Metal deforming – By use of closed-die and coacting work-forcer – Cup or shell drawing
Reexamination Certificate
1999-10-07
2002-07-09
Tolan, Ed (Department: 3725)
Metal deforming
By use of closed-die and coacting work-forcer
Cup or shell drawing
C072S462000, C072S476000
Reexamination Certificate
active
06415640
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a cast or molded tool for machining, sheet-metal parts, and a method of producing such a tool.
Tools formed by casting are used almost exclusively in the fields of large tools for producing auto bodies. This trend arose, among other reasons, because of the flexibility of casting construction. Upon observing the ribbing of such tools, one predominantly finds constructions in which the ribs are orthogonal to one another and orthogonal to the base surface of the tool. In technical language, ‘ribbing’ is the provision of the tool with recesses in an effort to reduce its weight. The embodiments known from the state of the technology are oriented in appearance toward welded constructions.
Welded constructions are assembled from individual plates that lend these tools their typical appearance. The outcome of this design is poor force courses. Especially in drawing tools, there are locations at the drawing-part flange, between the hold-down device and die plate, at which high surface pressures occur due to the ribbing underneath. In contrast, only a low surface pressure is present between the ribs, namely in the region above the recesses. For this reason, the drawing-frame surfaces of the hold-down device and the die plate must be thick to reduce the undesired side effects.
The primary objective of the die plate/hold-down device system is to clamp the sheet at the flange such that no folds are formed in the sheet due to the tangential compressive strains during the drawing-in process, and the material flow between the die plate and the hold-down device is optimally controlled by way of locally-varying surface pressures. The surface pressure cannot be too high, or cracks will occur. Folds are formed when the surface pressure is too low. Therefore, the sheet must be able to continue flowing within the two failure limits of “cracks” and “folds.”
Tests reveal that, with a relatively rigid bedplate and press ram, the horizontal bending resistance of the tool components has an insignificant effect on the surface pressure at the drawing-part flange. The vertical pressure resistance of the tool components is practically the only relevant characteristic.
The tool structures that have traditionally been used up to this time have a high pressure resistance in the surface of the hold-down device, above the ribs, whereas the pressure resistance between the ribs is very low. The objective is to attain the most uniform vertical pressure resistance possible; the horizontal bending resistance is inconsequential.
“Intelligent die cushions” are being used more and more frequently. These cushions permit the center-sleeve forces or levels to be controlled individually during the drawing process, and thus a purposeful influence of the surface pressure at the drawing-part flange. Tests of the function of the hold-down device show that only one hold-down device having a high vertical pressure resistance uniformly transmits the center-sleeve forces to a defined surface at the drawing-part flange. The bending resistance is also of little consequence here.
It is the object of the invention to provide a tool for machining, sheet-metal parts, the tool having a high, uniform vertical pressure resistance that is attained with a low financial outlay. Furthermore, a method for producing the tool is disclosed.
SUMMARY OF THE INVENTION
The above object is accomplished according to the invention by a cast or molded tool for machining sheet-metal parts, the tool having at least one component for exerting a force wherein at least one of the tool components possesses channel-like recesses that are arranged in a sort of pattern virtually parallel to the direction of the exertion of force by the aforementioned component. The channel-like recesses extend at least over a large portion of the length of the component, and have the same predetermined cross section.
The ribbing of the tool base body, i.e., the tool components provided with channel-like recesses, assures a high vertical and locally-uniform pressure resistance of the tool components, which is an essential criterion for the optimum distribution of the forces. Because of the relatively-small hollow spaces, and the thin walls between the hollow spaces, large amounts of mass can be omitted. This is also especially the case because a smaller thickness of the drawing-belt surface is necessary above the smaller hollow spaces due to the fact that far less bowing occurs in the smaller, unsupported regions. In addition, smaller hollow spaces can be better arranged for avoiding larger, non-ribbed regions in the edge zones. The better rigidity properties for a lower material cost significantly reduce the costs of producing the tool.
The uniform vertical rigidity that is already present greatly reduces the spot-grinding costs. Spot-grinding is a machining process in which a uniform surface pressure is attained through the manual removal of material from the tool. Because spot-grinding is highly labor-intensive, the reduction in the spot-grinding outlay can contribute greatly to cost savings.
The use of channel-like recesses having simple geometric base surfaces is ideally suited for computer-aided tool design, because the hollow spaces can be adapted into the tool base body by means of simple algorithms, which can save a considerable amount of weight.
The construction times for the tool base bodies can be drastically reduced, which is synonymous with large cost savings.
The cross section of the channel-like recesses is preferably circular. The use of cylinder-type hollow spaces as the channel-like recesses assures the simplest type of design of tool base bodies having a prismatic ribbing. As their cross section, the cylinders have circular surfaces of identical diameter, which can be inserted into the cross-sectional surface of the tool base body with very simple algorithms. This ribbing variation is therefore ideal for the automated design of the tool base body employing CAD (Computer-Aided Design).
Simple rules can be established for determining the dimensions and surface ratios in the ribbing cross section. For example, the minimum wall thickness between the hollow spaces corresponds to the respective material viscosity, or its ability to be cast.
The supporting surface component can be derived from the required tool rigidity. The diameter of the channel-like recesses can be determined with the minimum wall thickness and the supporting surface component.
The cylinders are drawn, perpendicular to the tool base surface, to an upper auxiliary construction surface that extends parallel to the surface of the drawing belt. Various amounts of weight can be saved, depending on the stamp contour and the cylinder arrangement in the cross-sectional surface.
In particular, the cross section can possess different diameters. The prismatic ribbing with cylinders of varying diameters is based on the same principle as the aforementioned ribbing variation with a constant diameter. In the automated ribbing of the tool base body, however, more complex algorithms are required for arranging the cylinders in the ribbing cross section. In principle, the number of cylinder diameters should be kept small to permit the use of simpler algorithms.
An ideal diameter for the largest cylinders can be determined for the required minimum wall thickness and the necessary supporting surface component. If smaller cylinders are to be disposed between the larger cylinders, there is a diameter for the smaller cylinders that permits a suitable utilization of the optimum minimum wall thickness. For the next-smaller cylinders, there is again a unique optimum diameter. On this basis, a unique and optimum graduation of cylinder diameters can be determined for the initial conditions of minimum wall thickness and supporting surface component.
Usually, two to three different diameters are adequate for further reducing the tool weight if the surface component that actually provides support lies above the minimum required supporting surface because of the minimum required
Daimler-Chrysler AG
Kunitz Norman N.
Tolan Ed
Venable
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