Pelton wheels are the driving force of the hydro-power industry, extracting energy from fast-flowing water to generate electricity. However, manufacturing a Pelton wheel is a challenge. Due to the wheel’s complex design, the impulse blades are usually machined separately and then mounted to the rim, which is time consuming and adds to production costs.
Bucket-shaped impulse blades are situated close together and taper towards the rim of the wheel, which makes it extremely complicated for the cutting tool to navigate between the blades without causing a collision. There are also special demands on the surface quality and topological precision of the organically-shaped blade cavities to ensure they extract the maximum amount of energy from the water.
On top of these issues, Pelton wheels are big and heavy, and stretch the physical limits of the machine. Such characteristics make it difficult to machine an entire wheel in an efficient and economically viable way. To do so, the machine tool must be able to navigate around the whole workpiece in a highly dynamic motion, efficiently cutting the wheel and performing high-speed, 5-axis, collision-free machining to shape the blades.
To offer Pelton wheel machining on its MU-V series of machining centres, Okuma soon realised it would need technically sophisticated tool-path generation and simulation software to master the special technical challenges. For this reason, Okuma teamed up with ModuleWorks to find a way forward.
Overcoming the sheer physical limitations means the wheel is machined in two stages. One half of the wheel is machined before being turned 180° to enable the tool to reach the other half. The ModuleWorks triangle mesh roughing modules were used to cut the rough shape of the Pelton wheel.
For creating the bucket-shaped blades, the ModuleWorks adaptive roughing strategy uses consistent tool-path engagement and a smooth tool-path motion to generate a dynamic pattern and a high-performance roughing process that reduces the overall costs of producing non-prismatic shapes. For optimal finishing, ModuleWorks geodesic machining is said to deliver a high-quality finishing pattern and comes with features for producing rounded corners using a morph or constant step-over pattern. This capability enables Okuma to meet the high demands of the workpiece and ensure optimal energy transfer from the high-pressure flowing water.
ModuleWorks 5-axis calculation strategies also extend the reach of the tool to provide enhanced tilting strategies on complex workpiece geometries and areas with deep grooves. As a result, the strategies are suitable for machining the narrow areas between the impulse blades, enabling the machine to work efficiently with the heavy workpiece to perform tool-path moves that optimise productivity and ensure an economically viable product. Combining the special tool-path generation strategies with the tilting and collision avoidance algorithms extends the reach of the tool to successfully navigate the extremely tight spaces.
Gouge-checking during the tool-path calculation followed by full 3D kinematic machine simulation ensure the validity and accuracy of the tool path before machining. Behavioural analyses of the segment length, feed rate and height allow the tool path to be refined for optimal finish and quality. Moreover, because the wheel is machined from a single workpiece, simulation and machine set up only need to be performed once, which further increases the efficiency and cost effectiveness of producing Pelton wheels.
At EMO 2019, Okuma will present a live display of Pelton wheel machining on one of its MU-8000V 5-axis vertical machining centre (Hall 27, stand D26). Also at the show, ModuleWorks will demonstrate the company’s specially devised Pelton wheel machining simulation (Hall 9, stand E01).
