Integrated Ball-Screw Based Upset Process for Index Head Rivets Used in Wing Panel Assembly

Abstract

A new high speed forming process for fatigue rated index head rivets used in wing panel assembly using ball-screw based servo squeeze actuation has been developed. The new process is achieved using a combination of force and position control and is capable of forming to 40,000 lbs at rates of up to 200,000 lbs/second whilst holding the part location to within +/− 10 thousandths of an inch. Multi-axis riveting machines often have positioning axes that are also used for fastener upset. It is often the case that while a CNC is used for positioning control, another secondary controller is used to perform the fastener upset. In the new process, it has been possible to combine the control of the upset process with the machine CNC, thus eliminating any separate controllers. The fastener upset force profile is controlled throughout the forming of the rivet by using a closed loop force control system that has a load cell mounted directly behind the stringer side forming tool. Panel assembly where the components are not pre-tacked is referred to as a ‘one-up’ process. This process requires that aircraft parts be rigidly and precisely fixtured, and that the fastening processes do not result in excessive part motion. The recently developed riveting process uses a separate position control loop and a position sensor to hold the location of the panel during rivet squeeze to within +/− 0.010′′. CITATION: Haworth, P., Peterson, D., and Hayes, C., Integrated Ball-Screw Based Upset Process for Index Head Rivets Used in Wing Panel Assembly, SAE Int. J. Aerosp. 8(1):2015, doi:10.4271/2015-01-2491. 2015-01-2491 Published 09/15/2015 Copyright © 2015 SAE International doi:10.4271/2015-01-2491 saeaero.saejournals.org MECHANICAL UPSET ARRANGEMENT Figure 1. Vertical fastening machine overall arrangement. The newly developed rivet upset process uses a multi-axis servo driven arrangement for fastener forming. The same axes used in machine positioning also serve as the fastener forming axes. Figure 1 shows the overall vertical panel assembly machine arrangement(2). Figure 2 shows the upset axis arrangement. Figure 2. Mechanical upset arrangement. Upset is primarily achieved by motion on the stringer side of the machine. However, servo motors controlling the skin side clamping head, and the skin side upset axis also move during fastener forming in order to control aircraft part motion. Rivet Process Overview The new upset process consists of the following stages; 1. Insertion. Using a high speed skip move which ensures accurate positioning in countersink. A sensor on the front of the upset tool detects when the rivet has bottomed out in the countersink (Figure 3). 2. Servo/Servo move. To ensure that all of the forming load is imparted into the rivet (and not into the surrounding panel surface) the servo driven pressure foot is lifted off the surface of the wing. Simultaneously the servo driven upset tool drives forward to maintain contact with the head of the rivet and ensures no part motion during this step (Figure 4). 3. Rivet forming. Forming is broken down into 4 stages. This is done so that the ramp rate can be varied at each of the 4 forming stages (elastic, plastic stage 1, plastic stage 2, final plastic) (Figure 5). 4. Dwell. The target forming load is maintained for a period of time defined as the dwell. 5. Ramp down. Load is ramped down to a zero load level. Figure 3. Rivet insertion and seating in the countersink. Figure 4. During the Servo/Servo move, the head is pinned into the countersink by the squeeze axis. Figure 5. Upset occurs from the stringer side, as the skin side moves to maintain part position. Figure 6 shows the key characteristics of a traditional, previouslyestablished rivet upset profile. The basic process parameters are forming rate, target upset force, dwell and ramp down rate. Haworth et al / SAE Int. J. Aerosp. / Volume 8, Issue 1 (September 2015)