From Raw Cutting Toward Precision Machining A History of Waterjet Technology First Edition by Unknown

Author:Unknown
Language: eng
Format: epub
ISBN: 9781040763131
Published: 2025-11-15T00:00:00+00:00

7.3.3 Stepped Mask Milling

One of the methods that was developed was the use of reusable stepped masks to just mask over the undercutting zone while minimizing the overall side wall taper. In this process, masks are sequentially replaced with successively larger masks to shadow the AWJ from the undercutting zones at the base of the pocket. Once the undercutting begins, the previous mask is removed, and a new mask is placed on the part. The new mask overhangs over to the edge of the undercut geometry. Once the new mask is applied, AWJ milling is continued until the undercutting appears again, and the process is repeated until the desired depth is achieved. Figure 7.17 shows an illustration of the stepped mask process. In this illustration, the hatched cross-sectional rectangles represent the cross-section of a mask.

Figure 7.17 Multiple masks to shadow the AWJ from the undercutting.⏎ One of the downsides of using this method is that the sizes of the pocket openings (mask overhang) must be calibrated to the erosion rates of the target material and the resulting wall taper that is created by the specific milling process and depth. In addition, the masks must be precisely positioned over the workpiece so that misalignment witness marks are not produced on the sides of the workpiece. Figure 7.18 shows a flat-bottomed glass sample that was produced with three masks. The corners were not matched well, the second mask was slightly misaligned, and an edge can be seen at mid-depth. This technique does show that the undercutting that is visible at the intersection of the two sidewalls has been prevented at the base of the pockets after the final masking.

Figure 7.18 Multi-masking approach to alter side wall channeling.⏎ Figure 7.19 shows a titanium sample milled to a depth of 18 mm. When viewing the part from one direction (left side of the figure), the corners of the pocket and side walls look great, and there is no sign of any undercutting at the base of the pockets. But when rotating the part 180 degrees and viewing the same pockets from the opposite side (right side image in Figure 7.19), it can be seen that the masks were slightly misaligned when the masks were exchanged, thus creating witness marks/edges. In both cases, there was no undercutting at the base of the pockets which shows the power of the stepped masking process.

Figure 7.19 Titanium sample milled to a depth of 18 mm with misaligned stepped masking.⏎ Figure 7.20 shows a 12.5 mm thick titanium sample that had a resized mask placed on the sample after about 80% of the depth was milled. In the image, it can be seen that the mask was not perfectly centered on the part. This resulted in one side of the pocket having more of a parabolic side wall geometry (left side of the pocket), and the other side more of a tapered side wall (right side of the pocket). Figure 7.21 is a silicon carbide mirror that is used with three masks during the milling process.

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