DMIS Programming Language Links CAD Data to CMM Inspection Routines

DMIS Programming Language Links CAD Data to CMM Inspection Routines

Standard is gaining acceptance with builders and users.

By Walter Pettigrew, President, LK Metrology Systems Inc.

Computer-aided design (CAD) systems and coordinate measuring machines (CMMs) have proved to be important assets in the production of large fabricated parts and assemblies. Both have contributed to improved productivity and quality in aircraft component manufacturing, automotive body assembly, and other discrete part manufacturing operations.

Efforts to combine these technologies for maximum benefit have been hindered by the inability to communicate efficiently between them. Only recently have software tools been developed that provide time- and cost-effective ways to program CMMs using data pulled directly from the CAD system.

The Dimensional Measuring Interface Standard (DMIS) was developed as the standardized programming protocol to make commercial bi-directional communication between CAD systems and CMMs possible. DMIS was developed by the Consortium for Advanced Manufacturing International (Bedford, TX) and approved in its current 3.0 version by the American National Standards Institute (ANSI) in February 1996.

Similar to APT Programming
The DMIS protocol provides a standard language that can be used to program inspection equipment in much the same way that the Automatic Programmed Tool (APT) language is used to program machine tools. In real-world applications, commercially available DMIS-based software is first merged with data from the manufacturer's CAD models and engineering release files. It then generates code used for programming part inspection routines in a CMM.

The code is either directly compatible with a DMIS engine built into the CMM, or is made compatible with the machine's proprietary software through a translator device. Because DMIS is a neutral format, DMIS code can be converted to any native CMM programming language. This is a major benefit for many national and international companies with a variety of inspection equipment.

Using DMIS-based programming software, there is no need to write extra code to support different CMM languages or different software upgrades. Moreover, once the code is developed, the standardized format helps prevent the obsolescence of parts programs

Another advantage of DMIS is that it is relatively easy to read, reducing requirements for programmer training and lowering life cycle costs. It also makes it easier to optimize inspection routines for factors such as clearances or probe speeds, which may not be effectively addressed in CAD programs used with the inspection software.

CMM Builder Support Required
The DMIS protocol is today supported by a growing number of CMM builders. Because the protocol is a standard, rather than a proprietary resource, advances on existing capabilities must necessarily come slowly by industry consensus.

The latest version of DMIS, released in 1996, is quickly developing a positive track record and gaining in popularity. Effective implementation of the standard, however, depends on the support of CMM suppliers and users. Before purchasing new CMM equipment, for example, it is important to determine the extent to which the standard is supported by the CMM supplier.

LY90 coordinate measuring machine from LK Metrology Systems Inc.

Another significant issue is whether the CMM uses a DMIS engine or translator. Translators are less efficient since mistakes can be made during the translation process that will have to be corrected. Translators also require maintenance of two files—both the original native language file and the DMIS file. With a DMIS engine, by contrast, there are no translations and only one file, saving significant programming time and cost.

Still other issues confronting the potential DMIS user include the CMM's compatibility with DMIS programs, whether the CAD interface connects directly to the vendor's software or to an offline programming package, and the required computer and/or network configuration.

Information for this article was supplied by: LK Metrology Systems Inc., 12701 Grand River, Brighton, MI 48116. Tel: 810-220-4360. Fax: 810-220-4300.

Edited by Jim Lorincz
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