Page 92 - MetalForming October 2019

The Science of Forming By Daniel J. Schaeffler, Ph.D.
Differences Between Tryout and Production Conditions
Die-construction standards spec- ifying important characteristics such as tool-component mate- rials, hardness and minimum wall thickness must be communicated to a metal stamper’s tool build source. Along with many other parameters, a stamper specifies maximum die weight, shut height and specific quick-die-change provisions to ensure that the tool fits within the press-line infrastructure. But even this communication may not result in quality parts as the product moves from build to production.
A good practice is to conduct a dimensional-layout study during pre- production runs prior to tool buyoff at the construction site, and use a statis- tically significant number of randomly selected sample parts to validate that the product meets print specifications and tolerances. Ensure that the tool- construction source produces these parts while the process runs in a con- tinuous mode for a minimum specified time or rate, perhaps 30 strokes/min. for at least 10 min. Do not accept data on a part produced from a single stroke.
Danny Schaeffler, with 30 years of materials and applications experi- ence, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solu- tions (EQS). EQS provides product-applications assistance to materials and manufacturing com-
panies; 4M teaches fundamentals and practical details of material properties, forming technolo- gies, processes and troubleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bach- elor of Science degree in Materials Science and Engineering from the Johns Hopkins University in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
Despite diligence on the part of a stamper during tool development and buyoff, numerous factors as described below can influence success when pro- ducing parts at production rates.
Material: Temperature Effects
Depending on the sheet metal and forming system, the 10-min. run-at rate performed during buyoff may not be long enough to display the effects of heat buildup. Higher-strength sheet metal grades tend to become notice- ably hotter with increasing contact over
“Despite diligence on the part of a stamper during tool development and buyoff, numerous factors as described below can influence success when producing parts at production rates.”
critical radii. Lubricant additives designed to improve metal flow may not be active at these elevated tem- peratures, thus worsening the problem. Differences in local metal flow change the strain distribution, leading to increased springback. Direct contact with the die surface may result in scor- ing as well as permanent tool defor- mation, as the hardness of some advanced high-strength steels approaches that of tooling.
Material: Simulation
Perform forming simulations using realistic inputs covering the full range of properties that you might receive based on the specific sheet metal grade ordered. Ensure appropriate distribu- tion of binder and ram tonnage at the
four press corners during simulation, tryout and production, and avoid over- loading at any one corner of the press. Evaluate the impact of allowable thick- ness tolerance on clearances and form- ing. Software allows for simulation of the manufacturing process as well, pro- viding the ability to check for interfer- ence at production speeds.
Process: Blank Edge
If a developed blank is needed for a part running with line dies, the blank die may be the last to be completed. Laser cut blanks, often used when blanking-die construction is not com- plete, allows for adjusting the blank contour between runs to accommodate lessons learned in tooling tryout. But blank-edge quality differs between sheared and laser-cut edges. Cut con- sistency also differs, with sheared edges affected by knife clearance, alignment and sharpness. Be aware of methods used to create the blanks in tryout and production, and the associated char- acteristics of the blank edges.
Process: Lubricant
Lubricant type, application method, amount and distribution all influence metal flow. Differences can be expected when comparing hand-applied shop lubricant during tryout with applica- tion methods used in production.
Ensure that the die-tryout shop uses production-intent lubricant, even if they cannot use a production-intent application method. Guide the tryout shop on acceptable and non-accept- able methods. For example, when com- paring use of a rust-preventive oil in a spray bottle against wiping a vanishing compound on each blank, one approach may better represent pro- duction plans. Note that neither approach lends itself to generating run- at-rate panels.
90 MetalForming/October 2019
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