Page 10 - Metallforming Magazine December 2020

Tooling by Design
By Peter Ulintz
Drawing on Experience
Deep drawing is considered the most complex of all sheet metal forming processes. It presents many process variables, some inter- acting synergistically, that impact a stamper’s ability to produce defect- free parts repeatedly.
The topic of deep drawing has been extensively researched and written about. Much effort also has gone into developing predictive software to assist with designing and troubleshooting draw dies. Still, I receive more questions by far on deep drawing than on any other topic. This is due in part to many small companies lacking access to forming-simulation software and, therefore, needing to rely on rules-of- thumb and past experiences.
This month I thought I would share a couple of questions that I recently received, along with my responses.
Calculating Tonnage
Question: We are trying to calculate the tonnage on a job that we are quoting. Our estimator has been quoting jobs for a great many years and has relied on cal- culations derived from the book “Con- densed Practical Aids for the Experienced Die Engineer, Die Designer and Die Maker.” The book offers this formula:
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
The 600-ton rating for this press occurs at approximately 1⁄2-in. above bottom dead cen- ter (BDC). Deep drawing requires working the material high in the press stroke where derated tonnage comes into play. In the example outlined under the Calculating Ton- nage subhead of this article, our 600-ton press has approximately 280 tons available at 2 in. above BDC.
Tonnage Capacity of a Mechanical Press
700 600 500 400 300 200 100
0
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00
Inches Up From Bottom
Tonnage Zone
π*d*t*s
Where
π = 3.14
d = Shell diameter (in.) at the center
of the stock)
t = Stock thickness (in.)
s = Ultimate tensile strength (psi) This is the same formula referenced
in your MetalForming December 2009 Tooling by Design column.
Recently, I also found an online cal- culator for estimating drawing tonnage. This calculator also considers the draw- reduction ratio multiplied by a coeffi- cient of some type.
Our problem is that the two methods produce very different results: 847 tons using the book formula and 376 tons using the online formula. The book for- mula puts this job beyond the capacity of our 450-ton press, but the online for- mula says that the press will suffice. Is the book formula providing peak draw- ing force, and the online formula cal- culating an “average” draw force?
Response: The estimated-force cal-
culation in the book assumes a worst- case scenario because it relies on ulti- mate tensile strength (UTS) in the for- mula. The deformation process (drawing) begins when the material reaches its yield stress, which can be significantly less than the UTS, and as the material work-hardens it increases in strength. Theoretically, the material could be worked up to its UTS, which is why it is used in the formula. But not all deep-draw parts reach this level of work hardening, so the actual draw- ing force could be considerably less.
The book formula also assumes that the maximum draw reduction is being taken. When the draw reduction ratio approaches 1.8:1, we have the near- maximum blank area in contact with the blankholder, which means that more friction must be overcome and a greater area of material must be deformed (the flange of the blank deforms by thickening). Smaller draw- reduction ratios certainly would require less force to deform the blank, but I
8 MetalForming/December 2020
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Available Tonnage