University of Washington, Seattle

Department of Mechanical Engineering

Computational Techniques in Engineering – ME 535

Percussive Riveting Analysis

Presented to: Professor Ashley Emery

Presented by: Wadih Zaklit

ContentsIntroduction.................................................................................................................................................2

Tools............................................................................................................................................................3

Methods......................................................................................................................................................3

Sensors....................................................................................................................................................3

Testing.....................................................................................................................................................4

Performance quantification.....................................................................................................................4

Results.........................................................................................................................................................4

Manual Projected Curves........................................................................................................................4

Rivet Gun and Bucking Bar Accelerations................................................................................................7

Simulation.................................................................................................................................................10

Conclusion.................................................................................................................................................11

References.................................................................................................................................................12

Introduction

Riveting is a forging process that may be used to join parts together by way of a metal part called a rivet. The rivet acts to join the parts through adjacent surfaces. Rivets have many uses, such as in the construction and sheet metal industries [1]. Figure 1 describes how a rivet is installed using a rivet gun and a bucking bar.

FIGURE 1: TWO MECHANICS INSTALLING A RIVET INTO THE BODY OF AN AIRPLANE.

To build an airplane, about 5 million rivets are needed, from which, 20% are done manually. The mechanics responsible of the riveting process are exposed to really high forces and vibrations which is causing them to have shoulder problems as well as Vibration White Fingers (VWF). In order to address the problem, a test bench was built in the Boeing Advanced Research Center at the University of Washington. The test bench is able to simulate the riveting process while collecting data that can later be analyzed. The goal of the project is to find the adequate tools that protect the riveters best.

In this report, the methods used to replicate the riveting process on the test bench are described. A mathematical analysis of each tool’s acceleration characteristics is done using the Fast Fourier Transforms. The rate of rivet formation is calculated and predicted by a Least Square Fitting. Parameter Estimation was used to estimate some system characteristics. Finally, a discussion of the results is presented with a few recommendations.

Tools

At Boeing, mechanics use a lot of different types of rivet guns. The most common ones were selected for the testing. The three rivet guns are the Atlas Copco 12P (AC12P) which is a recoilless gun, the Ingersoll-Rand AVC 27 (AVC 27), and a modified (mechanics add mass to it claiming that it feels better) Ingersoll-Rand AVC 27 (MERG). Three different bucking bars were also used and are as follows: a 3kg Steel bar (SBBG), a 7kg steel bar (SBBM), and a 7kg Tungsten bar (TBB).

Methods

A complete set of automatic tests installing rivets with the test bench and an identical set installed by Boeing mechanics. To compare three riveting guns and three bucking bars against each other, five rivets were installed with each permutation of the tools. This leads to nine permutations of tools configurations to test. Cumulatively, 90 rivets were installed during the testing. Data is collected using a National Instruments PXIe 1082 machine running a custom controller in LabVIEW and be processed in MATLAB.

SensorsData was collected using various sensors over nine different channels during automatic testing and six different channels during manual testing. A summary of these channels and of their respective rates of acquisition is presented below in Table 1.

TABLE 1: SENSORS USED TO COLLECT DATA WITH THEIR LOCATION AND SAMPLING RATE.

Test Bench Testing (TB) Manual Testing (M)

Rivet GunSample

rate (kHz)

Bucking BarSample

rate (kHz)

Rivet GunSample

rate (kHz)

Bucking BarSample

rate (kHz)

Shock Accelerometer 20 Shock

Accelerometer 20 Shock Accelerometer 20 Shock

Accelerometer 20Tri-Axial

Accelerometer 20 Load Cell 10 Tri-Axial Accelerometer 20 Load Cell 10

Load Cell 10 Laser Displacement 5 Microphone 20 - -

Laser Displacement 5 - - Flow Rate 10 - -

Microphone 20 - -

Flow Rate 10 - -

TestingAutomatic testing is conducted using the automated test bench located in the BARC on the University of Washington campus in Seattle. For manual testing, two experienced Boeing mechanics install an identical set of rivets as the Test Bench. This part of the test gives insight into how well the test bench replicates the riveting process.

Performance quantificationIn order to determine how well a riveting tool performs, the quality of the rivet is observed. For a rivet to be within specification, the tail (the end of the rivet on the bucking bar side) height and diameter must be within predetermined bounds specific to each type of rivet.

Results

To analyze the data collected, the method of least squares was used on the laser data and a Fast Fourier Transform was conducted on the acceleration of the bucking bar. Both methods are later discussed in detail.

Manual Projected CurvesThe method of least squares is a standard approach in regression analysis to approximate the solution of overdetermined systems, i.e., sets of equations in which there are more equations than unknowns. "Least squares" means that the overall solution minimizes the sum of the squares of the residuals made in the results of every single equation [2].

The first clear and concise exposition of the method of least squares was published by Legendre in 1805 [2]. The objective consists of adjusting the parameters of a model function to best fit a data set.

Using the laser, the rivet height was being tracked during the riveting process. Then the data was imported into Matlab and filtered using a low pass frequency filter in order to reduce vibration peaks. A least square fit was conducted on the filter and a second order polynomial was approximated. Figure 2 below shows an example of how the laser data looks like and how the second order polynomial fit approximates the data.

FIGURE 2: EXAMPLE OF A 2ND ORDER CURVE FITTING IN PREDICTING THE RIVET FORMATION WITH TIME.

During the manual testing, laser data was not collected, but only the cycle time and final rivet height. Using the 2nd order polynomial found for each tool permutation from the test bench, the predicted final height of the rivet was calculated, and an example can be seen below in Table 2.

TABLE 2 PERCENT DIFFERENCE BETWEEN THE ACTUAL BUTTON HEIGHT AND THE EXPECTED ONE CALCULATED WITH THE 2ND ORDER FIT APPLIED TO THE MANUAL TESTING.

Hole Operator Gun type

Bar type

2nd order fit coefficients

Actual Button

height [in]

Expected Button Height

[in]

Percent Difference

[%] H=at^2+bt+

c

1 Manual AVC-27

SBBM a = 0.0080 0.1805 0.1805 0.03

2 Manual AVC-27

SBBM b = -0.0691 0.1765 0.1832 3.78

3 Manual AVC-27

SBBM c = 0.3841 0.1815 0.1860 2.49

4 Manual AVC-27

SBBM 0.1770 0.1819 2.75

5 Manual AVC-27

SBBM 0.1845 0.1901 3.04

Finally, to compare automatic and manual installation times for each tool, the time to reach a button height of 0.200 in was calculated using the polynomial fit for the manual testing and the

laser data for the test bench. Figures 3 and 4 compare the formation rate of each permutation during automated and manual testing.

FIGURE 3 AVERAGE REQUIRED CYCLE TIME TO FORM RIVET TAIL HEIGHT WITHIN SPECIFICATION (AUTOMATED INSTALLATION).

FIGURE 4 AVERAGE REQUIRED CYCLE TIME TO FORM RIVET TAIL HEIGHT WITHIN SPECIFICATION (MANUAL INSTALLATION)

As expected, manual testing takes longer than the automated testing to get the rivets’ tails height into specification. From these results, the TBB and SBBM reduce cycle time relative to the SBBG. We can also see a slight decrease in cycle time in the TBB compared to the SBBM.

Rivet Gun and Bucking Bar AccelerationsA fast Fourier transform (FFT) is an algorithm that samples a signal over a period of time (or space) and divides it into its frequency components. These components are single sinusoidal oscillations at distinct frequencies each with their own amplitude and phase [3]. Figure 5 illustrates how a signal can be seen in the time and frequency domain using an FFT.

FIGURE 5 VIEW OF A SIGNAL IN THE TIME AND FREQUENCY DOMAIN.

Then the results have to be weighted according to ISO 5349-1(2001). The standard is about mechanical vibration: Measurement and evaluation of human exposure to hand-transmitted vibration [4].

All results are derived from acceleration data recorded using shock accelerometers mounted on the rivet gun as well as on the bucking bar.

Figure 6 below shows an example of the raw acceleration data collected from the shock accelerometer mounted on the bucking bar.

FIGURE 6 EXAMPLE OF THE SHOCK ACCELEROMETER DATA WHEN IT WAS MOUNTED ON THE BUCKING BAR.

In Figure 7, a graph of the power spectral density was generated using an FFT on the raw data.

FIGURE 7 POWER SPECTRAL DENSITY GRAPH SHOWING THE DOMINANT FREQUENCIES IN THE SIGNAL TO BE 750HZ AND 2500HZ.

Figure 8 shows the weighting curve of the ISO 5349-1 standard.

FIGURE 8 WEIGHTING CURVE OF THE ISO 4349-1 STANDARD SHOWING THAT LOW AND HIGH FREQUENCIES GET FILTERED OUT. AND FREQUENCIES AROUND 15 HZ ARE THE MOST SEVERE.

Using the Root Mean Square (RMS), and after weighting the accelerations, RMS values were calculated. The root mean square is defined as the square root of the mean square [5].

A similar analysis was done on all the runs of all nine configurations and the results are summarized below in figures 9 and 10.

FIGURE 9 ISO 5349-1 RMS SCORES FOR THE BUCKING BAR SHOCK ACCELEROMETERS WITH THE TEST BENCH TESTING.

FIGURE 10 ISO 5349-1 RMS SCORES FOR THE BUCKING BAR SHOCK ACCELEROMETER WITH MANUAL TESTING

The AC12P proved to be the superior gun in both vibration emissions and in rivet formation. It reduced gun vibrations by 89% ± 7% over a 95% confidence interval.

Tungsten is superior to steel for bucking bars. Within a 95% confidence interval, a tungsten bucking bar forms rivets 14.6% ± 7.1% faster than an equivalent mass of steel. A tungsten bucking bar decreases the vibration emissions to the mechanic by .66% ±4.7%, but this is not statistically significant.

Simulation

To predict the results without having to physically test different combinations a model of the gun and bucking bar is being simulated via Simulink. Simulink is a graphical programming environment for modeling, simulating and analyzing multi-domain dynamical systems [6]. The model is in its early stages and no concrete results have been established yet. The model can now predict the bucking bar’s position and rivet formation. Also, it uses parameter estimation to find the system’s characteristics: stiffness, damping, and friction. Parameter estimation is a branch of statistics that deals with estimating the values of parameters based on measured empirical data that has a random component [7].

Figure 11 shows the early stages of the model developed on Simulink.

FIGURE 5 SIMULINK MODEL USING THE SIMSCAPE TOOLBOX. THE MODEL REPRESENTS THE BUCKING BAR ONLY.

Conclusion

From the results, it is recommended that the standard gun used in the riveting process would be the AC12P. Its recoilless feature dampens out vibrations and absorbs a lot of the energy that would be transmitted to the mechanic.

The tungsten bar allows mechanics to work more efficiently than an equivalent mass of steel. Although the rate of vibration exposure is identical, the reduction in cycle time proportionately reduces their total vibration exposure at work.

Regardless of tool choice, the vibration emissions from the bar are still very high for the hand-arm system. Further research should investigate plastic-based bar grips that have a damping equivalent to the hand-arm system and a recoilless bucking bar.

References

[1] The Library of Manufacturing, thelibraryofmanufacturing.com/riveting.html. Accessed 1 June 2018.

[2] Wikipedia, https://en.wikipedia.org/wiki/Least_squares. Accessed 1 June 2018.

[3] Wikipedia, https://en.wikipedia.org/wiki/Fast_Fourier_transform. Accessed 1 June 2018.

[4] International Organization for Standardization, https://www.iso.org/standard/32355.html. Accessed 1 June 2018.

[5] Wikipedia, https://en.wikipedia.org/wiki/Root_mean_square. Accessed 1 June 2018.

[6] Wikipedia, https://en.wikipedia.org/wiki/Simulink. Accessed 1 June 2018.

[7] Wikipedia, https://en.wikipedia.org/wiki/Estimation_theory. Accessed 1 June 2018.