On-Machine Metrology Taking Off In Aerospace

Today's CNC manufacturing equipment is all about productivity – removing material from workpieces as efficiently as possible and producing parts requiring few, if any, additional operations. Spindle probes don't remove material, but many aerospace parts manufacturers have discovered that they can contribute to productivity by quickly and accurately aligning parts prior to cutting to reduce CNC machine idle time. They are also using spindle probes to perform in-process checks and inspection functions that save time and get parts out the door faster.

Of course, the mere presence of a spindle probe on a CNC machine is not enough to produce these advantages. Spindle probes need to be easy to use, unobtrusive to the manufacturing process, and provide fast measurements and analyses that immediately impact productivity and quality. The cumbersome macros and G & M code sequences users currently struggle with to program spindle probes do not provide that kind of functionality. On-machine metrology software does.

In addition, for spindle probes to be viable, the aerospace manufacturer is likely to need help from the software developer's application engineers. They can assist with on-machine measurement process development and provide contract-programming services prior to and for as long as needed after the installation of the on-machine metrology system.

Wilcox Associates has created two separate PC-DMIS NC products for on-machine gaging. During 2007, this company worked closely with a number of aerospace parts manufacturers, as well as with waterjet cutter manufacturers and systems integrators, to develop specific on-machine metrology applications employing these products.

The observations in this article are drawn from the experience of David Tackes, William Brooks, Sascha Jost, and Shawn Grady, applications engineers for Hexagon Metrology, the multinational parent company of Wilcox Associates, who worked on these projects in Japan, Canada, Germany, and multiple locations in the United States.

Server-Based or Interactive

Based on specific end user requirements, there are two major types of third party software that can be used for on-machine metrology with spindle probes. The primary difference between the two versions consists in the way they interact with the CNC machine.

Server-based on-machine metrology software allows users to develop and test their programs on standard off-line measurement programming stations and then post process them in the appropriate format for a specific machine tool. In turn, these measurement programs are loaded onto the machine tool's controller in the form of standard G & M codes.

During the probing processes, the measurement data goes to the server for data archiving, evaluation, analysis and reporting. Because of this architecture, a single server-based system can capture and analyze data from many CNC machines on a single network.

Engineer David Tackes, who has developed a number of server-based programs for both aerospace composites and engine parts applications says, "The beauty of server-based on-machine metrology is that measurement subroutines are resident on the CNC machine's controller. The operator runs them like any other program. He puts the workpiece on, calls up this program, and runs it. If the machine is executing an alignment program, it picks up the spindle probe, measures the appropriate features, the work offset gets updated, and the operator hasn't done anything but push another button. It's very fast and simple for the operator – two hours of training is overkill."

The primary rationale for putting on-machine software on a server was to maximize machine cutting up time by making measurement routines as unobtrusive as possible. Interactive, on-machine metrology software meets a different set of market requirements.

Interactive, on-machine metrology software actually assumes control of the CNC machine for its alignment and measurement processes. This means that operators can do almost anything on a CNC with a spindle probe that they can do with a coordinate measuring machine. This is particularly important when measurement programs must adjust themselves on the fly based on measurement results (e.g. iterative alignments of parts or fixtures, logical program branching, relative measurements, etc).

Application engineer David Brooks explains that one of the advantages of the interactive approach is that the software does not have to post-process the measurement code, nor does it have to be integrated with the manufacturing program on the machine's controller. "I can write the measurement sequences on my laptop and bring it down to the NC-machine on a jump drive. A PC at the CNC machine accepts the program and takes full control of the machine while the alignment or measurement program is being used. It's very clean."

Applications and Benefits

Both server-based and interactive on-machine metrology software have shown excellent results in a variety of aerospace CNC manufacturing applications. These include alignment of large parts, alignment and inspection of all sorts of parts on waterjet machines, as well as on-machine inspection of small composite parts and conventional prismatic parts.

Locating Large Parts – The application engineers report that a majority of current customers use on-machine gaging as a cost-effective approach for generating accurate workholding offsets. Much of this is driven by the airframe manufacturing sector, which has a tremendous need for composite structural components manufactured on large mills and waterjet cutters. Even with the aid of devices like portable arms and laser trackers, setting up large parts can be tremendously time consuming.

PC-DMIS NC on-machine metrology software is used to accelerate setup of parts up to 30ft long on waterjet systems manufactured by PaR Systems (shown here) and Flow. The productivity of a waterjet machine is nearly doubled by reducing two hours of manual setup to a 12 minute automatic part locating routine using the on-machine metrology software.

With on-machine metrology software, it is relatively easy to measure three tooling balls or some other known geometry on a workholding fixture and locate it with six degrees of freedom. As a result, a measurement routine that can be performed automatically in 10 or 15 minutes replaces two people spending two hours measuring the fixture and shimming it to locate it properly for the machining. Sometimes the fixture is permanently located on the machine tool. Even so, users have found it useful to measure the location of the part within the fixture to provide secondary offsets that improve the accuracy of manufacturing. This can be particularly useful in aerospace composite manufacturing applications where rapidly developing processes are producing inconsistent workpieces for laser machining. In these applications, interactive on-machine metrology software proves invaluable because of its ability to generate precise workpiece offsets by performing interactive alignments. This not only saves setup time but also ensures that critical features will be cut more accurately.

The engineers observe setup time savings for creating workholding offsets ranging from about 3:1 to 10:1, depending on the size and complexity of the workpiece. Plus, user studies confirm that the accuracy of these alignments are at least as good as, and are generally better than, those produced by meticulous manual part setup procedures. The bottom line is that setting up large workpieces using on-machine metrology instead of doing it manually can recover long stretches of non-productive time in which more actual machining can be done.

PC-DMIS NC on-machine metrology software provides an intuitive CAD-based programming environment so that users no longer have to write cumbersome G & M codes to govern the behavior of spindle probes on CNC machines. The software allows manufacturers to readily create metrology programs that aid in part setup, as well as perform in-process checks or on-machine inspections.

Powerful Parametric Programming features in PCDMIS NCi and NC Server make it easy to create programs for measuring multiple families of parts. For example, to measure different hole patterns in similar parts, the programmer simply inserts new hole location and size values in the program's parametric tables and a new program is generated automatically.

Small Airframe Components – Another engineer works with a manufacturer of small airframe components to develop PC-DMIS NC programs for validating production parts on its mills and waterjet cutting system (including a dual system alternative milling head). This company values the consistency of automatically setting up groups of parts, cutting them, and then measuring them on their machine tools automatically. In operation, the user locates the parts on the machine by moving the probe to a specific location and invoking a measurement program that locates a bushing in each of the holding fixtures. PC-DMIS uses this information to calculate offsets for the cutting program. Once cutting is completed, the operator calls another probing routine that automatically measures and validates each part as it is cut. The parts are now ready to ship rather than waiting to be sent off to various measurement systems for validation.

Table Driven Programming – In a similar application, an applications engineer further increases throughput by using table-driven, parametric programming techniques to measure families of parts quickly. As in the previous case, the goal is to integrate the setup and measurement of parts more closely with the cutting processes – since they are dealing with families of parts, they are able to take full advantage of PC-DMIS's capabilities of parametric programming. Developing a program for another member of a part family becomes a matter of changing a few entries in a table, resulting in a faster turnaround on part deliveries.

Jet Engine Parts - A manufacturer of highprecision, jet engine components wants to use on-machine metrology to reduce the load on its expensive, high accuracy CMMs while not compromising the quality of their parts. The solution is to use the PC-DMIS's SPC tools to continually compare the results coming from the CNC machines with the results coming from final inspections on the CMMs. By continually tracking the differences between them, they are able to confidently move in-process checks from their CMMs to their CNC machines. The end result is a dramatic reduction of the loads on the shop's CMMs and in the time spent moving parts from place to place.

Waterjet Setup – Flow and PaR Systems offer both PC-DMIS NC Server and NCi with their waterjet cutting systems. Manually setting up these systems to cut airframe components consumes enormous amounts of time, which is reduced by automating the process.

For example, an application engineer must program a PC-DMIS NCi to determine workholding offsets by measuring the position of bushings within a fixture for a carbon fiber part that looks like a long U-shaped channel. He notes that the automated probing routine took about 12 minutes compared to the two hours or more it took to locate the fixture using a laser tracker. Where the system used to occupy about half of its time in setup and the other half in cutting, it now spends most of its time actually cutting. The productivity of a machine that costs well over a million dollars is essentially doubled.

Growing Confidence

While many current users of PC-DMIS NC are employing on-machine metrology for both CNC machine setup and inspection, almost all of the users to date had a priority established for one or the other before they purchased their on-machine metrology software. Therefore, when they create their initial measurement programs, they focus almost entirely on either programs for setup or programs for on-machine inspection.

Once comfortable that they can meet their primary objectives, many begin to investigate and use on-machine gaging for other purposes. The application engineers surveyed confirm that the users who first implemented on-machine metrology for specific applications are finding more and more uses for it in improving both their cutting and measurement processes.