Page 44 - MetalForming May 2011

Tooling by Design By Peter Ulintz
AHSS Press Considerations
Tooling Technology
The application of advanced high-strength steel (AHSS) with ultimate tensile strengths of approximately 600 MPa (85 ksi) are well established within the automotive industry. Current trends indicate growth in applications requiring even higher strength grades up to 1000 MPa (140 ksi) and beyond. Stamping AHSS materials at these strength lev- els can affect the size, strength, power and overall configu- ration of every major piece of the press line, including coil- handling equipment, coil-feed
systems, straighteners and stamp- ing presses.
It is fairly well known that the ton- nage rating of a press represents the maximum load that can be exerted, in continuous operation, without causing damage to its structure or drive system. Tonnage rating typi- cally is determined at a slide position between 1⁄32 and 1⁄4 in. above bottom- dead center (BDC), depending on press design and the manufacturer.
Die-process engineers understand
that the available working force
(available tonnage) of a mechanical
press decreases as working distance
above BDC increases. This occurs
because the lever-arm angle—the
relationship between crankshaft
angle and the pitman—has reduced
higher in the stroke. Engineers must keep this relationship in mind when developing deep-drawing processes where draw- ing begins several inches above BDC.
Higher pressing forces (tonnage) obviously are required when stamping AHSS materials, but press energy is of prime concern. 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. Unfortunately, many die engineers do not fully comprehend and apply press-energy data.
Press energy—often expressed as in.-tons of torque,
Peter Ulintz has worked in the sheetmetal-forming industry since 1978. His background includes tool and die making, tool and process engineering, engi- neering management and product development. Peter also operates the website www.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 Ulintz
pete.ulintz@toolingbydesign.com www.toolingbydesign.com
expends with each press stroke. In combination, the fly- wheel stores and delivers the required work energy while the electrical motor restores depleted energy by maintaining flywheel speed and avoiding excessive slowdown. Forming and drawing processes can consume large amounts of press energy, due to their long working distances. For example, pushing 50 tons through 1 in. of forming (on mild steel) would require 50 in.-tons of energy; a 3-in. deep draw
requires 150 in.-tons of energy. Changing the material to an AHSS grade might require 100 tons of force working through the same 3-in. dis- tance, requiring a machine rated for at least 300 in.-tons of energy.
Simply matching the energy requirements of a stamping process to the rated energy capacity of a press may not be sufficient, partic- ularly with AHSS materials. Stamp- ing higher strength steels requires higher blankholding and punch forces, resulting in increased fric- tional forces and interface temper- atures. When interface temperatures reach a critical point, lubrication may break down and other forming and die-related problems, such as galling, can occur. During a pro-
duction run, it would not be uncommon to reduce the stroke rate of the press to reduce the process operating temperature and eliminate these thermal-related forming problems.
Consider, for example, the relationship of press speed and press energy for a 600-ton press (see table). This is summa- ry data only; the press manufacturer’s table provides complete data from 0 to 60 strokes/min., in 1-SPM increments. Full energy for this particular machine is rated at 35 strokes/min. When press speeds are reduced below this point, the avail- able energy diminishes rapidly. Slowing the press below this critical speed may eliminate thermal problems, but now there is not enough press energy to carry out the continuous stamping process.
Servo-Driven Presses Help Here
One way to address this “energy crisis” is to consider the benefits of a servo-driven mechanical press. In a servo-driv- en press, high-capacity servomotors replace the flywheel, motor and clutch/brake. A servo motor can provide energy and torque that remain constant even at a press speed of 1 stroke/min.
PRESS ENERGY CHART
Press Type: Flywheel (direct drive)
SPM
In.-Tons of Energy
5
5
10
19
15
43
20
76
25
119
30
171
35
285
40
285
50
285
50
285
mechanical advantage
42
MetalForming/May 2011
www.metalformingmagazine.com