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.
What makes these fasteners special is that they are installed and tightened from one side, much like pop rivets, although that would be a crude description of these high tech components. Each fastener has a hole drilled dead center through the head and shaft, then an actuating rod is inserted. When the fastener is installed, 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 of a special 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 fasteners during machining (hard turning and tapping). To accomplish this we had to address several challenges.
The Northfield Solution
Monogram stressed the importance of holding the parts concentric during machining. If not, the build up of stress factors on the fastener during use could cause it to fail.
Achieving this concentricity was complicated by the large size of the fastener head that the chuck jaws would have to grip over. In addition, because there were two head styles – flat head and button head – our chuck would also have to accommodate two different datums for axial banking. Adding a further complication was the fact that Monogram manufactures fasteners in literally hundreds of different sizes and lengths. This meant that our chuck would have to allow quick changeovers from one set-up to the next.
The chuck that we 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. The only special feature was a modification we made to the stop to allow 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. Again the work piece goes all the way into the chuck, as if it were a standard style banking. Then, 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. Using this system, we were able to locate the part with greater accuracy then had been possible before.
To accommodate the variety of diameters and lengths being machined, the chuck is equipped with a sub-jaw/insert combination. This allows Monogram to change from part to part by removing three screws which hold the three inserts and one stop pad.
In addition, the chuck has a flow control valve installed inside the chuck body that lets the operator adjust the speed of the pusher piston compared to the speed of the jaws for the button head family of parts. When the pusher is not in use, the flow control valve is set to cut off the air supply completely making the pusher/piston a fixed stop. 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 as mentioned earlier, repeatability is within 0.0001 inch.