“In designing for manufacturability, we took into account both the machining of complex prototypical parts and the methods used to produce them once the engine was in commercial production,” said Scott Lindahl, manufacturing manager for EPS. “A particularly challenging component is the propeller shaft that incorporates both gearing (splines) and bearing diameters in 17-4 PH stainless steel.”
According to Jared Manhart, CNC machinist/programmer for EPS, “We start out with a rough forging. With more than 20 gears in the engine, the decision was made to generate our own splines and gear teeth in-house using our lathe, which requires gripping on the ID in order to ensure proper runout. Prior to cutting the gears or splines, we face, drill and turn the part with multiple bearing diameters requiring a tolerance of ±7 µm.”
To achieve the precision required, while at the same time allowing for efficient part change capability, EPS chose a Hainbuch MANDO workholding system from Hainbuch America Corp. (Germantown, WI), which can quickly convert from OD collets to MANDO expanding modules for ID work. “I was aware of Hainbuch from previous experience and, after discussing the application with their people at IMTS, I realized that it was the best solution for us,” said Michael Fuchs, president of EPS.
Because the propeller will ultimately bolt onto the shaft, the use of a tailstock was undesirable. Also, space was limited on the machine chosen for turning operations. The Hainbuch chuck is designed to provide maximum gripping power on the external diameter and can be quickly adjusted to incorporate the MANDO insert for critical ID work. The clamping component of the MANDO, called a bushing, is a single component composed of steel segments bonded together by a proprietary vulcanized polymer. The large gripping surface of the bushing allows for maximum clamping force without marring the finished surface of the part, while providing a rigid workholding platform.
“This is especially critical when it comes to the gear hobbing operations,” according to Jared Manhart, CNC machinist/programmer for EPS. “Both pull back and non-pull back phases are used in the machining of the part, and any deviation or chatter would result in the scrapping of an extremely expensive component.”
According to Nicholas Mahoney, CNC machinist/programmer, “The workpiece is extending out 10” with no vibration and, without a tailstock, that is exceptional performance. Further, we’ve cut the cycle time on the propeller shaft to about one hour—less than half the original time required. A further advantage is the quick-change capability that the system offers. This ensures that, once the engine is FAA approved and we enter the production phase, we will be able to retain the desired tolerances while meeting our required production goals. Also, we can quickly change over to running bar stock without incurring significant downtime.”
The flexibility of the Hainbuch system has led to discussions on the possibility of utilizing rotational workholding in select milling operations, thereby generating increased machine utilization and improved efficiencies.
“Simultaneously meeting the twin goals of high precision and efficient manufacturing capability is typical of our way of working,” Fuchs said. “We believe that if you do something new, it can provide a great solution, but you have to be prepared to handle many issues at once. The success of our team in achieving this is what will ultimately make our engine, and our company, take flight.”