Strategies for Designing an Extraction Nut, Part I

Strategies for Designing an Extraction Nut, Part I


Editor’s note: Part II will appear in the May / June issue.

The molding process evolved in the huge industry we know today before the physics of metal formability and mold forming were understood and mathematically quantified. As a result, matrix design has historically been based on an engineering culture of experience, intuition, and trial and error.

For example, the manual Diemaking and Die Design by Franklin D. Jones, published in 1915, had almost no equations or discussions of strains and stresses in sheet metal. The design approach was purely from a geometric point of view. by Frank W. Wilson Matrix design manual, published in 1957, included some discussion of stresses and deformations, but without aspects of formability. David A. Smith’s 1990 third edition focused more on all three topics.

Since that time, finite element analysis incorporated into commercial software packages has become a dominant tool for the molding industry. Even so, in practice, the culture often remains to use experience, intuition, and trial and error to determine the size of the mold features, then try those designs using computer simulation.

In the automotive industry, virtual computer testing now happens much earlier in the vehicle development process, to the great benefit of the industry. But even when the virtual test detects some problem, the solution to the problem is still addressed by the old process of experience / intuition / trial and error, although the engineer has gained considerable “experience” from the simulation results.

The alternative method, described here, is to structure the problem to use the mathematical analytical formulas for sheet formability, friction and kinematics, but apply them as the design evolves, rather than as a check after completion. of the mold design. Problem structuring eliminates the mathematical complexities inherent in FEA and helps the designer to focus on the root causes of the problems.

This approach differs from numerical analysis with the finite element method, which involves calculating stresses, strains and forces for the existing mold configuration. In the proposed approach, the designer assigns the necessary deformations within the part to achieve the required performance (no cracks or wrinkles, minimal springback) and then, using a spreadsheet, works outward through the part and calculates from those deformations assigned the necessary stresses to be imposed on the cutting line of the piece. Then keep using the spreadsheet to calculate adjacent tool radii, arc lengths, draw wall lengths, line lengths through the winding, and draw beads to get the right amount of energy out of the press. and properly concentrate that energy in the sheet metal at each strategic point around the perimeter of the part.

Extraction nut elements

The basic elements of the sheet are shown with the die closed on it Figure 1. The diagram illustrates the common approach of using a double action die in a single action press; the punch is positioned at the bottom and the collector and die are installed at the top.

The red line represents a section through the drawn shape of the final product region. For simplicity, the product has a slightly curved shape and the pre-hem flange at the bottom has been explained to be a simple extension of the product surface to the cut line. The phantom blue line indicates the shape of the blank after the collector has closed on it (also known as the envelope) but before the punch has engaged the sheet. The purple line includes the punch perimeter radius, a nearly vertical straight section and the die perimeter radius. The total length of these three elements is greater than the length of the wrapping material adjacent to them (below them in the figure).

When the mold closes, it stretches these elements (along with any material that is stretched by the punch) just enough to create the force on the product sheet (red line) to force it in the right amount. These three elements are the pull nut energy management system. They draw the energy required by the press and concentrate that energy in the workpiece.

Displacement map

The designer’s job is to determine the size of the features in the purple, green, and blue lines shown in Figure 1. To do this, he must first determine what energies need to be imposed on the product sheet at strategic locations on the edge of the part. For this, he should create a wireframe abstraction of the product geometry, with the wires forming circular arcs connected butt-to-tail at true tangents. There should be all the arcs needed to approximate the actual curvature of the product. The wires of the wireframe will have a number of characteristics (see figure 2) which have the same parameters as the wall drawn in the purple line in Figure 1, except that the dimensions are fixed by the product design. The mold designer cannot change them.


A plan view of the wireframe (see Figure 3) must capture the geometry of the actual product. Two of the wires, shown in solid blue in the figure, must be neutral lines and will be approximately normal to each other within the limits of the part design. These show where the designer does not want the sliding motion of the sheet to slide across the face of the mold. The sliding motion, caused by stretching or simply dragging, should radiate away from these lines, although it can flow along the neutral line. There will be no sliding (shifting) in any direction where the neutral lines cross. This is the “zero” point in the die. The designer uses displacement mapping to determine where these wires need to be to form the part correctly. The wires must be spaced so that the sheet metal between them forms in conditions between that of the wires surrounding them.

In Figure 2, the length of the product characteristic from boundary curve A to boundary curve B is greater than the adjacent length of the winding (between boundary curve B and point C). The difference is drawn as the red and green arrows. For example, if the length AB minus the length BC is 15mm, then both the red and green arrows would be plotted to 15mm in length in the CAD data. The designer must decide whether he wants the metal to slide into the element from the direction of the red arrow, the green arrow, or part of each. His decision is his training strategy for that trait.

For example, if the product feature below the green arrow is 500mm long and the other (left) end is on a neutral line, it could provide the required 15mm of material by straining 3.0%. If this is what the designer decides to do, that will be his strategy. If he only intends to strain the material by 2.0%, he will only get 10mm on the green arrow side and he will have to get the remaining 5mm on the red arrow side. If this is what he wants, the 2.0% strain will be his target strain (training strategy) in that position. He has to design the purple, green and blue characteristics in Figure 1 to force the occurrence of the target strains.

This selection may depend on the sheet metal material and component requirements. For example, if the part is formed from extra deep drawing steel with excellent formability, further elongation can occur on the part side (green arrow), increasing the strength of the component through work hardening of the sheet. If the part is made of ultra high strength steel or aluminum alloy, the formability is more limited and a larger shift from the binder side can be planned. These considerations are addressed by the designer’s specification of the deformation torque to be imposed within the panel and usually at the zero point, where the neutral lines cross.

Another factor that plays a significant role is whether the part design is driven by strength or stiffness. In the first case, it makes more sense to stretch the sheet to obtain greater strength or to use high-strength steel. For stiffness driven parts, the optimum level of elongation can be achieved by using a rather small elongation where the material hardens significantly. The minimum thickness requirement is often considered for corrosion-critical applications, such as vehicle underbody components. Reversing the direction of travel creates a stroke-drawing boundary in the feature.

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