Thursday, 24 September 2009

Brake hanger, part 3.

I got my brake hanger back today after my friendly fabricator Indiana had shaved off some 6 mm, leaving the full 15 mm only where the brake caliper bolts to the hanger. These are my findings on the subject of laser/abrasive water jet-cutting of thicker alu pieces:

"Daws" mentioned that the program I'm using – Adobe Illustrator – is not very good at generating AutoCAD-files and that the resulting curves are not as easy for the cutter head to follow as curves resulting from a proper CAD-application. And he does have a point. Looking at the cut surface (see image 1) it's obvious that the cutting head/water jet hasn't described perfectly smooth radiuses (radii?). It looks a littble bit as if the cutter head has paused/started/wobbled around a bit. This may of course be a characteristic of the water cutting method, but I suspect that it has something to do with the file as well. He also cautioned me that the cut surfaces won't be 100 "parallel" but rather have bit of taper - also a characteristic of the cutting method. Again, he was correct. It wasn't super obvious though and I'm not sure I would have noticed if I wasn't looking for it.

So, is it a worthwhile method? Yes and no. It's a quick and relatively cheap way of turning out pretty complex shapes without having to resort to more complex CAD/numerically controlled milling. On the other hand, the resulting pieces are nowhere near as perfect as pieces done using a mill. I spent three hours cleaning up and smoothing/straightening the edges in order to make it look like I wanted (see image 2). The surface isn't perfect yet – I'll leave the final polishing to the anodizers.

Conclusion: I think I'll save this method for thinner pieces in the future. And I'll use the freeware application "Solid Edge 2D Drafting" that Daws recommended.

2 comments:

  1. Regarding using AI files and getting bad results:

    Adobe Illustrator makes horrible DXF files. You need to use AI files directly. Or, if the waterjet machine you are working with can't accept AI files, then use a 3rd party convertor such as that from Kandu Software to convert from AI to a CLEAN dxf.

    If you look at the Adobe created DXF, and zoom in really closely, you will see that it is not smooth curves, but instead zillions of lines. What you are seeing in the final part is the faithful reproduction of these lines.

    Regarding taper on the edge: This is an inherent property of waterjet cutting that is avoided with the more sophisticated machines by simply tilting the jet in the opposite direction, eliminating the taper. If you cut on a machine that does not have such a tilting head, then you can reduce (but not eliminate) the taper by slowing the process down and employing a bunch of other tweaks. However, a tilting head is really a must if you want a taper-free part.

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  2. "Once again, things that could've been brought to my attention YESTERDAY!!!"

    Adam Sandler, the Wedding Singer.

    Joke aside, this is a learning by doing project, and I actually enjoy stumbling upon the occasional problem. I wanted to try this technique out on something that wasn't too difficult or expensive to replace if it went wrong. Thanks to generous people like yourself, who are willing to share their knowledge, I now now a lot more about waterjet cutting than I did a week ago, and that's success in my book.

    Today I got the chance to observe a true master craftman forming a tank from aluminium sheet, just using an English wheel, a TIG welder and a hammer. I love that kind of stuff!

    /D

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