CAD Interface | CAD and CMM

Making The CAD To CMM Interface More Effective

The Dimensional Measuring Interface Specification (DMIS) was developed as a standard to allow inspection data to be transferred between computer-aided design (CAD) systems and dimensional measuring equipment such as coordinate measuring machines. An inspection program created on any CAD system, when formatted following the DMIS standard, can be downloaded to any CMM that has a processor for accepting programs in this format. Inspection data can also be transferred from the CMM to CAD systems with equal facility using DMIS.

Creating programs for a CMM at a CAD system has certain pitfalls, however, that are now being addressed by software developers. One of these pitfalls arises from the fact that the CMM programmer at a CAD system may not have access to vital information that ensures successful execution of the inspection routine. Likewise, the CMM operator has no assurance that an inspection program created on a remote CAD system takes into account essential details, such as fixture location or part alignment, that may not be available to the programmer.

An example of a product designed to help overcome this problem is EDGES, an Expert DMIS Graphical Editor and Simulator. This software product takes a DMIS file as its input and then graphically displays the CMM inspection program in 3D on the computer monitor. This display allows the CMM operator to visualize the intent of the programmer and verify aspects of the programmed inspection routines.

EDGES, as it first reads in the DMIS file, checks the actual wording or syntax. It will point out spelling errors or prompt for a missing command, a feature especially useful for shops that have "handwritten" DMIS programs. This step verifies for the operator that the DMIS program will run the CMM. Assuming the file does not have any "fatal errors," the display will proceed to show nominal features defined in the DMIS file and the path of the probe.

The next step is to check for possible collisions. A program created on a CAD system that is not located near the CMM might include a path for the probe to follow which intersects the position of the fixtures or even another part mounted on the CMM. The CMM operator who lacks the graphical means to display the inspection path must either blindly trust the CAD operator to have anticipated fixturing needs correctly or use a text editor to check the file for problems.

The simulation features of EDGES allow the operator to preview the path of the probe in animation or one step at a time. The view of the part may be rotated, translated, or zoomed to enhance visualization of the path. Editing features help the CMM operator add position points to move around a fixture, delete necessary position points to speed program execution, or change the probe. The system acts as the DMIS expert because it modifies the DMIS files by generating the necessary commands to achieve the CMM operator's intent.

Probably the most difficulty CAD users have had in programming CMMs has been in the area of alignments. This aspect of CMM programming is highly specific to the CMM and considerable variation exists from vendor to vendor. The CMM operator is generally more familiar with the peculiarities of the CMM in use from experience programming in the machine's native programming language and best understands the correct features to select for a given alignment. EDGES interprets the intended alignment of the CAD programmer, graphically displays the existing coordinate system and allows it to be modified easily.

DMIS defines formats for return of inspection results to the CAD system. EDGES can display simultaneously the nominal and actual values, and will highlight out-of-tolerance features in contrasting colors on a color monitor. This software runs on any IBM PC or clone with 512K of memory.

Applied Automation Technologies, Inc., 19852 Haggerty Road, Livonia, MI 48152.

PHOTO : CMM programs prepared on CAD systems using the DMIS format can be previewed by the CMM

PHOTO : operator using a software package called EDGES. Errors or possible collisions can be

PHOTO : detected and corrected more easily using this product.