Just as a great deal of time and research goes into engineering a quality product design, a proportional amount of time must go into determining the most cost-effective and streamlined method of producing the product. After a custom bicycle gear maker developed an intricate new chain sprocket design, it next looked to find the most efficient manufacturing process to maximize profit and throughput.
In the past, the company had used conventional methods to produce sprocket assemblies, requiring many machining operations and tools. Knowing that fineblanking provided tight accuracies and complex part design possibilities at a "part-per-stroke" rate, the company turned to one of the leaders in fineblanking technology, Feintool Cincinnati, Inc. (Cincinnati, Ohio), for help in producing its new sprocket.
Feintool suggested its new modular tooling system that combines precision forming with fineblanking and conventional blanking in one operational sequence, utilizing adjustable and controllable press forces. With the modular tooling system, all blanking and forming operations take place in one pass through the press.
Recently installed, the system produces the company's new sprocket with dimensional accuracy to ±0.001 inch and flatness to 0.001 inch per inch with each press stroke. No secondary machining is required, and the sprockets are ready for plating after minor deburring.
The system consists of single tool sets arranged in sequence. Unlike progressive tools, which incorporate forming operations, modular tooling provides complete separation of each operation. Parts are transported from one tool set to the next via a transfer system. This separation of operations simplifies timing and adjustment of the tooling during setup, and it reduces long-term maintenance. It also allows empty tooling stations to be included, providing flexibility to expand the number of steps in a process to accommodate future design changes.
The company offers two versions of its new sprocket assembly design, one for leisure and one for competition. The assembly for leisure bikes consists of three steel coaxial sprockets, while the racing version utilizes heat-treated aluminum alloy for the middle and large sprockets, noticeably reducing the weight of the assembly. Both versions possess a redesigned, intricate tooth profile and geometry which allow cyclists to smoothly shift to higher or lower gear ratios, under full load with total reliability.
In conventional production, producing a sprocket by non-fineblanking methods required nine operations and a number of different machine tools. In addition, work-in-process and transport between operations resulted in extra "hidden" costs. Forming and fineblanking with modular tooling resulted in cost savings of 80 percent. Furthermore, part quality and dimensional accuracy could now be relied upon to remain constant over the entire production
Fineblanking the sprocket not only reduced part production costs for the company, but it also provided increased gear tooth wear resistance through inherently "free" cold hardening. The fineblanking process involves both cold forming and shearing. Cold forming both shapes the part and contributes desired grain flow and orientation, resulting in pronounced hardening around the formed area of a part, increasing material hardness. The fineblanking process increases material hardness as much as 100 percent in the sheared zone.
If only the stresses in the material are considered, the fineblanking process appears more closely allied to deep-drawing, coining and cold-forming than to stamping. For this reason fineblanking, unlike stamping, calls for soft, easily cold-formed materials. About 90 percent of all fineblanked parts are mild or alloyed steels, about 8 percent are aluminum or aluminum alloy, and about 2 percent are copper, copper alloy or other materials.
Before heat treatment, C45 high-carbon steel possesses a ferrite-perlite structure that, if used in fineblanking or bending, produces deep cracks. Hot rolled, non-annealed sheets of such materials are, therefore, unsuitable for fineblanking or for forming by bending. The structure of carbon and alloy steel for forming and fineblanking must be of the annealed type. In carbon steels, this structure consists of a ferrite matrix containing 90 to 100 percent spherical cementite. In alloy steels, the structure must contain carbon as well as the alloying elements.
Only annealed materials possess tensile strengths within the range required for forming and fineblanking. For example, optimum annealing of C60 (carbon content 0.6 percent) non-alloyed steel would maintain a tensile strength no higher than 74,000 psi. When annealed to about 90 percent, this steel would show a tensile value of about 82,500 psi, while in the complete absence of annealing, this value would rise to 118,000 psi, far beyond the range suitable for fineblanking.
In certain cases the tensile strength of soft, non-alloyed, good cold-forming steels, such as SAE 1008, 1010 and 1018, can be improved by cold rolling. However, cold rolling also decreases ductility, so that steels made stronger by cold rolling become less suitable for fineblanking. For optimum process performance, the increase in tensile strength that can be achieved without compromising either part quality or tool life should be determined for each case.