Incremental sheet forming

Incremental sheet forming (or ISF, also known as Single Point Forming) is a sheet metal forming technique where a sheet is formed into the final workpiece by a series of small incremental deformations. However, studies have shown that it can be applied to polymer and composite sheets too. Generally, the sheet is formed by a round tipped tool, typically 5 to 20mm in diameter. The tool, which can be attached to a CNC machine, a robot arm or similar, indents into the sheet by about 1 mm and follows a contour for the desired part. It then indents further and draws the next contour for the part into the sheet and continues to do this until the full part is formed. ISF can be divided into variants depending on the number of contact points between tool, sheet and die (in case there is any). The term Single Point Incremental Forming (SPIF) is used when the opposite side of the sheet is supported by a faceplate and Two Point Incremental Forming (TPIF) when a full or partial die supports the sheet.

Advantages over conventional sheet metal forming

Because the process can be controlled entirely by CNC processes no die is required as is in traditional sheet metal forming. The elimination of the die in the manufacturing process reduces the cost per piece and increases turnaround time for low production runs because the need to manufacture a die is eliminated. However, for high production run the time and cost to produce a die is absorbed by the higher per piece speed and lower per piece cost. Several authors recognize that the formability of metal materials under the localized deformation imposed by incremental forming is better than in conventional deep drawing.[1] In contrast, there is a loss of accuracy with the ISF process.[2]

Implementation

The ISF process is generally implemented by clamping a sheet in the XY plane, which is free to move along the Z axis. The tool moves in the XY plane and is coordinated with movements in the Z axis to create the desired part. It is often convenient to retrofit a CNC milling machine to accommodate the process. Spherical, flat-bottomed, and parabolic tool profiles can be used to achieve differing surface finishes and forming limits.[3]

The machine employs a combination of stretch forming by drawing the sheet incrementally down over a die, with the CNC tool approach described above. This is said to produce a more even distribution of thickness of the material. The process is well suited to one-off manufacture though difficulties in simulating the process mean that toolpaths are complex and time-consuming to determine.

Ford Motor Company has recently released Ford Freeform Fabrication Technology, a two-point incremental sheet-forming technique being implemented in the rapid prototyping of automotive parts. Complex shapes such as the human face[4] and cranial implants[5] have been manufactured successfully using this manufacturing process. Advances in the technology are expected to increase adoption in the near future by other sheet metal-reliant manufacturers.

List of process parameters

The mechanics of the process is influenced by many parameters, including:

  • the transverse X-Y feed rate,[6]
  • the vertical Z feed rate or pitch,[7]
  • the (optional) tool rotation,[8]
  • the coefficient of friction,[9]
  • the tool shape (radius),[10]
  • the sheet temperature,[11]

Current research

Research is underway at several universities.[12][13] The most common implementation is to outfit a traditional milling machine with the spherical tool used in the ISF process. Key research areas include

  • Developing rolling tools to decrease friction.
  • Reduce thinning of sheets after forming
  • Increase accuracy by eliminating springback[14][15]
  • Develop novel uses, especially extending the process to new materials (e.g. composites) and to apply heating [16]
  • Improve surface roughness[17]
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References
  1. Strano, Matteo (31 December 2004). "Technological Representation of Forming Limits for Negative Incremental Forming of Thin Aluminum Sheets". Journal of Manufacturing Processes. 7 (2): 122–129. doi:10.1016/S1526-6125(05)70089-X.
  2. "Dieless NC forming". Retrieved on 2008-11-05.
  3. Examining Tool Shapes in Single Point Incremental Forming (Cawley et al, 2013)
  4. Behera, Amar Kumar; Lauwers, Bert; Duflou, Joost R. (2014-05-01). "Tool path generation framework for accurate manufacture of complex 3D sheet metal parts using single point incremental forming". Computers in Industry. 65 (4): 563–584. doi:10.1016/j.compind.2014.01.002.
  5. Duflou, Joost R.; Behera, Amar Kumar; Vanhove, Hans; Bertol, Liciane S. (2013-01-01). "Manufacture of Accurate Titanium Cranio-Facial Implants with High Forming Angle Using Single Point Incremental Forming". Key Engineering Materials. 549: 223–230. doi:10.4028/www.scientific.net/kem.549.223. ISSN 1662-9795.
  6. Hamilton, K.; Jeswiet, J. (2010). "ScienceDirect". Cirp Annals. 59: 311–314. doi:10.1016/j.cirp.2010.03.016.
  7. Golabi, Sa’id; Khazaali, Hossain (August 2014). "Determining frustum depth of 304 stainless steel plates with various diameters and thicknesses by incremental forming". Journal of Mechanical Science and Technology. 28 (8): 3273–3278. doi:10.1007/s12206-014-0738-6. ISSN 1738-494X.
  8. Davarpanah, Mohammad Ali; Mirkouei, Amin; Yu, Xiaoyan; Malhotra, Rajiv; Pilla, Srikanth (August 2015). "Effects of incremental depth and tool rotation on failure modes and microstructural properties in Single Point Incremental Forming of polymers". Journal of Materials Processing Technology. 222: 287–300. doi:10.1016/j.jmatprotec.2015.03.014.
  9. Lu, B.; Fang, Y.; Xu, D.K.; Chen, J.; Ou, H.; Moser, N.H.; Cao, J. (October 2014). "Mechanism investigation of friction-related effects in single point incremental forming using a developed oblique roller-ball tool". International Journal of Machine Tools and Manufacture. 85: 14–29. doi:10.1016/j.ijmachtools.2014.04.007.
  10. Carrino, L.; Giuliano, G.; Strano, M. (2006), "The Effect of the Punch Radius in Dieless Incremental Forming", Intelligent Production Machines and Systems, Elsevier, pp. 204–209, doi:10.1016/b978-008045157-2/50040-7, ISBN 9780080451572
  11. Fan, Guoqiang; Gao, L.; Hussain, G.; Wu, Zhaoli (December 2008). "Electric hot incremental forming: A novel technique". International Journal of Machine Tools and Manufacture. 48 (15): 1688–1692. doi:10.1016/j.ijmachtools.2008.07.010.
  12. "" Retrieved 2008-11-05.
  13. J Jeswiet: "Asymmetric Single Point Incremental Forming of Sheet Metal", CIRP Annals - Manufacturing Technology, 2005
  14. Behera, Amar Kumar; Lu, Bin; Ou, Hengan (2016-03-01). "Characterization of shape and dimensional accuracy of incrementally formed titanium sheet parts with intermediate curvatures between two feature types". The International Journal of Advanced Manufacturing Technology. 83 (5–8): 1099–1111. doi:10.1007/s00170-015-7649-2. ISSN 0268-3768.
  15. Behera, Amar Kumar; Verbert, Johan; Lauwers, Bert; Duflou, Joost R. (2013-03-01). "Tool path compensation strategies for single point incremental sheet forming using multivariate adaptive regression splines". Computer-Aided Design. 45 (3): 575–590. doi:10.1016/j.cad.2012.10.045.
  16. Walczyk, Daniel F.; Hosford, Jean F.; Papazian, John M. (2003). "Using Reconfigurable Tooling and Surface Heating for Incremental Forming of Composite Aircraft Parts". Journal of Manufacturing Science and Engineering. 125 (2): 333. doi:10.1115/1.1561456.
  17. Behera, Amar Kumar; Ou, Hengan (2016-12-01). "Effect of stress relieving heat treatment on surface topography and dimensional accuracy of incrementally formed grade 1 titanium sheet parts" (PDF). The International Journal of Advanced Manufacturing Technology. 87 (9–12): 3233–3248. doi:10.1007/s00170-016-8610-8. ISSN 0268-3768.
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