Page 48 - MetalForming January 2014

Tooling by Design
By Peter Ulintz
Solving Stretching and Drawing Problems
material in contact with the punch.
Drawing requires the opposite parame- ters, to prohibit mate- rial from stretching across the punch face. Keep the punch radius relatively small and leave the punch face rough to create
Peter Ulintz has worked in the metal stamping and tool and die indus- tries since 1978. He has been employed with the Anchor Manufacturing Group in Cleveland, OH, since 1989. His back- ground includes tool and die making, tool engi- neering, process engi-
neering, engineering management and product development. Peter speaks regularly at PMA semi- nars and conferences. He is also vice president of the North American Deep Drawing Research Group. Peter Ulintz
pete.ulintz@toolingbydesign.com www.toolingbydesign.com
Stretching and drawing operations require proper location of processing lubricant on the sheetmetal blank. Stretching oper- ations require lubrica- tion between the sheet- metal and the punch face to provide a low friction site for the material to slide and stretch. The material on the die surface should not be lubricat- ed, creating sufficient friction to inhib- it material from drawing-in from the
flange.
In deep-drawing operations, do not
lubricate the sheetmetal surface under the punch, to promote friction between the punch face and workpiece. The die surface and die entry radius must be lubricated to ease material flow.
Too often, press shops overlook these important lubrication conditions and apply lubricant to the entire sheet before it enters the die, with no con- sideration for the forming mode. Changes in lubricant quantity in criti- cal zones on a stamping can alternate
Tooling Technology
Perhaps the most challenging forming modes to isolate are stretching and drawing. Because the process variables that influence each mode can differ significantly, stampers must make accurate assess- ments before changing their tooling or stamping process.
Changing a tooling parameter that promotes material stretching will not solve a drawing-related problem; good examples include punch and die radii. In stretching operations, punch radius is critical. It must be very smooth and large enough to promote material stretching across the face. Then, opt for a relatively small die radius to restrict material flow over the die sur- face. In addition, establish a rough pressure-pad surface to inhibit material flow and force the
frictionbetween the punch and the sheetmet-
al. This
inhibits
material
stretching,
which forces the
material to draw in
from the flange
toward the die cavity. To
ease material flow into the die cavity, maintain a highly polished die face and radii, and make the die entry radius as large as possible without causing wrinkles.
Lubrication Concerns
the forming mode between draw- ing and stretching, which can complicate problem solving and contribute to inconsistent part quality.
Pesky Wrinkles and Fractures
Pressure pads serve different pur- poses for stretching and drawing oper- ations. During stretching, lock beads prevent material movement across the pad. Because such movement from the flange is restricted, material contacting the punch stretches.
During deep drawing, restraining forces are significantly lower to per- mit some material flow across the blankholder (a type of pressure pad), yet high enough to prevent wrinkle for- mation. The blankholder surface con- tacting the sheetmetal is usually smooth, and sometimes polished, to ease material flow. When deep drawing irregularly shaped parts, certain areas of the binder may be roughened to help balance material flow.
When wrinkles do occur, their loca- tions on the part can help differentiate between stretching and drawing modes. Wrinkles on or around the punch nose—sometimes called puckers —could indicate insufficient restrain- ing forces. Pressure may need to increase to as much as 10 times the punch force, depending on material type and thickness. To promote stretch- ing, stampers can increase the blank size, reduce lubrication between the sheet and the pressure pad, roughen the pad or add lock beads.
“When wrinkles do occur, their locations on the part can help differentiate between stretching and drawing modes.”
46 MetalForming/January 2014
www.metalformingmagazine.com