Tyrannosaurus rex - Splitting large models

Contributed by
Jacky Wan

Have you ever wanted to print a model that is way bigger than a build volume of your 3D printer? I know you have! Certain objects are simply designed to be so huge, that even the Ultimaker 2 Extended+ won’t be able to print it all in one go. Take the Naturalis Biodiversity Center's Tyrannosaurus rex bones for example. How can you bring one to life without compromising on the quality? Here’s a detailed guide to help you learn how to split, print and re-combine impressively large models.

What you need

Naturalis T.rex bone
1.2m long T.rex proximal femur print for Naturalis

The Naturalis Biodiversity Center will feature bones 3D printed on Ultimaker printers to complete the Tyrannosaurs rex skeleton they are currently excavating. By making 3D scans of the bones they do have they can mirror the model to recreate the missing side. One of the largest bones they are printing is the thighbone (femur). It consists of two halves, one of which is 1.2 meter and the other 0.8 meter long.

You can download the Tyrannosaurus rex thighbone model, which is based on the real-life 3D scans made available courtesy of the Naturalis Biodiversity Center, from YouMagine.

There are two versions, one set of the original Naturalis models for following this guide, and another set already split up and ready to go if you are just interested in printing one yourself.

For this tutorial we will show you how to split up each part of the thighbone into two pieces. The print used in Naturalis is much bigger and contains far more pieces, but the same principles would apply if you wanted to create a bone on that scale.

You will also need a 3D modeling program. There are many free programs that can accomplish this task, such as Fusion360 and Tinkercad. While each program operates differently and we won't explain how to perform the operation in each specifically, the basic concepts contained in this guide can be applied to all of them.

T.rex thighbone models

Begin by loading the top and bottom bones into your software. Note that when you import an .STL file, it may not retain all of the editing capabilities that you are used to for your platform.

First, scale all parts down 50% and orient them by rotating so they are easy to work with. You can work on one bone at a time to make it easier.

Find the function in your software that will cut a model in half. After determining how many parts you want to split your model, it is time to choose the best place to cut the model. This bone will fit comfortably on an Ultimaker 2+ in 2 parts, so we will find a good midpoint for our cross section.

T.rex thighbone slice options

In this picture, you see two options on where to cut the model. The left is the obvious split location. This is OK if you’re not familiar with the modeling software or just require a quick cut. But for those wanting to optimize the cutting process, the right example is a much better solution.

T.rex thighbone optimized slice

Here you see how the pieces will be oriented on the build plate. Since the cut provides a clean flat surface, the cut surface is the ideal orientation for touching the build plate and will provide good adhesion with a large surface. Thanks to the glass bed on the Ultimaker 2+ this cut will allow the pieces to be joined together nicely with minimal seam lines.

But most importantly this angle eliminates the need for supports. Because while it appears unintuitive to print the blue part at an angle, with this solution none of the angles in the model exceed a 45-degree overhang which means it will print well without support. This is the primary consideration for the angle of the slice.

Explaining support slice decisions

If we split the mesh at an angle (left), we eliminate all of the 90-degree overhangs. If we simply split it in half, we would get a rather large overhanging part which would require us to use support material. Supported surfaces are usually not as clean as unsupported, use more material and require cleanup after printing. So if this can be avoided by the angle of the slice it should be done so.

T.rex thighbone optimized slice 2

Here is the solution we’ve chosen for the second bone.

T.rex bone half done

Clean print, no supports or brims needed. Ready for assembly right off the printer! Be sure to wait for the print bed to cool down naturally for a nice flat surface on the bottom.

Key and slot

T.rex bone slot and key

After the meshes are split, we can optionally create a slot and key so that when we glue the two pieces together there is no guesswork as to the alignment of the parts. It will create a stronger joint and additional surface area for the glue to stick.

T.rex key and slot

In this example, we use an extruded hexagonal shape as seen in this picture. Using a boolean subtraction operation in the modeling software the red object is removed from the bone model, resulting in the slots in the prints that we will fill with the key. The green object is the key, printed separately and inserted in between the bone halves during assembly.

The red subtraction object is 0.2mm larger than the green key to ensure a tight fit, but not so tight that it is impossible to insert without sanding. The subtraction object tapers to a point so there are no overhangs inside the print. For simplicity, you can also use a box shape instead of a hexagon.

Assembling the t.rex thighbone

Printing

We suggest using Ultimaker pearl white PLA for this model. It is white with a slight iridescence and prints with a very good surface quality. It looks like a natural bone color when printed. PLA is generally a good material for artistic and display models. It provides a very crisp clean edge, and the lower temperatures and predictable characteristics make it a reliable choice for large, long prints. PLA also has notably low warping characteristics which makes it ideal for multi-part prints. You require the base to be as flat as possible to minimize the visible seam between the parts.

We also suggest using a 0.8mm nozzle to reduce print time. There are little to no details here that are smaller than 0.8mm so you will not lose quality yet still gain a dramatic increase in speed.

Suggested settings

Nozzle:0.8mm
Print speed:45mm/s
Infill:6%
Nozzle Temp:210°C
Layer Height:0.12mm
Top/Bottom Thickness:  0.8mm

A 6% infill with a 0.8mm top/bottom thickness is just enough to keep the top surface clean while using the minimum amount of material. The weight of the final, full bone is 887g.

T.rex bone slot and glue

After printing, test fit the key into the slot. Occasionally the bottom layer will expand a little bit as the material is pressed onto the glass and will require a little bit of cleanup before the key will fit. Apply super glue and press the pieces together.

Half T.rex thighbone

Repeat for the other side – and you’re done! A virtually seamless T.rex bone. Download the files and try it out for yourself now! As always, if you require any help or have questions about this tutorial feel free to visit the forum. And if you do print the T.rex bone, don't forget to upload them to the 3D Print section!

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