AutoCAD and Rapid Prototyping

By Terry Wohlers

A version of this was published in Vol. 10, No. 11, November 1995 issue of CADENCE
Copyright 1995 by Terry T. Wohlers

If you design and manufacture mechanical parts and assemblies, it's probably safe to assume that you've heard about or experienced the benefits of rapid prototyping (RP). For companies around the world, RP has dramatically reduced the time it takes to design, prototype, test and produce tooling for manufactured parts. RP has also helped to save these companies serious dollars, Deutsche Marks and yen.

RP machine sales and services began to expand significantly last year and it continues to grow. The reasons are many. First and foremost, manufacturing companies are becoming familiar with what the technology has to offer, an educational process that has taken years. Second, the range of RP processes that are commercially available have improved. Today, it's possible to produce relatively accurate parts in materials such as epoxy resins, ABS plastic, and glass-filled nylon. The list of possibilities continues to grow.

Third, companies are finding that they can justify the cost of RP on the basis of improving the quality of a design and avoiding expensive mistakes. Receiving a physical model of a proposed design early in the design cycle permits them to make improvements when changes are inexpensive. RP helps them to catch design errors long before production tooling. Oversights at this phase can cost tens, even hundreds of thousands of dollars. Also, it can delay product introduction, giving the competition a time advantage. A decade ago, you could get away with delays of weeks or months, but this is no longer true, especially with products that have a life of only a year or two.

CAD solid modeling

Advances in CAD solid modeling have also contributed to the growth of RP. Without a CAD model, it's impossible to produce an RP part, with the exception of data from CT scanners and 3D digitizing systems, although they represent a small percentage of the data driving RP machines. So when referring to RP as an aid for mechanical design, almost all parts are built from a CAD model, and nearly all of the them are solid models. For this reason, the RP market will only grow as fast as the solid modeling market.

Since RP and CAD solid modeling are complimentary technologies, they help to justify one another. I know of companies that purchased solid modeling systems, but not until they made the decision to go with RP. Also, companies are buying solid modeling systems because it secures them a future path to rapid prototyping. So they really do help sell one another. Some companies view the solid model as the soft prototype and the RP part as the hard prototype.

Requirements of RP

RP requires that you produce a fully closed, water-tight model. If you were to pour water into the walls of the part, it should not leak. For this reason, CAD solid modeling has become the most popular method for preparing model data for RP. Surface modelers enable you to meet this requirement, but most users will agree that using a solid modeler is far easier. Without even trying, a good solid modeler will automatically produce water-tight models.

The link between the CAD model and the RP machine is the STL file. An STL file is nothing more than a list of x, y and z coordinate triplets that describe a connected set of triangular facets. Also, it includes the direction of the normal vector for each triangle, which should point outward. CAD systems with an STL translator perform a surface tessellation and then output the facet information to either a binary or ASCII STL file. Binary STL files are much smaller and usually preferred, but ASCII STL files permit you view the contents of the file and even edit it if necessary.

Certain RP systems, such as machines from 3D Systems and Stratasys, require support structures which support the RP part as it is being built. While it is possible to produce custom support structures with AutoCAD, don't. Most owners of RP systems use support generation software that automatically creates supports structures for an STL file. It is many times faster and it does a better job.

Is AutoCAD a viable option?

AutoCAD Release 13 enables you to convert solid models to the STL format using the STLOUT command. From the File pull-down menu in the DOS version, you can pick Export and Stereolithography to issue STLOUT. AutoCAD asks you to select the solid and then indicate whether you want to produce a binary STL file. If you enter No, AutoCAD creates an ASCII STL file. In either case, AutoCAD automatically assigns STL as the file extension. A quick way to determine whether an STL file is ASCII or binary is to open the file using Notepad or Write in Windows, or by using either the TYPE or MORE commands in DOS to list the contents of the file.

In AutoCAD, the solid model should lie entirely within the positive XYZ octant. This means the x, y and z coordinates of the solid model must be greater than 0. If they are not, AutoCAD will display a warning message. Certain RP systems, such as stereolithography from 3D Systems, require that the model lie within the positive XYZ octant. If it doesn't, the operator of the RP machine can usually reposition it for you.

AutoCAD Designer does not offer an STL translator as part of the product. Since AutoCAD R13 can output STL files, Autodesk suggests a work around. Use the SAT file transfer method to convert a model created with Designer to a native Release 13 solid. From there, you can output an STL file. SAT files contain topological and surface information about models created using the ACIS engine. R13 can import and export SAT files using the ACISIN and ACISOUT command, respectively.

User experiences

I contacted a few companies that use AutoCAD for RP part production, hoping to get their impressions of AutoCAD as a front-end to rapid prototyping. Their comments ranged from "we're pleased with it" to "it's okay" to "it's junk and I want my money back."

Bill DeBerard, a mechanical designer at SyQuest (Boulder, CO), uses AutoCAD Release 12 and Designer to produce piece parts for disk drive products. Overall, he has been satisfied with the performance of the software. Since it's not possible to output STL files with Designer, he outputs SAT files and then sends them to Rapid Prototyping Corporation (Longmont, CO), a service bureau that offers design, analysis, and rapid prototyping using CAD and CAE tools and stereolithography. RPC uses custom software created in-house that reads SAT files and then converts them to an STL file.

Tim Pickles of Krone, Inc. (Englewood, CO), a German telecommunications company, uses Release 12 for Windows and AME to produce models for RP. So far, he has successfully produced 20-25 RP parts. Much of his work involves designs such as phone jacks and wall face plates. He uses the SOLSTLOUT command that ships with AME as a bonus feature to produce STL files. The results have been mixed. One part required 5 or 6 attempts before getting it right. One problem has been with AutoCAD inadvertently reversing the direction of the facet normal. Sometimes, he can correct the problem by setting a new mesh density using the SOLWDENS system variable. This causes AutoCAD to retessellate the surface of the model.

A long time user of AutoCAD, Ernie Marine of Edmar Technologies (Denver, CO), is using AutoCAD Release 13 and Designer. He said that he has outgrown AutoCAD and is considering another product, even after investing years with it. He was especially disappointed with R13. As for Designer, he would like his money back. He said the product is extremely limited. Also, he believes it is difficult to use and it produces enormously large files. He created what he considers a relatively simple part -- a connecting rod for an engine -- and the file grew to 6.5 Mb. He explained that it's not possible to use it for modeling assemblies, nor is it useful for producing core and cavity tooling from the solid model. Presently, he is looking at I-DEAS from SDRC.

Testing AutoCAD Release 13

Using AutoCAD R13c1 for Windows, I created a plant holder with a wall thickness of 0.125 inch by revolving a closed polyline. I then created a binary STL file from the solid model. I was able to view the STL data using special viewing software called SolidView to verify the integrity of the STL data. I discovered that AutoCAD did not properly create the binary STL file. In fact, it wasn't even close. I loaded the same solid model into 13c2 for Windows, and later R13c2 for DOS, and output new binary STL files. Both produced good binary STL files, so I concluded that the problem was with R13c1 for Windows.

Steve Stewart, president of Protogenic (Boulder, CO), said that his company has experienced similar problems with binary STL files produced with R13 for Windows. Protogenic is an established RP service bureau using an STL-250 and SLA-500 from 3D Systems (Valencia, CA) and a Solider 5600 from Cubital (Troy, MI). He said that AutoCAD (i.e., R13c1) usually does fine when producing ASCII STL files, but it produces bad binary STL files. My tests confirmed this, but you can produce good ASCII STL files with R13c1 for Windows.

Improving Surface Quality

Release 13's FACETRES (short for facet resolution) system variable adjusts the quality of the surface of an RP part produced from an AutoCAD STL file. FACETRES permits you to enter a value between 0.01 and 10. Using the default value of 0.5, I created an STL model made up of 2,428 facets stored in a 121 Kb binary STL file. Setting FACETRES to a higher value creates smaller triangular facets and an STL file that more accurately reflects the true size and shape of the solid model, although it increases the size of the file. I created an STL model with FACETRES set at 5.0. This created a 896 Kb binary STL file containing 17,916 triangular facets. Using a service bureau, a typical RP part can cost $500 to more than $5,000, depending upon its size, so you will want to produce the best quality part possible. This means creating relatively small facets.

FACETRES in R13c2 for both Windows and DOS works fine, based on my series of tests. However, it does not work reliably, if at all, in the Windows version of R13c1. Note also that R13c1 for Windows produces fewer facets, compared to the R13c2 versions of AutoCAD, even when FACETRES is set the same. R13c1 for Windows produced only 828 facets, while the two R13c2 versions produced 2,428. I expect that this is another flaw that Autodesk has fixed.

Summary

RP is growing at an impressive rate. The technology is helping companies justify the cost of implementing CAD solid modeling. Producing physical models and prototype parts from a CAD model has never been so easy. As the CAD solid model market grows, so does the RP market. The two are complimentary technologies and they help sell one another.

AutoCAD's R12 and R13 solid modeling options are being used to produce STL files for RP part production. The results, however, have been mixed. Some users are happy while others disappointed. If Autodesk wants to produce software tools for serious mechanical design work, they must consider both the positive and negative experiences of their customers, as well as the technical requirements of RP technologies. As lower price RP machines enter the market, Autodesk is in a position to ignite the emerging desktop RP market, but questions remain as to whether AutoCAD and its companion products are suitable for the job.

Industry consultant Terry Wohlers assists organizations with their selection, management, and development of technologies for CAD/CAM/CAE, rapid prototyping, and reverse engineering. He contributed monthly to CADENCE from 1986 to 1992.

Copyright 1995 by Terry T. Wohlers