The Phoenix Group Articles

How Does a Bead Create Blank Restraining Force? Friction, Bending, and Work Hardening

The stamping process allows us to consistently produce a permanent, new shape thru stretching and work hardening a sheet metal blank over a component’s die geometry. The Bead geometry plays a very important part in maintaining process stability. Beads produce restraining forces that control material flow from the binder to be drawn into the die work envelope; this control, helps prevent metal fractures while ensuring sufficient draw in to successfully achieve the part geometry. A combination of work hardening (from the sheet metal bending around the bead radii) and sliding friction generate the “restraining force” to control metal flow from the blank edge.

In the sketch above we understand that R1 and R2 radii influence bead setting forces as the bead is set to depth during binder closure. The smaller these radii, the greater the setting force. Increased bending forces will also produce a substantial increase in the friction generated by material sliding into the die to form the part shape. 

 

As the blank is drawn through the bead into the die addendum, the bending and unbending action alters (increases) the yield strength of the displaced sheet metal. The original Y.S. of the blank is lower than the Y.S. of the material as it bends around the Male Bead radius. The second bend and unbend action further elevates the Y.S. of the material and additional bending force, generating higher frictional forces, will be produced in holding the material down and sliding over R2. If R1 and R2 are the same size radii with identical surface finish, R2 will be responsible for a larger portion of the restraining force as the blank is being drawn into the part. (WHY?)

 

The amount of Increase in Yield Strength each radius will induce, is a function of not only the bead radius, but also the sheet metals N-value, or work hardening exponent. A higher “N-value will require less severe bead radii, and lower binder closure tonnage requirements. Larger radii and lower binder closure forces will translate into lower friction, and less maintenance from the associated reduction in bead wear.

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