Boeing, NIST help to take STEP-NC to new heights.
In the continuing quest for lower costs, manufacturing has seen a shift from large original equipment manufacturers (OEMs) as manufacturers to large OEMs as assemblers and managers of the supply base. Boeing established a virtual cooperation with several suppliers in order to produce the Boeing 777 airplane. Boeing designs, assembles, and markets the aircraft, while an international network of suppliers produces the components. The OEM specifies the product definition that is transferred to a supplier, which eventually machines the part. With OEMs and suppliers processing the part information in different ways, inadequate data exchange hampers productivity and can impair part quality. Ideally, a complete, unambiguous, and standardized data-exchange representation is required to make the shift in the manufacturing paradigm truly effective.
The prevalent international data-exchange standard is the ISO 10303, Standard for the Exchange of Product Model Data, or STER The parts of STEP implemented in software systems are called application protocols (APs). STEP AP238 is the "application interpreted model for computerized numerical controllers." AP238, or more commonly STEP-NC, is a standard for the exchange of comprehensive manufacturing data. STEP-NC offers accurate and complete product definition data from product design all the way to the machine tool. In addition to data exchange, STEP-NC offers a number of potential manufacturing advantages, including vertical integration, horizontal integration, comprehensive process model, flexible deployment, feature-based precision machining, unified part description, distributed machining, and adaptive machining.
The Boeing/NIST AP-238 work first concentrated oil the leaner manufacturing benefits related to using AP238 toolpaths. Presently, most five-axis CNC machines receive ISO 6983/RS274 data, defining each axis movement required in order to manufacture a part. This geometric data are referred to as machine control data (MCD). MCD provides a very low level of instruction: tool, axes positions, feed, and speed. This direct programming model means that the orientation axes are traversed as synchronized axes, and are tied to specific tool geometry.
The problem with MCD programs is that they are not portable or adaptable.
Portability is a problem, since unique axes-position data must be generated for each machine control combination (part, tool, and machine configuration) on which the part is to be run. Adaptability is a problem because no information is provided to the machine to help it adapt to real-time changes in machining dynamics (feed and speeds) or machine tool alignment (tool and wear offsets).
By comparison, tool center programming defines program geometry as cutter-movement data, instead of axes-movement data. Tool center programming is similar to robotic 6-D pose representation. Motion is defined as a 3-D tool-tip position (X,Y,Z) and a 3-D tool axis orientation (I,J,K). For each TCP (X,Y,Z, and I,J,K), the CNC controls the two rotation axes so that the tool is positioned and oriented as specified, in addition, the CNC controller performs tool-offset compensation along the tool axis according to the position of the tool tip in the proper position and orientation.
STEP-NC allows tool-center programming to define program geometry as cutter movement data, instead of axes movement data. STEP-NC also provides rich, high level information about the part features, materials, cutters, and dimensional tolerances. In the aerospace industry, tighter and tighter part tolerances are the expected norm so that the need for STEP-NC is pronounced. TCP can provide some direct accuracy improvements since each CNC will determine its tool tip position, as opposed to a CAM system generating static toolpaths as a series of axes positions. Since machine geometries can vary slightly even between identical machines, expected accuracy improvement should be significant.
The Boeing/NIST validation goal was to prove that AP238 TCP motion data is "machine neutral" and may be used directly by machines with different five-axis configurations. For the initial testing, the part selected was a half-scale National Aeronautics Standard (NAS) 979 5D part. commonly known as the circle diamond square (CDS), designed for runoff tests of new machine tools. The chart below shows the validation process in which a manufacturing part is sequenced through CAD/CAM and ultimately onto a CNC.
Boeing modeled the NAS 979 part in CATIA workbench. Using CATIA ver. 5, Boeing then generated a part program into legacy cutter location (CL) format. CL can represent angular cutter motions in a CNC configuration-independent I,J,K way, with the assumption that the underlying machine tool controller will translate the I,J,K into machine specific five-axis angular configuration.
Boeing wrote a program to translate the CL file into AP238 Part 21 file based on the AP238 toolpaths technology. STEP Part 21 specifies the data-exchange format for product data based on schemas defined in the EXPRESS language (ISO 10303-11). In our case the Part 21 file encodes machine workingsteps as TCP toolpaths based on the AP 238 Express schema, which is then suitable for the transfer between manufacturing systems. Boeing wrote another program, which translated the AP238 Part 21 into a controller-specific TCP programs. Boeing successfully demonstrated the ability to produce and run identical TCP programs on different brands and configurations of five-axis CNCs at Boeing/Tulsa. This demonstration alone is significant in that it reduces costs because of post processing to machine-specific RS274 part programs. If CNC vendor participation increases, the translation from AP238 Part 21 toolpath translation can be done transparently on the CNC.
With the goal of validating the TCP technology on a CNC in a different setting, Boeing asked NIST to also machine the NAS 979 part using the same TCP program. Upon receiving the Boeing TCP program, it quickly became apparent that there is more to part program portability than just geometries. An initial dry-run test of the TCP program proved this out as the NIST CNC expected coordinate transformations to be explicitly turned off during a tool change, which all Boeing controllers implicitly do, so that the NIST CNC ended up crashing into the tool magazine.
To allow portable TCP programs, it was determined that each machine should be required to host a set of standard STEP-NC infrastructure workingsteps (IWs) that are callable CNC subprograms from the STEP-NC-downloaded-part program, thus promoting portability by pushing machine dependent operation onto the machine. With machine-dependent infrastructure workingsteps embedded in the CNC, TCP part programs can be machine neutral and made compatible for machines within a given class such as milling or turning.
After establishing an agreed upon set of workingsteps, the half-scale NAS 979 5D part was simulated at NIST on a DeckelMaho Gildemeister (DMG) DMU 70 eVolution running a Siemens 840D controller. NIST wrote customized workingsteps for the DMG and the NAS 979 5D TCP program "worked great in simulation." However, before actual machining, NIST machinists reviewed the NAS 979 5D setup, and said the half-scale part was not really suitable for the DMG. The machinists said that the DMG did not have any 1" diameter tools or any l" cutter holders for the DMG, and that the specified 3.25" length cutters would have too much overhang and be prone to chatter or break. The NIST machinists further pointed out the supplied TCP feeds and speeds were not appropriate for the high-speed machining DMG, but could be used. After the realization of the process divergence, the NAS 979 5D part and tooling were scaled down to 1/4 size at NIST. The Boeing TCP program was successfully used to produce the NAS 979 part on the NIST five-axis DMG.
The collaboration between Boeing and NIST proved useful in exposing potential areas of non-portability of AP238 TCP programs. The difference in NIST and Boeing machining cultures highlighted a machining axiom that although CNC programs can be "data-neutral," they are not necessarily "process-neutral." The machining cultures between NIST and Boeing vary greatly. NIST is a small-batch job shop, in which machinists work with engineers to design and then produce CNC programs exclusively using Mastercam and then post and run the CNC programs themselves. Boeing and its subcontractors operate large production plants with 24/7 manufacturing, with extensive staff for CAD/CAM/CNC programming, post-processing, and machine operation. The disparity of manufacturing practice was illustrated by the differing machining process approaches: English versus metric, ways of fixturing, material hogging versus high-speed machining, and typical part sizes. Fortunately, the definition of a suite of standard STEP-NC infrastructure workingsteps appeared to resolve all of the portability issues by localizing machine inconsistencies.
Another finding was that even if TCP part programs are indeed portable, they are not necessarily optimal. The use of the most limited process parameters for feeds and speeds in the TCP program may not be practical for machining the same part on a 20-year-old Cincinnati gantry and then on a one-year-old, high-speed DMG. In spite of these issues related to portability, there is indeed a huge benefit to be gained by a large production facility with many machines of comparable capability, such as Boeing, to exploit TCP program portability and quickly move part production between machines of a similar chip-removal capability on a production floor.
The joint Boeing/NIST AP-238 work on proving TCP portability was just a first step toward leaner manufacturing, but the impact of STEP-NC technology on large OEMs, such as Boeing as well as their suppliers, can be significant. Boeing and NIST are cooperating on additional validation work in order to evaluate and publicize STEP-NC benefits and increase vendor participation.
The Boeing/NIST work attest to the need for standardizing STEP-NC workingstep infrastructure as a way to allow TCP program portability and handle the CNC machine dependent variations and options. Using AP238 toolpaths and a common working infrastructure, TCP cutter movement data were shown to be portable across different five-axis configuration CNCs as well as across manufacturing cultures with different machining practices. Boeing Company, www.rsleads.eom/ 702tp-154; NIST, www.rsleads.com/ 702tp-155.
S. Venkatesh and D. Odendahl, Boeing Company; X. Xu, University of Auckland, New Zealand; and J. Michaloski, F. Proctor, and T. Kramer, NIST.