Stretching sheet metal

Modern Machine Shop, August, 1993 by R.J. Rizzo

A wavy piece of sheet metal can be straightened by gripping it at two opposite sides and stretching it a bit beyond its elastic limit. It will come out straight, flat and undistorted. Sheet metal so treated by a mill is termed stretcher leveled, and is supplied to meet flatness requirements. It can come with a smooth satin finish, but because of the stretching, its temper will be slightly raised. It can be furnished in sheets of desired size, interleaved with heavy paper, if desired, in stacks of a specified quantity, and sandwiched between protective plywood panels for shipping, if so ordered. Such material is used for making precision stampings, particularly blanks for instrument bases, plates for electronic assemblies, as well as clock components, gears, cams and similar applications.

Parts blanked from aluminum and other soft metals can also be stretched if product designers and tooling designers agree that it is needed. In the operation sequence, one or more stations before the blanking station, a station is provided for impressing a grid of closely-spaced pinpoint indentations on both sides of the material. These indentations, although tiny, shallow and barely visible, have the same effect as stretching. They flatten and stiffen the metal in the area where the blanking will occur, so that the finished part will have the required flatness, yet with the indentations hardly noticeable.

Figure 1 depicts a shallow rectangular tray that, according to specs, must be stable and free from any "snap" or "cricket" action--a defect giving the tray a twisted appearance or causing it to rock annoyingly when resting on a flat surface.

The periphery of the tray has a short vertical rim and a narrow horizontal flange. When this part is draw-formed, an unequal compressive stress builds up along the rim and flange, and continues down into the bottom panel. This stress sometimes warps a part like this and introduces the objectionable "snap" action. To avoid this defect, the bottom of the tray is stretched a little in a way that cancels the compression stress there. This gives the metal a new granular orientation, eliminates the objectionable twist and allows the tray to lie steady on the table.

The stretching that is introduced could be designed into the drawing die, or it could be done as a second operation after the part is completed. In the first method, the blank is drawn and the bottom stretched in one operation. In the second, the drawing only is performed in the first of two dies. The stretching is done later in another die. Both methods produce the same results. Depending upon the ingenuity and discretion of the die designer, the stretching could comprise a pattern of shallow rectangular depressions pressed into the bottom panel to stretch the excess metal, remove stress, and prevent it from twisting.

Unfortunately, it is the nature of these procedures to render unpredictable results, so they must be worked out experimentally in each case.