Daniel Schaeffler Daniel Schaeffler
President

New Column Tackles Pressroom Challenges

January 11, 2022


Four years ago, I had the privilege of taking over MetalForming magazine’s The Science of Forming column, crafted for decades by Dr. Stuart Keeler, the father of the forming-limit diagram (FLD). His were big shoes to fill, and I have tried to do justice to the memory of Dr. Keeler and the great knowledge he shared over his long and storied career.  

Dr. Keeler passionately brought science to the pressroom floor, and his articles reflected just that. Nearly every topic he discussed had its origin in a classic textbook or something observed in a laboratory. He had a knack for stating theoretical concepts in a manner understood by experienced practitioners.

Some of my writings attempt to cover similar topics. I also have incorporated concepts related to common issues observed while working with global metal producers and manufacturing companies, small and large. With the evolution of content, the MetalForming magazine editorial staff and I decided we should retire the Science of Forming moniker and create a new column—Metal Matters—which more accurately reflects the wider spectrum of topics now covered.  

Several of my recent articles have spurred remarkable conversation among the readers. Among them: An article from 2018 titled “Golf Shots and Manufacturing Variability,” which notes that while every company has certain strengths which they play to their advantage, many do not fully grasp the nuances of the production and supply of different metal grades and how these affect stamping-process optimization. The article discussed how process-control differences at three metal-production mills leads to each supplying a product in full compliance with the ordered specification, however, the different property ranges impact stamping performance.

Also from 2018, an article titled “Sims Aren’t Everything” discussed how metal forming simulation has advanced tremendously during the last 20 years and saves days of hands-on trial-and-error work, helping stampers evaluate what-if scenarios associated with variables such as sheet metal grade and thickness, binder and addendum designs and blank shape. However, simulation results only are as good as the simplified estimates of reality used as inputs.

For example, imported CAD files are accurate prior to tool construction, but sheet metal flow deviates from simulation once a grinder changes the tool surface, especially at a draw bead. Simulations may assume a rigid tool, ram, bolster and press, but production stampings reflect a different reality. Tool recuts to address forming or dimensional issues remove mass from the tool, and therefore contribute to a loss of rigidity not considered in the simulation. These are just a few scenarios that require the need for additional hands-on analysis of physical parts.

We’ve also discussed the FLD, noting in a five-part series in 2019 that many stamping companies require a passing FLD as a go/no-go gate to proceed to the next stage of tooling buyoff. That series also covered strain analysis, the forming-limit curve (FLC), and the associated thinning-limit curve, where we highlighted the differences between the FLC and FLD and how the FLC provided by the sheet metal supplier can help stampers assess the robustness of a stamped part with an FLD. Importantly, these articles guide the practitioner on techniques to promote measurement accuracy, critical since incorrect measurements or their interpretation may lead to acceptance of a substandard tool and process, or result in unnecessary, costly and time-consuming tool work. Also included are the procedures for using an ultrasonic thickness gauge to obtain nearly all of the information provided by a full circle-grid strain analysis.

Material-Selection Science

Selecting the right sheet metal alloy for a particular application has implications far beyond the stamping press. In my January 2020 column, “Material Selection: The Rest of the Story,” I describe how the development of higher-strength steels has led to equipment upgrades at steel mills and at metal forming and processing companies. In fact, even first responders need new equipment and modified extrication techniques to address the properties and characteristics of new alloys. Automotive-accident repair has even become more challenging since welding and flame straightening adds unwanted heat, resulting in degraded local properties in parts made from advanced steels. And, increased use of aluminum requires greater consideration of the higher scrap value, in terms of its minimization as well as its capture.

I followed up in February 2020 with an article titled “Getting More Than You Ordered,” discussing how sheet metal suppliers may choose to upgrade an ordered material to help improve their own production-scheduling efficiency.  Steel mills cast and roll grades of similar thickness and width together to achieve a steady-state production environment, which maximizes their processing efficiency. For example, the commercial steel grade CS-Type A allows the carbon level of the supplied steel to range from the ultra-low carbon levels typical of the most formable grades, to as high as 0.10-percent carbon—associated with some high-strength steels. Allowing such a wide berth likely means that this is one of the least expensive grades. If the width and thickness of an order fit best with a mill campaign for a ULC order, the steel mill may choose to give a no-cost upgrade and supply this value-added version. However, tuning the dies to accommodate the more formable side of the potential range of CS-Type A properties may lead to problems when receiving steel at the other end of the spectrum, which might occur if there is a supplier change. Steel shipped from either side of the range complies with the order, but the steel received may not be sufficient for the existing tool and process.

Troubleshooting

That brings us to 2021, during which several articles in this space focused on troubleshooting sheet metal forming problems. Fault may lie, we’ve noted, with the sheet metal itself or with the stamping process, or more likely with a combination of the two. Check the mill certs for yield strength, tensile strength, total elongation and n-value, I’ve preached, and look for any trends between these properties and stamping success.  Mill certs come from samples taken at the end of what might be a mile-long coil, or even once per 300-ton heat.

To best characterize the problematic sheet metal, test a coupon from the troublesome lift. Ensure that the stamping recipe hasn’t changed from when production launched. Binder and ram pressures at the four corners and the stroke rate in the press are among the variables that should be the same as those used during the start of production. Do not assume that the settings are the same; check the data and records.  Confirm the proper orientation and flow characteristics of the lubricant nozzles. Compare the draw panel saved at the start of production with the current panels.  Differences in scoring and sheet metal pull-in around the periphery and can identify locations where plant personnel can focus corrective actions.

Lastly, in my May 2021 column, “Approach Stamping Problems Like a Crime Scene Investigator,” I urged metal formers diagnosing quality issues to take the time to compare tensile properties of incoming sheet metal with that of prior shipments. Check the underside of parts for scoring, which occurs as the sheet metal flows over a tool radius. A scoring pattern looking rough and abraded may indicate restricted sheet metal flow. In the case of splits initiating at a flanged edge, compare the cut-edge condition against the edge condition from early buyoff panels. It may indicate improper clearance for the material grade and thickness, or possibly wear or chipping of the cutting knives. Gathering and analyzing evidence about the metal and the part helps to define the root cause of the problem. MF

Industry-Related Terms: Alloys, Blank, CAD, Case, Draw, Edge, Forming, Gauge, Periphery, Ram, Scrap, Spectrum, Stroke, Surface, Tensile Strength, Thickness, Ultrasonic
View Glossary of Metalforming Terms

 

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

Technologies: Materials

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