Cutting Masks and Parts

The printers currently on the market all works pretty much the same way. The material to be cut is placed on a sticky “cutting mat”. The cutter moves the mat in one dimension, and moves the cutting blade in a perpendicular dimension (in some cases the cutter also rotates the blade). The result is complete two degrees of freedom, and naturally the blade is also lifted off the material when needed.

Using a cutter takes some getting used to, and you have to find “what works”, particularly when it comes to choosing materials and how much pressure to put on the cutting blade. The basic workflow is simple:

1. Prepare artwork.
2. Import artwork into the cutter program.
3. Adjust layout so that the parts fit the material at hand.
4. Cut!
5. Separate results from the cutting mat.

I have used a Cricut Maker cutter, but what I describe below roughly applies to other cutters as well. But remember, caveat emptor, and your mileage may vary.

First comes the preparation of the artwork for the parts to be cut. The Cricut Design Space software has limited capability of creating the exact shapes you might need, unless you only need basic geometric shapes like circles and rectangles. I use Adobe Illustrator which is pricey but gives you incredibly good control over what you create. I have found that the best graphics format is SVG (Scalable Vector Graphics) because not only can Illustrator save and edit SVG documents, but the Cricut software can directly import this format. SVG is what we call an “vector graphics format”, and this means it can be scaled without any loss of precision, unlike “rasterized” or bitmap formats (like JPEG, for example). Illustrator also gives you precise control over the size of the artwork, and this means that when you create your artwork in the correct size (for the end results), there is no need to scale the artwork once you import it into the cutting program.

Once you have imported your artwork, you need to lay out the separate pieces to be cut. The software will do this for you, but does not understand about cutting masks (where you need to leave enough material around whatever hole you are cutting). I have found that using the “attach” function in the Cricut program makes a single shape from multiple shapes, and the constituent parts of that shape are not moved with respect to one another when the program performs the layout. This technique lets you place holes already in your graphics program to be sufficiently far from one another, resulting in usable masks.

When you cut masking tape or film, you effectively always get two masks: a piece that can be used to cover, say, a window, and a mask with a similar hole in it, which can be used when painting on top of already applied paint (say, when painting white circles on top of which to apply roundels). This "figure & ground" consideration is useful and gives you options for how to actually use the masks.

Simple Example

Here is a simple example of using the cutter. I needed new wheel well doors for my Me 262 project. Not only should the doors have been thinner than what came in the kit, I was converting to a variant with a modified nose wheel arrangement and thus needed different doors than what the kit gave me.

First, I scanned Me 262 scale plans and used those to trace the gear doors. These formed the outer surface of the doors. I used Adobe Illustrator's "offset path" function to get another set that is slightly smaller than the original; these formed the inner surface (see the last photo). I save my artwork as an SVG file; you can download it here.

My artwork for normal and U4-variant gear doors.

Next, I uploaded the SVG file into the Cricut Design Space software, and then used my Cricut Maker -cutter to cut the parts from 5 thou styrene sheet. Cutting plastic this thin is easy, I only needed the cutter to score each cut twice. For thicker plastic you need more pressure and many more cuts (these are all configurable in the cutting software). If you look at the photo of the actual cutting process, you can see that I removed the regular nose gear doors before cutting; this is possible in the cutting software itself.

Finally, I assembled the parts by gluing together each larger part with the corresponding smaller part. I first made a mistake and got a mirror image of the nose gear doors, but since I could cut more parts, this was not a problem. This is one of the real benefits of a computer-controlled cutter: you can get consistent and predictable results no matter how many copies you need to cut.

Cutting gear doors for the U4-variant. I used 5 thou styrene sheet.

Gear doors assembled.

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