Incremental Sheet Forming
Incremental sheet forming (ISF) is an umbrella term for a range of processes in which a sheet is formed incrementally by a progression of localised deformation. The key advantage of ISF over conventional sheet forming processes is that specialised dies are not required; a wide range of shapes can be achieved by moving a spherical-ended indenter over a custom-designed numerically controlled tool path. Hence ISF is ideally suited for small-batch-size or customised sheet products. The wide range of potential applications includes bespoke medical implants, architectural features, specialised laboratory equipment and parts for specialist vehicles.
ISF is considered a sustainable manufacturing process because a relatively small and light-weight machine can be used to form sheet metal whilst avoiding the material expense of specialised dies. In addition, ISF can be used as a local production process, avoiding long transportation distances. Re-work or re-forming of old products are further capabilities offered by ISF, which is less energy-intensive than re-melting the material.
One of the most widely researched forms of incremental forming is called ‘single-point incremental forming’ or SPIF, in which a sheet is clamped rigidly around its edges but unsupported underneath and formed by a single spherical-ended indenter. Other variants of the process exist and are widely referred to as ‘two-point incremental forming’ or TPIF, in which the sheet is formed against full or partial dies using one or more indenters. Further variations include the use of a water jet or a combination of water jet with shot as an alternative to the solid indenter, as demonstrated by Iseki. Although TPIF offers the advantage of improved control of the deformation and hence improved geometric accuracy through physical constraint of the sheet, it is less flexible than SPIF because specialised tooling is required. Hence the research at Cambridge University has focussed on improving the capabilities of SPIF, therefore avoiding the use of specialised tooling.
|(a) Single-point incremental forming (SPIF)||(b) Conventional deep drawing with male and female dies|
|Comparison of single-point incremental forming (SPIF) (a) and deep drawing (b).|
Research Topic – Incremental forming of sandwich panels
The application of incremental forming to sheet metal has been widely researched. However, until recently the application to other classes of materials had not been investigated. This lead to the following research questions:
To what materials other than sheet metals can incremental forming be applied?
What mechanical properties of the material are required for incremental forming to be applicable?
Sandwich panels are an attractive material for weight-saving applications where high strength and stiffness are required. A range of sandwich panels exist which can be formed by conventional sheet forming processes such as pressing and hydroforming. Broadly, these can be summarised as two types:
Metal-polymer-metal sandwich panels
Metal-metal fibre-metal sandwich panels
Although the success of forming these materials using conventional sheet forming processes has previously been reported, it cannot be assumed that ISF will also be successful due to the different method of application of the deformation: localised deformation vs. wide-spread deformation; deformation in shear vs. deformation in plane-stress.
Tests were therefore carried out to investigate the applicability of ISF to a range of sandwich panels by forming two simple tool paths: a straight line across the centre of the sheet; and a spiral path in a horizontal plane.
|(a) Line test||(b) Spiral test|
|Tool paths to test mechanical feasibility of ISF of sandwich panels: (a) line and (b) spiral.|
Cross-sections were examined to look for failure modes including delamination of face plates, fracture of the core, fracture of the face plates and local indentation.
From this research, the following conclusions were made:
- Incremental forming is applicable to sandwich panels with metal face plates and a continous polymer core.
- Sandwich panels with metal face plates and metal fibre cores are not suitable for structural applications when formed by ISF because they exhibit crushing of the core (leading to loss of stiffness) and fracture of core fibres (leading to loss of core shear strength). However, ISF may be a suitable method for non-structural applications such as decorative features and housings.
- The mechanical properties which are the most important for successful application of ISF are ductility and incompressibility of the core and face plates, allowing the sandwich to sustain a localised deformation without damage.
Research topic – The mechanics of incremental sheet forming
The mechanics of the deformation of ISF (i.e. how the sheet moves under the action of the indenter) is fundamental to research of the process. The deformation mechanism influences the forming limits that can be achieved and should be considered when designing finite element models of the process.
Despite the importance of this topic of research, a literature review revealed that the deformation mechanism had never previously been experimentally measured and numerical simulation results were contradictory. The deformation mechanism had sometimes been described as plane-stress, i.e. analogous to that of pressing, and on other occasions had been described as pure shear in the plane normal to the direction of tool travel, i.e. analogous to shear spinning. However, without experimental evidence these claims could not be made and it was not possible to conclude what was the deformation mechanism of ISF.
|a) SPIF||b) TPIF||c) Shear spinning|
Idealised deformation mechanism of pure shear that has sometimes been associated with ISF drawn for various tooling configurations: a) SPIF; b) TPIF; and c) shear spinning, for which this mechanism was originally measured by Kalpakcioglu (1961).
The aim of the research was to establish through experimental methods what is the deformation mechanism of incremental forming.
The experiment involved separating a copper plate into two-halves with a grid-pattern machined on one side of the cross-section. The two halves were joined together by brazing, formed by incremental forming and then separated by heating in a furnace. The mechanism could be investigated by measuring the deformed grid pattern on the cross-section.
The results of the research can be summarised as follows:
- In both SPIF and TPIF the deformation is a combination of stretching and shear that increases on successive laps, with the greatest strain component being shear in the tool direction.
- Shear occurs perpendicular to the tool direction in both SPIF and TPIF, which is more significant in SPIF resulting in a piling up of the material at the centre of the plate
- The deformation mechanism is inherently different for SPIF and TPIF, and therefore the two processes should be distinguished in future discussions of their deformation mechanics.
Research topic – Tool forces in incremental forming
A specialised rig for incremental forming has been developed at Cambridge University which incorporates a real-time tool force measuring system. The system is the first in the world to enable tool force to be measured simultaneously with tool position, which allows maps of tool force to be drawn against the position of the tool in three-dimensional space. The system allows the unique opportunity to correlate changes in tool force to features in the forming process, and hence offers the potential for a real-time process control system using force and position feedback to be developed. The initial topic for research was to measure components of tool force during a forming process and to correlate this to features and trends in the shape of the product. This was carried out for the copper plates which were described above in the experiments for measuring the deformation mechanism.
The findings can be summarised as follows:
- All components of tool force increase on successive laps in both TPIF and SPIF, which is an indication of increasing work and is consistent with the measurements of increasing strains
- Tool forces are greater for SPIF than TPIF, which is also consistent with the measurements of strains for the two processes.
|Tool force components vs. tool position in the x,y plane for (a) SPIF and (b) TPIF: (i) diagram showing the direction of the tool path; (ii) force parallel to the tool direction (Fr); (iii) force perpendicular to the tool direction (Fθ); and iv) axial tool force component (Fz).|
A list of publications on our work to date on incremental sheet forming can be found here.