Die Science: Keys for drawing rectangular or square shells

To design and build a drawing die to form a box shape, you need to understand metal flow and stretching principles. Square and rectangular shapes have metal flow patterns that are difficult from simple axial symmetrical cups or cylinders.

First Things First

Before any processing takes place, you have to determine what the material is. Different materials have distinctly different behaviors that affect formability:

• Thickness—thicker metals stretch farther than thin metals of the same type.

• Tensile, yield, and elongation—These are the basic forming, rupture, and stretching abilities.

• Hardness— Many steel and nonferrous metals come in different hardness levels, such as quarter-hard, half-hard, three-quarter hard, and full-hard.

• Grade— Advanced high-strength steels (AHSS), such as dual-phrase, have unique behaviors that you must compensate for.

Metal Flow Basics

To understand the metal flow patterns in a square drawn shell, consider how a cylinder cup is drawn.

A cylinder starts out as a simple round blank. For the larger-diameter round blank to transform into the smaller cylinder shape, it must compress radially. In other words, the metal must flow inward toward the centerline of the cup simultaneously as it compresses together (see Figure 1 ).

Controlled compression results in a part with a flat flange. Uncontrolled compression causes a severely wrinkled flange (see Figure 2 ). The key point to remember is that metal in compression has a great resistance to flow.

Radial Compression in Square Draws

The corners of the square draw are essentially one-quarter of a round draw (see Figure 3 ). The radial compression that produces a round draw causes a resistance to flow in the corners of a square draw. This type of deformation is known as draw deformation, or compressing the metal as it flows inward intension.

If too much metal surface area is forced into radial compression, it will cause a great resistance to flow, resulting in excessive metal stretching and possible splitting. However, if the blank is close enough to the radial profile of the punch, the metal will compress and flow inward, resulting in less metal thinning. This basic relationship between the blank edge and the draw punch is called the limiting draw ratio (LDR).

Shell Side Walls

The side walls of a drawn box are in bend-and-straighten deformation. With little or no compression during this deformation mode, the metal flows inward with very little resistance. You can increase resistance by applying more blank holder force in this area or adding draw beads.

This type of deformation is known as plain strain, or simply stretching the metal in one direction. If too much metal flow in this area, it may cause the side walls of your part to be bowed outward.

Surface Area Displacement for Compressive Deformation

During compressive deformation, the flat sheet metal’s surface area is displaced into a radial shape that must consume nearly equal amounts of surface area. In other words, if the metal flows inward in the corners of the draw 1 linear inch, the shell will get taller than 1 inch.

Consider that when producing a round drawn cup, the outer blank edge is a much larger diameter than the drawn cup. For the blank edge to become the edge of the finished cup, the surface area contained in the flat blank must be displaced into a smaller profile corner. The only way the smaller diameter can consume the surface area is to get taller (see Figure 4 ).

Blank Size and Shape

Whenever feasible, use the smallest blank possible to make the part.

The closer the blank is to the draw punch, the more the metal will flow. When using a square blank to make a square shell, consider nesting it 45 degrees in reference to blank. This puts greater resistance to flow in the side of the shell, where more tension is desired, and less material in the corners to help maximize flow in the radial profile.

Square Draw, Round Blank?

Yes.

In the side walls, there is very little resistance to flow; in the corners, resistance to flow is greater. Combining this factor with the draw-in-to-height ratio differences from the side wall to the corners results in a “roundish” blank (see Figure 5 ).

Draw Spotting

Often referred to as a running spot, the draw holder or die face with respect to the compressive thickening and tensile thinning that occurs during the drawing process.

In the corners of a draw, the metal will be in compression. Compressed metal trapped between the die face and a blank holder will thicken during the drawing process. Grinding the die face or blank holder with respect to compressive thickening and tensile thinning that occurs during the drawing process.

In the corners of a draw, the metal will be in compression. Compressed metal trapped between the die face and a blank holder will thicken during the drawing process. Grinding the die face or blank holder with respect to compressive thickening will help reproduce the resistance in the corners of the drawn shell.

The typical thought is that if a part is flat, a perfectly flat blank holder can be used. Flat blank holders are fine as long as they are draw spotted to metal flow conditions in a process known as tuning the blank holder.

Figure 6 shows a hard mark, resulting from compressive thickening in the corners, that must be ground away by draw spotting.

These are just a few concepts and ideas that will help you to successfully draw a shell. Until next time… Best of luck!