Metal sheet drawing - Importance of deformation modes

Sheet metal stamping is an operation that produces complex, non-developable parts, unlike the simpler operations of bending, rolling or cold forming. This widely-used process is used to manufacture car body parts, household appliances, kitchen utensils, metal packaging, mechanical parts, etc.

In addition to the shape of the tool, which depends on the complexity of the part to be obtained, the success of the operation depends on a number of other parameters: on the one hand, those linked to the process, such as press settings, stamping speed and lubrication, and, on the other, those linked to the qualities of the sheet itself and its capacity for forming, also known as formability, which is the subject of this article. Great progress has been made in measuring the mechanical characteristics of sheet metal, and in interpreting their relationship to formability. The same applies to the understanding of the stamping operation, not least through numerical simulation, which now enables us to virtually visualize the behavior of the metal in the tool. Deformation measurement systems, particularly optical systems, are also tools that enable quantitative analysis of real parts.

As we suggested in the previous edition of this article (in 2002), numerical methods for stamping calculations and tool design have made such progress that they are now used almost systematically. It's fair to say that there are very few stamping specialists – and even fewer tool designers – who don't possess at least one calculation code. This has completely changed the profile of the profession, where specialists with extensive experience have been replaced by operators more familiar with the use of computers than with the behavior of metal.

This change is undoubtedly highly beneficial in economic terms: parts and tools are designed more quickly and with far fewer failures than previously. Modifications that used to be carried out as the actual tool was developed, by removing material or adding it (reloading deposits), are now tested on the computer, much more easily.

There's only one regret about this development: technicians are losing the habit and the ability to think seriously about the problems they encounter (it's easier to just run a calculation), and we sometimes find ourselves at a loss when a problem arises on the shop floor. The expert capable of analyzing the cause is often no longer around.

What follows is mainly focused on the material. We will try to show which characteristics of metal sheets are influential with regard to shaping, particularly in relation to local modes of deformation, which play a major role here.

This text deals mainly with thin sheet metal, i.e. in practice sheet...