Stampers often overlook the influence of the sheet metal surface on material flow. When viewed with the naked eye, the surface often looks flat and smooth. Higher magnification tells a different story, revealing peaks and valleys (Fig. 1).
We can characterize sheet metal surfaces with several parameters, with average surface roughness (Ra) and peak count (PC) as the most common. To simplify, think of Ra as the height of the trees in a forest and PC as the number of trees. Changes in Ra and PC lead to changes in the interaction between the workpiece, lubricant and tool surface.
As one of the last steps in the rolling mill, coils of sheet metal pass through large work rolls having engineered surface profiles. In addition to incrementally reducing the sheet thickness, the work-roll texture presses into the sheet surface.
|Fig. 3—Aluminum-alloy surfaces
Roll surfaces continually wear. Rolling mills sequentially process orders requiring a restricted surface profile, in order to achieve the proper surface. They process other orders when they can schedule appropriate matches for grade, thickness and width. The rolling campaign continues until reaching the mill control limit, typically 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 campaigns between roll changes lead to coils of varying profiles depending on when they were rolled. Note that, unless stated in the sheet metal specification, 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 interlock unless the stamper applies a barrier lubricant. Conversely, with smooth surfaces (B) lubricants will squeeze out, leading to galling and scoring. To minimize 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-discharge texture (Fig. 3). Carbon-steel surfaces are slightly rougher and usually nondirectional.
While a smooth tool surface may be appropriate for forming carbon steel or aluminum sheets, it may prove beneficial to use a rougher tool surface when forming smoother stainless steel. In addition, when encountering wrinkles 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
Hear Danny Schaeffler speak about Specifications, Properties and Applications in Metalforming at the April 2020 Sheetmetal Forming and Joining Technology Series, sponsored by PMA. Visit pma.org/calendar/event.asp?productID=83275694 for more information.
See also: Engineering Quality Solutions, Inc., 4M Partners, LLC
Technologies: Lubrication, Materials, Tooling