Originally published in:

DESIGN APPLICATIONS

DESIGN NEWS

From the Regional Editors - 12/21/92

Vented Door Boosts Aircraft Reliability

Cast-steel door replaces solid aluminum model, allowing ventilation of fighter-plane's cannon

William Leventon, Eastern Technical Editor

Middletown, NJ-Too much heat and gas were building up around the automatic cannon on Grumman's F-14D Tomcat during sustained firing. Grumman sought to fix the problem by adding vents to the cannon's solid access door. The replacement door designers envisioned was complex, with louvers, contours ' and thin, double walls. These features would make standard machining and welding techniques prohibitively expensive.

Grumman's F-14D Tomcat now has a vented access door to its
automatic cannon. The door lets heat and gas escape from the firing chamber.

Working closely with experienced casting and manufacturing engineers, designers modified the new door so that it could be cast as a single piece. Besides the cost advantage, casting allows designers to incorporate features like small radii to enhance the integrity and performance of the part. "It was more functionally operative as a casting than it would have been as a weldment," explains Walter Dubovick, general manager of Engineered Precision Casting Co., Middletown, NJ, the door's manufacturer.

Designers and manufacturers met repeatedly during the design process. Among the things the group discussed were materials for the door, wall thickness, tolerances, and contour control. Once the doors were made, close dimensional checking and straightening in a unique fixture ensured that they would fit properly in their frames and thus meet thei cost and performance constraints.

One of the main concerns of the design team was maintaining the door's strength while adding louvers to it. Aluminum, which was used for the solid door, was ruled out because of its relatively low strength-to-weight ratio. Instead, the team decided on 17-4PH, a high-strength stainless steel. With two 0.090-inch-thick walls, the door is stronger than the solid aluminum door, Dubovick says.

Since the designers wanted to limit machining to that which was required for assembly, they had to come up with a reliable way to inspect the casting. They decided to provide target points from which critical features could be measured using geometrical tolerancing.

For example, to check the door's edge profile, Grumman engineers made a plaster cast of a door that fit in the existing access-door frame. From this, manufacturing engineers made a fixture that duplicated the door frame and added contour gauges and "go/no-go" pins to check that the cast doors were within tolerance. After casting and softening/annealing heat treatment, each door was inserted into the fixture to see if its edges would line up with the frame within tolerances as close as ± 0.0 IO inch.

If a door edge fell outside the tolerance limits, it was straightened with a fixture developed by the manufacturer. The fixture features six, five-ton hydraulic presses positioned at strategic locations around the door. Connected to a manifold which allows them to be activated individually or in any combination, the presses operate normal to the curved door surface, straightening the door where needed. After a return to the tolerance fixture, the doors receive an aging heat-treatment to bring them up to their design strength and hardness.

All of the 700 doors made so far have passed inspection. "Since we started using the straightening fixture, not a single door has been sent back to us," says Dubovick. "More important, the new doors cleared up the gun problem."

For additional information on

INVESTMENT CASTINGS

Application brochure covers eight case histories that illustrate the potential and highlight the advantages of investment casting. Selection includes links for endless track, electrical cable connectors, machine guide shoe, and other components, most cast ready to use and some requiring only simple secondary machining. Guide outlines applications and shows parts, and has helpful tips for specifying investment castings.

OTHER APPLICATIONS

· Electronic cases

· Impellers

Back to EPCO Home Page