I had a question on how to use 3d files from the internet with the STL importer. I have been doing this for so many years I had forgotten that it is not obvious and something that I had to learn in the beginning.
Firstly, 3d models use several different file types. The most common is ".obj" but there are many, some being proprietary files systems as well. For example: ".obj", ".3DS", ".max", ".dae", ".34d",...etc. You get the idea. There are a bunch of different file types and they are not compatible with each other. The format of the files are different. The ".stl" format is just one of the different ways to store 3d information. Because the file formats and structure of the files are different, the software that interprets that information as a 3d model has to also be compatible or be able to read that format.
The STL importer add-on is only compatible with ".stl" format files. It only reads ".stl" files.
To use one of the other file format files, such as ".obj:, ".dae", in a program that only uses ".stl" like the STL importer you have to convert it first. There are several programs that can do this. Not all file formats are convertible, or at least not easily.
So to use any of the other format types you have to convert the format to ".stl". I use Blender 3d can convert ".obj" to ".stl", but there are several programs that can do it as well. There are many YouTube videos that show how to do this in Blender.
File > import Wavefront obj > Export Stl
Caution: Blender 3d is a pipeline 3d modeling animation package that is way overkill for basic cnc pattern development/construction.
3d models are built for different end results. 2d rendering, games, animation, simulations, etc. Not all 3d models make good cnc patterns.
Depending on the 3d model, you may have to clean up the mesh before exporting to ".stl" so it can be carveable. The STL importer does not like broken meshes. A mesh is a bunch of dots (vertices) that are mathematically connected to generate a plane. If the mesh has missing dots in a middle of a plane in visible locations, the STL importer tends to crash for me. So when importing a 3d model that tends to close (crash) designer you probably have a bad mesh somewhere.
There a probably millions of 3d models on the web. Only a small percentage of them can make good cnc patterns "as is" and the vast majority will also be in different formats from stl. To use them with the STL importer you will first need to convert them. They will likely also need some post work after importing. Many will be game assets. Meaning they were optimized with the lowest possible mesh geometry to reduce the memory needs for a game. Low number of dots, low number of memory and computer processor needed to display in a game. They were not made for cnc pattern generation. Doesn't mean you can't do a little work and make them a decent cnc pattern but it will not be a simple push button A and get a masterful cnc pattern. It will take work, knowledge and skill.
For example:
Here is a wonderful 3d model shared by luxox_18 for personal use.
https://free3d.com/3d-model/german-shepard-22904.html
It is in 3DS and OBJ formats. You need to convert it first to the STL format before importing into designer. But then we have an issue.
It is a game asset. To minimize the size in memory and processor needs they made it as small as possible. You can make a pattern from it but it will look horrible. So it needs post work.
The model was also split into sections so a picture of the dog could be painted on it. It is a way to visually add detail without using additional geometry. It tricks the human eye but not the computer. The computer is only looking for the geometry for our needs. So first thing is to merge all the sections into one object.
Next are all the triangles. Not a huge deal but triangles in tight areas make for points and can detract from the model smoothness. If not visible or something that is not exaggerated you can leave them. But I like to convert to quads (squares) for a smoother pattern.
Next there is not enough geometry to get a smooth model. So I subdivide the mesh to generate additional geometry. The more geometry the less the computer has to "guess" on how the planes need to be deformed. The software has about a 100 meg limit so you can't go crazy. The more geometry, the larger the memory needs.
Now you have a 3d model that can be imported into designer using the STL importer. Great! But you are not done yet. We are converting from 3d to 2.5d. The difference is in 3d you can look at the model in any angle. In 2.5d you can only look at the model in a single angle and since you are doing all this work to port to designer you are limited to the depth of your lumber dimensions and bit depth.
Again not all 3d models make good cnc patterns even after all that work.
So again some post work may be needed to push and pull the 3d mesh to optimize the port to designer. The Pattern Scultpor add-on is meant for this function.
You may only want a section of the pattern. The STL importer add-on does this function. It can slice and cut the pattern at different angles to port into designer. You can stack the cuts so can build a larger carving.
Enclosing the stl model of the dog for your stl importer practice.
Good luck.