Page 80 - MetalForming October 2011

The Science of Forming By Stuart Keeler
Tooling Technology
Two More Tools for the Troubleshooting Toolbox
Demand for dimensionally consistent stampings is increasing. Drivers include more accurate alignment for laser welding, stacking variation of multiple stampings, minimizing visual gaps and robotic processing. Stampings of thinner-gauge materials with increased strength for weight reduction predictably have greater springback and reduced stretchability. These higher-strength materials also generate localized gradients (narrow peaks) of deformation, accompanied by localized reductions in thickness. Springback and deformation gradients are lead- ing causes of dimensional variations that must be minimized and controlled.
An obvious first approach is to tighten controls on the material properties and the other 40 or so inputs to the forming process. Next, stampers should implement tighter process-control procedures, although the job of establish- ing control charts and monitoring 40-plus inputs can prove daunting, as most difficult problems arise due to the inter- action of three or more variables.
For example, consider the change in material movement from the binder area over the die radius, caused by the inter- action of die wear, sheetmetal and lubricant thickness and temperature in the deformation zone. While the change in each individual input might be within normal specifica- tions, it is the synergistic interaction that can cause sheet- metal flow to range from free flow to lockup.
A useful tool for troubleshooting problems with dimen- sional variation is the portable (now available as battery-oper- ated handheld units) ultrasonic thickness gauge (UTG). With the correct UTG frequency, probe size and delay line, sheet thickness can be measured to within ±0.0001 in. Examples of how stampers can use a UTG:
1) Checking the thickness variation of incoming coils or blanks. A small length of steel at the head and tail of a coil may have a different thickness than the remainder of the coil, depending on the specific processing at the producing mill or service center. Some press shops remove these coil ends
Stuart Keeler (Keeler Technologies LLC) is known worldwide for his discovery of forming limit diagrams, development of circle-grid analysis and implementa- tion of other press-shop analysis tools. Keeler’s metal- forming experience includes 24 years at National Steel Corporation and 12 years at The Budd Company Technical Center, enabling him to bring a very diverse background to this column and to the seminars he provides for PMA.
Keeler Technologies LLC
P.O. Box 283 | Grosse Ile, MI 48138 Fax: 734/671-2271 keeltech@comcast.net
before the material reaches the press line. The UTG provides a quick read of thickness anywhere in the coil, particularly where micrometers can’t reach. At the other end of a press line, a UTG can measure critical thickness locations on fin- ished stampings and internally record several thousand readings for further data analysis. Sometimes the UTG will detect a wedge-shape thickness profile from one blank edge to the other.
2) Circle grids can be etched in the blanks and used to dis- play deformation patterns throughout the stamping. The con- stancy-of-volume rule says that a given combination of sur- face deformation must result in a specific amount of change in material thickness. A UTG can track variations in thickness deformation to flag changes in surface deformation. Circle grids are not needed until more detailed problem solving is required.
3) Draw beads are very effective in controlling material flow through the binder and over the die radius. Constant draw-bead restraining force is required to maintain a con- sistent flow. Research by the Auto Steel Partnership shows that draw-bead restraining force is proportional to the percent reduction in sheetmetal thickness measured between the material entering and exiting the bead. While a UTG cannot provide the absolute magnitude of the restraining force, it can detect changes in the force. As a real-world example, consider a right/left symmetrical die set with identical draw beads on both sides of the die. Solder strip tests show that the beads in both sides of the die had identical clearances and depths in the unloaded state. However, under production loading the bead on one side of the die caused the sheetmetal to tear, while the bead on the opposite side created loose material and buckling. The UTG measured 35-percent reduction for the torn side and only 5 percent for the loose side. Further investigation uncovered that the die was bending (breathing) under load. A cushion pin was located under the draw bead at the torn side, and no cushion pins were located near the loose side. Using data collected from the UTG, the stamper relocated the die to a zone with balanced bead-restraining forces.
4) High peak-deformation gradients often are overlooked, due to the need to apply circle grids. Instead, the stamping can quickly be scanned with a UTG until detecting a narrow band of decreased thickness. Then, a circle grid can be applied to that location to obtain surface-deformation data for corrective action.
5) When not solving forming problems, a UTG can be used to measure and track internal corrosion or abrasive wear inside piping in the press shop.
78 MetalForming/October 2011
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