Switch from casting to hog-out for complex aerospace reduces lead time by 33 per cent

Citation

(2000), "Switch from casting to hog-out for complex aerospace reduces lead time by 33 per cent", Aircraft Engineering and Aerospace Technology, Vol. 72 No. 3. https://doi.org/10.1108/aeat.2000.12772caf.004

Switch from casting to hog-out for complex aerospace reduces lead time by 33 per cent

Keywords Hamilton Sunstrand, Fuel systems, Machining

By switching from casting to hog-out for producing fuel metering unit (FMU) bodies for jet engines, Hamilton Sundstrand has reduced lead time by 33 per cent while improving material properties and reducing weight. In the past, the high cost and lead time required to produce tooling for aluminum castings meant that work could not begin on prototype components until the design had stabilized. Hamilton Sundstrand now contracts with Remmele Engineering, Inc., New Brighton, Minnesota, to machine the complex FMU body from a solid block of aluminum. The new process eliminates the need for long lead-time tooling and makes it possible for prototypes to be constructed at various stages of the design process. Machining also avoids the formation of residual stresses and porosity that can occur during casting, providing more consistent material properties. The four-times-tighter tolerances that can be held during the hogging-out process make it possible to reduce wall thickness, often allowing for reductions in weight to the FMU.

Hamilton Sundstrand traces its roots to the founding of the Sundstrand Corp. in 1905 and Hamilton Standard in 1919. Headquartered in Windsor Locks, Connecticut, the company's various business units operate around the world to design, manufacture and support aerospace and industrial products for numerous markets. Annual sales exceed $3 billion and the company employs approximately 18,000. For nearly 50 years, Hamilton Sundstrand has been a world leader in jet engine control systems, which have logged more than one billion flight hours. Since the firm introduced the first mechanical fuel control in 1950, Hamilton Sundstrand has been providing FMUs that accurately control the rate of fuel flow to the engine. These units are custom designed and built to meet the needs of a wide range of commercial, regional and military aircraft.

Complicated internal passages

FMU housings are very complex aluminum components that can range in size from 2lb - measuring 6in x 3in x 2in - to about 28lb - measuring 18in x 12in x 12in. These units contain a large number of internal flow passages that are critical to their performance. In the past, they were always produced as aluminum castings that were later machined to final tolerances. The advantage of this approach is that a casting gets the part to near net shape and reduces the amount of machining required. Casting also greatly simplifies the process of generating complex internal flow passages.

Yet there also are major drawbacks to casting these components. The biggest is the cost and lead-time required to produce casting tooling. These factors make it impractical to produce prototypes during the design process. Instead, engineers must wait until they are certain that the design has stabilized. The need for the design and prototype stages to take place sequentially rather than concurrently increases the overall length of the product development cycle. Another problem is that it is difficult to predict and control the material properties of a casting. The invariably unbalanced flow of material through the mold and the difficulty of controlling the cooling process mean that residual stresses and porosity are bound to form in some areas of the casting, causing its properties to vary. One final difficulty is the fact that the casting process is only able to hold tolerances on the order of +/- 0.040 inch. This means that wall thickness must be maintained at relatively high levels, increasing the weight of the part.

Trying a new approach

Several years ago, in an effort to address these issues, Hamilton Sundstrand engineers decided to attempt to hog-out a body for a particular model of FMU. This model was a very large, complex housing which could not have been cast within the time frame required by the customer, according to David Winkler, Senior Engineer for Hamilton Sundstrand. The firm found a supplier that was capable of machining the complex housings from a solid block of material to the required tolerances. Without having to melt and freeze the material, the hog-out approach provided more uniform material properties throughout the component, improving part quality. This combined with the ability to hold machining tolerances of +/- 0.010 inch made it possible to specify significantly thinner walls throughout the housing.

Machining challenges

Hamilton Sundstrand management and customers were extremely pleased with the benefits of the hog-out approach and, as new FMUs were developed, began converting the bodies to the new technology. As expected, issues arose with the new process that required Hamilton Sundstrand and the supplier to team together and resolve. "These are extremely difficult components to produce", Winkler said. "There are many places where small diameter tools must perform long reaches into the part to machine internal passages. Some of these machining operations put a heavy side load on the cutter, which increases the possibility of tool chatter or breakage. These parts also require a significant commitment of the supplier's resources. Once the machining process is refined, time on machine for one housing can range from 90 minutes for the smallest bodies to as much as 30 hours for the largest."

Lengthy experience

Remmele has more than 50 years of experience in providing precision machining services to clients all over the world in such industries as computers, medical equipment, defense, aircraft, machinery and aerospace. Today Remmele Engineering, Inc., with four divisions and six facilities, has grown to more than 500 employees and annual sales exceeding $100 million. Remmele provides: custom-designed, computer-controlled automation systems, precision machining of complex components, and design and build tooling and assembly fixtures, all for leading manufacturers worldwide. The company has six manufacturing facilities in Minnesota that are electronically linked to provide real-time sharing of engineering data and production information. It frequently works with the full spectrum of special materials such as aircraft grade aluminum, stainless steel, graphite, titanium, and INVAR.

Remmele engineers worked with Hamilton Sundstrand to optimize their design for machining from a solid. They used CAD/CAM software to efficiently design fixtures and create cutter paths directly from the SDRC Ideas part geometry. They verified the CNC programs electronically to identify any issues that could delay development of the machining process. High-speed machining methods were applied to speed the metal removal process. Five axis machining capabilities were applied to minimize the number of setups required. Flexible machining systems were used to eliminate machine setup and facilitate unattended operation while processing multiple pads on an interchangeable basis.

"Remmele's project management system provides a single point of contact to support our design team needs", Winkler said. "This approach allows them to respond quickly and accurately to design changes. Once the programming of the part is completed, and a few prototype pieces are produced, everything usually runs very smoothly. They have one fully manned shift during the day on weekdays and a second lightly manned shift in the evening. Overnight and on weekends, the plant operates on an unmanned basis. With four to six pieces set up on fixtures in five-axis machining centers, they can turn out a quality product with minimal operator intervention. The machines are set up to beep the operators at home if they require a tool change or experience some type of problem. Their ability to meet our delivery schedules while maintaining consistent quality for the last several years has helped us convert all of our new products to the hog-out process."

For further information contact Remmele Engineering, 10 Old Hwy 8 SW, New Brighton, Minnesota 55112, USA. Tel: +1 612 635 4100; Fax: +1 612 635 4168; Internet: www.remmele.com