Turbine Root Form Grinding
Advantages of Turbine Blade Root Grinding
Inefficient production processes once increased the lead time required to fulfill specialized orders for aerospace components. Machine shops relied on milling to shape cutting-edge materials like heat-resistant super alloys (HRSAs) and composite materials. Milling involves a variety of processes to shape individual components, including:
Operators may need to switch between fabrication strategies like broaching, lapping, or honing to create turbines and other pieces. Often, they have to closely watch machines to avoid breakdowns or wear. Additionally, the high heat and speeds can result in plastic deforming and other defects.
However, recent advances in turbine blade grinding have allowed shops to meet the requirements more easily. Grinding systems have proven to be more effective and efficient, crafting turbines out of HRSAs with greater precision. As a result, manufacturing companies have begun to embrace the advantages of agile, integrated grinding systems.
A Return to Grinding Turbine Blades
Several years ago, aerospace manufacturers began to replace grinding with high-speed production processes. While they saw some success, the inefficient operations, high costs, and increased tool wear impacted their workflows. Further, the use of HRSAs required extreme strength, heat, and power that many machines couldn't handle.
That said, HRSAs such as titanium, iron, and nickel alloys offer countless benefits for the demanding industry, such as:
Heat and corrosion resistance
Enhanced surface integrity
High strength-to-weight ratio
However, the same characteristics that attracted aerospace developers also make them difficult to machine. As a result, tool life can drop, and lead times, costs, and inefficiency may increase dramatically without the proper systems.
Luckily, new research on grinding systems has revolutionized the HRSA fabrication process. Using a grinding machine to shape super alloys allows operators to bring them closer to dimensional tolerances more quickly. A grinding wheel results in a more reliable finish and can dramatically reduce cycle time. Tests have shown that using a grinding system can reduce setup time by 95%, while cycle time may decrease by as much as 66%.
While manufacturers always aim for a more efficient machining cycle, the main advantage of using a grinding system is the reliability of the finished products. Turbine root form grinding improves the surface integrity of the jet engine turbine blades by applying less residual stress. A reduction in stress results in extended fatigue life, allowing ground turbine parts to run faster and for longer periods than milled or broached products.
Perhaps the most exciting aspect of reintroducing grinding machine systems to manufacturing processes is the far-reaching effects. Alongside the aerospace industry, manufacturers in the automotive, medical, energy, tooling, and gear industries have turned to grinding wheel packs for more efficient, reliable, and compliant components.
Addressing the Root of the Problem
When manufacturers craft them through the use of milling and broaching techniques, jet engine turbine blades require near-constant monitoring. Because of the irregular shape of their "fir tree" root forms, shop operators needed to carefully oversee each cycle to prevent a potential breakdown or workpiece defect.
Machine shops have recently teamed up with suppliers to determine a solution to several key factors that influence turbine blade manufacturing, including:
The Use of Super Alloys
Heat-resistant superalloys represented a significant change to the aero-turbine manufacturing process. Since their introduction to the market in the 1940s, gas turbine blades have relied on their strength, durability, and corrosion resistance.
While superalloys have increased the lifespan of turbine blades, they require more machining power. As a result, standard tools break down more quickly, while tooling costs soar.
Broaching, one of the key machining processes manufacturers use when crafting a turbine blade, requires specialty tools. Unless they switch to grinding systems, many manufacturing companies will need to replace or rebuild their current broaching machines to accommodate higher-powered cutting tools.
Varying Needs of Manufacturing Systems
As aerospace engineering continues to evolve, the demand for improved weight-to-thrust ratios and better fuel performance increases, too. To fulfill these demands, manufacturers must craft complex, highly sensitive turbine blades made of super alloys, which, as we mentioned above, require state-of-the-art tools to fabricate.
The Cost of Advanced Tools
The cost of producing the modern turbine blade directly reflects the three factors we discussed above. Aging machinery simply cannot adapt to be able to efficiently craft blades for gas turbines. This results in an increased demand for modern broaching tools. The complex and hard-to-find tools have a limited lifespan as well.
With these factors in mind, manufacturers have developed new methods to standardize the cutting of root form slots for turbine blades. Traditionally, companies coated the turbine blades in a soft alloy that allowed the broaching machine to hold them more easily. However, the coating process requires considerable time and must be managed by operators as well.
In contrast, engineers have recently created a fixture that secures the fir tree root form of the blades. The fixture loads the blades automatically and even features a robot mechanism to change the grinding wheels when necessary.
Many manufacturers opt for electroplated cubic boron nitride (CBN) grinding wheels, especially when working with the nickel alloys that aerospace manufacturers commonly use. CBN displays high heat and corrosion tolerance, resulting in less wear and tear while grinding the HRSA.
While companies must upgrade their systems to work with CBN grinders, the added strength, endurance, and speed can ultimately save companies millions of dollars each year, based on their average blade production levels.
The Turbine Blade Root Form Grinding Process
The grinding of turbine blades requires an exacting process. Each blade must meet specific dimensions. Further, the root form slots in the turbine rotors require careful contouring. The slightest mistake can render a rotor completely useless.
With this in mind, manufacturers have developed several different root form grinding processes, some more efficient than others. Below, we'll take a look at some of the more common fabricating processes that manufacturers use to grind root forms and other aerospace parts.
Creep Feed Grinding
Manufacturers typically use the creep feed method when working with depths of 20 millimeters or less. The deeper cuts result in higher material removal rates and machine efficiency despite the method's lower speeds. Additionally, creep feeding can usually finish milling in a single pass compared to other methods.
The powerful, efficient method works well for mass production. Despite its slow speed, creep feeding can remove as much as 80 mm3 (cubic millimeters) of material in seconds, with a workpiece rate of up to one meter per minute. Further, creep feeding typically results in less tool wear and plastic deformation. Some creep feed grinders even feature continuous dressing mechanisms, which sharpen and trim – or "dress" – the grinder while cutting the root forms.
As many would imagine, vertical grinding involves an up-and-down and side-to-side motion. First, a vertical machine holds the workpiece upright. Then, a wheel drops from overhead and begins to shape the piece.
This method often results in improved shape with less distortion. Additionally, a vertical grinder can typically finish the job in a single pass, minimizing the production cycle.
Also known as "Very Impressive Performance Extreme Removal," the VIPER method quickly overtook creep feeding after its introduction in the 1990s. The VIPER method injects a high-pressure coolant onto the grinder wheel. The force of the wheel moves the coolant forward, cleaning the wheel while cooling the surface of the grinder at the same time.
The VIPER method has continued to grow in popularity, especially with the introduction of HRSAs. By keeping the cutting area cool, the VIPER method reduces the risk of damage or deformation without slowing the production process.
Improving Production Efficiency Through the Blade Root Grinding Process
Advances in aerospace machining have changed the production game for many shops. However, as operators and engineers will agree, there is still room to improve.
Even so, utilizing advanced grinding systems and cutting-edge technology can significantly decrease the production cycle of turbine root forms. In addition, by upgrading or replacing aging systems, manufacturers can incorporate specialized grinding tools that:
Improve material surface quality
Eliminate waste from defects
Fabricate dimension-tolerant HRSA turbine parts in minutes
Many aerospace manufacturers expect the world's aircraft fleet to double by 2040. The industry's growth represents countless opportunities for manufacturing companies. Yet, with the increased number of possibilities come challenges as companies work to comply with regulations.