Draw Forming, Part VII
Setting the processing variables is different between the net shape and non-net shape processes. While non-net shape variables have to be programmed constantly to change the tool's position in space to follow the unique shape of the part being made, the shape of the part for the draw forming net shape process is built into the die.
Editor's Note: This is the last 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 six 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, metrics, connectivity, geometry, remoteness, and redistribution.
This article addresses the significance of nonuniformity during the forming process and summarizes the main focus of this series of articles.
The Non-Net Shape Situation
In the non-net shape processes, the energy applied to the workpiece is kept as constant and uniform as possible.
In machining operations, the cutting depth, cutting feed rate, and cutting speed are kept constant. The shape is achieved by positioning the cutting tool, and the cutting parameters are geometric, with the same metrics and conceptual meaning as the geometric description of the desired final product.
The same is true of welding: The welding tips (if spot welding) must be maintained to a specific shape and condition, and the force on the tips and the current through them are maintained for every spot, regardless of the spot's position. A similar example could be made for arc welding or any of the non-net shape processes. The shape of the product does not enter into the determination of the application of the "shaping" energy.
The Net Shape Situation
The situation for draw forming (as with all the net shape processes) is quite different. The forming energy is applied from a remote source and through the workpiece material.
As the energy travels through the workpiece to the area of transformation, it is reduced through friction or through work being done to other areas of the workpiece. The net result is that the design of the energy-affecting tooling geometry (for example, the addenda and the wrap and draw beads) at every point around the product shape must be calculated and calibrated to account for all the variables.
The shapes of the energy-affecting elements cannot just be located in a handbook. They must be calculated for each location for each die.
The approach to setting the processing variables is quite different between the net shape and the non-net shape processes. While the non-net shape variables have to be programmed to constantly change the tool's position in space to follow the unique shape of the part being made, the shape of the part for the draw forming net shape process is built into the die.
For draw forming, the processing variables—binder force, lubrication, blank size, press shut height—are set once and maintained at those set points as the part is formed. Although the actual shape of the part being draw-formed is not directly used to determine the set values for the processing variables, the shape of the product indirectly influences their values.
The processing variables must be set so the energy-applying die geometries will work correctly. Once the energy-applying die geometries have been designed, the values of the processing variables are established. Usually preconceived (or facility-specific standard) values for the processing variables are assumed in the computations of the energy-applying die geometries and used unless the calculations show the process will not work at those values.
The Main Idea
The main point to consider from this series of articles is that the net shape processes need their own unique theory. They differ from the non-net shape processes in the areas of:
- Level II processing.
Net shape process engineering is intensive in material science and mechanical engineering, whereas non-net shape process engineering is intensive in technical memory and industrial engineering. These differences affect how engineering resources are allocated and managed and the training needed within the resource groups.
A primary part of the theory is that the energy-affecting elements cannot be derived directly from the final part geometry. They must be derived in a two-step process:
- Step 1—Redefine the product requirements into the strains and displacements (transformation characteristics) that must be induced.
- Step 2— the energy-affecting elements to impose those strains and displacements.