Stream Finishing Turbine Blades For Maximum Surface Performance
With operating combustion temperatures typically exceeding 1000 degrees Celsius, gas and aircraft turbines exert enormous demands on their components in harnessing the power generated. Whether producing the thrust of a jet engine or the energy to feed electrical power grids, the turbine blade is vital to efficiency – its shape, edge profiles and surface roughness are key determining performance factors.
Turbine blades can be forged, milled, cast and additively manufactured in varying shapes and sizes and from many different materials. To achieve their performance goals all blades will need surface finishing as there is high initial roughness and/or edges are left overly sharp from the manufacturing process.
Two key aims of surface finishing are:
- Homogeneous smoothing – normally to Ra < 0.4μm, but more often even less than Ra 0.25μm is desirable. Smoothing by hand is still common. Leaving aside the high costs associated with this method, its main disadvantage is the variability in finishing quality achieved. Robots too reach their limit as soon as the turbine blade’s geometry becomes complex – say when made up from guide vane segments which also need to be smoothed.
- Rounding the edges to a defined measurement – this is traditionally done on a CNC machine or by hand. As the thickness of the blade can change along the edge, this is a difficult process to achieve accurately with any repeatability. A typical problem that arises with these methods is that the blades can become over rounded at the corners.
To achieve the two key aims of homogeneous smoothing and defined edge rounding Fintek have installed the latest stream finishing machines developed by Otec Präzisionsfinish GmbH. This relatively new process offers turbine blade manufacturers vastly superior smoothing and edge rounding to exacting aerospace standards with absolute repeatability. More so the two aims are achieved in a single process cycle that is able to deal with complex geometries in much less time than traditional methods.
How Stream Finishing Works
A turbine blade is clamped to a special work holder. The blade is then immersed in a rotating drum filled with an abrasive medium. Choice of the medium, angle of immersion, speed of drum rotation and process duration all have a determining effect on the quality of surface finish. The direction and angle of contact can be precisely controlled to optimise the medium flow acting on the surfaces and geometry of the blade.
Usually a wet stream finishing process is used. Here an additive together with water is slowly mixed with the media to help carry away the fine particles removed from blade during processing. The water is then filtered and recycled.
Depending on the combination of factors described, repeatable quality of smoothing to up to Ra 0.1μm is easily achievable with complete accuracy. Commercial goals are also important considerations – so processing times often vary between two and thirty minutes to achieve a required quality.
Higher productivity is made possible by clamping up to five blades at a time in one machine and moving to automatic loading and unloading using a standard robot cell. Fintek work closely with Otec and can also customise the stream finishing machine and process to suit most requirements.
A significant advantage of the selective direction of the process stream means that both the leading and trailing edges of the turbine blade avoid the common problem of being over rounded as can occur with traditional methods. The blades are clamped in a way that ensures the corners are at the top and almost emerging from the process medium. The blade is directed toward the stream and swings to and fro at a specific angle (eg +/- 30 degrees C) to ensure a homogeneous surface finish.
Subcontract or Buy?
With over 30 years’ experience Fintek can develop the right process for your turbine blade that meets both your quality and commercial aims. AS9100 and ISO9001 certified, we provide a comprehensive subcontract service. We are also the UK agent for Otec and will help you find the correct machine, media and process cycle to use in-house. You can also try our free trial part processing service.