Page 21 - MetalForming Magazine June/July 2022 80th Anniversary Issue

Tooling by Design
By Peter Ulintz
Progressive-Die Timing
Progressive dies feature many moving components, including the stock strip, stripper plates, pressure pads for forming, stock guide- rail lifters, cams, and punch-gagging and part-ejection devices. The timing of events must be correct for the specific stamping and its material thickness in order for the tool to work properly.
Die Kinematics
As the die begins to close, the upper stripper plate contacts the stock strip and any guide-rail lifters used to raise the stock strip above die level—usually to facilitate feeding over form stations. The stripper face pushes the stock strip down until it is held
toward bottom dead center (BDC) of its stroke. At BDC, the forming pads have bottomed out (but not the stripper), and any stencils in the die have penetrated the strip surface.
The moving components in this simple example:
• Stripper plate and its pressure system
• Forming pad and its pressure system
• Cutting and forming punches
• Stencil.
All four components must work in
a predetermined, timed manner to ensure that the stripper holds the mate- rial with adequate pressure before any
due to the vertical heights of the cams.
Understanding Travel Distances
Work travel describes the distance that the punch works on the part mate- rial. For drawing operations, work travel equals the distance from punch contact with the blank to the bottom of the punch stroke. For cutting operations, work travel is the distance that the punch travels past the part material to the edge of the die steel.
With the die in the open position, the pressure pad and strippers must travel beyond the punch steel. Other- wise, the working components of the die will contact the part material on
against the lower-die steel. The guide-rail lifter has over-trav- eled slightly to ensure the presence of a gap between the top and bottom surfaces of the lifter (Fig. 1).
As the press ram
continues downward,
the stripper plate and
pressure pads for
forming (if any) start
to compress and
build pressure. This is
followed by cutting
and forming as the ram continues
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His back- ground includes tool and die making, tool engi- neering, process design, engineering manage- ment and advanced product development. As an educator and technical
presenter, Peter speaks at PMA national seminars, regional roundtables, international conferences, and college and university programs. He also pro- vides onsite training and consultations to the met- alforming industry.
Peter Ulintz, Technical Director, PMA, pulintz@pma.org
Springs must have enough preload to
compress lower springs
Strip Floats
18 MetalForming/June/July 2022
www.metalformingmagazine.com
Fig. 1—Ensure that the strip floats between the top and bottom lifter-rail surfaces.
cutting or forming of the strip. The cut- ting punches enter the die matrix to the proper distance—usually 0.030 to 0.040 in. below the die surface—and the stencil marks the part appropriately when the die reaches its home position. In addition, the set block has been set properly to avoid over-hitting of the die, which could result in excessive tonnage and possible die damage.
The use of cams, especially aerial cams, further complicates the timing sequence due to their travel distance, stripper-contact timing and increased travel distances of other components
Push down an extra 1/16 in. to simplify timing
Stop Block
the downstroke, before the pressure pad and strippers securely hold the strip with the correct amount of pres- sure. On the upstroke, the pressure pad and strippers must remain in contact with the part material until the punches fully retract. This means that the pres- sure-system travel must always exceed work-travel distance.
High-Speed Applications
Small parts—for the electronics industry for example—almost always feature complex forms, and those forms generally have tight tolerance