Page 41 - MetalForming February 2014

tested each day for five days. Changes in room temperature, humidity, oper- ator, die wear, location of material sam- ple in the coil and other variables can bias the results.
A lot of steel is temper-passed to eliminate yield-point elongation and apply a consistent shot-blast pattern on the steel surface. Bare steel, hot- dipped coated and galvanealled steels usually receive the same degree of tem- per passing. In one statistical set of draw-bead COF tests, these surfaces had values between 0.10 and 0.15. Elec- trogalvanized coatings do not receive a temper pass after coating and have COF values between 0.08 and 0.35 depending on the coating line. These COF differences can affect strain dis- tribution and forming severity.
Some press shops have personnel that like to replace or modify lubricants at every opportunity. Consider, for example, a metalformer experiencing constantly changing strain patterns and final stamping dimensions from one press/die throughout the day. It applies lubricant through four sprayers aimed at the die cavity before placing the blank, and then applies lubricant to the blank before the die closed. The sprayers receive lubricant from a reservoir along- side the press. A troubleshooting team notes large changes in the spray density during the shift. Secretly, the team takes hourly samples from the reservoir and measures the lubricant’s solid density. During the shift, density varies between 25- and 70-percent solids. Some of the employees believe that the lubricant is too thick, so they’ve been adding water, while others believe the lubricant is too thin and have been adding random amounts of solids.
To avoid such elusive problems, the team should install a huge lubricant reservoir inside at the back end of the plant, with piping running from the reservoir to all areas of the press shop and then to each press. It should use the same lubricant for every stamping, and not trust anyone—locate the fill pipe outside in back of the plant and
lock the cap. The lubricant suppler can then fill its tanker truck with the proper ratio of solid and water—only the deliv- ery driver has the key to the filler cap.
In a 1988 SAE paper titled, Forma- bility Criteria for Selecting Sheetmetal Forming Lubricants, the authors state:
“A single lubricant should be select- ed for use in a press shop. The control
of metal flow should be dependent on the design of the tooling, the blank size and contour, the binder force, and the draw beads. A change in the lubricant, much less a dab of lubricant on the blank, should not be the method to modify material flow within the tooling.”
Think about your friction—not only
The Science of Forming
about your lubrication.
M F
Make Smarter use of Your Energy... Through Storing
VISIT & Applying Pressure!
VISIT BOOTH 1455
BOOTH 80816
Our desig of more th comprehe and relate
• Custom engineered Piston & Bladder Accumulators
• Optimize system performance
• Reduce energy cost
• Increase efficiency
• Provide smooth,
more reliable operation
• Special material, porting,
and options available
...with Tobul, the pressure is always on!
EconoLator ® tobul.com
61 Innovation Drive l Bamberg SC 29003 USA l 803.245.2400
www.metalformingmagazine.com
MetalForming/February 2014
39