Custom Program Instead Of A Custom Fixture

In five-axis machining, if the programmed tool paths are free to move, then it may be possible to avoid expensive fixturing.

For example, some CNCs can adjust the location and orientation of the X-Y-Z coordinate system. The part is set up reasonably close to its expected position, with the machine using the spindle probe to measure the part’s actual location and orientation. The control then adjusts the machining program’s coordinate system accordingly, translating and rotating the coordinate axes to adapt the program to the actual part position. Thus, there is no need to use complex, expensive custom fixturing to hold the part precisely where the program expects to find it.

However, relatively few machines that run five-axis parts have access to this kind of control capability. Large aircraft parts, for example, are often made on older machines with long service lives. Is custom fixturing a fact of life for these parts?

Not necessarily, says Ed Cavallero, founder of Numerical Control Software Solutions (Covington, Washington). The company’s “NC Transformer Plus” software provides the freedom to shift an NC program to match the part’s position, even though the coordinate system may be fixed.

This software may take advantage of probing, too. It may also be used with any other measurement device able to precisely locate reference or datum features of the part. Once these measurements are input to the software, it uses the difference between the actual and expected locations to completely rewrite the machining moves. The result is a new NC file tailored specifically for the current workpiece as it sits on the machine. It is a custom program instead of a custom fixture.

Because the software works with the posted NC file, the choice of CAM software is irrelevant. The user inputs this posted file, along with the expected locations of the reference features. The software then calculates linear and angular offsets based on the actual positions of these features, using these offsets to calculate new linear and rotary axis moves.

Safety features are built in. The software flags a warning in cases where the new tool path seems likely to exceed a travel limit for a rotary axis. Also, the software flags M and G codes that it does not recognize. This latter feature allows for the fact that an unfamiliar code might represent an uncommon function that the user will want to reevaluate in the context of this new NC file. (The user can update the list of valid M and G codes with the shop’s own preferences.)

There are certain program elements that cannot be translated. Mr. Cavallero cites an obvious limitation: canned cycles. Because these cycles do not make the actual toolpath moves apparent within the NC code, the NC Transformer Plus software is unable to reinterpret them.

“Only point-to-point programming will do,” he says. “Planar” functions such as canned cycles won’t work, because the new NC file will have a different relationship to the coordinate planes.