Page 44 - MetalForming January/February 2022

Metal Matters
mizes 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—asso- ciated 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 prob- lems. 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 pro- duction 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 con- dition 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 ana- lyzing evidence about the metal and the part helps to define the root cause of the problem. MF
FOLLOW US
@MetalForming
42 MetalForming/January-February 2022
www.metalformingmagazine.com