September 12, 2006
The relationships among part geometry, tool geometry, and control of the processing inputs are significantly different between the net shape and non-net shape processes.
Editor's Note: This is the fourth in a series of seven articles that identify and define the need for a new theory on 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 three 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; and finally how those disconnects must be connected.
How does geometry contribute to the differences between the net shape and the non-net shape processes? How does this additional difference help drive the nature of the net shape processing theory?
The tool for a net shape part is a die or a mold, and the shape of the tool is the shape (negative impression) of the part to be made. The shape of the tool is not created to facilitate the focusing of energy onto the workpiece. Instead, it is a faithful copy of the finished shape of the part to be formed and is contrived either for a pleasant appearance or to meet some function of the final consumer product.
The tool (in this case, the draw die) must somehow apply energy onto the workpiece; otherwise, the workpiece will not take the shape of the tool. In draw forming, the energy is applied by pulling on the edges of the sheet metal, which are outside of the product. A subsequent trimming operation removes those pulled edges from the part.
The features needed to do that pulling must be designed and built into the die. The shape of the part is created by hardware. If the hard construction of the die does not have these features properly contrived, the die will not pull on the edges of the part correctly and thus will not form the part properly.
These energy-affecting die features are in the addenda and binder geometric elements of a draw die. These elements therefore must be calculated from the energy requirements of the part being made. There is no one correct addendum or binder for every part, or even for every point in a single part. Both the amount of energy to be applied and the shape of the part into which the energy must be applied are likely to change around the perimeter of the part.
The geometry of the energy application elements is directly coupled to, and derived from, the geometry of the part being stamped. And the geometry of the part is determined by the shape of the tool.
An exception occurs when the energy applicator shapes are internal to the part, such as in a common beverage can. The stretching of the can side wall actually generates the forces that form the can. Even then, the binder restricts the material that is moving from a flat plane into the cylinder of the can's side wall to ensure that the exact correct forces will be generated.
In these situations, the design of the stamped product must incorporate the needs of the energy application. Most draw formed parts do not have the luxury of being cylindrical or of having decades to refine the design as the beverage can has.
In contrast, the tool for making a non-net shape piece part is designed solely to control the application of energy onto the workpiece; rarely is the shape of the tool influenced by the shape of the workpiece.
There are some exceptions. For instance, sometimes a milling cutter must be of a certain diameter that is smaller than the cavity being cut into the workpiece. Sometimes formed cutters are used in lathes to turn a form onto a cylinder by simply plunging the cutter rather than manipulating the cutter to create the form. Even in those instances, the influence of the product shape on the tool is limited and, when incorporated into the tool's shape (as for the formed cutter for the lathe), generally detracts from the tool's ability to focus energy.
An example is the shape of the tool bit in a lathe. The tool bit is configured to maximize the concentration of energy onto the point where molecules are being ripped apart to carve away unwanted material. The final shape of the part being turned in that lathe has no bearing on the shape of the tool bit.
The product shape is determined by the manipulation in space of the tool bit by the other process input variables: the workpiece rotational speed, the axial location of the cross slide, and the radial location of the tool post on the cross slide. The manipulation of the process inputs is driven purely by part geometry and is totally decoupled from any energy requirement of the process.
The shape of a non-net shape part is induced through the active control and manipulation of the processing inputs, not the tool. By manipulating the rotating speed, axial cross slide position, and radial tool position on the cross slide of a lathe, an operator can turn any number of various shapes with the same tool.
The product shape is created by software. That software can be the mind of the operator who is manually turning cranks, or within a computer that is moving the position controls with servomotors.
The shape of the tool is derived from the tool material, the workpiece material, and sometimes the cutting speed, but not by the shape of the part to be made.
The relationships among part geometry, tool geometry, and control of the processing inputs are significantly different between the net shape and the non-net shape processes. Although the examples in this article are draw forming and lathe turning, the different roles of the part and tool geometries and processing inputs can be described in similar terms for all net shape (molding, casting, and forging) and non-net shape (milling, drilling, welding, and assembly) processes.
These differences are some of the reasons that the process and tooling design must be approached quite differently for the net shape processes.