The Evolution of a Layered Manufacturing Interchange Format

Over the last several years we have developed the Berkeley Solid Interchange Format (SIF) for layered manufacturing data exchange. By building both design software that outputs SIF as well as manufacturing software that processes the SIF input files, we gained insights into the concerns of both sides of data exchange – insights which often led to major changes in successive versions of the format. In this paper, we share some of the most important lessons we learned (many of which are applicable to all geometric data exchange, not merely for layered manufacturing) and explain how they shaped SIF. INTRODUCTION Designers who want to make prototypes of solid threedimensional parts directly from CAD descriptions are increasingly turning to a class of technologies collectively referred to as layered manufacturing (LM) or rapid prototyping. These technologies include stereolithography (SLA), 3-D printing, fused deposition modeling (FDM), selective laser sintering (SLS), and laminated object manufacturing (LOM)(Beaman97). In all these processes, a triangulated boundary representation (b-rep) of the CAD model of the part is sliced into horizontal, 2.5-D layers of uniform thickness. Each cross sectional layer is successively deposited, hardened, fused, or cut, depending on the particular process, and attached to the layer beneath it. (For technologies such as SLA and FDM, a sacrificial support structure must also be built to support overhanging geometry.) The stacked layers Corresponding author. form the final part. The computer representation of the part plays a central role in this process. While a human can easily interpret a shaded 3D surface model or a display of a wireframe model with dashed hidden lines, a solid model that unambiguously defines the region inside of the part is necessary for layered manufacturing. For a b-rep such as the STL format that has become the de facto standard in the LM industry, the boundary must be watertight, oriented, and not self-intersecting. Unfortunately, STL files commonly contain errors such as cracks and penetrating or extraneous faces. Service bureaus that manufacture LM parts typically massage and clean up these files to produce sanitized and consistent models which then are then used as input to the fabrication machine’s software. If the original file is highly inconsistent, then the manufacturers have to make an educated guess as to what the intent of the original designer was and what the desired geometry might have looked like. They will then try to approximate that geometry as best possible with a clean STL description. In this context we developed the Berkeley Solid Interchange Format (SIF) (McMains99) to serve as a replacement interface between designers and fabricators of LM parts. During this project we built both design software that outputs SIF as well as manufacturing software that processes the SIF input files. Building this software helped us gain insights into the concerns of both sides of data exchange – insights which often led to major changes in successive versions of the format. In this paper, we share some of the most important lessons we learned (many of which are applicable to all geometric data exchange, not merely for layered manufacturing) and explain how they shaped SIF. 1 Copyright  2002 by ASME