Keywords

1 Introduction

The way companies communicate has changed. Companies send most of their information digitally instead than on paper and it is considered as a great step forward in reduced time and money. One of the ways to save money is the business mindset of Product Lifecycle Management (PLM).

Inside the PLM system, the data from which the information is extracted, are heterogeneous by their type (photo, video, text file, CAD file, etc.); the CAD data can be heterogeneous by their formats: legacy formats (CATIA, CREO, NX, etc.), neutral formats (STEP, JT, IGES, STL, etc.), but also from the expertise that created the data or which it is intended for. Thus, the links and the dependences, which appear between heterogeneous CAD data, are becoming more complex during the daily activities of users. When products are developed in 3D for the engineering domain, the data are initially stored in the legacy file format of the used CAD software. If this 3D CAD data is to be made available to people who do not have this software, neutral 3D formats are needed. Some file formats are: 3D PDF from Adobe, X3D, U3d from Web3D consortium, JT and STEP [1]. The proprietary nature of CAD vendor specific data limits the possibility of open-ended analysis of the 3D data and creates restricted universality of such tools since they work only for the format or API of the particular application [2]. The access to the information becomes a laborious task due to the unstructured data and the lack of information sharing capability. “How can we overcome dependence on legacy formats in order to access geometric and topological information? How can we allow any user to query and retrieve geometric and topological information to a desired granularity?” This raises the need to analyze 3D neutral formats to see their distinctive features view and PMI (product & manufacturing information).

2 Product Lifecycle Management

PLM is made to be composed of digital product definition and delivery. The tasks are based on computer models processed with computer assistance. According to Terzi [3], most of the product data are generated at the beginning of life (BOL). The software owned by a company, particularly 3D CAD systems, have to be able to communicate with each other. This type of tool is developed to help product designers and to provide a traceability support of the evolution of an artefact from the requirements to the product use and even recycling. The digital product definition forms the origin and core of PLM. Product definition has evolved from engineering drawings to computer-aided X (CAx) as mentioned in [4]. Most CAD systems enable exchange of the shape models with CAx (CAE, CAPP, CAM etc.) systems. Tools are being developed to optimize the design subject to conflicting requirements in one pass. This enables designers to transfer physical testing and simulation to digital simulation and prototyping for much of the design.

For PLM to be successful, data formats of digital products must be able to convey design intent, be machine interpretable, and lose as little information as possible in the translation process. From McHenry [5], some problems arise when companies have different file formats and due to format conversions and companies outsourcing have missing, collapsed, inverted faces, models that do not form closed solids such as those with surfaces and edges that do not connect. The major problem at hand is the interoperability between 3D CAD systems. Companies can have an issue even in-house if multiple 3D CAD systems are used.

3 Research Background of Some Interoperable 3D File Format

3.1 Studies of Interoperable Files

The Research Triangle Institute at the National Institute of Standards and Technology has performed a research to determine the differences between STEP and lightweight file formats in 1999. This study analyzed the transfer of models from one CAD package to another through STEP and an outdated standard, IGES [6]. It was very simple and maybe effective for 1999, but the technology, the standard and other variables have improved and changed since then. More recent studies performed on lightweight file formats like lightweight Formats for Product Model Data Exchange and Preservation [7]. However, this study only covers the basics of model fidelity, metadata support, security, features, file size, software support, and it is important since it compares basic differences between many lightweight formats. Academia has performed studies to try and to evaluate the differences between STEP, JT, 3DXML, and U3d. This study was performed to demonstrate all the differences regarding the functionality of part registered inside these formats [8]. ProSTEP iViP Association is an international association that helps drive the development of vendor-neutral file standards and validates the quality of software solutions for interoperability. ProSTEP performs research on the benchmarking of STEP and JT formats. ProSTEP published a comparison in 2010 between 3D Formats in the field of Engineering, in which JT, STEP, and 3D PDF were analyzed using multiple CAD systems and several attributes. The CAD systems used were CATIA V5, CREO, and Siemens NX [9].

It is clear that an in-depth multi-level comparison should be done with the STEP standard and other new coming file formats such as JT, because the design in context is one of the most important processes which could be based on JT and structured information (STEP AP242XML [10] - Fig. 2) coming from various 3D CAD systems. The latter two file formats include the annotations and attributes associated with the edges and faces of the CAD model in order to detail the product geometry and specifications from a manufacturing perspective. Also, non-geometric data such as surface texture specifications, finishing requirements, process notes, material specifications and welding symbols are include [11] (Fig. 1).

Fig. 1.
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ISO roadmap for JT and STEP AP242 [1].

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3.2 Features of 3D File Formats

Different CAD systems have different ways of representing a CAD model, which is determined by the system’s modelling kernel. Some CAD systems use the same modelling kernel as other systems. For example, ACIS by Spatial Technology Corporation and Parasolid by Siemens. Other companies use completely proprietary modelling kernels that are not shared. The modelling kernel determines how the CAD model is mathematically represented, semantically represented, and its internal accuracy of the geometric definitions [12].

The issue of legacy information has brought forward interoperable and neutral (lightweight) file formats into existence. The problems between those formats are mainly perceived when we look at the features (the shape, the appearance and the scene) of the models in each neutral file format and also the PMI.

  • The shape of a model is often stored as a set of 3D points or vertices. The surfaces are stored as a series of polygons or faces that are constructed by indexing these vertices. This kind of format is known as the tessellated format. If truly smooth surfaces are required, at any scale, then a convenient option is the use of Non-Uniform Rational B-Spline patches (or NURBS). These parametric surfaces are made up of a relatively small number of weighted control points and a set of parameters known as knots. B-Rep provides explicit representations for the geometrical elements such as vertexes, edges, faces, etc. Designing a shape can be done along the lines of constructive solid geometry (CSG) that uses Boolean operations on simple shape primitives such as cubes, cylinders, spheres, etc.

  • The appearance, in its most common form associated with materials, entails applying an image or texture to the surface of the model. A model that does this must store these texture coordinates within the 3D data file. Most 3D file format support texture mapping. There would be three dimensions for three-color components corresponding to color spaces, such as RGB. Many file formats are supported to store material properties. However, an application that loads material properties usually ignores many of these properties when a user is manipulating the object.

  • The scene describes the layout of the 3D model in terms of cameras, light sources, and other nearby 3D models. The camera is defined by some parameters as magnification and principal point, location, the direction the camera is facing and an arrow indicating which direction is up.

4 Case Study

Due to the ephemeral nature of CAD file formats and the applications that work with them, the migration of CAD information into lightweight formats could be a solution for preserving, exchanging and recovering information in the future. So, the need to correctly visualize the model, access the geometric and topological information with the geometric definitions and tolerances, entails the analysis and choice of some standard formats; but it also needs to answers the following question: Which format are we going to choose? Which free viewers are available? Have the formats been recognized by some standard organization? What about the data exchange and the visualization?

Based on the formats supported by the community, those used by most of the companies and the CAD packages and third party software available to our use, we selected STEP, JT, 3D PDF, and STL in order to evaluate their discrepancies with respect to some chosen criteria.

The chosen criteria are:

  • File size: Since we are dealing with the so-called lightweight file format, the data size is an important criteria. This criterion could affect the opening of the model. A smaller file could take less time to manipulate in a PLM system.

  • PMI: It includes the geometric dimensioning and tolerancing (GD&T), which is used to communicate permissible deviations of the product to manufacturing. It is provided by the CAD system to describe linear and diametral dimensional tolerances, and geometric tolerances of flatness, perpendicularity, position, surface profile, and run-out [13].

  • Documentation: Is there a structured document able to contain the 3D model and access the PMI?

  • Data exchange: In a PLM environment, Companies want to share and exchange information. The standardized methods aim to improve the performance of data exchange. The standardized formats can also be used to convert CAD format to layers for 3D printing.

  • Visualization: The translation from one file format to another can cause some issues. It is important to understand if the tree view’s representation is missing, if the parts’ names are the same and if the formats support geometric dimensioning and tolerancing.

4.1 Software and Hardware

CATIA from Dassault Systèmes and CREO from PTC were used to perform the test. CrossManager (dkt) software from DATAKIT was used to convert legacy CAD file format to STEP, STL, JT and 3D PDF. CAD Assistant from Open Cascade was used to analyze the completeness of the interoperable file formats and the integrity of all the CAD models. It could not read file formats as STL, 3DPDF at the time of testing. So, the tested file formats were STEP and JT. For visualizing 3D PDF component, we used adobe reader and for visualizing STL component, we used the “window 3D viewer”.

4.2 Survey Data Analysis

Figure 2 shows the set of models used. The approach is, by taking one 3D model designed in CATIA and CREO with the same characteristics, same features and converted to STEP, STL, 3D PDF and JT formats through the means of CrossManager, what could be the difference in terms of the chosen criteria? Are the file formats influenced by the translation process or by the legacy CAD systems?

Fig. 2.
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Set of models used.

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NIST_ctc_02_asme1_ap242 is used for observing the discrepancies with respect to the same model translated to other formats.

Figure 3 shows the synopsis of the translation process. AXE_COUV represents an assembly model. Surface is a surface part. The same 3D models are represented in CATIA and CREO in order to be analyzed and to observe the differences.

Fig. 3.
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Direct translation from legacy files format to another through Crossmanager

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Cat_dkt_stp and Prt_dkt_stp are the STEP file format got, using directly the translation of the legacy file format through Crossmanager.

Cat_dkt_JT and Prt_dkt_JT are the JT file formats got, using directly the translation of the legacy file format through Crossmanager.

NIST_ctc_02asme1_ ap242_JT is the original file format from NIST translated in JT.

Cat_dkt_PDF and Prt_dkt_PDF are the PDF file formats got, using directly the translation of the legacy file format through Crossmanager.

Cat_dkt_STL and Prt_dkt_STL are the STL file formats got, using directly the translation of the legacy file format through Crossmanager.

The file formats are analyzed and compared. The data analysis is based on the preservation of information. The results are compared to each other in various ways to help determine possible trends. If changes occurred within 3D models after translation processes, then those changes are determined using percent change where applicable. Percent change is determined by the relative change between the attribute of the newly translated 3D model and the original attribute of the 3D model in the legacy CAD.

Regarding the area and the volume, the comparison is just performed using STEP and JT thanks to the information obtained in CAD assistant. Therefore, we compared the area of legacy file format with respect to JT and STEP by doing:

$$ {\text{Delta}}\_{\text{Area}} = \left( {{\text{Area}}\,{\text{ of}}\,{\text{ the}}\,{\text{ translated }}\,{\text{file}}\,{\text{ format}}\,/\,{\text{Area}}\,{\text{ of}}\,{\text{ the }}\,{\text{legacy }}\,{\text{file }}\,{\text{format}}} \right)\text{ * }100 $$
(1)
$$ {\text{Delta}}\_{\text{Volume}} = \left( {{\text{volume}}\,{\text{of}}\,{\text{the}}\,{\text{translated}}\,{\text{file}}\,{\text{format}}\,/\,{\text{volume}}\,{\text{ of }}\,{\text{the }}\,{\text{legacy }}\,{\text{file}}\,{\text{format}}} \right)\text{ * }100 $$
(2)

During our analysis, we have observed an increase of the 3D model’s areas from CREO during the translation process to other formats, while those from CATIA decrease their areas in comparison to the original ones. The volumes of the STEP files are almost equal to the legacy formats while those from JT are a little bit different. The difference regarding the volume and the area is around 4% on average. Consequently, we have a slight difference between regions and volume of objects in the two files.

4.3 Results Assessment

PMI

STEP AP242 and JT integrate Table 1 and cover many computable representations for several types of 3D model data, including geometric definition and tolerances. The STEP and JT characteristics allow 3D PMI module to represent product information that is machine-readable. The module uses XML and EXPRESS schema languages to define product data model. CAD assistant shows the material converted in STEP and JT. In addition, the color of the part is visible. STL is dedicated to printing format, which has been used to transfer information from CAD software to printing hardware. The STEP and JT files keep the characteristics of precise geometry, tessellated geometry and tolerances as we can see in Fig. 4-a and b.

Table 1. Translation path material property preservation
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Fig. 4.
figure 4

NIST model in (a) STP; (b) JT; (c) 3D PDF; (d) STL

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Additional features of lightweight formats not part of STEP specification and 3D PDF, as advanced material lighting properties, level of detail mechanism, some level of compression or many STEP features not available in the lightweight formats as from features, construction history drafting capabilities. Some possibly features, not available by default need to be extended manually in the GT2Go. Consequently, JT and STEP are suited for this task.

SIZE

Regarding the file size of the different 3D models, the test results were found using the file size of the legacy CAD. STEP and STL are the ones much sized and JT file sizes are the lowest in most cases. By doing some calculations with respect to our samples, JT files are almost 60% lighter than STEP files and, it is the best one in terms of size. The size of the data is determined by the data content instead of the format itself. The fact that JT is less size that another format could make JT file format, the ideal candidate suited in a PDM system because its opening could be faster than the others.

DOCUMENTATION

Because of its document-oriented structures, 3D PDF is an excellent solution for this purpose and any device that allows adobe reader can read it. STEP and JT can also be suited for this since they have been standardized ISO, so they provide a very good container for 3D models.

With respect to PLM context, 3D PDF is the most suited because it allows 3D information to be represented together with other information. All the data are contained in a single document or file.

DATA EXCHANGE

Because of the numerous applications, STEP seems best suited for that since it matured long time ago. JT is also suited to exchange information, but compared to STEP, it needs to mature. Regarding 3D PDF or STL, because of the lack of exact representation and the lack of some information, they are not suitable.

VISUALIZATION

With the translation of files through Crossmanager, we can obtain either a faceted or a tessellated representation. In addition, the capacities of the available viewers make JT (Fig. 4-b) and 3D PDF (Fig. 4-c) ideal for visualization. In fact, with JT and 3D PDF, we can notice in the tree view, the parts forming the assembly of the legacy file translated, while with STEP, it is not possible.

In terms of metadata, all elements constituent the assembly structure in these formats are displayed at the level of the tree view regardless the viewer. So, the metadata are visible. The name of the shares (parts, sub-parts, assembly etc.) is as described in the legacy CAD system. Consequently, JT is suited for the visualization case thanks to the access to the geometry, topology and PMI.

5 Conclusion

Due to the examination methodology selected for this study, we used CAD systems CATIA, CREO and, third party software CAD assistant and JT2Go. After analyzing the different 3D neutral formats, it is clear that depending on the specific use case, one file format could have an advantage over another. However, given the interest which our study has shown in focusing on the geometric and topologic information and transfer of PMI (color, material, tolerances, etc.), it could be said that the JT file is very promising. Despite the translations from one file format to JT file format, it is always possible to access the PMI correctly and visualize the geometric and topological information. Also, the view of the part tree in the JT neutral file format is done unambiguously, with a clear representation of the components.