Metal Stamping & Steel Stamping Company

Metal Stamping Design Guidelines

Material Selection | Tolerancing | Chemistry | Blanking, Trimming and Perforating
Deep Draw | Cosmetics

The Process

Deep draw refers to the process of pulling a flat “blank” of material over a radiused die edge and into a cavity, producing a closed bottom, round or irregularly shaped cup or cylinder. It should not be confused with stretch-forming. The blank is actually forced into a plastic state as it is dragged over the die radius and down into the die. This process is done under calculated and very controlled conditions involving blank-holding pressures, punch and die radii, punch speed and lubrication.

Anatomy of a Deep Draw

The 2 stages of a draw are cupping and drawing. When the punch first contacts the blank, the nose of the punch initially embosses the material into the die. Some stretching occurs at this point and produces what is known as a “shock line”. This is a pronounced area of thinning around the radius at the bottom and just up into the straight wall of the shell. Depending on the shape of the bottom, the material may still be near original thickness across the bottom face (flat bottom) or thinned out by a stretching action (spherical bottom). As the blank is pulled into the die, the material at the circumference gathers and the wall progressively thickens. As the blank is pulled in to near shell diameter, the material thickens to as much as 10% over the original thickness. Clearance must be provided for this thickening to occur so that the material will not get bound up between punch and die. In addition, the punch must be tapered so that the finished shell can be stripped off. Therefore, a drawn shell will taper from bottom to top. It is possible to minimize this through subsequent sizing operations, but not eliminate it entirely.

The blank used to produce a shell is cut from rolled strip material with a grain structure elongated across the blank in the direction of rolling. Since this cross-grain does not pull into a drawn shape evenly from all directions, great stresses are induced in the shell wall. Due to these uneven stresses, a drawn shell will not be perfectly round. A flange added to the top of the shell will minimize this, but the smaller the flange, the less strength it has to keep the shell round.

Specifying a Drawn Shell

Since the original blank is so altered by the deep draw process, the wall thickness cannot be specified in terms of mill tolerances. Depending on application, the three ways of specifying the thickness of material in a shell would be to call out the thickness of material to be used, the minimum wall thickness or the maximum wall thickness. Wall thickness can be specified in more detail, but only after development work has been done with the draw process. Since the material is formed around the punch, shells are typically dimensioned to the inside diameter, with taper allowed from bottom to top. Alternatively, the shell can be dimensioned to the outside diameter with the maximum size found at the top, and tapering down to the bottom.

If a straight shell with no flange is required, the shell will be “pinch-trimmed” – that is trimmed flush with the outside diameter. Since the shell has a radius at the top, the remaining trimmed edge will have a partial radius from the inside, abruptly ending in a somewhat sharp outer edge. Also, since the die must have enough clearance to accept the shell, there will be a slight flare at the top of the shell. The bottom of a shell can be pierced out in a similar manner to produce a tubular part, but the same pinch-trim principles apply to the inside diameter. If a straight, cut-off edge is desired, it would require a secondary machining or cut-off operation and should be specified on the part drawing.

Flatness

Raw Material: Coiled strip material by nature is not flat. As material is unwound off the coil it retains some of that curve shape along its length, called coil set. In addition, the width of the strip usually has a slight arc to it. This is called crossbow. Coil set can be minimized or removed by material handling equipment at the beginning of the metal stamping process. But crossbow is much tougher to remove and generally survives to affect the flatness of the finished stamping.

Stamping Process

As described earlier, the metal stamping process places compressive forces on the raw material. As the top edge is rolled into the cut, the bottom edge tends to turn slightly also. This distortion at the edges affects the flatness of the finished part, being minimal in thinner or milder materials, but becoming severe in heavier stock or tougher materials such as stainless steels and high-strength alloys. When flatness is critical, tooling can be designed to minimize distortion but may require extra stations or secondary operations.

For the same reason, perforated or trimmed features that are placed too close to each other or the material edge tend to roll the material between, producing a distorted or thinned edge. The rule of thumb in stamping design is to leave a minimum of 1-1/2 times material thickness between trimmed or perforated features. Also, the stretching and compression of forming can distort holes adjacent to a form or bend. Holes are best kept at least 2 times material thickness beyond the radius of a formed feature. If this is not possible, the hole should be designed with sufficient clearance to allow for distortion.

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