Page 47 - MetalForming March 2020

While a smooth tool surface may be appropriate for forming carbon steel or aluminum sheets, it may prove ben- eficial to use a rougher tool surface when forming smoother stainless steel. In addition, when encountering wrin- kles on steel or aluminum stampings, a rougher tool surface (in the right area) may restrict sheet metal flow enough to keep the stamping wrinkle-free.
Adapt your tool-polish strategy to
the characteristics of each type of sheet metal formed. Using the same tool roughness for all parts may limit sheet metal flow. MF
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Fig. 3—Aluminum-alloy surfaces.
either processing a certain number of coils or rolling for a defined number of hours. At that time, the mill swaps out the work rolls and sends the used set out for refinishing. Depending on the volume and grades rolled, this swap may occur every few hours or days.
Just as stamping plants can improve uptime by minimizing die-change time, steel-producing mills improve uptime by extending work-roll life. Long cam- paigns between roll changes lead to coils of varying profiles depending on when they were rolled. Note that, unless stated in the sheet metal speci- fication, no restrictions exist on the surface condition of supplied coils.
Surfaces and the Influence of Different Metal Alloys
Fig. 2 illustrates the interaction between the sheet metal surface and the tool. If the two surfaces have similar high roughness (A), their peaks will inter- lock unless the stamper applies a barrier lubricant. Conversely, with smooth sur- faces (B) lubricants will squeeze out, leading to galling and scoring. To mini- mize friction and help ensure good sheet metal flow, strive to have one surface smoother than the other (C).
Typically, stainless steels exhibit a relatively smooth surface with a low Ra. Aluminum-alloy surfaces typically exhibit either a directional mill-finish or have a non-directional electro-dis- charge texture (Fig. 3). Carbon-steel surfaces are slightly rougher and usu- ally nondirectional.
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