VOL. 18 March ISSUE YEAR 2017
in Vol. 18 - March Issue - Year 2017
Flying High – How Mass Finishing Helps The Aerospace Industry Extend Fatigue Life And Save Fuel Costs
Photo 1: Helicopter gears requiring a "super finish"
Photo 2: Surface profile after different finishing stages. Note that after super finishing, the oil retention "valleys" still exist.
Photo 3: Landing gear
Photo 4: Blisk
Photo 5: Blisk being treated in special rotary vibrator
Photo 6: Deburring of the root of turbine blades (Critical area highlighted in pink)
Photo 7: Deburring of the root of turbine blades in a "Surf Finisher"
Photo 8: Turbine disk with finished dovetail slots
Strong competition forces aerospace manufacturers to constantly improve the performance, safety and cost efficiency of their products. These efforts focus mainly on fuel consumption and fatigue life of key components. While the development of new lightweight materials has brought significant progress, innovative surface finishing methods also play a key role in the overall improvement of the performance characteristics of new as well as MRO components.
Why is high quality surface finishing so important?
Mass finishing can be used for many tasks like deburring, edge radiusing, surface smoothing and polishing. And each one is important for a wide range of aerospace components.
For example, deburring and edge radiusing greatly reduces the risk of stress cracks and leading edge corrosion. The surface smoothing or "super finishing" offers a slew of different advantages. On airfoils, it reduces airflow resistance and improves the TSFC metrics (specific fuel consumption). Smoother surfaces also reduce the risk of deposits sticking to the airfoil surface. In addition, not to forget, they result in lower engine operating temperatures allowing a greater margin between the actual exhaust gas temperature and the engine "red line", also known as the Maximum Exhaust Gas Temperature (MEGT). This helps extending the service life of an engine, before it must be removed for overhaul.
Aerospace surface specs are the most stringent in the world. To avoid costly weld repairs, the required finishes must be achieved with a minimum of material removal of max. 0.0005" (0.013 mm).
New, innovative mass finishing systems meet these entire requirements and ensure that the finishing results are achieved consistently within narrow tolerances.
Shot peening and mass finishing go hand-in-hand
Shot peening is used for many engine and structural airplane components to induce a residual compressive stress that helps extend the fatigue life by preventing micro cracks and stress corrosion. Nevertheless, peening usually creates rougher surfaces, which can drastically increase friction. That is why, for many work pieces, the shot peening step should be followed by a super-finishing process to reduce the surface roughness to the specified Ra values, which for airfoils can be 0.2 μm and for gears as low as 0.05 μm.
The following example shows the development of the surface profile of a gear (photo 1) after grinding, shot peening and super finishing. In this case the Ra value could be reduced from 1.5 μm after shot peening to under 0.06 (photo 2) without affecting the oil retention capability of the work piece!
A technology for finishing many airplane components
Mass finishing methods are used for a large variety of different components, for example, for surface finishing of
• All kinds of airfoils like low pressure fan blades, low and high pressure compressor blades, vanes, blisks, turbine disks, and propellers for turboprop airplanes
• Air frame components like wing spars, fuselage parts and landing gears
• Different kinds of gears and bearing components (for example, helicopter gears)
Below are a few examples of components, for which mass finishing made the difference:
Deburring and edge radiusing of landing gears (photo 3)
These components, made of titanium, are machined on CNC machine tools, which can leave heavy burs. Prior to shot peening, the burs must be removed and all edges radiused. After shot peening, the parts can be painted and assembled.
Surprisingly, this deburring and edge radiusing operation is still frequently done by hand, requiring up to 80 man-hours and producing erratic finishes.
In contrast to this archaic treatment, landing gears can be processed in single-piece operations in large tub vibrators with specially formulated ceramic media. Treatment times can vary between 60 – 90 minutes.
Surface smoothing of blisks (photo 4)
Since they are less costly to manufacture, blisks are increasingly used in the compressor section of a variety of turbines. Blisks, usually made from titanium, can be produced with different manufacturing methods like investment casting or CNC milling. They can even be made from powdered metal or with 3D printing. Irrespective of the manufacturing method however, the blade section must be smoothed from Ra values of 1.0 – 3.0 μm down to 0.1 – 0.7 μm).
The finishing of blisks with diameters of up to 1,200 mm usually takes place in special rotary vibrators without inner dome (photo 5) filled with special finishing media and with the component firmly attached to the bottom of the work bowl.
Deburring of the roots of turbine blades (photo 6)
After machining, the roots of some turbine blades may have to be deburred and the edges rounded to a radius of 0.1 mm. This requires the finishing of certain surface areas of the work pieces, namely the root sections, while other surface areas must remain untouched.
For such tasks, the recently developed, so-called "Surf Finisher" (photo 7) is the ideal machine: A 6-axis buckling arm robot picks up the work pieces and guides them through a bed of media in a cylindrical work bowl that rotates with up to 500 RPM. This system produces consistent finishing results in surprisingly low cycle times.
Deburring and edge radiusing of the dovetail slots on turbine disks (photo 8)
The machining and broaching of the dovetails on turbine disks leaves burs and very sharp edges. To remove the burs and create a smooth radius on the dovetail contours requires an intensive deburring and edge radiusing process.
This goal can be achieved in high-performance drag finishers. After a cycle time of just a few minutes, the dovetail slots are perfectly rounded.
The above examples have hopefully provided a glimpse of the many applications of mass finishing technologies in the field of aerospace components. The various machine types with their practically endless material handling capabilities paired with hundreds of different finishing media offer numerous possibilities to treat a wide variety of aerospace components, from very small to very large and from simple deburring to super finishing of targeted surface areas!
How OEM’s can improve their surface finishing operations
Surprisingly, many OEM’s leave their surface finishing operations to job shop sub-contractors, with sometimes, little control over the finishing process and its costs. Working more closely with qualified suppliers of finishing equipment and media can help the OEM’s save hundreds of thousands of Euros or $. Intensive consultation between OEM and finishing specialist will in all likelihood produce better and less costly finishing methods. Additionally, it may lead to minor product changes by the OEM, which will facilitate the finishing process resulting in higher finishing qualities as well as lower costs.
The specialist suppliers of finishing equipment and consumables maintain comprehensive test labs staffed with qualified and experienced process engineers. Why not use these facilities and talents to optimize the finishing process for aerospace components?
by Eugen Holzknecht
Contributing Editor MFN and
Rösler Oberflächentechnik GmbH