CAD Meets Rapid Prototyping

By Terry Wohlers

Published in Vol. 11, No. 4, April 1992 issue of Computer-Aided Engineering
Copyright 1992 by Terry T. Wohlers

Understanding the interface between CAD and rapid prototyping equipment allows you to get the most out of this powerful technology.

Thanks to service bureaus, the power of rapid prototyping (RP) is readily available to virtually any firm, providing the ability to create RP parts for design review, fit and function testing, and molded tooling and investment casting. CAD files can be sent to any one of about 40 RP service bureaus for prototype part production. They will give you a quote on the cost of making an RP part, and most will build it in a couple of days.

To take advantage of these services, however, you need to interface your CAD system and the rapid prototyping equipment. Of particular concern is the file format required by the rapid prototyping machine and output by the CAD program. Understanding these formats and how to set one up is the key to quickly turning your CAD models into physical models that match the original geometry.

The STL Connection

If you have a 3D CAD system, there is a good chance you can create an STL file. This is the file type accepted by nearly all RP systems. The STL file is, by far, the most common way of sending a CAD model to an RP system. 3D Systems developed and published the STL format in 1987 for moving 3D CAD models to its Stereo-Lithography Apparatus (SLA).

Today, STL translators are available for more than 30 CAD and modeling products, from companies such as Autodesk, Cadkey, Computervision, Intergraph, Parametric Technology, and SDRC. A few companies offer the translator as a standard part of their product. Others treat it as a separate product and make you pay for it.

Third-party software developers also offer STL translators. For example, Enigmatic Associates offers a $995 STL translator for AutoCAD called Enigma-AS/SD. In addition to the translator, the package includes a program that enables you to convert an STL file to a DXF file. This is useful for viewing and debugging STL files with AutoCAD. Autodesk plans to ship its own STL translator with AutoCAD's Advanced Modeling Extension (AME) version 2.0. The extension lets AutoCAD users create solid models with AutoCAD Release 11. The optional AME costs $500.

C-TAD Systems is one of two companies that offer an IGES-to-STL file translator. It is part of a product line called The Integrator. The translator accepts an IGES file from any CAD system and converts the file into STL format. It supports all IGES surfaces, including trimmed surfaces or faces. The product, priced at $9,975, offers color rendering, dynamic rotation, and basic editing of STL files. This enables you to verify the part and make minor corrections before sending it to an RP machine. Brock Rooney & Associates also offers an IGES-to-STL file translator, priced at $4,995.

It is technically possible to develop an STL translator for almost any CAD system that creates solid models. Surface models work too, but they must be closed so they are "water tight." This means if you were to fill the volume of the model with water, it should not contain holes that would allow it to leak. It is not natural for users of surface modelers to create fully closed models, and it can be a major hassle to close them. Therefore, most CAD users that create STL files use solid modelers.

Closed models produce closed 2D contours. The contours are the horizontal cross sections that drive the layering mechanism of the RP machine. If a model contains a hole, one or more contours will contain a gap. These gaps cause the RP machine to choke because geometric information is missing.

For SLA customers, 3D Systems offers a fix to the problem. Version 3.82 of its SLA system software can detect small holes and cracks in STL models and patches them. The software is particularly useful to those using surface modelers. In addition, users of certain solid modeling products have reported tiny cracks in their solid models. The problem occurs when the solid modeler tessellates the model. The polygons in the mesh don't always accurately meet. Steve Stewart, president of Protogenic, says he has experienced cracks with Bravo and AutoCAD models but has never experienced the problem with Pro/Engineer models. Protogenic is an SLA service bureau in Boulder, CO.

Supporting the Part

CAD models that contain cantilevers (overhanging features) may require special attention, depending on the nature of the RP system. Systems from 3D Systems, Quadrax, and Stratasys require that you secure the cantilevers with a support structure. The structure is like a bottom-up crutch that supports the overhang as the machine fabricates each layer. A technician removes the supports after the part is complete. RP machines from Cubital, DTM, and Helisys do not require supports because material surrounds and supports the model as the machine builds it.

The preparation of the CAD model, therefore, may require the creation of supports. The shape and location of the supports depend entirely on the shape of the part. If you use RP service bureaus, they usually prefer to create the support structures for you because they are more familiar with the characteristics of their RP machine. You send them an STL file of the CAD model, without supports, and they take care of the rest. STL file slicing occurs at the RP system, so the operator of the RP system will take care of this as well.

About 80% of all RP system installations are SLA machines from 3D Systems, motivating third-party software developers to develop support structure software for use with the SLA. For example, Enigmatic Associates offers a library of support structures with its Enigma-AS/SD software. The package saves time because you can select a structure from the library rather than creating one from scratch.

Solid Concepts has developed a popular product called Bridgeworks. It examines an STL file and automatically creates supports where they are needed. The software stores the supports in a separate file, as required by the SLA. The company claims its software creates supports faster and better than alternative methods.

Plynetics general manager Eric Dremann supports this claim. Dremann says his company, an SLA service bureau in San Leandro, CA, is extremely satisfied with the product. It generates support structures 80 to 90% faster and has paid for itself in about a month. Without Bridgeworks, according to Dremann, it would take from 15 minutes to more than two hours to create the supports, depending on the complexity of the part. With Bridgeworks, it takes just five minutes. In contrast, Protogenic's Stewart chooses to create his own supports using Pro/Engineer.

Inside an STL File

STL translators create a file with an STL file extension. Most translators will create either ASCII or binary files. Binary files are about four times smaller, according to Stewart, so most people create binary files.

If you look inside an ASCII STL file, you will see groups of three x,y,z coordinates. Each group defines a single triangle. Together, the groups define a connected set of triangular facets. The facets describe the shape of the CAD model. Many RP users view the facets as a limitation of the STL format because the facets approximate the surface of the design. In fact, you can usually see the facets on the surface of an RP part.

Dave Albert of the Albert Consulting Group created the STL format about four years ago. He chose to use the facet approach because you can create a faceted representation from most CAD systems that offer surfacing and solid modeling.

The uneven surface caused by the facets may require extra hand sanding or bead blasting, depending on how you plan to use the part. If it's going to be used as a pattern for making a molded tool, for instance, you will want to produce a smooth finish, which can take hours. Plynetics president Frost Prioleau says he dedicates two full-time people to finish parts from his single SLA-250. Note, however, that surface finish is less important if the RP part will be used primarily for design review and feedback.

Most STL translators make it possible to adjust the size of the facets. Reducing facet size produces a smoother and more accurate part, but at the expense of file size. Pro/Engineer permits you to specify the facet size when making the STL file by setting a quality index. With AutoCAD, you use the SOLWDENS system variable to adjust the size of the facets. Entering a high value, such as 10, for the variable produces a smoother surface than a low value, such as 4.

It's important to note that other factors can affect the quality of the surface. For example, the thickness of the layers that make up the part affect the appearance and smoothness of the surface. Relatively thick layers (0.0150 in.) produce a rough surface, especially in areas that are near horizontal. Also, the RP process and materials can affect the surface finish. For instance, parts made from DuPont's SOMOS 2100 resin are smoother than parts made from most other RP materials.

STL Alternatives

The STL format has been a topic at RP meetings and conferences over the past couple of years. RP system users say they want RP system developers to improve or develop an alternative to the STL format. CAD users who can create smooth, precise surfaces (nurbs) want a translator capable of accommodating these surfaces. Also, users believe the file format is inefficient in its present form. An STL file is often larger than the CAD file from which it was created. The actual size depends on the size of the facets you choose to use and the shape of the part. An STL file containing many planer surfaces can actually be smaller, claims Protogenic's Stewart. A rectangular plane requires only two triangles to describe it. STL files with many curved surfaces, however, require a large number of triangles. These files can be 11Ú2 to 3 times larger than the CAD file.

Cubital has taken a step forward. The company has developed an alternative format called the Cubital Facet List (CFL). It uses facets to describe the model, yet the files are several times smaller than equivalent STL files, according to Reuven Scharf of Cubital. This compactness stems from the fact that the CFL format stores the facet descriptions without storing redundant information. An STL file unnecessarily repeats information, such as sets of coordinates and strings of text.

CFL files contain information not contained in STL files. For example, a file header can optionally include a description of the file, the date, and a brief description of the CAD system used to create the file. Also, it can include comments from the file creator as well as their name, address, phone, and fax numbers. The CFL file sets aside special fields for the name and version of the operating system used to create the file and the unit of measure (in., mm, etc.) used when creating the CAD file. STL files do not contain this information.

So far, only a few CAD packages offer a CFL translator, such as Duct, SilverScreen, and Solution 3000, according to Scharf. He claims third-party developers have created CFL translators for CADDS 4X, Catia, I-DEAS, and Personal Designer. Cubital's Solider 5600 accepts STL files, but not directly. An operator must first convert them to the CFL format using a converter that comes with the Solider machine.

3D Systems vice president Dennis Medler says his company is planning to offer its own alternative to the STL format. Sometime this year, possibly this summer, 3D Systems will distribute software that enables you to send IGES and DXF files directly to the SLA. The software will create the contours necessary to drive the movement of the laser, without first creating an STL file. What's more, if the surfaces in the source file are smooth, the contours and final part will also be smooth.

According to Medler, the software will permit you to view wireframe and surface models as they come in from the IGES or DXF file. You will be able to rotate and translate the model, and examine it for holes and cracks. The software will offer editing tools that enable you to patch models that are not closed and water tight.

Medler does not view the software as a replacement to the STL format. CAD users that have STL translators will continue to use them. However, if you don't have an STL translator, or if it is substandard, you will have an alternative. This development is particularly good news for those who want to take advantage of smooth surfaces, such as Bezier and nurbs.

Industry consultant Terry Wohlers writes, speaks, and consults on rapid prototyping technologies, applications, and markets. Wohlers Associates, OakRidge Business Park, 1511 River Oak Drive, Fort Collins, CO 80525, 303/225-0086, fax: 303/225-2027, CompuServe 73417,1465.

Copyright 1992 by Terry T. Wohlers