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Accuracy and resolution used for the specimen reproduction depend on the scanner and its proper use, and the skill of those operating the CNC/rapid prototyping machines (Kadobayashi et al., 2004; Remondino et al., 2005, 2008; Bitelli et al., 2007). Specimen features measuring less than 1 mm will be lost due to improper scanning or editing (mainly if the scan left many and large holes to fill in the post-processing, something that can be avoided by doing more scans from more different angles). Good scan data can be belittled by improper use of the CNC/rapid prototyping machine.

As scientists, teachers, and museums have different requirements concerning costs and accuracy, the proper reproduction technique must be chosen depending on their intended use. For numerous analyzes in functional morphology or anatomy, highly accurate models are needed, and error margins resulting from inaccurate models have to be considered. Especially when studying such delicate and complex structures as diplodocid cervical vertebrae, accuracy should be considered more important than costs.

3D-printing is not only more accurate (0.08 mm compared to 0.1 mm in CNC-milling), but also better suited for producing models with complex and delicate structures such as vertebrae or skulls (Figs. 4a and 4b). The problem that the 3-D Z-Corp. printer at our disposal had a maximum build space of 300 mm x 400 mm x 300 mm, was circumvented by subdividing the vertebrae into smaller parts. Furthermore, in the case of the Z-Corp. machine, rapid prototyping machines are much less noisy and unclean than CNC-milling devices, and the remaining unused powder can be reused.

Advantages of the CNC-milling process are that the machines do not have such a limited building space as 3D printers. Especially the more elaborated 6- or 7-axes devices are thus very well suited for the reproduction of large objects with no or only minor undercuts (Fig. 4c). Moreover, as this method can be used with very durable and cheap materials like wood or metal, costs of models produced in this way are much less fragile and expensive and therefore ideal for educational use in schools or museums (Deck et al. 2007).

The complex shape of the cervical vertebrae of SMA 0004 required a large number of polygons to properly describe them. A 3D printer capable of generating slices small enough to replicate those polygons was therefore preferred in this case study. Using CNC-milling, the machine output would not have shown all the specimen features required for further studies.


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