Overcoming challenges of prototyping with single point incremental forming through formability and geometric accuracy analysis

Abstract

With recent developments in rapid prototyping technologies, the automotive industry has been able to move away from costly and inefficient methods of prototyping. In fact, rapid prototyping techniques now exist for nearly all the components in a car, meaning time and money is saved in product development. One exception to this trend, despite their ubiquity in automotive applications, is formed sheet metal components.

Single point incremental forming (SPIF) is a sheet metal forming technique with a fast turnaround that uses little to no custom tooling. It is a promising method for filling the gap in rapid prototyping capability for sheet metal components. However, despite significant research over the last two decades, barriers to industrial viability still include the key issues of fracture occurring in the sheet metal, or a final part being rejected due to unacceptable dimensional error. These two issues are affected by SPIF process parameters, but the extent of their influences are not well understood. By investigating the effect of process parameters on material formability and geometric accuracy, this thesis seeks to address these issues.

Case studies emphasise the impact of formability and geometric accuracy on prototyping automotive components with SPIF. Also emphasised is the importance of effective support walls and optimal design of the forming surface that is used to generate toolpaths for forming components.

A systematic review of the literature regarding the first key issue, formability in SPIF, highlights significant inconsistencies in published research about the effects of process parameters. A hypothesis to explain this result presents the idea of non-linear effects and parameter interactions, which is supported by original experimental work. This shows the difficulty of empirical prediction of formability when, for example, a small change in one parameter may interact with another to significantly influence the outcome of the final part.

Identifying and following safe formability limits will minimise the likelihood of fracture for the forming surface of a component. Research in this thesis looks at the thickness distribution of variable wall angle conical frustum (VWACF) parts as a basis for defining a safe formability limit. However, experimental results show this is not viable due to irregular trends in the thickness distribution close to the fracture point of the VWACF.

The second key issue of geometric error in SPIF is approached by focusing on a single mode of error, namely `wall bulge', or springback in flat walls of components. Experiments studied how a variety of tool shapes and sizes affected its severity, and found a trade-off with `pillowing', another mode of geometric error. At the same time as flat-ended tools reduce pillowing in the base, the experimental results show an increase in the amount of bulging in the walls.

The findings of this thesis demonstrate the impact that a single parameter change can have on multiple aspects of a component. Also highlighted are the complexities of the SPIF process that remain as barriers to industrial viability. This work contributes to overcoming these barriers and achieving efficient rapid prototyping of sheet metal components.