Page 14 - MetalForming Magazine March 2022

Tooling by Design
By Peter Ulintz
Pressure-Plate Strippers—Part 1
Die Shoe
Keeper Block
Stripper Plate
The term “stripper” applies to the stripping plate, keepers or retain- ers, and pressure system (springs, gas, rubber, etc.), which all aid in the stripping of stock material from around the punch steel.
Most die design rules of thumb rec- ommend that stripper pressure con- stitute approximately 10 to 30 percent of the cutting force. In general, softer materials and tighter die clearances require higher stripping force. But, every rule of thumb involves some degree of tradeoff for the convenience of applying the rule. In the case of strip- ping forces, the tradeoff is die wear.
Frictional forces inside of the die influence the amount of pressure required for stripping. In high school physics I learned that “Friction is FμN,” a convenient way to remember that friction (F) is the product of the coef- ficient of friction (μ) times the normal force (N).
The frictional forces associated with stripping force directly relate to the rate at which the cutting edges of the punch and die components wear. As a result, a primary design objective should be to reduce the value of the μ and N factors to produce more stamped parts between sharpening.
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
Factors that influence μ include: a) lubrication, which breaks down with process heat; b) punch surface finish, which contributes to adhesive wear; c) punch hard- ness, which often degrades over time due to poor sharpening practices; and d) the type of material being punched.
Factors influencing N
include: a) punch-to-die
cutting clearance, which increases the normal force as the clearance is reduced; b) the ratio of hole size to stock thickness; c) the spacing between adjacent holes; and d) the cutting-edge conditions on the punch.
Stripper-Design Guidelines
Stripper construction plays a sig- nificant role in die reliability and dura- bility. Consider these important design guidelines:
1. Avoid stripper bolts, which may become loose and tend to break at the undercut of the threads.
2. Use keeper blocks or spools in
Fig. 1—Keeper-block system.
place of stripper bolts.
Advantages of using keepers: a)
increased rigidity and reduced tipping due to more surface area, which pro- vides improved guiding; b) a correctly designed keeper will not bend or deflect and cause jamming, whereas stripper bolts bend easily; c) keepers provide more strength under high strip- ping pressure, whereas large strippers would require far too many stripper bolts; and d) keepers are the best choice for long stripper-plate travel due to better guiding (Fig. 1).
Advantages of using spools: a) reduced space required on the die shoe
Spring can
Pad spools (different size)
Nylon screw
Pad
Pad
12 MetalForming/March 2022
www.metalformingmagazine.com
Fig. 2—If the die requires spools of more than one length, use different spool diameters.