Pelton wheel machining in one

But manufacturing a Pelton wheel is a challenge. Due to the wheel’s complicated design, the impulse blades are usually machined separately and then mounted onto a rim, which is time consuming and adds to the production costs.

In a new collaboration project, Okuma and ModuleWorks have devised a solution that enables the entire Pelton wheel to be machined from a single block of metal. This new procedure accelerates production and reduces manufacturing costs.

Machining a Pelton wheel from a single workpiece is notoriously difficult. The 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 this, Pelton wheels are big and heavy and soon stretch the physical limits of the machine which makes 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 to efficiently cut the wheel and perform high-speed 5-axis collision-free cutting to shape the blades.

To offer Pelton wheel machining on its MU-V series of machining centres, Okuma – whose machines are available in the UK from NCMT – soon realised they would need technically sophisticated toolpath generation and simulation software to master the special technical challenges. This is why Okuma and ModuleWorks teamed up to find a solution.

To overcome the sheer physical limitations, the wheel is machined in two stages. One half of the wheel is machined and then the workpiece is turned 180° to enable the tool to reach the other half. The industry-proven ModuleWorks triangle mesh roughing components were used to cut the rough shape of the Pelton wheel.

For creating the bucket-shaped blades, the ModuleWorks adaptive roughing strategy uses consistent toolpath engagement and a smooth toolpath motion to generate an impressive dynamic pattern and high-performance roughing process that reduces the overall costs of producing non-prismatic shapes.

For optimal finishing, ModuleWorks geodesic machining delivers a high-quality finishing pattern and comes with advanced features for producing rounded corners using a morph or constant stepover pattern. This enables Okuma to meet the high demands on the workpiece to 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. This makes the strategies ideal for machining the narrow areas between the impulse blades and enables the machine to work efficiently with the heavy workpiece to perform toolpath moves that optimise productivity and ensure an economically viable product.

Combining the special toolpath generation strategies with the advanced tilting and collision avoidance algorithms extends the reach of the tool to its full limit to successfully navigate the extremely tight spaces.

Gouge-checking during the toolpath calculation followed by full 3D kinematic machine simulation ensures the validity and accuracy of the toolpath before machining. Behavioural analyses of the segment length, feedrate and height allow the toolpath to be refined for optimal finish and quality.

Moreover, because the wheel is machined from a single workpiece, simulation and machine set-up only needs to be performed once which further increases the efficiency and cost-effectiveness of producing the wheel.

A special combination of adaptive roughing, geodesic and classical machining strategies cracks the challenge of the Pelton wheel and opens new machining possibilities by extending the reach of tools for safe operation in extremely tight areas.

For Pelton wheel machining it means new design options by using new, innovative machining processes that boost productivity and cost-effectiveness.

NCMT www.ncmt.co.uk

ModuleWorks www.moduleworks.com