Innovative sheet forming processes
Flexible forming can be broadly defined as the forming of a wide range of shapes using simple tooling. In this group of processes, incremental sheet forming (ISF), invented in the 1990’s has been a subject of a large volume of research. However, this research has mainly concentrated on the initial process configuration and the process has not been widely accepted in the industry.
The aim of this project is to expand from the first inventive step that led to ISF, into the wider area of CNC sheet forming, and to develop novel flexible sheet metal forming processes with applications in industry.
In addition to the original ISF process configuration, many other unexplored process configurations exist and other, older craft processes could potentially be automated. Some examples of unexplored process configurations by Allwood (2008) are shown in figure 1.
Therefore the first question that needs to be answered is:
How can we evaluate and compare these processes?
An approach is proposed analogous to time domain control theory, where the impulse response of the physical system characterises a control system. We can represent a CNC forming process as a feedback control loop as in figure 2, where the transfer function (the controlled plant) is the forming process itself.
The transfer function and impulse response function contain the same information about the system (forming process). To obtain the impulse response, the system is excited with an impulse input and the resulting response is measured. In this case, the impulse is the change in sheet geometry created by the shortest realistic application of the controlled tool (Figure 3).
Three processes are investigated: Incremental sheet forming, power hammering and English wheeling (Figure 4), using Al 5251 aluminium alloy sheets. The sheet shape is measured using a stereovision camera.
|Fig. 4 – Three investigated processes|
Initial comparison reveals the following:
- In ISF the tool has a highly localised effect on the sheet, making it suitable for forming fine features in sheet metal parts
- The English Wheel and power hammer both have a similar, more global effect on the sheet, which suggests that these two processes are suitable for larger features in sheet metal parts.
Ideally, the system (the forming process) would be linear with a constant impulse response regardless of sheet thickness, prior deformation and the location of the impulse – and the controller design problem would thus be straightforward. In reality, for most flexible sheet forming processes, the impulse response will vary with location, input material and prior history, so the impulse must be measured and contrasted appropriately. Therefore the next question is:
How does impulse response vary with operating conditions?
Fig. 6 – Impulse response under various operating conditions
From the information in the graphs above, we have information about two aspects of the processes: the influence of the processes on the sheet, and the control of the processes.
Influence of the processes on the sheet:
- ISF is suitable for fine features
- The English Wheel and the power hammer are similar
- All three processes become more localised as the sheet is deformed
Control of the processes:
- The impulse response of ISF changes with all tested operating parameters
- The responses of the English wheel and the power hammer do not change with thickness or prior deformation
- Therefore, control of English wheel and power hammer is potentially easier than ISF
Application to online control of the ISF
The impulse responses can be used for closed-loop feedback control of flexible forming processes. This is demonstrated by forming a process model from a set of spatial impulse responses, which allows online feedback control of product geometry. This is applied to the incremental sheet forming process .
The process setup is shown in Figure 7a on the right. The control software runs in Matlab and a stereovision camera is used as the online shape sensor.
The target shape in this case is an axisymmetric 45° cone, formed in a 1mm thick Al 5251 H22 alloy sheet. The results shown in Figure 7b demonstrate that parts can be made to within ±0.15 mm of the target geometry using this feedback system.
A novel test method has been proposed and tested. Trials suggest that this method can characterise a process usefully, and could therefore be used for evaluation and comparison of various process configurations. Furthermore, the same concept has been applied in online control of ISF and it has been demonstrated that a significant improvement in accuracy can be achieved.
However, this is only the first step towards designing and implementing novel flexible forming processes. The next step will be to design a new process; using the approach outlined above, processes with desirable characteristics can be selected from a large number of configurations obtained by either brainstorming or a structured search; this will be followed by more detailed design of the process, and the building and testing of a new flexible forming machine.
The project is sponsored by Ford Motor Co., Novelis, Cummins Generator Technologies, Metal Spinners and a Dorothy Hodgkin studentship from the EPSRC.
Allwood, J.M.: A structured search for new metal forming processes, International Conf. on Technology of Plasticity (2008).
Allwood, J.M., Music, O, Raithatha, A., Duncan, S.R.: Closed-loop feedback control of product properties in flexible metal forming processes with mobile tools, CIRP Annals – Manufacturing Technology (2009), Vol. 1 (accepted manuscript)
A list of publications on our work to date on innovative sheet forming processes can be found here