Monogram Aerospace manufactures highly sophisticated aerospace fasteners. If you have ever flown in a commercial airliner, you have probably seen Monogram fasteners dotting the wings of the plane. Unique because they are installed and tightened from one side. Much like pop rivets, although that is a oversimplification of these high tech components.
Each fastener has a hole drilled dead center through the head and shaft before an actuating rod is inserted. During installation, a ratchet tool is used to twist the actuating rod. This causes the end of the tube to deform outwards and hold the fastener in place. Monogram’s fasteners are made from an alloy that deforms to a specific size and shape. The actuating rod is snapped off, and the fastener is secure.
Monogram asked Northfield to develop a chuck to hold these aerospace fasteners during machining. The process requires hard turning and tapping. To accomplish this we had to address several challenges.
- Monogram stressed the importance of holding the parts concentrically during machining, otherwise the the build up of stress on the fastener could cause it to fail during use. The large size of the fastener heads the chuck jaws need to grip over, make achieving the required level of concentricity complicated.
- The two head styles, flat and button, presented an additional challenge. Two different datums for axial banking need to be accommodated by our chuck.
- Finally, Monogram produce fasteners in hundreds of different sizes and lengths. This means that our chuck also needs to allow rapid and accurate changeovers from one size set-up to the next.
The Northfield Solution:
The chuck provided satisfied all of the challenges while providing a part-to-part repeatability of 0.0001 inch.
For the flat-head style fasteners, the gripping operation is fairly straightforward. The chuck jaws open wide enough to clear the head of the fastener. The part is pushed into the chuck until it reaches a standard fixed stop. The jaws close and the part is ready to be machined. A modification made to the stop is a special feature. It allows for a continuous chip to exit through the stop and out one side.
The gripping operation for the button head style fastener is more complex. The work piece goes all the way into the chuck, as if it were a standard style banking. Then in a second step, the jaws begin to close in sequence with a pusher piston mounted on the face of the chuck that pushes the part back out so that the button head banks right on the back side of the jaws. The parts can be located with greater accuracy then previously possible before using this system.
The chuck is equipped with a sub-jaw/insert combination to accommodate the variety of diameters and lengths being machined. This allows Monogram to change from part to part by removing only three screws which hold the three inserts and one stop pad.
This custom chuck has a flow control valve installed inside the chuck body. It allows the operator to adjust the speed of the pusher piston compared to the speed of the jaws for the button head family of parts. The flow control valve is set to cut-off the air supply, making the pusher a fixed stop when not in use.
Using our chuck, Monogram is able to run their machines twenty-four hours a day, seven days a week. Cycle times are measured in seconds and repeatability is within 0.0001 inch.