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INTRODUCTION

 

Access to original material or specimens is limited for palaeontologists due to a number of reasons. Fossil material is rare and difficult to access (e.g. distance, museum policies, intense scientific interest). Fossil bones can be heavy or fragile to transport, or the desired element is mounted and on public display in a museum. Three-dimensional reproduction techniques therefore offer a great solution for archiving significant data of paleontological objects (Breithaupt et al., 2004; Remondino et al., 2005). Whereas in the early years of paleontology, physical casting of fossils has proved a valuable alternative way for researchers to undertake their investigations, as casts are usually lighter, less fragile, and easier to handle than original material, in the last 15 to 20 years alternative technologies have been developed to produce digital copies of original specimens (see Zollikofer and Ponce de León, 1995; Chapman et al., 1998; or Andersen et al., 2001 for some of the earliest attempts in Paleontology). Laser, x-ray, or magnet scanner, mechanical digitizers and photogrammetry methods transform matter into bits and bytes to create, manipulate, recreate and change objects assisted by a computer with editing software. For paleontological purposes the digital models can be stored and used for scientific research, spread via internet, or can be displayed virtually in museum exhibitions (Johnston et al., 2004; Mallison et al., 2009). Entire specimens can be measured providing accurate results for further research (Deck et al., 2004). Using simulation methods, bones can be articulated with each other without having to handle them manually – and thus without subjecting them to possible abrasion or damage due to real bone to bone contact (Chapman et al., 2001). They can be subjected to retrodeformation methods in order to re-establish a shape that is supposed to be closer to the original morphology (Motani et al., 2005; Kahzdan et al., 2009; Tschopp and Dzemski, in review now). Moreover, digital models of skeletal elements can be virtually connected with tendons and muscles to reconstruct the entire animal and to study in vivo motion patterns and biomechanical hypotheses (Walters et al., 2001; Bimber et al., 2002; Dzemski and Christian, 2007, 2010).

On the other hand, scientists often stress the importance of studying three-dimensional physical objects to manipulate them by hand and realize the dimensions of the dynamic elements. Physical models can furthermore be used for education and in museum exhibits, and have a great potential to indirectly protect the original objects (Mallison, 2007; Remondino, 2007; Schlader et al., 2007). Instead of the traditional casting process described above, there are now several ways to transform a digital object to a physical model. They can be subdivided into additive (layer by layer “printing”) and subtractive (e.g. carving, milling) techniques.  The additive method used for this case study (3D printing), as well as CNC-milling as example for a subtractive technique will herein be described, and their usage, advantages, and possibilities will be compared and discussed regarding the reproduction of the neck of the diplodocid sauropod SMA 0004 for research purposes.

 

Abbreviations: CAD: Computer Aided Design; CAM: Computer Aided Manufacturing; CNC: Computer Numerical Controlled; SMA: Sauriermuseum Aathal, Switzerland; STL: Standard Triangulation Language (a file format for 3D models).

 

METHODOS

 

In order to  reproduce a physical copy from the digital model, the following steps are necessary: scanning the object with a 3D-scanner, prepare the digital objects with 3D-software for the reproduction process, and then transform the digital model to physical models.

 

Scanning
 

Numerous ways to to create a digital model of the fossil have been reported (see Wilhite, 2003; Hohloch and Mallison, 2005; Mallison et al., 2009; Möller et al., 2009; Dzemski and Christian, 2010). The most commonly used techniques, as well as their advantages and disadvantages, are listed in Table 1.

In order to obtain proper physical models based on scanning data, it is crucial to be able to scan the original material with great accuracy (preferably below 1 mm, otherwise minute structures like small foramina would get lost). The structured light scanner Atos I from GOM, Germany, has proved a highly valuable tool in various research projects (Dzemski and Christian, 2007, 2010; Christian and Dzemski, 2007, 2011), and was thus also used for the scanning of SMA 0004. This industrial optical measuring machine is based on the principle of triangulation. Projected fringe patterns are observed with two cameras, mounted on the sides of the projector (see Fig. 1 for working setup). 3D coordinates for each camera pixel are  calculated with high  precision retrieving  a polygon mesh of the object’s surface.


 

Method

Subcategories

Advantage

Disadvantage

structured

light scanner

Beamer

Triangulation

Very fast scanning process and accurate measurements
Max accuracy: ~0.05mm

High price

Laser scanner

Volumetric Method

Hand-held laser
Laserline (software based)

Accurate measurements

Relatively low price

Max accuracy: ~0.3mm

Inappropriate for lucent surfaces

Mechanical digitizers

Physical contact measurement

Relatively low price

Relatively Slow

Inappropriate for organic structures, physical contact necessary

Computer

tomography

X-ray

Neutron

Accurate

Scans through hard material (stone, fossil)

Reveals inner structure

Micro-CT max accuracy: ~0.00007mm

Very high price

Limited availability

Magnetic resonance tomography

Magnetic field

Accurate

Ideal for soft (living) tissue

Clinical environment

Max accuracy: ~1mm

High price

Limited availability

Inappropriate for fossil structures

 Tab. 1: Selected scan methods with specific advantages and disadvantages.

 

Fig 1: Scan setting with the structured light scanner ATOS 1 from GOM. The scanner uses a single light source to project various light patterns on the object. Two cameras record the object and a software calculates the 3D matrix of the object by triangulation. For the specimen SMA 0004 30 scans from different directions per vertebrae were necessary in order to create the point cloud for the digital model.

 

The measuring system has a resolution of 800'000 points with a distance between two points of 0.12mm. Due to hidden surfaces depending on the view, around 30 scans from various angles were made of any single bone. The single vertebrae of SMA 0004 were placed on a rotating table that was turned after each scan by the same angle. In order to complete the scan from the other side, the rotating of the table had then to be repeated with the vertebrae placed in a different way. The scan of the 14 vertebrae and the skull of SMA 0004 took approximately 12 hours of scanning, and 6 hours of post-processing to create the digital model. The scanner software allows combination of single scans instantaneously. In the case of diplodocid cervical vertebrae, with their numerous cavities, it can be difficult to keep track of the areas the scanner did not reach yet. Thus, having a preliminary version of the 3D digital model in real time allows the immediate recognition of scanning holes. The objects to be scanned can then be orientated accordingly in order to achieve a more exact and less time-consuming procedure. Remaining holes have to be filled during the post-processing, which was done in the freely available software “Blender” for our case study (Fig. 2a). This step can be done automatically by the software, which provides good results for holes not exceeding the size of the polygons building the mesh to a considerable degree. However, larger holes should be treated manually, and the consulting of the original material or at least pictures of them is advisable in order to obtain an accurate digital model.

 

Preparing for reproduction

Depending on the technique to be used for the reproduction, the digital models have to be prepared in different ways. The STL file format is the standard format for CAD software and the transfer format to both CNC-milling machines and 3D printers.

 

For CNC-milling. In order to prepare the bones for CNC-milling, it is advisable to split them into two halves to avoid undercuts. Undercuts are areas of the object that are hidden by other portions of the element, and thus cannot be reached by the drill of the CNC-mill. The prepared elements have then to be transferred to a milling program that calculates the pathways of the CNC-machine (e.g. “DeskProto”, see Fig. 2b).

 

 

Fig 2: Software used to generate the 3D-files for CNC-milling and 3D printing. A: Blender with two camel vertebrae. B: DeskProto with a T-rex mandible and milling paths. C: Zprint with cervical vertebrae of SMA 0004.

 

CNC-mills (Fig. 3b) are classified in devices from 3-axes up to 7-axes. The more axes a machine has, the more degrees of freedom are possible: 6- or 7-axes CNC-mills are capable of milling a 360 degree object without interruptions, and are thus ideal to produce precise replicates out of wood, synthetic material, or metal for educational and scientific purposes (Deck et al., 2007). If one has only a 3-axes device at his disposal, an extra step in the preparation of the digital models is needed: the limited mobility requires to split the virtual model in several parts and load it into a control program (e.g. “WINPC-NC”) to control the CNC machines. Subsequent to the milling process, the different parts of the models have to be assembled in order to conclude the 3D replicas (Fig. 4c).

For Rapid Prototyping. As 3D printers are assembling the models layer for layer, the digital 3D object has to be divided into thin, printable slices. For this, the 3D-printer software Zprint from Z-Corp. was used        (Fig. 2c). Given the limited size for a printable object in the Z-Corp. device (Fig. 3a), larger objects have to be subdivided into smaller parts for this process as well – or they can be reproduced at a smaller scale. Especially for research purposes it can be very handy to have small physical models of large bones. This is particularly useful for elements of sauropod dinosaurs, and was therefore also done in the case of the vertebrae of SMA 0004. The digital models were rescaled by 1:4 before slicing them virtually, and loading them into Zprint. However, one has to take into consideration that by scaling elements down, small details on the bones get lost.

 

Reproduction process

The subtraction process: CNC-milling. CNC-mills (Fig. 3b) can perform the functions of drilling, bevelling, painting, cutting, as well as other tool tasks. These functions thus subtract material from a larger block in order to obtain the desired shape. With very advanced machines, a resolution of 0.1 mm can be achieved, but this depends as well on the thickness of the chosen drill. Any usual material (e.g. wood, metal, plastic) can be used, and the produced models are therefore very durable and well suited for educational use (Fig. 4c; Deck et al., 2007). Different drills specialized for the various materials are available on the market,

The additive process: 3D-Printer. 3D-printers are rapid prototyping devices using a layer by layer printing technique (Fig. 3a). The resolution of available printers goes from 0.2 mm to 0.08 mm.

A Z-Corp. printer was used for the case study. It reproduces slice by slice with one layer of polymer powder and a layer of binder. The cycle is repeated until the entire geometry has been processed. The device allows printing the 3D-object with colors to highlight important areas, or label the object directly on the surface. Finally, the obtained physical model has to be freed from the residual polymer powder by cleaning it with air and a brush. The printing results in a very fragile compound of polymer powder and binder. The object has then to be post-processed with a liquid plastic such as polyurethane or acrylic used in the prototyping business to get a solid surface.

 


Fig 3:Hardware used to reproduce the digital objects. A: Zprint 510 from Z corp. Build size 254 x 356 x 203 mm, layer size 0.08 to 0.2mm, equipment dimensions 107 x 79 x 127 cm, weight 204kg. In the background the air cleaning station can be seen.

 

 

Fig 4: Production results from 3D-Printer (A and B) and CNC-mill (C). A and B: 1:4 models of the vertebrae of SMA 0004, scanned in the Sauriermuseum Aathal, Switzerland. The high resolution of the 3D-Printer models allows a scientific benefit for morphometric analysis and functional anatomy. C: The CNC-milled Amargasaurus model was produced for a science center. In this case, the CNC-mill the best choice, because the reproduced objects have large dimensions, the level of details of each single bone is less important, and the technique is more cost effective.

 

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