
volvement in the design and tooling of the casting, the new bumper was implemented into the production design months ahead of schedule. A key result of the new design
was how it eliminated the machin- ing once required to drill and tap mounting holes, reducing costs. The company can also ship more products at a time, because the new design doubles the amount of castings that can be packed onto a skid going to the assembly line, and
increased the number of containers on a truck load from 21 to 26 skids. This reduced costs as well. The two-part bumper lowered
the weight from one 67-pound unit to two that are 32 lbs. each. That was accomplished without compromising the material needed to achieve balance on the mower. And by changing the geometry on the part, the center of gravity was redistributed, improving the center of gravity of the machine. The two-piece bumper also
improved production efficiency, as casting three parts on a smaller, ver- tical molding machine allowed for faster production. This part is pro- duced using transition iron, further reducing the cost to the company. The two-piece bumper system is also safer and more ergonomic for assembly and replacement at the factory and the dealership level. Crucially, the durability was enhanced by eliminating the weak point in the center of the one-piece bumper design, which reduces warranty claims and the need for replacements. And finally, scraping the bottom of the bumper during trailer loading and unloading was eliminated by reducing the overall profile that existed with the previ- ous design.
Table 1. Converting a Bumper for Landscaping and Grounds Care Before
After Weight
Total Castings in One Mold Chamber
Machining Required Bumpers Per Skid 67 lbs. One Yes 12
Two 32-lb. units (64 lbs.)
Three of each half or 1 1/2 bumpers per mold.
No 24 bumpers (or 48 pieces) Mar/Apr 2017 | METAL CASTING DESIGN & PURCHASING | 49
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