Warm Rolling for Aerospace Fasteners

The shortage of aerospace quality fasteners has resulted in production delays on new aircraft programs and highlighted the need for new and better production processes. As a result, Kinefac directed its expertise to determine how it might improve the way in which the threads were rolled on them.

For thread diameters 3/8" and greater, the rolling has generally been performed in rolling machines with three cylindrical dies. These machines have been preferred for cylindrical die rolling because they use a fixed vertical rolling axis and the dies are actuated uniformly to the rolling centerline, which eliminates the need for a work support blade. In addition, the vertical rolling axis provides a stable blank loading position.

However, the maximum die diameter that can be used in a three cylindrical die configuration without the dies clashing 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 degree that is generally insufficient to carry the required rolling load for hard materials. It also reduces the available working surface of the dies and thus limits die life.

For these smaller threads, the rolling has generally been performed on flat-die machines. These flat-die machines are of a traditional design with virtually no calibrated adjustments. In addition, the matching process must be done by a skilled operator, and since they are continuous cycle machines, the operator must place the part in the machine at the correct point in time for the thread to successfully be rolled.

To deal with these problems, some manufacturers attempt to roll small bolts on cylindrical two-die machines, which are easier to set up because the dies, once matched, remain in match throughout the rolling cycle. With constant match and single cycle capability, there is no special point in time for the operator to introduce the blank into the rolling position to roll a good thread.

The two cylindrical die configuration eliminates the matching and blank introduction problem and allows the use of larger dies, but none of these two-die machines have a vertical rolling axis, which is easier to feed when rolling headed blanks. In addition, since their rolling axis is horizontal, the rolling has to be done on a fixed blade, with the blank centerline located well below the die-to-die centerline, so the growth of the OD during thread rolling does not cause the part to pop out of the dies before the thread is fully formed. However, if the match is done incorrectly, it results in crest deformation and possible flank laps.

The three-die machines generally in use have been virtually unchanged for more than 50 years. They have a complex structure that makes setup and rotary match difficult to replicate from job to job, and the spindle structure lacks rigidity necessary to achieve diametrial repeatability with varying blank diameter and hardness. In addition, these machines still use sleeve bearings in the spindles, which wear out quickly under the high load required for high temperature operation.

None of the current rolling machines lend themselves to the automation of the warm rolling operation. Finally, many of these fasteners are made from high strength alloys that require the application of heat to the blanks prior to thread rolling in order to facilitate metal flow and improve rolling die life.

Based on analysis, a new system approach was necessary, which provided an opportunity to apply the MC-15 FI CNC PowerBox Kine-Roller as part of a modern, automated warm thread rolling system, simplifying and facilitating the production of aircraft fasteners. Kinefac concluded that the system should consist of a rolling machine, a centering rolling support system, part feed and handling equipment, a location for a heating device, a control system, and safety guarding, which has the following basic elements and characteristics:

• A rolling machine with two cylindrical dies up to 6¾" in diameter capable of rolling aircraft headed fasteners from 4mm to 25mm.
• A fixed vertical rolling axis to facilitate manual, pick-and-place or robot part handling.
• An actuated work support system that holds the blank, up to the start of rolling action, against a super hard, low-friction rolling support element, and then stabilizes it on the centerline after the rolling is complete.
• A 5-axis CNC programmable robot part handling system with a 24" reach and 35" vertical lift capable of handling parts up to 5 lb with a pneumatic gripper and related electronic controls.
• A high-stiffness, rolling, load-carrying structure that fully surrounds the rolling operation and guides the rolling spindle systems and is actuated with a single hydraulic die actuation cylinder.
• A direct-acting rolling die actuation system using a hydraulic cylinder in which the piston rod is in line with, and its diameter exceeds, the die rolling surface length to provide very high stiffness in the taper plane, minimizing effects on pitch diameter (PD), straightness of any off-center rolling load or blank taper.
• A numerically-controlled hydraulic die actuation system capable of applying up to 54,000 lb with servo valve positioning of the spindle support structure to a resolution of 10µm (0.0005").
• A CNC spindle drive system in which each spindle is driven by its own servo motor, with a minimum backlash, high-reduction worm gearbox and an encoder on each motor shaft.
• Rotation of each spindle controlled by the Kine-Trol CNC control system capable of automatic rotary die match (after mismatch measurement), and precision 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 storage for signature error notification.
• A die performance analysis system that captures and stores the radial die load for each individual die-toblank contact, compiles the radial die load history for each set of dies, and calculates the integrated value of radial die load per part-to-die contact, providing for future failure prediction and the analysis of the effects of blank material, rolling cycle length, die material and other die-load-producing variables on die life.
• Load and unload positions as required by the user's blank supply system.
• A location for mounting the user's heating coils.
• A safety guarding system covering the system and all of the robot motion areas.
• A Kinefac control system, which integrates the controls of the Kine-Roller, the Kine-Mat part support and feed system, the cycling of the heating unit, the protective functions of the safety guarding and the demand of the load and unload system process.
• The MC-15 FI Kine-Roller design with its Power-Box structure (left), which already provides for many requirements defined above.

The ‘Robo-Roller' is used in a cell and provides inspection for the blank diameter and other head and shank features prior to rolling. It is manually loaded and unloaded.

After evaluating additional enhancements, a system was built that included all the other elements. This "Robo-Roller" system provides the user with unsurpassed rolling process versatility when rolling threads, knurls, splines or other rolled forms, and other aerospace parts.

The integrated handling and rolling system greatly simplifies part handling in cells for transferring parts from a loading station into the rolling position, supporting it there, and then moving it to the output station. It can also be used to feed the rolling operation from a bulk storage unit, output track or part transfer tray or carrier.

In either case, a blank heating station can be added between the pickup and rolling station, while precision grippers, the robot feed system, can handle very small parts.

The Robo-Roller allows easy integration of automatic preheating of rolling blanks for such other operations as the blank diameter inspection, the chamfering of blanks, and even the assembly of locking devices prior to the thread rolling operation. The Robo-Roller also provides opportunities for subsequent operations on rolled parts while they are still under the control of the robot.

In the standard Robo-Roller system for aircraft fasteners, the robot is directly parts up to approximately 12" long with diameters up to 1½". Typical rolling cycles are 2 seconds or less and the dies start and stop for each cycle. The overall process time is limited by the requirements for heating, part transfer and other processes that may be performed while the part is in the system. Special systems are available to handle larger parts and provide a longer robot reach for increasing the number of pre- or post-rolling operations.

In all cases, the basic CNC machine control system, including all feed and process functions, is provided. In addition, the MC-15 FI Kine-Roller is provided with the Kine-Trol CNC rolling control system, which automates die match, maintains rolling cycle programs and provides error message information and other operator convenience functions.

Depending on the application, totally enclosed, wire mesh, or light curtain operator safety guarding is provided. The integrated robot and cylindrical die rolling machine, including all controls, is mounted on a common base with three-point support and is easily movable without affecting the program relationships of the robot to the rolling position.

The Robo-Roller System applied to aircraft fastener manufacturing represents a truly important advance in the processing of high-quality, high-strength threaded fasteners. The automation of blank heating, the repeatable warm blank handling time, the unique Kine-Trol CNC control and Die Saver system, and the outstanding rolling capabilities of the MC-15 FI CNC Power-Box Kine-Roller provide a new and more efficient way to roll warm, or at room temperature, aircraft quality threads and knurls.