Page 30 - MetalForming May 2010

Tooling Technology
Peter Ulintz has worked in the sheetmetal-forming industry since 1978. His background includes tool and die making, tool and process engineering, engineering management and product devel- opment. Peter also operates the website ToolingbyDesign.com, a source for the transfer of modern metalforming and tool-and-die technology, and which promotes the use of “Performance-Based Die Engineering Strategies.”
Peter speaks at PMA seminars and roundtables focusing on tool and die design, die maintenance, deep drawing, stamping simula- tion, tooling for stamping high- strength steels and problem solv- ing in the press shop.
Peter Ulintz pete.ulintz@toolingbydesign.com www.toolingbydesign.com
Last month a reader submitted a question regarding press tonnage calculations. The reader had com- mented, “On several occasions we had to move our dies to a larger press because the press we selected did not have enough power. In fact, the press slowed down noticeably during production. When we moved the die to a larger ton- nage press the problem went away.” His concern was that his engineers were using incorrect formulas when calcu- lating the required press forces.
As it turned out, the force calcula- tions that were used had some flaws; primarily, not including the force required to counteract the draw pad forces. Nor was the reduction in avail- able working force (de-rated tonnage) considered as the distance above bottom of the press stroke increased. Still, the inaccurate calculations were close enough to get the job assigned to a press with adequate tonnage capacity. Why, then, does the company continue to have problems (i.e., slowing down of the press) running this particular job? And, why does the problem go away when the die is moved to a press with greater tonnage capacity?
Simply stated, this company has an energy shortage. If they plan to deep draw similar parts from higher strength materials or some of the new advanced high-strength steels (AHSS), they will find themselves in the midst of a full-
PETER ULINTZ
blown energy crisis.
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. The energy rating of a press deals with the product of the applied press load and the distance through which the load must be applied. Since energy is expended with each stroke of the press—and this ener- gy must come from somewhere—atten- tion should be focused on the main drive motor and the flywheel.
The main motor is the only source of energy for the stamping press. The motor must have sufficient horsepow- er to supply the demands of the entire stamping operation. The press flywheel, by virtue of its mass and rotational speed, serves as the energy storage device. The energy in the flywheel is expressed as in.-tons of torque. In com- bination, the flywheel stores and deliv- ers the required work energy while the electrical motor restores depleted ener- gy by maintaining flywheel speed and avoiding excessive slowdown.
Deep-drawing operations consume large amounts of press energy due to their long working distances, some- times beginning several inches above the bottom of the press stroke. For example, pushing 50 tons through 1 in. of deep drawing would require 50 in.-tons of energy while a 3-in. deep draw requires
TOOLING BY DESIGN High-Strength Energy Crisis
Tonnage
Drive System
Strokes/min.
In.-tons*
110-ton
Non-Geared (Flywheel)
80
50
110-ton
Single-Geared
40
130
150-ton
Non-Geared (Flywheel)
80
70
300-ton
Non-Geared (Flywheel)
60
300
28 METALFORMING / MAY 2010
www.metalformingmagazine.com
*Check with your press manufacturer for specific values