Page 107 - MetalForming October 2015

Fig. 3—This graph can be calibrated from the data in Fig. 2. Note the constant FLC for negative minor strains.
Note: Thickness strain readings can rapidly be obtained with an ultrasonic-thickness (UT) gauge, and then plotted onto graphs (such as Fig. 3) for immediate severity deter- mination.
Many sheetmetal-thinning problems result from highly localized deformation called strain gradients (Fig. 4). Local- ized stress causes deformation to concentrate in one area that usually has a large major surface strain and a long perpen- dicular axis with zero minor strain. For example, consider an embossed character line. As the surface gradient grows with increasing major strain, the thickness of the gradient must decrease to comply with the constancy-of-volume rule. Often, gradients are ignored until a fracture occurs. Howev- er, the gradients easily are found in a stamping by using the UT gauge to scan potential areas. A high magnitude of thin- ning should alert one to a potential failure zone requiring fur- ther analysis. Why wait until failures start during a produc- tion run?
Sheetmetal bending is a little more complex. A pure bend operation will not cause any change in workpiece-material thickness. The tensile thinning of the outer surface is com- pensated by the compressive thickening of the inner surface, again due to the constancy-of-volume rule. Metalformers can
refer to charts providing outer-fiber major strain based on the ratio of punch radius (R) to material thickness (t). Compare this value to the total elongation of the material to determine relative severity. However, many times a tensile force is applied to the ends of the bend during forming. These forces cause stretch through the full thickness of the sheet and add deformation to the outer strain, causing premature fail- ure. The UT gauge can measure such extra thinning and add that component to the R/t stretch ratio obtained from the charts.
UT gauges in metalforming applications can:
1) Deliver very short measurement times, which allow for repeat measurements for improved accuracy;
2) Read thickness to 0.0001 in.;
3) Be 10X more accurate than circle-grid strain for 0.030- in. thick material;
4) Verify die setup conditions after die transition;
5) Find any deformation changes at the end of a run to mark the die for downtime correction;
6) Monitor critical nonstable deformation areas in a stamping; and
7) Determine edge-center-edge thickness of a coil without removing the banding MF Stuart Keeler and Peter Ulintz will speak at PMA’s Higher
Strength Steel seminar on October 27-28 in Cleveland, OH.
Planning. Precision. Possibilities.
DESIGNERS OF PROGRESSIVE, LINE AND TRANSFER DIES.
3D Solid Die Designs
Concept Drawings FEA/Formability Studies
Surfacing
Machine Design
Processing & Product Manufacturability Analysis
Transfer System Coordination Program Management
3D Designs using Catia and Unigraphics Simulations using PamStamp 2G and AutoForm
Ph: 616-365-2454
Fx: 616-365-1135
3258 Clear Vista Court NE Grand Rapids, Michigan 49525
www.diecadgroup.com
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Fig. 4—As stretch increases, the gradient becomes more local- ized in width.
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