Grinding In Lean Operations: Setup Makes The Difference

Kent Westergard, project manager at Parker Aerospace, Control Systems—Commercial Division (Ogden, Utah), says that his company’s goals of achieving lean manufacturing, single-piece flow and cellular manufacturing haven’t happened overnight.

However, Mr. Westergard says that some cells in the facility are “textbook” examples of lean principles at work. “We have cells processing families of parts, operating with a single part going from one machine to another to another, using minimal floor space and requiring minimum travel time and movement,” he says. “Often overlooked in discussing lean operations is the critical role of setup time.”

The Control Systems Division produces primary hydraulic flight controls for commercial aircraft—Boeing, Emery, Canadair and Fairchild.

“We have just about every category of machine tool: HMCs, turning centers, engine lathes and a large battery of Studer grinders for precision OD and ID grinding. The components made here are flight critical, meaning they are crucial to the safe take-off, maneuvering and landing of an aircraft. It also means that we have no margin for error in our processes or the tools we use.”

Mr. Westergard explains that there are two parts to a hydraulic control system: “the brain” and “the muscle.” “The pilot moves the control yoke, which sends a signal, either hydraulic or electrical (fly-by-wire), to the control module—the brain—which then directs hydraulic fluid at a certain volume and pressure to the proper cylinder and piston—the muscle—that then cause movement,” he says. “Inside control modules are electro/hydraulic servovalves that require highly precise grinding.”

A valve is made of two cylindrical parts, a slide and a sleeve, the former of which is designed to travel within the latter. Slides and sleeves range from 0.375 inch in diameter and 2 to 3 inches in length up to 1.250 inch in diameter and 9 or 10 inches in length. These part families are processed in three cells, each cell handling a particular segment of the family size range. Materials are Nitroloy, 52100, 440C, 4340 and 155 stainless. The diametral tolerances on the slides and sleeves range from 0.00008 to 0.0004 inch, and they must be straight and round within 0.000030 inch. Many of these flight critical parts may have some 30 to 40 process operations, so adhering to the process is mandatory.

Mr. Westergard says his company’s relationship with Studer grinders goes back to the late 1970s or early 1980s, when the Hydraulics Division (Irvine, California) acquired two early models.

Mr. Westergard was doubtful at the proposition of grinding threads on an OD machine. He says that he’d grown up with thread grinders and was skeptical when it was suggested that one could grind a thread on a machine with a grinding wheel with an axis that was parallel to the part.

“Thread grinders have a wheel that’s kicked over at an angle to form the helix of the thread,” he says. “I simply didn’t believe it was possible to grind a thread on a machine with a straight wheel in the same axis as the part. I didn’t think it could be done and produce a quality thread, and do it repeatably.”

A team proposed some tests be performed and went to United Grinding Technologies. The team brought its own parts, thread gages, comparator screen layouts and metrology equipment. It ground threads on a Studer S40.

“We spent days measuring those threads every which way,” Mr. Westergard says. “We sectioned them, we checked them with ring gages, with snap gages. We simply could not find anything wrong with the threads. Threads, flats, hexes, edges—all on an OD machine. I became a believer.”

Also central in Parker’s cellular manufacturing is the elimination of waste. Mr. Westergard explains that when working with manual grinders, changeover often required hours to complete. Now, many of these operations are programmed into a single machine.

“Because of our relatively low production rates and fairly small lot sizes, changeover still remains an important issue and can shred the gains of operating lean,” Mr. Westergard says.

With the QuickSet package, changing parts involves calling up the part number at the control and letting the machine set itself based on previously entered parameters. If the new part is within the working range of the workhead and tailstock, there is little to change over.

“In some cases,” Mr. Westergard says, “setup time for changeover approaches zero. I know some will take issue with the idea of zero setup time, but when you compare the minute or so required now with what was required not so long ago, the time saved in setup is very, very dramatic.”