in Vol. 13 - March Issue - Year 2012
Does Size Matter?
Long vibratory trough for finishing aircraft structural components
Large fan blade
Many large/long aircraft structures and components are deburred in vibratory trough machines producing a uniform edge radius
The vibratory mass finishing technique is widely used and commonly known as a cost effective method for finishing small to medium sized components, but did you know that the technique can be applied to very large and long components?
Typically vibratory mass finishing applications are applied to mass-produced components that generally fit into the palm of your hand, and are produced in large quantities. These components are often very difficult (or not cost effective) to finish by hand or by other methods.
The mass finishing technique can be successfully applied to very large or long components or profiles especially if the part in question should be surface finished in a uniform manner. Component types in this category include large aero-engine fan blades, aircraft wing spas and stringers, landing gears and even aircraft cabin floor panels.
Special vibratory trough machines have been developed to handle very large components, and depending on the component length and weight, determine the type of vibratory drive system to be installed on the machine. Direct drives and vibratory groups ensure uniformity of the mass movement; this is the key to a completely consistent and repeatable surface finishing which is a prerequisite especially in the aerospace industry.
Vibratory mass finishing, whether for aircraft components or any other parts, is a multi-functional technology. It is not only used for deburring or edge radiusing but also for surface smoothing and finishing on a wide array of aircraft parts. Deburring and edge radiusing reduce the likelihood of stress cracks in components, and smoothing and polishing surfaces such as large fan blade airfoils reduces air flow resistance and improves fuel efficiency as measured by specific fuel consumption (SFC), which calculates the amount of fuel needed to maintain a specific power for a specified period of time.
The smooth surfaces of a fan blade reduce the risk of deposits sticking to the airfoil surface. Smoother airfoil surfaces result in lower engine operating temperatures, allowing a greater margin between the actual exhaust gas temperature and the engine "red line"--maximum exhaust gas temperature (MEGT). This temperature reduction lengthens the time that an engine can remain in service before it requires overhaul and repair.
Large structural fuselage sections and wing spars are produced mainly from aluminium alloys and more recently some in composite materials can in some cases be processed in a large vibratory mass finishing machine; for these parts the treatment is deburring and edge radiusing after machining. Parts free from burrs and with smooth radii as opposed to sharp edges have higher resistance to fatigue crack initiation and propagation. Paint adhesion is also improved, especially on part edges.
Some very large components may require to be processed in a protective fixture. This will prevent the possibility of any damage or nicking of delicate edges, the fixtures are simple to operate, and can be loaded and unloaded in a few minutes, in some cases more than one component can be loaded into the fixture.
Considering the sensitivity of aircraft components with regard to precisely controlling material removal and edge profiles, and risk of possible component failure in service, their manufacture, surface treatment and surface finishing is subject to numerous stringent process and quality control procedures, including Nadcap, the European Aviation Safety Agency (EASA) and U.S. Federal Aviation Administration (FAA) regulations. (Formerly the National Aerospace and Defence Contractors Accreditation Program), Nadcap is now a global cooperative standards program for aerospace and related manufacturing.
In light of these stringent quality requirements, it should come as no surprise that manual deburring and surface finishing processes will soon stop completely in the aerospace industry. The finishing process must be repeatable and completely documented. This requires a high degree of mechanization and, preferably, automation with very little or no human process intervention.
All surfaces must be uniform and repeatable; this cannot be achieved by hand finishing regardless of the skill and experience of the person carrying out the treatment, as it is inevitable that inconsistencies will occur.
by Paul Rawlinson
Contributing Editor MFN & Business Development Manager, Aerospace International for Rösler