Gas station without pumps

2021 August 23

Dragon key holder

Filed under: Uncategorized — gasstationwithoutpumps @ 15:18
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In 3D printed names, I showed a key holder for my home and office keys, which finally failed after 3 years.  My family has been bugging me to replace it anyway—pointing out that it is stupid to have one’s name on a key holder, as it makes it easy for anyone to figure out what doors the keys are for if they get lost.

broken-name-key-holder

The key holder cracked at one of the bolt holes. It was still sort of usable (the keys were still constrained by the bolt and the cover plate was not yet coming loose), but it looked bad and would sometimes snag when I was trying to get it out.

I decided to make a new key holder with a dragon motif, based on a sweater from Past Times that has gotten too moth-eaten to wear, but that has a pattern I like.

Past-Times-sweater

I took only the dragon part of the pattern, which is 18 pixels by 109 pixels (20 by 111, if you include a 1-pixel white border).

My first job was to transcribe the knitted pattern onto squared paper, then type it into the computer.  I had hoped that OpenSCAD’s “surface” function (which I used successfully for the quantum dot pendant) would let me convert this image easily to a 3D relief, but it did a terrible job, as each black dot became a sharp peak.

I ended up using the Pillow fork of the PIL Python package to manipulate the image and export it in PNG format.

    im_5=im.resize([5*(num_rows+2), 5*(num_cols+2)], Image.NEAREST)
    im_max=im_5.filter(ImageFilter.MaxFilter(3))
    im_big=im_max.filter(ImageFilter.SMOOTH)

I resized it by a factor of 5, ran it through a “max” filter to spread out the black, then smoothed it.

dragon5_smooth

The PNG file is the negative of original image. Unfortunately, the checkerboard grid does not print well at the size I needed to make the image.

hollow-dragon5_smooth

I edited the pixels manually to get a somewhat more printable shape, then did the same sort of spreading and smoothing.

I initially printed just the dragon on a little oval, to see how it would come out, before adding the rest of the key-holder design from the original key holder.  Because OpenSCAD produces huge STL files when the “surface” function is used, I simplified the STL files with https://myminifactory.github.io/Fast-Quadric-Mesh-Simplification/

3-test-samples

Here are the 3 samples. The top one tried just expanding the pixels to 5×5, the second one did the smoothing, and the third one got rid of the checkerboard patterning.

failed-prints

The first two prints were failures. The top print was done face-down on the glass bad (as I had done the previous key holder), but there was too much spreading on the first layer. The second print failed because the glass plate rotated during the print—I had to add hot glue to it again, to keep it in place.

dragon-keyholder

The final key holder is ok, but it would really look better if I had a smaller nozzle (the 0.4mm nozzle limits the horizontal resolution).

dragon-key-holder-with-keys

Here is the finished key holder with keys—I expect that it will last another 3 years (though I did use 50% fill this time, to make it a little stronger).

I’ve uploaded the design to https://www.thingiverse.com/thing:4939976.

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