On Making Cookie Cutters

Tux Cookie Cutter - solid model

Tux Cookie Cutter – solid model

Someone asked about how to convert a PNG file to a cookie cutter; she was stymied by some of the terminology and didn’t have a good overview of the process. I thought my reply might be useful to someone else.

trying to understand how to create an STL file

The key to understanding 3D printing is to realize that an STL file is just an intermediate step along the way from an idea to a plastic object. The real challenge is to create a 3D (aka “solid”) model of the object you want; after that, the rest follows more or less automatically.

The overall process goes like this:

1. Create a solid model (more on this below)
2. Export the model as an STL file, usually by a menu selection
3. Convert the STL file to G-Code using the printer control program
4. Extrude plastic!

Now, each step has many sub-steps, but that’s the Big Picture.

You really don’t care about the STL or G-Code files, because they’re generated from the 3D model.

take a png clipart image

Because a PNG image represents a 2D (flat) drawing, it can contain grayscale (or color!) information that you may not want. Let’s start with just a simple black-and-white outline drawing that shows the outline of the cutter.

The CAD program then “extrudes” that flat image into a 3D shape with a known height; I use OpenSCAD, but any 3D program should be able to do that trick. If you started with a PNG file of a circle, the extrusion will produce a 3D ring. If you start with an outline of Tux, you end up with an oddly shaped 3D ring.

(Note that the term “extrusion” has two meanings. The CAD program extrudes the flat 2D image into a 3D model and the printer extrudes molten plastic to form the object. Gotta love the language!)

Now that you have a basic 3D shape, you can fancy it up with thicker areas and handles and whatnot, but you could just print the shape and have a simple cookie cutter.

Dr Who Cookie Cutters

Dr Who Cookie Cutters

If you’re making a cookie press, similar to those in the Dr Who cutters, then you start with a grayscale PNG (or JPG) and create a “height map” where the grayscale intensity determines the extrusion height: black = high and white = low (or the other way around). Again, any CAD program should be able to create a height map from a grayscale image.

In fact, a black-and-white outline is just a simple version of a height map: it’s either tall (black) or short (white), with no levels in between.

The height map becomes a 3D rectangle with one wavy side corresponding to the image. You join it with another rectangle to set the minimum thickness (you don’t want holes where the image was white), then add handles and suchlike.

That’s how the Dr Who cutters work: a height map generated the flat press part and an outline generated the hollow cutter surrounding the press. The settings I used to print my copy may be helpful.

You may have already seen my blow-by-blow description of converting an EPS drawing to the Tux cutter that starred in the movie.

The process had far more complexity than it should, mostly because that old version of OpenSCAD had a few bugs that prevented me from using 2D-to-3D extrusion as I described above. The overall process was similar, though: start with a 2D shape, convert it into a long 3D rod, slice off a suitable length, then punch a hole in the middle.

So, by and large, in order to make cookie cutters, you must master the “extrusion” part of 3D modeling. After you get a suitable 3D model of the cutter, then the rest will be easy! [grin]

Hope that helps get you started…

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  1. #1 by Cristin (Pinkie) of Cristin's Cookies on 12-October-2012 - 02:53

    I’m still at a loss how to do this in SketchUp. I have the cookie cutter itself pretty much figured out, but the press is still a complete roadblock for me. I think you should become a master of SketchUp, not OpenSCAD, and then create a tutorial video showing exactly how to make a cookie press and send the video to me. Yep, that’s what I think. There could be cookies in it for you Ed. Be the hero. :-) Cristin

    • #2 by Ed on 12-October-2012 - 08:46

      As nearly as I can tell, Google Sketchup is the wrong hammer to create a 3D model that you intend to produce with a 3D printer. It’s not actually a 3D program, because it works with surfaces, rather than volumes, and it’s remarkably easy to produce a model with missing / misaligned / incorrect surface facets. Such models simply can’t be converted into a proper STL representation for conversion into G-Code.

      A quote from Sketchup Support that I used in a talk for the local ACM chapter (screen 43) sums it up pretty well:

      SketchUp is not the same as Computer Assisted Design or CAD. CAD applications are designed specifically for representing concrete information, while SketchUp is for exploration and design of concepts and ideas (though you are not prohibited from designing models that are as concrete or accurate as those designed in CAD).

      I think you’ve just discovered that on your own… [grin]

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