Keeping up increasingly broad and difficult Environmental Protection Agency standards does not come easy for any engine manufacturer. The challenge is compounded when the company's philosophy is to always exceed the given standards to provide customers with a quality level beyond the industry norm.
Such was the issue facing Terry Beyer, project engineer at Cummins Engine Co., in Columbus, Indiana, as he considered alternative manufacturing processes for the rocker levers used on the Cummins N-14 liter six-cylinder engine. Mr. Beyer had to find a production process that would meet the "Cummins Standard" while keeping the per-part cost low in what is a very price-competitive industry.
The first option was to stay with the existing process—30-year-old technology consisting of five machines, the part passing through each machine in a batch operation. Such outdated technology would make it difficult to hold the strict tolerances they required and still remain competitive in the pricing spectrum.
Another option involved using two machines: an in-line transfer machine and a center column indexing machine. This method would require dedicated tooling, plus a second chucking operation that could allow for unacceptable deviations from part tolerance standards.
The final option was for Cummins to implement a Turmat machine (Turmatic Systems, St. Louis, Missouri) offering a complete machining approach, improved tolerances, improved tool life, and an ability to meet their high volume production requirements.
"We decided to use the Turmat multi-axis rotary transfer machine after weighing it against the other two methods," explains Mr. Beyer. Following its implementation, a new sequence in the machining process emerged.
In the new machining sequence, castings are loaded into the machine via an automatic conveyor system. The casting then undergoes the following operations: boring, gundrilling, drilling, facing, finish boring, and tapping. Rocker levers are processed every 24.5 seconds, that is, 2.5 pieces per minute. Then random statistical process control inspection of every tenth piece ensures that parts are within 70 percent of specified part tolerance (1.33 Cpk).
After quality inspection, parts go through a process that Cummins Engine Co. describes as "synchronous flow" manufacturing. Here, the parts are deburred, washed, dried, and then transported to a nonsynchronous automatic assembly system.
The Turmat meets and, in some cases, exceeds the strict demands outlined by Cummins. The single chucking setup is attractive because it ensures the demanded level of high quality. The special design of the machine provides for easy accessibility to the tooling units, making tool changing an easier task. The design also allows Cummins to take advantage of some of the latest advancements in carbide tooling, including a modular tooling package. And in an another quality assurance measure, Turmatic Systems was able to engineer a fully automated loading and unloading material handling system, reducing secondary handling operations to a minimum.
According to Mr. Beyer, Cummins' implementation of the Turmat has led to several things. First, there has been a 30 percent reduction in tooling costs. Secondly, they have adopted a just-in-time system by eliminating work-in-process and finished goods inventory. They also have an ability to do two shifts in the time they used to do three shifts and now meet or exceed the Cpk 1.33 requirement.