Printing 3D objects with interlocking parts
Introduction
3D printing is an additive manufacturing technology, facilitating convenient and rapid fabrication of physical objects of almost any shape. It has a wide range of practical applications, from fast product prototyping, product development, 3D visualization, to distributed manufacturing of larger-sized objects such as machine parts.
However, 3D printing has an intrinsic limitation: a 3D printer cannot directly print an object whose size is greater than the printer's working volume. This practical limitation has been pointed out recently by Luo et al. (2012), who proposed a solution to partition a given 3D object into parts for 3D printing and then assemble the printed parts together to reconstruct the object. Besides addressing the intrinsic limitation, this approach has several other advantages. First, it facilitates cost-effective maintenance since we only need to print a replacement part for a corresponding broken part rather than reprinting the entire object. Second, this approach is good for storage and transport since we can disassemble a large 3D object to save space and avoid breaking it during the transportation. Lastly, similar to Lego bricks, we could reassemble an existing 3D object, change some of its parts, and even reconfigure it for alternative designs and appearance.
To connect 3D printed parts, there are some common approaches, e.g., by male and female connectors (Luo et al., 2012, Lo et al., 2009) and by glue (Shapeways, 2014). For the case of male and female connectors, though they are generally practical, they may not provide sufficient structural strength to sustain the parts connections. Moreover, for these tiny printed features, they may be broken easily during the object assembly or the transportation. For the case of glue, though it could tightly join the printed 3D parts together, it is a permanent connection, discouraging object reassembly, cost-effective maintenance, and reconfiguration. In sharp contrast, we take a 3D interlocking approach (Xin et al., 2011, Song et al., 2012) to construct and connect printed 3D parts to form an object assembly. By this, we can overcome the above mentioned issues.
Connecting parts by 3D interlocking has several advantages: i) first, parts connections are achieved by the parts' own geometry without requiring the creation of extra tiny connectors such as male and female connectors; ii) the assembled object can be repeatedly disassembled and reassembled, facilitating cost-effective maintenance, storage, and transportation; iii) 3D interlocking is known to be strong, evidenced by its usage in long-standing architectural wooden structures; hence, this connection method allows us to achieve stronger 3D parts connections, which are in fact supported by inter-parts blockage with their geometry; and iv) lastly, it enables us to produce 3D parts with clean and smooth surface without hole drilling and protrusion.
In this paper, our goal is to partition a given 3D object into interlocking parts for 3D printing and object assembly. This is a highly challenging and unexplored problem, since we have to enforce not only the complicated 3D interlocking requirement (Xin et al., 2011, Song et al., 2012), but also the geometric and dimensional requirements on the printed 3D parts, as well as maintaining the 3D object appearance after its assembly. The requirements to be considered when we develop the computational method are listed as below:
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3D interlocking: the printed 3D parts should interlock one another, yet can be assembled and disassembled;
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Printable parts: while being not too small, each printed 3D part should fit into the working volume of the target 3D printer;
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Structural soundness: we should avoid thin and/or weak features on the 3D parts since such features could be easily broken during the object printing, assembly, or transportation;
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Strong connection: we should achieve strong 3D parts connections by ensuring strong blockage among the 3D parts in the 3D interlocking assembly; and
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Aesthetics: lastly, we should avoid having cutting seams that pass through salient regions on the assembled object surface since this affects the object appearance.
To achieve the above requirements, we develop a novel voxelization-based approach to construct interlocking 3D parts from a given 3D model. Our technical contributions are three-fold. First, we develop a new framework that can create interlocking 3D parts from 3D models of general shape, where novel ideas include voxelizing the 3D model with local shape analysis, employing internal voxels to create initial interlocking parts, and attaching boundary voxels to initial parts while retaining the 3D interlocking. Second, we propose to deform the input model surface subject to the voxelization, which helps to avoid fragmented and disconnected shape features on the generated 3D parts. Lastly, we propose the shape and saliency connection graphs encoding local shape information to guide the parts geometric construction for achieving the parts structural soundness and aesthetics requirements. We have demonstrated the effectiveness of our approach on 3D models with a variety of shapes, and fabricated some of them by 3D printing to validate the parts connection capability, as well as their structural soundness.
Section snippets
Related work
3D fabrication A number of research works in computer graphics have studied different aspects of 3D fabrication, e.g., deformation behavior (Skouras et al., 2013), mechanical characters (Coros et al., 2013, Ceylan et al., 2013), articulated models (Bächer et al., 2012, Calì et al., 2012), spinnable fabrication (Bächer et al., 2014), 3D shape balancing (Prévost et al., 2013), and printing material reduction (Wang et al., 2013, Lu et al., 2014). Specifically, some of them focus on structural issues in 3D
Overview
In this section, we first discuss the challenges of partitioning a given 3D shape into interlocking 3D parts, and then give an overview of our approach.
Challenges of 3D shape partitioning For a given 3D shape, there exist different ways of partitioning it into parts, for example, plane – (Luo et al., 2012), curvature – (Hao et al., 2011), and voxelization-based (Medellín et al., 2007, Zhou et al., 2014) approaches. This work employs the voxelization-based partitioning approach to create interlocking parts since i) cubical voxel
Voxelization and shape analysis
This section presents the various preparation works we have before constructing the geometry of interlocking 3D parts from an input 3D model. This includes voxelization, local shape deformation, and the generation of shape connection graph and saliency connection graph.
Voxelization method Given the input watertight 3D model, we first voxelize it by (Nooruddin and Turk, 2003), which casts parallel rays through the model and uses parity count to classify voxels as interior or exterior. We improve this method by
Generating 3D interlocking parts
Given the various inputs we prepared in Section 4, there are four major steps in generating 3D interlocking parts:
Step 1: Generating initial 3D interlocking parts First, taking the voxels in the internal volume as input, we employ (Song et al., 2012) to construct an initial set of interlocking 3D parts, see Fig. 2(c). Here we give a short description of the randomized algorithm in (Song et al., 2012), which takes a general voxelized shape as input and iteratively extracts puzzle pieces from it to create interlocking puzzles. A formal model is
Results
Our method enables us to create 3D interlocking parts from object models of various shapes, see Fig. 9. All the generated 3D parts are well-connected by interlocking upon the parts assembly, see Fig. 10 for three more examples.
Implementation and performance We implemented our method in C++ and ran our experiments on a desktop computer with a 3.4 GHz CPU and 8GB memory. For a given 3D model, users can specify the number of parts to be generated. Although our method allows the creation of a large number of 3D parts from a
Conclusion
This paper presents a novel voxelization-based method to partition a given 3D object model of general shape into interlocking 3D parts such that the 3D object can be printed with smaller parts that are connected by 3D interlocking. We first voxelize the input 3D model and analyze the local shape within each voxel, where a local deformation strategy is developed to avoid voxel fragmentation in the voxelization. Second, we differentiate internal and boundary voxels according to the local shape
Acknowledgements
The authors would like to thank Yael Friedman, a jewelry designer, for her various helpful comments on designing the 22-parts interlocking ring. The project is partially supported by the National Natural Science Foundation of China (61403357, 61222206), the Fundamental Research Funds for the Central Universities (WK0110000044), One Hundred Talent Project of the Chinese Academy of Sciences, and MOE AcRF Tier2 funding (MOE2011-T2-2-041 (ARC 5/12)), Singapore.
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