Saturday, 25 May 2019

3D printing a crocodile

I have to say I was surprised how enthused everyone was for the 3D printed crocodile. It was the most interacted tweet I have ever done, and as promised I am ensuring all of the files are available so if people are keen they can do their own:

All files will print on a print bed that is 20x20cm, and are all scaled to the same size. However do check and scale up or down by 10 if required.

Forelimb (L - R)
Posteriormost Caudal vertebrae
So for anyone wondering how I put it together, it was fairly straight forward, albeit time consuming. If you don't have a set of 3D files (in this case ours were generated from CT scans and segmented in Mimics), there are an abundance of online 3D scans now in places like Morphosource, or and even thingiverse (the Makerbot repository where I've uploaded my bits), but I strongly encourage you to check the specifics with regards to the use of their scans and to only print things you are allowed to.

The next steps involve tidying up your meshes, e.g. decimation of your mesh to make it smaller/easier to use, and remeshing to standardise triangle sizes etc. I recommend Meshlab for a lot of this as it is free. Google is also full of how-tos, but I am happy help. Feel free to reach out here or Twitter.

I then separated the sections I wanted using the select tool and deleting the unwanted bits. This could, and should, be done in segmentation if you have the scans, but I have to admit I wasn't sure how I was going to separate things when I first started. You will also likely get 3D models online that need separating, so not bad practice. In the case of the vertebral column where I separated the different regions, holes were created in the mesh (where the vertebrae were connected as a single unit). I fixed these in Blender (again a free download) by selecting the nodes in Edit Mode and making new faces to fill the holes.

For sections I wanted to fuse together I created a cylinder where an end was in a bone of interest.
Right lower leg and foot with cylinders shown between all the bones.
In the case of the osteoderms, I created a scaffold of cylinders that would sit on the vertebrae, and then vertical cylinders to attach to the osteoderms:

Scaffold for the dorsal osteoderms (from shoulders to pelvis) in the foreground, with complete scaffold with fused osteoderms for pelvis in the background.
Originally, I used the Boolean tool in Blender to union join the various parts and the cylinder:

but I found that this was not necessary for our printer/software (printing done on a Ultimaker 3 extended, Cura software) so later parts just had cylinders and bones saved together as a single obj. Your experiences may vary!

I printed all of the model on the Ultimaker 3 as I said above, using a normal 0.15mm layer thickness, automated support in the zig-zag shape with chunks enabled, and either a raft or brim support using generic PLA (2.85mm thickness) purchased from Amazon. We have the option of using 2 materials, and this probably would have been better to print using soluble material (PVA) for the support as it took a long time to manually remove the supports but as this was a test it was cheaper and far quicker to print just using the one material. It is not a perfect method and some of the smallest bits have really thin supports and broke during separation so some superglue was used to reattach them. I suspect something like a soldering iron (anything that gets up to 200C) to quickly remelt the ends could also work but please use caution and common sense!

Because our model was printed in chunks (due to the 20x20cm plate), but the original animal was obviously fully connected I created some supports for the dorsal vertebrae to get everything to the appropriate height and to ensure a realistic look. This was achieved by creating a cylinder or cube in Blender, and scaling it to be about 1.5 times wider than a single vertebrae, and a bit longer than 2 vertebrae. I then boolean subtracted the vertebrae from the cylinder, creating a vertebrae cradle. The concave surfaces of the cradle (where there are overhangs) were deleted in the edit mode (and new faces were made to fill the holes) so that the print could be slotted into the cradles.
Vertebral cradle
A disc at the horizontal level of the sternum was created and then a cylinder (other shapes available) created to connect the two (same method as for connecting bones). For the support of the vertebrae over the sternum I created a hollow where the sternum was so that the support would sit flush on the ground with the sternum passing between it by boolean subtracting a slightly enlarged sternum from the underside of the disc (the same procedure as for the vertebrae and cylinder except now for the disc).
In place in the model showing off the front support also has a gap for the sternum.
For our printer there is less support created if you change the shape up to whatever the minimum overhang angle (45-60 degrees depending on the machine) so other shapes may be better for print times/reducing support structures.

I hope this all makes sense, let me know if it doesn't and I'd love to see what you all achieve. If any of the meshes misbehave, let me know and I will try to upload corrections.

1 comment:

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