Page 26 - MetalForming August 2010

Tooling Technology
Stuart Keeler (Keeler Technologies LLC) is best known worldwide for his discovery of forming limit diagrams, development of circle grid analysis and implementation of other press shop analysis tools. Stuart’s sheetmetal forming experience includes 24 years at National Steel Corporation and
12 years at The Budd Company Technical Center, enabling him to bring a very diverse background to this column and the many seminars he teaches for PMA. His most recent project is technical editor of the AHSS Application Guidelines—Version 4.1, which now is available for downloading free from www.worldautosteel.org. Keeler Technologies LLC
P.O. Box 283
Grosse Ile, MI 48138
Fax: 734/671-2271
E-mail: keeltech@comcast.net
Imagine a road with zero friction. A car could not use its tires to initiate motion and, once moving, the car would require wind resistance or some other external force to slow down and stop. Driving up or down slight inclines would be impossible.
In the metalforming world, many consider friction a nasty word. It caus- es so many problems that press shops would love to eliminate friction. Yet, the world of zero friction would make it very difficult to form most stampings due to lack of control.
Fortunately, press shops (both phys- ical and virtual) can incorporate friction as one of their tooling design options. Friction would be great if the coefficient of friction (COF) was constant. Howev- er, the variability of COF caused by dif- ferences in lubricant thickness, tooling temperature, water/solids ratio, material parameters and other forming inputs is the culprit. Several decades ago, the ability to hold water (no tears) was the main goal of press shops. Compensation for variations in friction was possible by keeping the amount of sheetmetal stretching below the failure level (edge of the deformation cliff).
Today, the demand for product dimen- sional consistency means even the stamp- ings that hold water and are safely removed from the edge of the cliff can be severely affected by changes in the COF. Control of process variables now must be improved by one or two orders of magnitude.
A major contribution to the COF is the surface topography (morphology) of the sheetmetal surface. For this reason, the surface peak height and peak count, measured by a profilometer, often is specified for painted surface appear- ance and COF. Certain topographies have a low COF, while others have a
STUART KEELER
high COF. A graphic description of a low COF is a forest from a high flying airplane. Complete surface coverage of clumps of equal-height trees (rounded hemispheres of metal) creates a lower load per unit area. The valleys between the trees (groves) contain the lubricant.
In contrast, a high COF resembles the Garden of the Gods in Colorado. Tall, sharp pointed spears create high local- ly concentrated loads on the die. Flat mesas also rise above the lubricant level to present a dry, flat surface to the die.
Cold-rolled steel coils are given a temper pass as the last stage of pro- cessing. The temper pass eliminates yield-point elongation and imparts the specified standard surface topography necessary for identical painted appear- ance of different groups of stampings.
How does the COF change when the cold-rolled coils are galvanized? Gal- vanized steels have a thin layer of zinc on their surface. The prime purpose of the zinc, for many industries, is corro- sion protection. Types of galvanized products include hot-dipped galvanized steel and electro-galvanized (electro- zinc) steel. A coil of hot-dipped galva- nized steel is heated, run through a pot of molten zinc and then pulled up toward a high ceiling to allow solidifi- cation of the molten zinc. The final processing step is a temper pass similar to that given bare steel. This temper pass imparts a similar topography to both the bare and the hot-dipped galvanized steels, resulting in approximately iden- tical COF with a given lubricant.
In stark contrast, studies have shown very large variations in COF for electro- galvanized steels. For example, lubricant X might result in a COF range of 0.10 to 0.14 for hot-dipped galvanized steel, but the range for electro-galvanized
THE SCIENCE OF FORMING Friction Effects of Galvanized Steels
Stuart Keeler’s next seminar is “Sheet Metal 101—Under- standing the Metal” scheduled for September 16 in Cleveland, OH. Check www.pma.org for this and other seminars.
24 METALFORMING / AUGUST 2010
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