Two More Tools for the Troubleshooting ToolboxOctober 1, 2011
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 leading 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 establishing control charts and monitoring 40-plus inputs can prove daunting, as most difficult problems arise due to the interaction of three or more variables.
For example, consider the change in material movement from the binder area over the die radius, caused by the interaction of die wear, sheetmetal and lubricant thickness and temperature in the deformation zone. While the change in each individual input might be within normal specifications, it is the synergistic interaction that can cause sheetmetal flow to range from free flow to lockup.
A useful tool for troubleshooting problems with dimensional variation is the portable (now available as battery-operated 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 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 finished 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 display deformation patterns throughout the stamping. The constancy-of-volume rule says that a given combination of surface 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 consistent 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.