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This section will discuss the experimental setup and approach. The same simplified cross-member structure assembly from chapter 4 is again investigated, however an 8 clamp assembly configuration is explored. A purpose built experimental jig, consisting of locating pins, manual toggle clamps, and a hand held weld gun mounted on linear slides for accurate weld placement, was used for the assembly process. This is illus-

64 CHAPTER 5. EXPERIMENTAL COMPARISON OF CLAMP SEQUENCES

trated in Figure 5.1, where the clamps are also identified A through H for description of clamping sequences. The remaining of this section will present the two clamping se- quences investigated for the experimental trials, the key component variation mode that is used to represent typical process variation in stampings, the measurement points that are extracted from the scan data, and finally the approach used to select component samples for the experimental trials.

Figure 5.1: Experimental assembly rig.

5.2.1 Experimental and FEM clamp sequences

Preliminary clamp sequence trials on the experimental assembly rig were conducted to determine two clamp sequences to explore in detail. This involved the application of a range of clamp sequences on an example set of components (without welding) to give an indicator of the assembly response to these different clamp sequences. From the range of clamp sequences explored in this preliminary trial, two clamp sequences which exhibited noticeably different assembly shapes were selected for further investigation with both experimental and FEM assembly setups. One sequence applies the outer clamps and then the middle clamps, and the other sequence applies the inner clamps and then the outer clamps. The sequences according to the clamp indicators noted in Figure 5.1 are as follows:

• Clamp sequence 1: firstly the middle clamps are applied, followed by the the outer clamps. This corresponds to the following clamp sequence as indicated in Figure 2: F-G-C-B-A-E-H-D.

5.2. EXPERIMENTAL CLAMPING STUDY 65 • Clamp sequence 2: firstly the outer clamps are applied, followed by the the middle clamps. This corresponds to the following clamp sequence as indicated in Figure 2: D-H-E-A-B-C-G-F.

5.2.2 Component variation

An artificial variation mode, negative bow in the top hat, was fabricated to simulate the type of variation that could be present in such a component due to variations in the stamping process. The nominal hats (without bow) were fabricated by folding and laser cutting at pieces of sheet metal. A bow in the hat was then created using a simple hydraulic press setup with tooling made from aluminium plates (see Appendix A.2). All top hats had the same level of bow, a middle de ection of 5 mm. This level of bow is probably more severe than what would be observed in an industrial setting, but it was selected so that any differences in assembly shapes would also be more severe, and therefore more easily discernable to the naked eye: this enabled a greater intuition to be developed about the assembly response to different clamp sequences. It can be expected that any observed trends would also hold at less exaggerated levels of bow. An example top hat with a 5 mm bow is illustrated in Figure 5.2. It should be noted that only hat section variation is considered (and not variation in the at bottom plate). This is because the hat is the primary structural member, and has a much greater in uence on final assembly variability than the bottom plate.

Figure 5.2: Sample top hat highlighting the bow variation mode of 5 mm. Colour map indicates the displacement between the bowed and nominal assembly (mm)

5.2.3 Measurement point extraction

The advancement of optical measurement techniques has allowed for more rapid and accurate dimensional inspection of parts. For this study, a 3D laser scanner was used to capture point cloud measurements of the sheet metal stampings and assemblies. Measurement points (MP's) were extracted from scanned data to give data points similar to that obtained by traditional CMM measurement procedures. Here, a surface

66 CHAPTER 5. EXPERIMENTAL COMPARISON OF CLAMP SEQUENCES

measurement point on a ange or the roof was taken as thez-displacement of a specified co-ordinate in thex-y plane, and a measurement point on a side-wall was taken as the

y-displacement of a specified co-ordinate in the x-z plane. Due to the measurement capabilities of the scanner, 1000's of measurement points were extracted in 5 mm grid formations across the anges, side-walls, and roof. Figures 5.3 (a) and (b) show 3D scan data, and corresponding MP's sampled at 5mm grid spacings along each surface. It should be noted that while the final assembly consists of the top hat and at bottom plate, only the top hat is measured for analysis purposes due to measurement access restrictions, and as it was determined to hold the features of most interest.

(a) (b)

(a) (b)

Figure 5.3: Figure 5.3(a) displays example point cloud data from a 3D laser scan. Figure 5.3(b) shows MP's that were extracted from the scanned data. Some areas could not be measured due to physical obstructions (ie, locating pins and clamps).

5.2.4 Component sampling

A population of 30 top hats with a bow of 5mm were fabricated. These samples were then plotted in a multivariate reduced dimensional space, which allows for most of the information in the 6000+ MP data set to be visualised in only 3 dimensions. The steps for performing a Principal Component Analysis decomposition of this 6000+ MP, 30 sample data set can be seen in section 3.3.2. The 30 measured experimental samples were plotted in a 3D PCA space in Figure 5.4. The first purpose of this plot was to identify outliers. The outliers would not be used for assembly as this study was concentrating on the in uence of clamping sequence, not initial differences in part shapes. The second purpose of this plot was to split the population into pairs of nearest neighbours: one member from each pair would then be built with clamp sequence 1, and the other member built with clamp sequence 2. This was done to ensure that similar populations were being assembled by each clamp sequence, in order to minimise the in uence of differences in initial component populations on any observed shifts

5.3. EXPERIMENTAL RESULTS 67