Robustness

Robustness is an important topic in the sheet metal forming industry. Traditionally, companies have focused on stamping robustness during the manufacturing phase, through their production and quality departments. Today, modern simulation software allows companies to also address stamping robustness issues during the earlier phases of product and tooling design. In other words, companies can now develop the best product designs and the best tool designs for a robust stamping process.

In robust analysis, the stability of the deep drawing process is analyzed under predefined process conditions. In everyday production, it is possible for parts to be produced smoothly one day, and the next day problems arise even though production conditions have apparently not changed at all. This is due to noise and variations in the forming process.

Under real production conditions, there are important but unavoidable and uncontrollable variations of drawing parameters. These variations may be divided into two classes:

  • Noise in forming process parameters such as, for example, draw bead restraining force, fillet radii due to tool wear, variation of blank holder pressure due to press state, fluctuations in lubrication, etc.
  • Noise in material properties such as, as for example, yield strength, tensile strength and r-values, which vary from coil to coil and from supplier to supplier

Robust analysis is carried out to analyze the influence of noise variables on the forming process. The user defines a variation for every noise variable in the form of a mean value and the associated standard deviation. Based on this variation, multiple simulations are carried out. All simulations on hand are then analyzed, using an analysis identical to that carried out in the sensitivity analysis. However, the analysis is now based on the variation of the noise variables and not on the basis of the design parameters. In this way, a quality function, which is dependent on the noise variables, is calculated. In robust analysis, it can be verified whether a forming process provides stable results under the influence of common noise of various parameters.

Noise variables for a robust analysis
Input diagram of noise variables for a robust analysis
Robust process window in the robust analysis
Robust analysis allows for the definition of a stable and capable process

If the influence and sensitivity of noise variables are known, the forming process can be designed accordingly so that:

  • Noise does not affect the desired quality of the result.
  • Par rejects are minimized and at the same time efficiency of production is increased.
  • Tolerance limits for the quality control of materials can be defined.

The result is used to predict the stability and capability of forming processes in terms of the selected noise variables. Robust analysis allows for the user to determine the robust process window, in which the best forming conditions considering noise variables are taken into account.

Addressing stamping robustness is important as potential stamping problems can be solved earlier in the vehicle development cycle saving more time and resources. This means faster time-to-market for new car models, with obvious advantages.

Further information on robust analysis at AutoForm:

Robust Production at Ford

Designing for Robust Stamping – During Product and Tooling Design

Software for Sensitivity Analysis, Optimization and Robustness