Even in the age of advanced CNC machining and conventional finishing machines, some hand finishing is seen as an optimum process for turbine blades. Key to this has been the need to retain a precise and uniform profile, especially on the leading and trailing edges of the blade where rounding is essential to add strength, integrity and to increase blade performance.
The increasingly tight target tolerances to which blade manufacturers are working means that even hand finishing is failing. Though a highly-skilled hand finisher can accomplish the required tolerances and deliver what appears visually correct, subsequent testing reveals a far from uniform blade profile – particularly along the vitally important leading edge of the blade. Even if manufacturers can find sufficient numbers of skilled hand finishers, the time it takes to achieve an acceptable result is excessive.
This is the experience several aerospace manufacturers have been sharing with Fintek, who has been applying to the problem knowledge gained from surface engineering research work with gear manufacturers for F1 teams, where the pinnacle of performance is derived from continual and rapid development.
With over 30 years’ experience in surface engineering, Jonathan Dean, Fintek’s, managing director, explains: “Surface finishing is an abrasive and subtractive process. Producing the target surface smoothness across the blade is relatively straightforward, while precise edge rounding presents a much greater challenge to ensure that the blade profile is not compromised.”
Smooth operator
The gamechanger has been the introduction of highly-controllable stream finishing systems by OTEC Präzisionsfinish. This has allowed Fintek, who represent OTEC in the UK, to develop a special process to improve the efficiency and safety of surface finishing engine blades.
Smoothing the air foil, the blade body, has a positive effect on blade efficiency. Depending on the required result, the surface can be smoothed in a new generation stream finishing machine to required values of Ra < 0.2μm in just a few minutes, however, an Ra < 0.1μm or less is commonly achieved. During the process, a minimal amount of material is removed evenly from the surface.
Smoothing and edge rounding the turbine blades takes place in a single stream finishing operation. The blades are clamped into the machine and lowered into a container of abrasive media. Processing is carried out by both the precision-controlled rotation of the container and the movement of the blade in the media flow. The flow to the blades in the machine is clocked so that the alignment angle of the blade changes at frequent intervals. This means processing can be precisely aligned to specific points on the blade, achieving a smooth surface and precise rounding, simultaneously, without altering the profile.
Another important benefit is the ultrashort processing times compared to conventional methods. Depending on the size and initial condition of a blade, the aerospace manufacturers were used to a single blade taking 40 to 50 minutes to hand finish. Working with Fintek to achieve the optimum process parameters, the rate of blade finishing could be reduced to as little as three minutes per blade. Furthermore, they found the process could eliminate three previously needed CNC operations. For smaller blades, one manufacturer reported this could potentially save the deployment of eight machines on the shopfloor.
Flashing blades
Clamping the blades individually in stream finishing mitigates any risk of damage occurring during the processing and all steps are carried out in the one machine – minimising handling. The OTEC SF-5 stream finishing system for example, processes up to five engine blades at once, ensuring high output and cost efficiency. An additional benefit is that the process is effective at removing engineering machining lines and reducing residual stresses produced by previous manufacturing stages. This helps create a more durable blade that is less prone to stress cracking.
The stream finishing process is also the ideal surface preparation before hard coating are applied to components, as is often required in aerospace applications. Also, it can be run post coating to remove any residual hard droplets, such is the precision and ability to minimise material removal. Due to their inherent hardness these droplets can cause havoc in moving parts engineered to exceedingly tight tolerances.
Fintek offers both a subcontract service to aerospace component manufacturers and can also supply OTEC machines for inline production combining deburring, grinding, smoothing and polishing into a single process. The versatility of the machines also enables the reconditioning of cutting tools and great strides are being made in processing additively manufactured parts.
“Achieving quieter, lighter and fuel-efficient propulsion systems are major goals of high-bypass turbofan aero-engine primes, such as Rolls-Royce, GE and Safran,” Dean concludes. “Saving fuel while producing the same or better thrust from a smaller package on wing that is safe, reliable and easy to maintain are the main drivers in the quest for improved surface engineering that can meet stringent tolerances, improve productivity and do it repeatably. To meet these challenges, surface finishers need to become surface engineers and work ever more closely with their customers.”
