The SA2 machine is the latest in the Low Voltage Electromagnet Riveter (LVER) family of automated fastening equipment from Electroimpact, and it is being used on the Airbus A320 aircraft.
LVER family machines are purpose-built five-axis types designed to fasten a vertically oriented wing skin and stringer. LVERs use a pair of actuated fastening tables that drive forward to rigidly clamp the wing and stinger together between two feet so subsequent drilling and fastening can be carried out. The skin-side fastening table carries the majority of the work tools, which are carried on a movable axis that allows for the transfer of tools.
The rivet-fastening cycle is as follows:
• Clamp up and measure the stack thickness
• Shuttle to drill spindle—drill hole
• Shuttle to EMR (electromagnetic riveting head)—insert rivet, check rivet length, and form rivet
• Shuttle to shave spindle—shave skin side rivet head flush to panel
• Unclamp and move to next fastener position.
The SA2 machine actively controls about 20 axes and about 70 pneumatic valves. It incorporates a number of major design revisions intended to increase productivity and reliability. One of the revisions involves the process used to move the rivet slug from the feed tube to the panel.
The previous designs for rivet feed used a pneumatic gripper located on the front of the EMR. The rivet feed path included a drop down chute called the U-turn, which was located on the side of the clamping foot. This chute only lined up to the rivet gripper while the shuttle table was at the drill position. This often resulted in a delay in the cycle; the drill had completed its job, but the rivet had yet to reach the rivet gripper. This delay was especially prominent in thin stacks and small-diameter rivets for which drill time was at its minimum.
The new injector design features a reduction in the frequency of adjustments and the elimination of delays in the cycle due to a fixed feed position—without a loss in functionality from the original design. The solution to the fixed feed position required the new system to be located on the moving portion of the shuttle table. Pneumatic grippers were replaced by simple spring-loaded rivet fingers, and the U-turn was replaced with a new rivet injector.
The injector transfers the rivet from the feed tube to the rivet fingers. The rivet-finger design is similar to that used on other fastening machines; the only change is that the new design involves holding of rivets in a horizontal vs. vertical orientation. The purpose of the rivet fingers is simply to hold a rivet directly in front of the rivet die so that when the EMR drives toward the panel the rivet is inserted into the prepared hole. The entire device has no pneumatics or electronics; it relies on springs to hold the rivet and simple slots cut into the EMR driver for alignment.
One of the major design aspects of the rivet injector is the axial gripping of the rivet. This feature actually helps to eliminate nearly all of the alignment requirements between the injector and rivet finger by allowing some flexibility and tolerance to misalignment. If both the injector and fingers held the rivet on the sides, the rivet would be over-constrained during the transfer. This would require a tight alignment between the two assemblies. Gripping the ends of the rivet releases this over-constraining problem. In addition, axial gripping also pushes the rivet toward the rivet die, setting a fixed distance from the rivet die to the end of the rivet. This is opposed to setting the fixed distance on the panel side of the rivet. Putting the set distance on the die side reduces the length of the fingers since it is always known where the end of the rivet will be located. This also reduces the chance of marking the shank of the rivet because the fingers do not need to slide down the length of the rivet before contacting the rivet die.
Special considerations were given to rivet marking. To reduce cycle times, the rivet is blown through the feed tubes at very high speed. Whenever the rivet contacts a metal surface, a small mark on the end of the rivet is produced. This mark is simply the removal of the rivet coating, but there is some concern that problems might develop over the long term. So, all surfaces that the rivet contacts in the new system are made of plastic and are all easily replaceable.
In the testing process, Electroimpact determined that an increase in airflow through the feed tubes was necessary. However, engineers found that rivets traveling at higher speed bounced off the end of the rivet chamber farther and farther as air speed was increased. Longer rivet settling times increased over the cycle period. But by cutting long vents in the rivet chamber, the rivet bounce disappeared regardless of rivet speed.
The SA2 machine at Airbus’ plant in Broughton, U.K., achieves 10 cycles/min and up to 12 rivets/min in speed tests. Previous machines achieved no more then 8.5 rivets/min. The improvement was achieved in part by decreasing the rivet feed time from about 2.5 s with the previous machine to about 1.82 s with the SA2 machine. And the rivet fed is always the correct length because it is selected based on the actual stack measurement and not pre-fed based upon the previous fed rivet. Maintenance downtime for rivet feed hardware has been reduced from approximately 1 h per machine per month to 0.25 h per machine per month.
Information for this article was provided by Christopher Buchheit, Project Engineer, Electroimpact.