Triumph Aerostructures, a subsidiary of Triumph Group, Inc., is a leading global manufacturer of aerostructures for commercial, military and business jet aircrafts. The company has full fabrication capabilities and its available products include fuselages, wings, empennages, nacelles and helicopter cabins.
The company’s customer base consists of the world’s leading aerospace OEMs. Operating in 70 locations worldwide, Triumph designs, engineers, manufactures, repairs and overhauls a broad portfolio of aerostructures, aircraft components, accessories, subassemblies and systems. One of the sites, located in Nashville, TN, produces individual parts and sub-assemblies for Airbus, Gulfstream, Cessna, and Lockheed. With approximately 900 employees spread over two million square feet of work space, the Nashville site has 10 large CNC gantry mills, nine large assembly riveters, and a variety of smaller CNC equipment. In the machining area, there are more than 35 CNC spindles making chips.
“The Nashville facility focuses on parts that are ‘long and large’. Mostly everything we do is longer than 30 ft,” said Numerical Control (NC) Manager Kevin Chandler. “In the past, we made thousands of small parts – parts you could hold in your hand, but those are gone. At one time Nashville had a staff of 32 programmers, now only four remain. We do not have the luxury of multiple try-outs anymore. That is where Vericut has been a life saver – and a job saver.”
Despite changing the ownership several times, the facility has a long history of using the NC simulation software. It first began using the Vericut software for material removal simulation in 1991, when it was Textron Aerostructures. In 1996 it was purchased by The Carlyle Group and in 2003 it became part of the Vought Aircraft Industries. Vought Aircraft Industries, Inc. was acquired by Triumph Group, Inc. in June 2010 and was renamed as Triumph Aerostructures – Vought Aircraft Division.
Vericut is a software program that interactively simulates and displays the material removal process of an NC program. NC programmers use Vericut to verify the quality and accuracy of their NC Programs while its 3D simulation for CNC machines checks for collisions. But the goal of simulation is not simply a collision-free and efficient NC program.
The first goal is an NC program that produces the correct workpiece. Vericut’s accurate model tells the NC programmer whether or not his NC program makes a correct part. For example, many NC programs use circular interpolation. Vericut emulates the circle motion and creates an as-machined cylinder feature that can be measured to ensure its correctness. Most internal simulations do not emulate circle motion, but instead divide the circle motion into a series of linear motions approximating the cylinder. These segments are not measurable as a cylinder.
CNC machine simulation
Full CNC machine simulation of the actual machine takes verification to another level. “Machine simulation has been the major key to everything we do here,” said NC programmer Bill Gwinn. “Our machines have gotten much more complicated and the risk of collision between vises, bolts, tool changers, and other components is greater than ever. Simulation is so much more than cutting the part.”
Chandler and Gwinn are two of the most experienced Vericut software users in the world. Combined, they have more than 40 years experience using Vericut. “Experience has shown us that the more we can simulate on the screen, the less problems we will have down the road. We both agree on the advantages provided by machine simulation,” Gwinn said.
Machine simulation detects collisions and near-misses between all machine tool components such as axis slides, heads, turrets, rotary tables, spindles, tool changers, fixtures, work pieces, cutting tools, and other user-defined objects. A user can set up near-miss zones around the components to check for close calls, and detect over-travel errors. Vericut is designed to support advanced control functions including look-ahead or 3D cutter compensation, tool tip programming and tool length compensation, gage length reference point programming, canned cycles and fixture offsets, rotary axis pivot points; variables, subprograms, and macros; subroutines, looping, and branching logic.
Before Vericut was implemented, the Nashville facility was using Catia V4 and programming in APT, which had a slow check process using a flat-bed plotter. Then they machined the first few parts out of foam, or some other non-production material. Chandler said, “With the introduction of Catia and Vericut, our first time useable part started out at 90 per cent. With the introduction of machine simulation we improved even more dramatically. A tested and proven result of this happened in 1995 when a new 4-axis machining center was purchased for small parts. Using Catia solid models, Vericut, and Auto-Diff, we were able to get 96 per cent first time usable part programs, and 90 per cent of those were never modified past Issue 01. Since then, all programs are required to be run through Vericut Machine Simulation before they are released to the machine. As a result, we are experiencing upwards of 98 per cent good, first time parts. Since that first test, it has been very easy to convince the management that Vericut is a necessary tool to invest in. It cuts the machining hours down to a fraction of what it would have been.”
Gwinn added, “When a program is ready, we do not even go out in the shop anymore. Once it is passed through Vericut, there is nothing major that can go wrong. We know it will be cut correctly.” For example, a new high-speed 5-axis Handtmann PBZ milling machine is being employed to machine stringers that are over 60 feet long. To hold the stringers in place during machining, the Triumph Aerostructures Numerical Control group designed dovetail-shaped fixtures that they refer to as ‘scuff plates.’ “We call it a scuff plate because it is okay to hit it with the cutter. We insert a value of -0.022" in Vericut’s Collision Tables, between the cutter and scuff plates. All the components in this machine get really close—at times there is only 1 mm of clearance between the shank and scuff plate,” said Gwinn. “Those cannot collide, so they have a zero collision value.”
NC program optimization
The Triumph Aerostructures NC group had a two-fold challenge: they needed to quickly update legacy programs for the new Handtmann UBZ panel milling machine, while in the process of upgrading from Catia V4 to V5. By using Vericut and its NC program optimization feature, OptiPath, the NC group was able to create NC programs for the new machine – without re-programming them in Catia V5. First, they re-processed the Catia V4 programs for part location and cutter changes, then re-posted the resulting APT source files for the new machine. Next, they simulated them in Vericut to ensure they would be collision-free, and finally used OptiPath to update the feeds and speeds for the target machine.
“Rather than re-inventing the wheel, we took some of our old programs and ran them through OptiPath to get more out of them,” said Chandler. “In addition to shorten machining times, we also saved several hundred hours that would have been required to re-program the parts in the new CAM system.”
OptiPath optimization software works by analyzing the NC program (G-codes or native CAM output) and cutter contact with the ever-changing workpiece. It then divides the motion up into smaller segments to determine what conditions would benefit by increased feed rates, and where the feed rate needs to be reduced to protect the cutter. Since the software knows exactly how much material is being removed at each segment, it is able to determine the ideal feed rate. The toolpath trajectory is never altered.
Optimized feed rates maintain a more consistent chip load, which reduces wear on the cutters. The wear on the cutter is also minimized owing to the reduced amount of time required to cut each part. NC programs with optimized feed rates maintain a more constant cutting pressure between the NC machine tool and the workpiece. The machine is subject to less wear and tear not only because of the reduced machining time, but also because of the constant load. An optimized tool path also produces a better finish as a constant cutting pressure causes little or no variation in cutter deflection. Work piece finishes in corners, edges, and blend areas are improved significantly.
Another major benefit for the Triumph Aerostructures NC group has been Vericut’s Auto-Diff feature, which enables the programmer to compare a CAD design model to a Vericut simulation model and automatically detect differences. According to Gwinn, “All new programs, especially those generated from Catia V5, have solid models. We did not always have those in the past using Catia V4, but now we try to make sure all new programs start with solid models, and we use those same models to compare against our NC tool path. We have a slang saying around here that has stuck and holds true: ‘Vericut doesn’t lie!’ We have come to trust that what you see on the screen in Vericut is what you will get on the machine. It is virtual reality!”