A method for designing lightweight and flexible creep-age forming tools using mechanical splines and sparse controlling points

Abstract

Creep-age forming (CAF) is a proven forming technique in the aerospace industry for the production of large integrally stiffened panels. One of the most urgent issues to be addressed in CAF is the development of flexible tooling. Flexible tools already have a long-standing reputation for the economic impact they have brought to the aircraft industry. However, with the rising need to establish comprehensive springback prediction models for CAF, the need for flexible CAF tools is now stronger than ever. In this article, an existing state-of-the-art CAF tool is described followed by the introduction of a novel design concept for flexible tooling. Based on the proposed design method, which utilises mechanical splines and sparsely spaced controlling points, a proof-of-concept prototype is built and characterised using corresponding analytical and finite element models that have been developed. Three parameters that can influence forming surface error: (i) the number of control points, (ii) spaces between control points and (iii) spline thickness are identified and optimised. Finally, an integrated optimisation process for tool offsetting is introduced, and its use is demonstrated. It is confirmed that this design method can be used to make flexible CAF tools with less than ± 1 mm error (defined as vertical difference from prediction) in the forming surface. In addition, this error can eventually be compensated and thus eliminated from CA-formed parts by using the developed optimisation technique. This article provides CAF tool designers confirmed advices for making new flexible CAF tools. Lightweight and flexible CAF tools can now be constructed through the use of mechanical splines and sparse controlling points.

Effect of pin arrangement on formed shape with sparse multi-point flexible tool for creep age forming

“…The effect of forming pin arrangement on formed shape was not mentioned. Lam et al [10] developed a theoretical method to design forming pin arrangement according to the shape accuracy after loading by SMPF tool. However, no corresponding experiments was presented for verification.…”

“…The test blank has the same shape as the spline during CAF. The shape accuracy (or deviation from the target shape) of loaded blanks by SMPF tool has been used as a criterion for designing pin arrangement by Lam et al [10], however, the stress and strain distributions in the loaded blank will directly influence the CAFed shape and should be considered as well.…”

“…SMPF tool was then proposed to reduce the weight and complexity of conventional MPF tool, in which the continuously distributed forming pins are replaced by sparse ones and a die sheet is added between the blank and sparse forming pins to maintain desired tool shapes, such as SMPF tool 1 in Table 1 [9]. Another type of SMPF tool was developed for large panels in CAF process, which replaces the die sheet by narrow splines for each array of pins [10]. Among these flexible tools, SMPF tool combines advantages of low weight and complexity from rib board tool and high flexibility from conventional MPF tool, making it a preferable choice for forming processes of large panels, such as CAF process.…”

An accelerated springback compensation method for creep age forming

“…After optimised tool shapes for both cases were determined, the depth of each forming pin was adjusted to generate corresponding tool surfaces [32]. The splines, rubber sheet and test plate were then assembled into the prepared flexible tool for subsequent CAF test, as shown in Fig.…”

Material modelling and its application to creep-age forming of aluminium alloy 7B04

“…Flexible forming surfaces were on both sides of the plate with an additional elastomeric sheet layer between the plate and lower flexible surface. Further details about the method of CAF tool design and test setup can be found in [5] and [6] respectively. The main steps of the forming test include:…”