Tip-entry machining of blisks can be practical for shorter blades. Machining blades in a single operation improves efficiency and reduces head movements and chance of collisions.
KENNETH J. KORANE Managing Editor
In most cases, the plane you fly in is older than the car you drive. Over the next 20 years, 40% of the current flying fleet will be retired and 20,000 new commercial aircraft are expected to be built.
Key projects include the 555-seat Airbus A380 and fuel-efficient Boeing Dreamliner. And major defense contracts include the $200 billion Joint Strike Fighter program and 620 Eurofighter aircraft slated for production.
As production ramps up, aerospace manufacturers are facing demands to hold the line on costs, improve quality, and speed delivery. This presents an excellent opportunity for aircraft builders and subcontractors to profit from recent manufacturing advances. These include techniques such as high-speed machining, and CAM software for machining complex shapes, according to officials at Delcam plc, a CAM software specialist based in Birmingham, England (delcam.com). Here's their take on some of the latest trends.
Engine and fuel-system components typically feature complex surfaces at various angles, containing many holes of different sizes. Positional five-axis machining has proven to be well suited for such parts. The process aligns and fixes rotary axes before cutting begins. "Because it only uses the three linear axes for cutting operations, it's quite easy to introduce high-speed machining techniques," explains Peter Dickin, Delcam's public relations manager.
Positional five-axis operations permit shorter cutters with higher speeds and longer tool life. As a result, it improves accuracy, lowers vibration, and produces a better surface finish. "And because cutting operations have a fixed orientation, programming is relatively simple," adds Dickin.
Another particularly useful aspect that's often trivialized, says Dickin, is five-axis drilling. CAM systems can recognize hole features defined in CAD models and quickly orient the drilling head for each hole in a complex part. Fewer fixtures are needed, which saves time and money. And mistakes are eliminated because there's the potential for error each time the operator repositions a part.
Positional five-axis machining can also generate most parts in two or three setups, making it practical to machine short runs from solid stock, rather than castings. With short runs and prototypes this makes the tactic particularly cost effective.
However, five-axis machining places more importance on the quality of the CAD model. A five-axis machine attempts to orient the head and find a way into gaps and small overlaps that three-axis motion tends to ignore, cautions Dickin. Thus, poor trimming (such as gaps between mating surfaces) of models is more likely to cause gouging. One source of problems can be data transfer, where tolerances of the system sending data are different from the one receiving it, he explains. This can result in trimming errors and small gaps in the model.
Some components push machine tools to their limits. For instance, machining blisks (bladed disks) from solid blanks is gaining favor over machining individual engine blades and assembling them on disks and shafts. One-piece blisks eliminate tolerance stackups and tend to be lighter than conventional rotor assemblies. And since blisk blades cannot shift or move from centrifugal force or blade load, aerodynamics improve and overall part performance can be more efficient.
Blisks generally require five-axis machining, but positional five-axis methods tend to produce less-than-satisfactory results, says Dickin. Disadvantages include the need for multiple operations, long machining times, extreme machine orientations, and a risk of collisions. The process may not remove all material, and it is difficult to smoothly blend between surfaces machined with different operations.
Thus, continuous five-axis machining, in which both rotary and linear axes move during cutting, is usually preferred. Because head orientation changes continuously when machining, precise control of cutting conditions can improve tool life, accuracy, and surface finish. It can mean fewer cutter passes, faster cycles, and better access to undercuts. And a single setup means fewer fixtures and positioning errors. Generally, high-speed machining is not possible because rotary axes move too slowly, although machine tools are improving in this regard, says Dickin.
Blisks are often machined in two operations, with tool access from the top and bottom of a blade. This keeps the head further from the bed, avoiding potential collisions. Tip entry can be practical for shorter blades machined in a single operation. It improves efficiency and reduces head movements and chance of collisions. Another new technique, flowline machining, plunges the cutter down a surface rather than milling across it, which can give more-efficient metal removal and fewer passes.
Complex, five-axis machining operations seem to get the most attention, but aircraft also include many simpler parts. The move to outsourcing means many of these parts will be made by small, sub-subcontractors that will never own or use complex CAM software, says Clive Martell, Delcam's director. In addition, repairand-replace operations in remote regions need simple parts quickly and consistently.
Feature-based CAM systems provide a ready solution, says Martell. The software identifies CAD features such as holes, pockets, slots, and bosses, then automatically determines rough and finish operations, selects tools, calculates feeds and speeds, and generates toolpaths and NC code.
For instance, Delcam's FeatureCAM software generates programming from a machiningattributes (rules) library. This eliminates data-entry errors and results in more consistent programs and machining operations, says Martell. The software accepts a wide range of CAD files, supports many different milling, drilling, and turning operations, contains an extensive tooling database, and simulates toolpaths before machining begins. He says that feature-based CAM can speed toolpath generation by as much as 500% for simple parts.
Both Boeing and Airbus plan to significantly increase their use of composite materials. For instance, about 50% of the Dreamliner's airframe and 25% of the A380 structure will be advanced composites.
According to Chris Edwards, business development manager, CAM software tailored for these materials can:
- Create or import part designs.
- Convert part models to pattern designs.
- Machine patterns for layup and molding.
- Convert 3D part models into 2D shapes for plies and cut shapes.
- Finish machining molded components, including trimming, drilling, and removing flash.
Recent developments include new methods for generating 2D patterns for prepregs (composite sheet) from 3D CAD models, and improved nesting to ensure the most economic use of materials cut from sheet.
Delcam's PowerShape, for example, is based on unwrapping techniques first developed for the footwear industry to cut leather shapes from shoe designs. The software createsprepreg designs needed for complex, curved shapes, then arranges the 2D patterns to use material most cost effectively.
A range of five-axis machining methods is essential for efficient composites manufacture and finishing. For instance, five-axis engraving of scribe lines defines component boundaries on layup tools, and five-axis profiling is used for trimming molded parts. Five-axis machining produces large patterns more quickly because shorter tools can cut faster. Finally, new five-axis drilling methods give faster drilling and more comprehensive hole recognition from a wider range of CAD systems.