A Numerical Study of Stretch Forming over a Sparsely Populated Reconfigurable Die

  • John Kelly

Student thesis: Doctoral Thesis


In this thesis a newly proposed design of pin tips for the stretch forming of sheet metal over a reconfigurable die is examined. The new pin tips are mounted on ball studs at the tops of the reconfigurable pins and allowed to pivot freely. Since the tops surfaces of the pin tips can slide against the bottom surface of the interpolator, during the stretching process, their final position is dependent on the overall curvature of the die. In phase one, of a two-phase numerical study (designed to identify a suitable pin arrangement and pin tip geometry for the employment of the new pin tips in a coarser bed of pins), where semi-hemispherical pin tips are used, several features are identified that are detrimental to the production of smooth parts. Hexagonal close packed matrices aligned in either of two directions, and a rectangular close packed matrix aligned diagonally to the stretching direction are shown to offer superior performance. In phase two of this study the geometry of the new pivoted pin tips is examined. Although it is possible to produce parts with superior smoothness using part-specific pin tip geometry, it was found here that using circular pivoted pin tips with flat mating surfaces (in a HCP pin matrix) offers a better global solution, with a clear advantage over their commonly used semi-hemispherical counterparts. A second numerical study is presented here to identify a suitable interpolator for use with this new system, where smoothness targets of 0.1 mm, for stoning, and ± 0.2 mm, for overall dimensional accuracy, are used. It was found that, with the new pivoted pin tips, the required interpolator thickness is no longer equivalent to the diameter of the pins, as it is with semi-hemispherical pin tips. An interpolator thickness of 30 mm is shown here to produce parts with acceptable smoothness, with 70 mm diameter pins. It was also found that using a three layer of interpolation system, where a softer polyurethane material is sandwiched between two layers comprised of a harder polyurethane material, further reduces dimpling, and hence produces parts with superior smoothness.
Date of AwardOct 2017
Original languageEnglish
SupervisorDesmond Brown (Supervisor), Alan Leacock (Supervisor) & D Mc Cracken (Supervisor)


  • Stretch Forming
  • Pam Stamp
  • Reconfigurable
  • Interpolator Pin Tips
  • Pin Geometry

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