The Science of Forming


 

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A Successful Stamped-Product Launch: Know What Goes Into Your Tooling

By: Daniel J. Schaeffler, Ph.D.

Danny Schaeffler, with 30 years of materials and applications experience, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solutions (EQS). EQS provides product-applications assistance to materials and manufacturing companies; 4M teaches fundamentals and practical details of material properties, forming technologies, processes and troubleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bachelor 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 Tel. 248/66-STEEL E-mail ds@eqsgroup.com: or Danny@learning4m.com

Thursday, July 25, 2019
 

Your part-development process results in quality stampings if your production process remains in control and the sheet metal resides within ordered tolerances. Metal stampers should know their processes well enough to control them. Accounting for the wide range in potential metal properties when using only one or two shipments during tryout, though challenging, must be addressed prior to tooling buyoff in order to produce high-quality, low-cost stamped parts in needed volumes. Strain analysis provides this guidance, but first, you must know what initial materials shipments contain.

Ideally, the material received for tryout will come from the same sheet metal production mill to be used when a part undergoes full production, thus minimizing a potential source of variation. This is not always possible, since either the purchasing department has not yet selected a production supplier, or the material is purchased from a service center that buys material on the open market. The risk of using different suppliers: While they all should meet the specified property limits, each supplier might take up a different portion of the allowable tolerance. Tuning your process to a limited range associated with one supplier makes it challenging to accommodate product from another supplier, even if that product also falls within allowed limits.


Gauge-thickness definition varies with sheet metal—as does tolerance.
A Certification of Material Properties document should come with each shipment received. These metal certs list at least the chemistry and perhaps also the tensile properties of the different coils that make up the shipment. Testing requirements for some orders at the sheet metal production mill may be such that only one tensile test is needed for a 300-ton heat, even though the coils produced from this melt are each processed differently and, therefore, will have different tensile properties. At best, a tensile test was performed on the coil applied to your order, but even here the information shown on the cert sheet comes from a sample taken from either the head or tail end of a mile-long coil. The development of a new product takes multiple years and hundreds of thousands of dollars. Minimize the risk of being misled by the certs and spend a few hundred dollars to confirm the properties of the metal fed into your simulation and placed into your dies.

Sending a sample to an accredited testing lab provides valuable information about the sheet metal, and also generates the stress-strain curves needed to accurately simulate metal flow during stamping. Metal properties change relative to the rolling or grain direction, so have tests performed in the longitudinal (0-deg.), transverse (90-deg.) and diagonal (45-deg.) orientations. Be sure to ask for more than the YTEs, or yield strength, tensile strength and total elongation. Uniform elongation, the plastic anisotropy ratio (r-value) and the strain hardening exponent (n-value) all help characterize different aspects of metal flow. The more details available, the more accurate the forming simulation.

Specify Important Properties When Ordering

Sheet metal orders include requirements such as thickness and grade. The properties to be satisfied are a function of the specification against which the order is placed. SAE/AISI 1008 steel is not the same as ordering steel to ASTM A1008. Either one will provide a steel with chemistry complying with defined limits for carbon, manganese and other elements. However, unless ordering a high-strength steel, there are no required tensile properties that must be satisfied prior to shipment from the steel mill.

If you think that it’s good enough to order based on chemistry only, think about two paperclips. Bend one of them a few times. Though the chemistry remains identical, one has more inherent formability remaining after this cold working operation. This formability loss is seen in coils unintentionally produced with surface-flatness issues such as waves or buckles. Strength increases and ductility decreases during reprocessing to eliminate these surface defects. The sheet mill can ship this coil to satisfy any order calling out only chemistry requirements. In the example of low-carbon steels, ordering to SAE Specification J2329 ensures a sheet product that falls into defined ranges of tensile properties as well as chemistry. Similar challenges are seen with other sheet metal types.

Thickness Tolerance and Cutting Clearance

Measure the sheet metal blank thickness before conducting the first tryout hits. Do not rely on the metal certs for thickness—the document shows only the minimum or nominal thickness, and does not represent the coil on your shop floor. If blanking, trimming or cutting clearance is determined as a percentage of thickness, realize that these percentages change depending on the choice of actual, minimum or nominal thickness as your reference.

The way you specify sheet thickness influences the product received. When ordering by gauge number, know that the numerical gauge designation denotes a different thickness depending on the metal described, as well as if the metal exists in sheet or wire form, or if it’s been hot or cold rolled (see Variation in Gauge Thickness table). Also changing: the allowable thickness tolerance, which could reach ±10 percent of the nominal value.

As an example, when ordering 16-gauge galvanized steel sheet, the sheet metal received may exhibit thickness anywhere from 0.0575 to 0.0695 in. If your goal is to set cutting clearance to 10 percent of metal thickness, cut-edge performance will change significantly depending on your use of nominal, minimum or maximum thickness as a reference, and on where any given shipment falls within the allowable spectrum of metal thickness.

If cut edges influence your ability to produce quality stampings, order sheet metal to an appropriate thickness tolerance and measure each shipment.

Are there topics you’d like to see covered in future Science of Forming columns? Let me know at ScienceOfForming@EQSgroup.com. MF

 

See also: 4M Partners, LLC, Engineering Quality Solutions, Inc.

Related Enterprise Zones: Materials/Coatings


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