Draw forming, Part V
The remoteness of the energy-affecting elements contributes to the differences between the net shape and non-net shape processes. This additional difference helps drive the nature of the net-shape processing theory.
Editor's Note: This is the fifth in a series of seven articles that identify and define the need for a new processing theory for the net shape processes (of which draw forming is one) and that explain the general content and configuration that new theory must have.
The first four articles in this series discussed how draw forming (a net shape process) differs from the non-net shape processes (such as machining, assembly, and welding) in the areas of the level II processing functions, the disconnect between the metrics of the product requirements and the metrics of the processing input variables, how those disconnects must be connected, and how the part shape (geometry) must be dealt with.
This article continues the series with a look at how remoteness of the energy-affecting elements contributes to the differences between the net shape and the non-net shape processes, and how this additional difference helps drive the nature of the net shape processing theory.
The term remoteness refers to the physical positioning of the energy-affecting elements in respect to the workpiece material being transformed from one shape into another through the application of that energy. In all shaping processes, some geometric feature of the forming tool is contrived, or designed, to apply energy to (or extract energy from) the workpiece to change it into the desired shape.
In general, the actual amount of energy exchanged is the direct result of the workpiece material's resistance to being formed. The tool merely focuses the energy onto the workpiece.
The press or machine is an infinite energy source. In some net shape processes such as die casting, the energy applicators (such as the water cooling lines) must be properly calibrated to remove the correct amount of energy. For draw forming and lathe turning (the examples used in this article), the material draws the required energy from the press or lathe as long as the press or lathe has sufficient energy. The tool simply focuses that energy onto the workpiece.
Geometric features of the energy-affecting elements of the tool for the non-net shape processes focus the energy directly onto the point where the shape of the workpiece material is being transformed. In contrast, the geometric features of the energy-affecting elements for the net shape processes are quite remote from the location within the workpiece material where shape transformation occurs.
The non-net shape lathe turning of a shaft is illustrated in Figure 1. The tool has a number of geometric features, such as: 1
- Cutting edge angle.
- Side rake.
- Back rake.
- Front clearance.
- Side clearance.
All the geometric features of the tool are designed to focus the energy directly onto the point where the chip is being torn off the workpiece. All other aspects of the system serve to ensure adequate rigidity and positioning.
Although the shaping energy is transmitted through the workpiece, the workpiece is not being changed by that force and so does not experience an energy-induced transformation except where the chip is being peeled off.
Similar examples could be made for the other non-net shape processes such as welding, milling, grinding, and assembly. Processes such as painting, electroplating, and heat treating are not shaping processes.
In the net shape processes, the tool is not shaped to apply energy at the place where the material in the workpiece is being transformed. In the case of draw forming, the transformation is plastic deformation, which is commonly called stretching.
For a flat sheet of metal to take on the desired shape, the surface area must be rearranged. A simple example is the automobile hood shown in Figure 2. The section of analysis for the hood is detailed in Figure 3.
The section through the hood shows the actual hood to be 700 millimeters wide from the center to the trim line. The section also shows that a small part of the edge will be flanged down. That flange will later be hemmed onto the inner panel.
The 700-mm portion of the hood is shown to be an arc with an included angle of 5 degrees. That arc would have an 8,000-mm radius if it were actually a single, true radius.
If the sheet material is 0.8 mm thick and the flat blank were to be simply wrapped over the punch (see Figure 4), the outer and inner fibers of the sheet metal would be strained only 0.005 percent, which is well in the elastic range. So once the wrapping forces were removed, the sheet metal would return to a flat plane.
To force plastic straining into the sheet to make it take the shape of the die, the die must be contrived to pull on the material edges, stretching it into the plastic regime. The energy-affecting elements are the features shown in Figure 3 (wrap, R1, R2, and point X), plus another feature, not shown in the figure, which is a draw (or lock) bead located between point X and the outermost edge of the blank to the right of point X.
These features of the draw die work together to provide the pulling force on the edge of the part to stretch it into plastic deformation and set the shape. All the material in the original blank that forms into the features outside the trim line is discarded as scrap after trimming.
Unlike the lathe turning operation, in which the material's shape was being transformed through chip removal at only one point on the part surface at any time, all 700 mm of the hood must be strained (stretched) in the draw die at the same time. The energy to accomplish the shape transformation must be applied at the edge of the part (the trim line in Figure 3) and transmitted all the way through the part material to the center.
Suppose, for example, that the designer has decided he needs 3 percent strain across the 700 mm of the hood in the direction of the section shown in Figure 3. It already has been shown that changing the flat blank into the 8,000-mm radius arc will stretch it only 0.005 percent (0.00005 X 700 = 0.035 mm) if the ends are held from moving. Stretching of 3 percent will mean that 21 mm (700 X 0.03 - 0.035 = 21 mm) will have to slide off the punch of the die, past point A (see Figure 5) and into the S-shaped feature consisting of R1, Y, and R2.
So the S-shaped feature (normally referred to as the addendum) must be equal to the length of the wrap between point A and point X, plus the 21 mm of material that will be sliding into R1. The length of the material in the S-shaped feature then will have to be increased to accommodate the additional stretching of the 21 mm of material as it bends and unbends under tension, sliding into and out of R1. Then the length of the S-shaped feature must be increased even more until length Y stretches enough to generate all the force needed to overcome friction, induce the bending and unbending, and stretch the 700-mm-long element to 3 percent straining.
When all of these calculations have been made, the die geometry for R1, Y, R2, and the wrap can be designed into the computer-aided design (CAD) data, locating point X in space.
So the energy-affecting elements in a draw die are remote from the material being transformed into some new shape. Unlike the non-net shape example of the lathe peeling off a chip and hence changing the shape of the workpiece, the draw formed part is pulled on its edges by elements outside the part. Plus, the energies must be projected through some portion of the workpiece to change the shape somewhere else.
Another significant point is that the detail shapes and the dimensions of those shapes for the energy-affecting elements of the draw die must be carefully calculated for each point (or at least several strategic points) around the perimeter of the part. The shapes and sizes of the energy-affecting elements must be derived from the needs of the part being formed.
The process is in contrast to that for establishing the non-net shape lathe tool's shape and dimensions, which are derived only from the material being machined, the tool material, and possibly the cutting speed. The shape of the part being made is not involved. Any variety of parts made from the same combination will use the same tool and the same cutting speeds and feeds.
Finally, because the application of energy to the workpiece is from a remote source, there is diffusion of the energy from any one applicator, and there are interactions between the energies being applied from various applicators. All these diffusions and interactions result in a specific energy being applied at any one point in the subject part, and the amount of straining at that point is the result of what energy actually is applied at that point.
The significance of the remoteness of the energy applicators is one of several reasons that the net shape processes must have their own theory for die design and part production. Practices and procedures borrowed from the non-net shape processes are not adequate.
Finally, the impact of these differences is not limited strictly to engineering calculations. The differences have an impact on policies, staffing, organizational structure, management style, job definitions, training, and even budgets.
Note:1. Albert Wagner and Harlan Arthur, Machine Shop Theory and Practice (Princeton NJ: D. Van Nostrand Company Inc., 1950), p. 75.
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