Daniel Schaeffler Daniel Schaeffler
President

Don't Forget About the Surface

February 27, 2020
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Fig. 1—Peaks and valleys on sheet metal surfaces.
Picture this scenario: A metal stamper goes months without any issues, but then experiences a 20-percent scrap rate on a handful of shipments. Stamping conditions have not changed, as the shop has continued to use the same recipe for die setting, gauging and tonnage. The YTE properties (yield and tensile strength, and elongation) on the metal certifications remain in compliance with the ordered specification, leaving the stamping company no closer to resolving the issue.

Often, a metal stamper will rely only yon the yield strength, tensile strength and elongation listed on the metal certs—a common mistake since the sample used to generate the certified properties was obtained from the end of a mile-long, 20-ton coil. Depending on the ordered requirements, the steel mill may pull a tensile test only once for a 300-ton heat, from which 15 coils were produced, all of which processed at separate times under different conditions.

I recommend that stampers send out a sample coupon, taken from the problem area, for testing at an accredited lab experienced in testing sheet metal. When experiencing splits, test for more than YTEs—also look at uniform elongation and n-value, obtained during a standard test and determined from the stress-strain curve. R-value testing, which may incur an additional cost, can prove worthwhile if the part looks like a deep-drawn cup and you have a reference for what you typically receive.

Tensile properties represent only one component of the forming system that affects stamping success. Friction characterizes the ease of material flow and quantifies the local interaction between the sheet metal, tool surface and lubricant. A different lubricant type, quantity or distribution will change material flow. When quality issues arise, and assuming that the stamper has not changed the lubricant, dilution or dispersion method, check for plugged or misdirected nozzles. In most cases, sloshing on more lube only creates a mess and wastes time and money.
Fig. 2—Sheet metal and tool surface roughness and peak count should be different to minimize friction.
Tool surfaces wear over time. Working with higher-strength sheet metal increases the risk of altering the die-surface topography. Geometrical changes of the tooling can occur if the tool hardness is not optimized for the sheet metal characteristics. Bring the local radii and tool surface finish back to nominal conditions when stamping problems occur.

Sheet Surface

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.

Industry-Related Terms: Scrap, Stainless Steel, Surface, Tensile Strength, Texture, Thickness, Nominal, Die, Forming, Carbon Steel
View Glossary of Metalforming Terms

 

See also: Engineering Quality Solutions, Inc., 4M Partners, LLC

Technologies: Lubrication, Materials, Tooling

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