As discussed in the ScienceDirect article “Seven Principles of Toolpath Design in Conventional Metal Spinning,” metal spinning is a complex process that requires precise toolpath design to avoid workpiece failure. Despite advancements in CNC spinning technology, the design of new parts often relies on a trial-and-error approach, resulting in wasted time and resources. In contrast, skilled hand spinners have successfully created new parts on the first attempt, leveraging years of accumulated expertise. This raises the question: Can human skill be captured and parameterized to improve automation in industrial metal spinning?
The Challenge of Toolpath Design in Metal Spinning
Metal spinning is one of the oldest metal forming techniques, with roots tracing back to the European Middle Ages where it was used to produce everyday items like saucepans and vases. Despite its long history, industrial spinning has not achieved the same level of automation as other forming techniques like drawing and stamping. This is mainly due to the complexities involved in designing the multi-pass trajectory, or toolpath, that the spinning roller follows to form the part.
An ideal automatic toolpath design algorithm would use initial conditions of the blank and mandrel to set parameters for a multi-pass roller trajectory that minimizes defects such as wrinkling, tearing, and excessive thinning of the workpiece. However, creating such an algorithm requires a deep understanding of the principles that skilled spinners use, which are often intuitive and based on judgment rather than formalized rules.
Seven Principles of Toolpath Design
Through an innovative approach, a haptic spinning system was implemented to capture and parameterize the skills of six experienced hand spinners. Their actions, along with their verbal insights during over 70 experimental trials, were recorded to develop a database of toolpaths. This analysis led to the formulation of seven fundamental principles for toolpath design in metal spinning:
- Take Small Bites: Gradually deform the material by taking small amounts at a time to prevent excessive stress and failure.
- Stay on the Mandrel: Ensure that the toolpath keeps the workpiece pressed against the mandrel to maintain control and prevent wrinkling.
- Use Forward and Backward Passes: Employ a combination of forward and backward movements to distribute forces evenly and achieve a smooth finish.
- Push in the Right Place: Apply force strategically, with higher pressure near the center and gradually decreasing towards the edges.
- Go at the Right Speed: Control the speed of the tool to match the material’s response, avoiding rapid movements that can cause tearing.
- Keep the Flange at the Right Angle: Maintain the correct angle of the flange relative to the roller to ensure consistent shaping.
- Use a Draft: Incorporate a draft in the design to facilitate the release of the workpiece from the mandrel without damaging the part.
Quantitative Toolpath Parameters
These principles were further quantified using parameters such as the average plastic strain per tool pass, the fraction of workpiece shape on the mandrel, and the feed ratio. Trials on thinner and larger blanks demonstrated that these parameters could predict success, suggesting that factors beyond simple displacement control—such as force and shape control—are essential for optimizing toolpath design in automated metal spinning.
The Future of Metal Spinning Automation
While hand spinning manuals, modern spinning guides, and academic literature provide valuable insights, they often lack the quantitative data needed for fully automating toolpath design in CNC machines. By leveraging the expertise of skilled artisans and formalizing their techniques into quantifiable parameters, the industry can move closer to achieving efficient and automated metal spinning processes.
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Article with all rights reserved, courtesy of ScienceDirect.