Warm rolling of aerospace fasteners

The shortage of aerospace quality fasteners is causing production delays for new aircraft programs and highlighting the need for new and better production processes.

The shortage of aerospace-quality fasteners led to production delays for the new aircraft program and highlighted the need for new and better production processes. As a result, we applied our expertise to determine how to improve the way threads roll on them.

For thread diameters of 3/8" and larger, rolling is typically done in rolling machines with three cylindrical dies. These machines have been the preferred choice for cylindrical die rolling because they use a fixed vertical rolling axis and the dies are driven uniformly to roll the centerline, eliminating the need for work support inserts. In addition, the vertical rolling axis provides a stable blank loading position.

However, the maximum die diameter that can be used in a three-cylinder die configuration without die collision is only about five times the diameter of the thread to be rolled. For threads 5/16" diameter and below, this limits the spindle diameter to a level that is normally insufficient to carry the rolling loads required for hard materials. It also reduces the available working surface of the die, thus limiting die life.

For these smaller threads, they are typically rolled on flat die machines. These flat die machines are of conventional design and have little to no calibration adjustment. In addition, the matching process must be done by a skilled operator, and because they are continuous cycle machines, the operator must place the part in the machine at the correct point in time to successfully roll the threads.

To solve these problems, some manufacturers have tried rolling small bolts on cylindrical two-die machines, which are easier to set up because the dies, once matched, remain matched throughout the rolling cycle. With constant matching and single-cycle capability, operators can roll good threads without special timing points to bring the blank into the rolling position.

The two cylindrical die configuration eliminates matching and blank introduction problems and allows the use of larger dies, but neither of these two-die machines has a vertical rolling axis, making it easier to feed when rolling headed blanks. In addition, because their rolling axes are horizontal, rolling must be done on fixed inserts with the blank centerline below the die centerline, so growth of the outside diameter during thread rolling does not cause the part to eject from the die before the thread is fully formed. However, incorrect matching can result in crest distortion and possible flanking rings.

The commonly used three-die machines have remained virtually unchanged for more than 50 years. They have complex structures that make setup and rotation matching difficult to replicate from job to job, and the spindle construction lacks the rigidity needed to achieve radial repeatability at different blank diameters and hardnesses. In addition, these machines still use sleeve bearings in the spindle, which can wear rapidly under the high loads required for high temperature operation.

None of the current rolling machines are suitable for automation of warm rolling operations. Finally, many of these fasteners are made of high-strength alloys, and they require heating of the blanks prior to rolling to promote metal flow and improve the life of the rolling dies.

Based on the analysis, a new system approach was needed, which provided the opportunity to streamline and facilitate the production of aircraft fasteners as part of a modern automated warm thread rolling system. The system should consist of a rolling machine, centering rolling support system, part feeding and handling equipment, heating unit position, control system and safety guards with the following basic elements and features.

✺ A rolling machine with two cylindrical dies up to 6¾" in diameter, capable of rolling aircraft head fasteners from 4mm to 25mm.

✺ A fixed vertical rolling axis for easy manual, pick-and-place, or robotic part handling.

✺ A driven work support system that holds the blank on an ultra-hard, low-friction rolling support element before starting the rolling action, and then stabilizes it on the centerline after rolling is complete.

✺ A 5-axis CNC programmable robotic part handling system with a 24-inch reach and 35-inch vertical lift capable of handling parts weighing up to 5 pounds with pneumatic clamping and associated electronic controls.

✺ A high stiffness, rolling, load-bearing structure that operates completely around the roll and guides the rolling spindle system and is driven by a single hydraulic die drive cylinder.

✺ A direct acting roll die drive system using a hydraulic cylinder where the piston rod is in line with the length of the roll die surface and has a diameter that exceeds the length of the roll die surface to provide very high stiffness at the taper surface to minimize the effect on pitch diameter (PD), any eccentric rolling load or straightness of the blank taper.

✺ CNC hydraulic die drive systems capable of applying up to 54,000 lbs. of pressure with servo valve positioning of the spindle support structure with a resolution of 10µm (0.0005").

✺ A CNC spindle drive system where each spindle is driven by its own servo motor with minimum backlash, high reduction worm gearbox and encoder on each motor shaft.

✺ The rotation of each spindle is controlled by a CNC control system capable of automatic rotational die matching (after mismatch measurement), as well as precise control of rolling die penetration and all other rolling cycle elements.

✺ A system for rolling load monitoring and overload protection, rolling cycle signature recording, and signature error notification storage.

✺ A mold performance analysis system that captures and stores radial mold loads for each individual mold in contact with the blank, compiles radial mold load history for each group of molds, and calculates integral values of radial mold loads for each part in contact with the mold, providing future failure prediction and analysis of the impact of blank material, roll cycle length, mold material, and other mold load generating variables on mold life.

✺ Loading and unloading positions as required by the user's blank supply system.

✺ The location where the user's heating coil is installed.

✺ A safety guard system covering the system and all robot motion areas.

✺ A control system that integrates the control of the part support and feed system, the cycle of the heating cell, the protection functions of the safety guards and the needs of the loading and unloading system process.