Page 52 - MetalForming May 2017

Tooling by Design
By Peter Ulintz
Blank-and-Draw-Process Problems
Irecently received this question from
a reader: We’re in the process of
designing a combination blank and
draw die. This is not the first time we
have designed and built a die like this,
but I don’t think we have been using
the right formulas in the past to cal-
culate press tonnage. On several occa-
sions we had to move our dies to a larg-
er press because the press we selected
did not have enough power. In some
cases the press slowed down noticeably
during the production. When we moved
the die to a larger-tonnage press the
problem went away. All of our stamping
machines are flywheel-drive mechanical presses. I don’t want to repeat the same mistake so I was hoping you could recommend some formula(s) to use for our tonnage cal- culations.
My answer: When a mechanical stamping press slows down during continuous operation, the problem generally lies with press energy, not tonnage. However, deep drawing in mechanical presses presents some unique problems relat- ed to press tonnage; thus, both topics must be addressed.
The tonnage rating of a press is the maximum load that can be exerted in continuous operation without causing damage to the machine structure or its drive system. But it is important to realize that the force available to carry out work (blanking, drawing, forming, etc.) decreases in mechanical presses as the working distance above bottom- dead-center increases. This occurs because the lever arm angle—the relationship between the crank/eccentric angle and the pitman—has reduced mechanical advantage higher up in the stroke. This commonly is referred to as de-rated tonnage.
Before de-rated tonnage issues can be addressed, make sure that the blanking and drawing tonnage-calculation meth-
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His background includes tool and die making, tool engineering, process design, engineering management and advanced product development. As an educator and technical presenter, Peter speaks at PMA national seminars, regional roundtables, international confer- ences, and college and university programs. He also provides onsite training and consultations to the met- alforming industry.
Peter Ulintz
Technical Director, PMA pulintz@pma.org
ods are valid. The two formulas that the reader supplied were:
Blanking = blank thickness x tensile strength x blank circumference / 2000
Drawing = blank thickness x yield strength x drawing circumference /2000 Let’s look at the blanking forces first. Although not technically correct, the formula would probably be okay to use as is. Substituting shear stress for ten- sile strength into the formula would provide more accurate results. But because shear-stress data are hard to acquire, the use of tensile strength will provide a margin of safety by over-pre- dicting the required tonnage. The safety margin would also compensate for the stripping forces (about 5 percent of the punching force) that were not included in the reader’s
calculations.
For illustrative purposes, let’s assume that a 4-in.-dia.
cylindrical cup is drawn to a depth of 2 in. in a single-station blank-and-draw die (Fig. 1). The blank material is 0.080- in.-thick (t) mild-steel with 35,000-psi shear strength (Ss) and 56,000-psi tensile strength (Ts).
First, calculate the tonnage required to produce the 8- in.-dia. blank using the following formula:
(t x Ss x blank circumference) / 2000 lb. + stripper force
[0.080 in. x 35,000 psi x (3.14 x 8.00 in.)] / 2000 lb. + stripper force
Blanking force = 35 tons + 1.75 tons stripper force (5 per- cent of punch force) = 36.75 tons
Next, calculate the tonnage required to draw the cup.
For deep drawing, it is important that tensile strength be used in the calculations rather than yield stress because work-hardening continuously increases the force required to deform the blank material as it is worked. Since tensile strength can be double the yield strength—or more for some materials—the results derived using yield strength may underestimate the require drawing tonnage by a significant amount. Tensile strength will over-estimate the required force but it is a much safer number to use as a press-selection criteria.
Another component missing in the reader’s formula is the addition of a blankholder force (BHF). The assumption for BHF is generally 30 percent of the calculated drawing force. This force is significant and it must not be overlooked.
Find the drawing forces as follows:
(t x Ts x drawing circumference) / 2000 lb. + BHF
8.00-in. dia.
4.00-in. dia.
2.00 in.
Fig. 1—Dimensions for a cylindrical cup drawn in a single-station blank-and-draw die.
50 MetalForming/May 2017
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