Direct generation of extended STL file from unorganized point data

Abstract

An extended STL file format is presented in this paper. Differing with existing solutions, it proposes a new format to produce and store triangles. The format uses a cluster unit composed of several triangles. The main advantages of the format are that it contains both geometry and topological information and has improved storage capability. Direct generation of the extended STL from the scanned data has a great advantage in that it can reduce the time and error in modeling process. In order to obtain the format from unorganized point cloud, a new triangulation algorithm was introduced. The algorithm is based on reconstructing the relative Delaunay triangulation of the sample points on the surface. Other advantages of the extended STL format were also presented in this paper.

Introduction

Since the life cycle of products is getting shorter due to the rapid industrial development, the efficient reduction of the new production development time and the manufacturing process should be the significant issue. To cope with customer’s diverse needs and aesthetic design, which are considered in design process, reverse engineering has become important these days in real workshops. Reverse engineering refers to the process of creating design data from existing parts [1], [2], [3], [4]. It recreates or clones an existing part by acquiring the surface data of an existing part using a scanning or measurement device. It is useful in recreating the CAD model of an existing part when the design engineering design is lost or other conditions. The CAD model developed by the reverse engineering (RE) process can be converted to the physical prototype using a rapid prototyping (RP) technique. Generally, in RP, physical parts are fabricated layer by layer. It uses additive manufacturing processes, which do not need a tool compared to the subtractive techniques used in the traditional machining operation. Although, different fabrication methods exist for RP, nearly all use the same geometry input format, called STL (Stereo Lithography), which consists of a list of triangular facet data. The STL format has advantages due to its sample structure and ease of use, but it also has serious drawbacks [5].

The shortcomings of the STL have been discussed by many researchers for the last several years [6], [7], [8], [9], [10], [11], [12], [13], [14]. Chandrajit [6], proposed an automatic surface reconstruction algorithm to obtain surfaces and fields from 3D scan data. In [7], Yan and Gu categorized the common sources of errors in RP and state that the approximation used in the STL format as one of the major errors. Makela and Dolenc [8] and Rock and Wozny [9] also discussed STL file problems. Stroud and Xirouchakis [10] summarized the deficiencies of STL as following. There is redundancy in the format since the facet normal can be determined in two different ways. The vertices are ordered counter-clockwise round the facet, this determines the facet normal, so the normal specified in the STL format line is redundancy, unless the facet is very small in which case the normal calculated from the vertex positions is inaccurate. Another deficiency is the lack of coherence between the facets. There is no information except for the vertex positions, which means that it is necessary to search for matching vertices if the model is to be recreated. This is potentially time consuming and dangerous, since the geometric point positions are inherently inaccurate and it is necessary to use a tolerance to test for coincidence. Finally, technological information could not be attached since the facet–face relationship is not defined and therefore, it is not possible to identify the faces of the object in the approximation. Also, Kumar and Dutta [11] summarized the shortcomings of STL, the intrinsic problems, the problems in conversion to the STL format and the problems in process planning due to STL. The deficiencies of STL are well known, this is merely a summary of some of the known problems. Cazals and Giesen [12] presented a Delaunay triangulation algorithm to reconstruct surface from data points. Hur [13] introduced a STL file generation algorithm based on data reduction. Yong-bo [14] gave a Delaunay-based algorithm to reconstruct surface from unorganized sampled points. Kumar and Dutta [11] also presented the alternatives to STL that have been proposed in the literature. It is clear that STL should be replaced by a suitable, more sophisticated format such as STEP standard, which adequately reflects RP’s needs as a manufacturing method of growing importance. The use of STEP has already been proposed by [15], [16], [17]. However, this possibility is difficult because of the well-known problem of introducing new standards. Such that, based on STL format, some researchers presented its extensions [10], [18].

The extensions to STL format should avoid the deficiencies mentioned above and can exchange/transfer with standard STL and other formats easily. In this research, we developed a new extended STL format. The motivation behind our extensions is the voronoi graph. This paper proposes the new extended STL format based on layered manufacturing (LM) system shown in Fig. 1. Given a set of unorganized points, which lie approximately on the boundary surface of a physical object, for which there is no a priori information about the topology of the given data points. Direct generation of extended STL file from data point points is pivotal in the system. The extend STL file is then sliced to generate a series of layers for RP fabrication.

In Section 2, we present a new algorithm for triangulation of data points. In Section 3, an extended STL format is introduced in detail. Section 4 outlines a method for generation of rapid prototyping slice files. Section 5 is devoted to discuss the conversion between the proposed extended STL and other formats.