TechArchive

Original article date: September 1999

Even if you’ve devoted your career to studying sheet steel, you’ll find a new handbook from Swedish Steel an invaluable reference. We certainly loved it here in the office.

The extraordinary Sheet Steel Handbook produced by Swedish Steel has become one of the ‘must have’ references for designers over the last few years. The massive, detailed technical guide to designing with sheet steel has recently been re-issued as ‘Edition II’, but it’s a spin-off publication which we think is as likely to generate demand.

The new Sheet Steel Forming Handbook is based on the ‘Fabrication’ chapter from its larger parent. This was easily the most interesting section to designers, and Swedish Steel took the decision to reprint that section on its own some time ago – many of you may already own a copy. The new Handbook goes much further however: not only does it seem to cover every angle, but it tackles a lot of theory which seems way beyond the call of duty. That said, it’s so well presented that no subject seems the slightest bit esoteric.

Chapters cover material properties (including plastic forming, effect of strain rate and temperature on yield stress), size shearing (angles, clearances, deviations) and plastic forming (bending, spinning, flexforming). Other properties covered are formability and material behaviour, tooling materials and surface treatments. These are followed by an extensive ‘Materials Data’ section, designed to be updated as materials change – this gives all the data so crucial for setting up finite element (FE ) analyses.

Many phenomena which occur in metal forming operations are covered in depth in the handbook, though mainly from the point of view of how they may be tackled on the press, rather than at the design stage. Nonetheless, all the advice remains extremely valuable to designers, in that it may instill greater understanding of and, therefore, confidence in the metal forming process itself.

For example, springback, which is caused by the stresses arising during forming and their release after the load is removed, is more complicated in press operations than in bending, because stretching, drawing and bending are often involved. Anti-springback measures possible in drawing are to reduce punch edge radii to ensure coining and to optimise blankholder forces.

Compressive stresses arising in deep drawing can cause wrinkling, especially if the workpiece is not properly restrained relative to the tooling. Wrinkling and buckling can occur in two different positions, sometimes on flat sections and sometimes in areas not in contact with the tool. Reduced sheet thickness and increased strength usually mean a greater risk of creasing. There are several ways in which the risk of wrinkling can be reduced. One is to reduce blank holder force. However, the increase must not be too great, otherwise there is a risk of fracture. Another way to minimise the risk of creasing in large, flat parts is to use larger edge radii, which makes it easier to stretch the material over the edge onto the punchface area. A third method is to provide drawing beads to retain and control material flow in sensitive areas.

• Swedish Steel

September 1999