Rapid Prototyping by Single Point Incremental Forming of Sheet Metal

Martin Skjødt

Research output: Book/ReportPh.D. thesis

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Abstract

Single Point Incremental Forming (SPIF) is a sheet forming process with a simple setup. The forming tool is a rod with a spherical end and the sheet is mounted in a rig which allows forming of the sheet into the cavity of the rig. SPIF is dieless forming since no dedicated dies are being used. The process is incremental forming since plastic deformation takes place in a small local zone underneath the forming tool, i.e. the sheet is formed as a summation of the movement of the local plastic zone. The process is slow and therefore only suited for prototypes or small batch production. On the other hand it allows high strain compared to normal sheet forming processes, cheap tooling and a short product lead time. The work in this project can be divided into the following topics all containing novel contributions to the science of SPIF: Tool path generation in SPIF. A program has been developed which transforms a profile tool path into a helical tool path. This is an advantage since using a profile tool path results in scaring of the finished part and peaks in vertical forming force. The use of a helical tool path eliminates these problems. SPIF using a dummy sheet. This is a new variant of SPIF which has been invented in this project. Two sheets are formed at the same time instead of one. The top sheet is just a dummy and protects the bottom sheet from the sliding of the forming tool. The use of a dummy sheet setup eliminates wear, increases surface roughness, improves visual appearance and only causes a small reduction in formability. This setup also allows forming of soft aluminium sheets without damage to the sheet surface. Theory of SPIF and prediction of formability. Using membrane equilibrium equations a set of formulas have been developed for calculating principal stresses in the plastic zone. Using these it is demonstrated that the growth rate of accumulated damage in SPIF is small compared to conventional sheet forming processes. This combined with an explanation why necking is suppressed is a new theory stating that SPIF is limited by fracture and not necking. The theory explains a lot of experimental observation seen in the literature. SPIF of tailored blanks produced by friction stir welding. It is demonstrated that SPIF of tailored sheets produced by friction stir welding is possible and a promising way of combining two innovative manufacturing processes. Multi stage SPIF. A multi stage strategy is presented which allows forming of a cup with vertical sides in about half of the depth. It is demonstrated that this results in strain paths which are far from straight, but strains are still limited by a straight fracture line in the principal strain space. The multi stage process has been simulated and good correlation is found between simulated and experimental results.
Original languageEnglish
Number of pages184
ISBN (Print)978-87-89502-81-6
Publication statusPublished - Dec 2008

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