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Hole Extrusions--Part 1

By: Peter Ulintz

Saturday, October 01, 2011
 

 Fig. 1
A hole extrusion may be designed into a metal stamping for any number of reasons. The inside diameter of an extrusion may serve as a bearing surface, a pivot point or an interface for a press-fit with another component. Most often, hole extrusions find use as screw attachments, usually cut or formed with threading taps or self-tapping screws.

Some people incorrectly refer to deep-drawn hole-tapping features as extrusions. The deep-drawn features are produced by gathering a predetermined volume of material into a bubble, then incrementally reducing the outside diameter while simultaneously increasing the wall height through multiple redraw stations. Depending on the process design, deep-drawn features can have wall thicknesses that are less than, equal to or greater than the original sheetmetal thickness. They also can achieve greater wall heights than possible with hole extrusions.

Hole extrusions start with a punched hole in a flat surface of the sheetmetal. Then an extruding punch expands the hole to the required ID, usually in one hit. The extrusion wall height is the result of the punched hole diameter, the edge quality of the punched hole, the diameter of the extrusion and the amount of allowable wall thinning.

Consider, for example, a hole extrusion designed as a bearing hub in a 3.0-mm-thick part (Fig. 1). The product designer requires an 8.2-mm ID and a minimum wall thickness of 1.5 mm., with a minimum 3.0-mm extrusion height (h). Is it possible?

 Fig. 2
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.

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.

  Fig. 3
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.

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

 

Related Enterprise Zones: Tool & Die

 


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