To approximate extrusion height, apply the constancy-of-volume rule: “Material volume is neither created nor destroyed by deformation.” This is analogous to forming a hamburger—squeezing the patty reduces its thickness and increases its diameter. The volume of beef doesn’t change; the material is merely displaced or rearranged. The same holds true for hole extrusions—as the extruded wall thickness decreases, wall height increases.
| Fig. 2
Using the constancy-of-volume rule, calculate the volume of material (VM) available to work with:
VM =[(½ A)2 - (½ B)2] π t, where:
A, inside diameter of the extrusion; B, punched hole diameter; t, material thickness.
Assuming the smallest possible hole diameter that can be punched in the workpiece is equal to the material thickness:
VM = [(8.2/2)2 - (3.0/2)2] π3.0
= 137.225 mm3
To determine the volume of the extrusion (VE), we treat this as a cylinder with open ends and a constant wall thickness:
VE = [(½ D)2 - (½ A)2] πh
The outside diameter of the extrusion (D) is equal to the inside diameter plus two-times the wall thickness, or 11.2 mm. The desired height (h) of the extrusion is 3.0 mm.
VE = [(11.2/2)2 - (8.2/2)2] π 3.0
= 137.131 mm3
If VM > VE, there is enough material volume to form the extrusion. In this case, VM slightly exceeds VE, so the height of the extrusion can be made greater than 3.0 mm. If the resulting extrusion height were too high, simply reduce VM by increasing the diameter of the punched hole.
Once the correct volume of material has been established, the quality of the punched hole becomes a prime concern. All hole extrusions start as hole expansions (Fig. 2), where a punch is forced into a blanked hole, causing a circumferential elongation or stretching of the cut edge. Absolute values of expansion limits depend on the material, tool design, lubrication and edge quality of the punched hole. Fig. 3, illustrates the negative impact that cutting-edge damage can have on hole expansion and hole extruding.
| Fig. 3
If the amount of stretching required to form the hole extrusion exceeds the residual stretchability of the cut edge after punching, consider of one several options available for restoring edge stretchability:
• Improve the quality of the original cutting operation;
• An additional cut of higher quality; or
• A shaved cut and deburred break edge.
Next month I’ll present some proven hole-punching methods and extrusion-punch designs for forming extruded holes in sheetmetal stampings. MF
Technologies: Quality Control