Peter Ulintz Peter Ulintz
Technical Director

Hole Extrusions—Part 1

October 1, 2011
1
Comments

 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
Industry-Related Terms: Case, Edge, Form, Forming, Point, Surface, Thickness
View Glossary of Metalforming Terms

Technologies: Quality Control

Comments

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Deryl M. FRI, JAN 29, 2021 @ 8:14 AM

How did you determine your wall thickness? If material is 3mm thick and your wall has to be 2 times wall thickness, then your wall thickness would be 1.5mm to end up at 11.2 total with ID. So how did you derive at 1.5 wall thickness. Is it always assumed half the material thickness for this formula?


Deryl M. MON, APR 5, 2021 @ 9:24 AM

Thanks Pete, I read the article twice and somehow missed that point in Paragraph 4. May I ask which formula you use to calculate your pre punched pilot hole? I have found several different ones online & am not sure which is the best to use

Peter U. FRI, JAN 29, 2021 @ 9:29 AM

Hello Deryl. The wall thickness was an engineering requirement for the extruded hole. The premise of the example was based on the parameters described in paragraph 4: "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?" Knowing these three parameters, we can determine what the outside diameter (dimension D in the illustration) and the pre-punch hole diameter need to be. A different wall thickness could be substituted to find the resulting wall height (assuming the part O.D., part I.D, and pre-punched hole diameter remain the same). Basically, an extrusion has five variable parameters: O.D., I.D., wall hieght, wall thickness, and pre-punched hole diameter. Knowing any three of these allows you to solve for the other two using constancy of volume calculations. Keep in mind, the calculations determine only if there is a sufficient volume of material to produce the desired extrusion. Other parameters determine if the extruding process will be successful, including material type and grade, lubrication, punch nose profile, and most importantly the edge condition of the punched hole. I hope this helped. Pete

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