# Patent

## Title
Sheet metal hem forming process

## Abstract

A method for hemming an inner panel and an outer sheet metal panel together. The inner panel has an outwardly extending perimeter flange comprising an end surface, an inboard surface, and a beveled surface located between the end surface and the inboard surface. The outer sheet metal panel has a peripheral edge comprising a bend portion, an intermediate portion, and an end portion. The method provides a reduced radius hem by providing the beveled surface, assembling the inner and outer panels together, prehemming the peripheral edge to a spaced relationship relative to the beveled surface, and hemming the peripheral edge of the outer panel into engagement with the perimeter flange of the inner panel.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a division of U.S. application Ser. No. 10/707,126 filed Nov. 21, 2003, now U.S. Pat. No. 7,241,073, issued Jul. 10, 2007, which is incorporated by reference herein.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved sheet metal hemming process.
2. Background Art
Hemming is a production process for joining an outer panel to an inner reinforcement panel. Conventional hemming processes are accomplished by bending a peripheral edge of the outer panel back onto the inner panel. A multiple step process is used to form a hem. In a first step, a peripheral edge of an outer panel is formed to extend substantially perpendicularly relative to the body of the outer panel. In a second step, an inner panel is assembled to the outer panel with a perimeter flange being inserted inside the peripheral edge of the outer panel. In the third step, the peripheral edge of the outer panel is pre-hemmed. Pre-hemming may be performed by forming the peripheral edge to an acute angle of approximately 45° to extend inwardly over the perimeter flange of the inner panel. Alternatively, the edge may be pre-formed in two steps to 60° and then 30°. In a final step, the peripheral edge is formed to extend parallel to the body of the outer panel and engage the perimeter edge of the inner panel.
Hems in highly visible areas such as hoods and doors are key to assessing a vehicle's overall craftsmanship. This is especially true when two closure panels are located adjacent to each other as in the case of a gap formed between two doors or between a hood and a fender. The radius of the hem visually impacts the appearance of the gap, or margin, between the two panels. Larger hems tend to make the margin look larger, while smaller hems produce a crisper cut line and smaller perceived margin. In the prior art, as the thickness of the inner panel increases, the hem radius increases. This, in turn, increases the perceived gap between matched panels. In conventional hemming processes, the final hem radii is a function of the thicknesses of the inner and outer panels. With a conventional hem, the radius is equal to one-half the inner panel thickness plus the outer panel thickness.
Recent developments in the field of hem forming have led to the development of reduced radius hems that improve the appearance of the fit of adjacent panels by reducing the perceived margin between adjacent panels. One example of such a hemming method is disclosed in U.S. application Ser. No. 10/063,757, filed in the name of Samant et al., and assigned to Applicant's assignee. The Samant application describes a method of manufacturing a hem with a reduced radius hem. The method and apparatus discussed in the Samant application produces a hem with a radius sharper than half the total stack height of the inner and outer panels and can significantly improve the craftsmanship of closure panels.
A marriage gap is defined as the space between the outer edge of the perimeter flange of the inner panel and the bend of the peripheral edge of the outer panel. The marriage gap is provided to avoid interference between the two panels. Interference between the panels occurs when, during the pre-hemming pass, the outer panel peripheral edge is being formed into an acute angle to extend over the perimeter flange of the inner flange and improperly comes into contact with the inner panel.
Referring to FIG. 1, a prior art hem flanging operation is illustrated in which an interference occurs between a relatively thick inner panel I and the peripheral edge P of the outer panel O during pre-hemming. A hemming tool R is shown contacting the peripheral edge P of the outer panel O during a pre-hemming pass. A top corner C of the inner panel I interferes or makes contact with the inner surface of the peripheral edge P. This interference may cause distortion in the hem and limit reduction of the hem radius. Interference may cause splitting or tearing of the hem. The marriage gap can be increased to prevent the interference to a limited extent. For conventional hems, the marriage gap is generally approximately 2 millimeters. If the marriage gap is too large, the hem may be weakened and the edge of the panel will become prone to bending or deformation.
Recently, lightweight inner panels have been introduced that are cast or otherwise formed of magnesium, aluminum and other alloys, polymers, or similar materials. These panels are generally thicker than conventional metal inner panels. Conventional steel panels are approximately 0.7-0.8 mm. Conventional aluminum panels range between 0.9 and 1.2 mm. Inner panels made of other materials may be of greater thickness. For example, a cast magnesium alloy inner panel may be 3 to 4 times the thickness of a steel inner panel. Such thick inner panels increase the radius of hems which increases the apparent gap size and adversely effects overall quality and craftsmanship.
The use of thick inner panels creates unique hemming challenges. For thicker inner panels, a 2 millimeter marriage gap is not always sufficient to avoid interference between the panels when using a reduced radius hem. A conventional solution to eliminate interference is to increase the marriage gap. Increased flange length is required to cover the larger marriage gap. This solution may be acceptable in flat-straight areas of the panel but can cause problems in hems on curved edges or surfaces.
Short flange lengths must be provided in areas where there is either cut line or surface curvature. In these areas, splits in either the edge or flange can be caused due to the stretching or compressing of the flange during the bending operation. A conventional solution to solving hemming problems in such areas is to reduce the length of outer panel flanges to mitigate this problem. It may be necessary to reduce outer panel flange lengths to 6 millimeters or less to accommodate cut lines and surface curvatures.
Thick inner panels when hemmed with a reduced radius hemming process may result in edge run out. Edge run out is an unacceptable condition that occurs if the hem edge is not long enough to cover the marriage gap. This problem is particularly difficult with hems in panels having curved cut lines and surface contours.
There is a need for an improved reduced radius hem and hemming process that can accommodate thick inner panels while maintaining the size of the marriage gap and minimizing flange lengths in critical contoured areas. The disadvantages and shortcomings of the prior art are addressed by this invention as summarized below.
SUMMARY OF THE INVENTION
The present invention provides a method of hemming an outer sheet metal panel having a peripheral edge to an inner metal panel having a perimeter flange. The method comprises removing a top corner of the perimeter flange of the inner panel to form a beveled surface across a portion of the perimeter flange. The inner panel and the outer panel are then assembled together. The peripheral edge is formed in a pre-hem pass with a hemming tool to bend the peripheral edge adjacent to the perimeter flange of the inner panel. The peripheral edge is formed to be in a spaced relationship relative to the beveled surface. The peripheral edge of the outer panel is then formed into engagement with a portion of an inboard surface of the perimeter flange of the inner panel.
These and other features and advantages of the invention will be more fully understood from the following detailed description of the invention in view of the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross-sectional view of a prior art hem wherein interference occurs between the outer and inner panels;
FIG. 2 is a schematic cross-sectional view of an improved hem made according to one embodiment of the present invention;
FIG. 3 is an end elevation view of an inner panel peripheral flange made in accordance with one embodiment of the present invention; and
FIGS. 4-6 are a series of schematic views showing the formation of a hem flange assembly progressing from a 60 degree pre-hemming step, a 30 degree pre-hemming step and a final hemming step.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
Referring to FIG. 2, a hem assembly 10 for a vehicle closure panel is shown. An inner panel 12 and outer panel 14 are joined with a peripheral edge 16 of an outer panel being folded back over a perimeter flange 18 of the inner panel 12. The perimeter flange 18 is tightly gripped by the outer panel 14.
The perimeter flange 18 of the inner panel has an inboard surface 20, a beveled surface 22 and an end surface 24. The peripheral edge 16 of the outer panel 14 has a distal portion 26, a intermediate portion 30 and a bend portion 32. An inner surface 34 of the peripheral edge 16 at the intermediate portion 30 of the outer panel 14 is adjacent to the beveled surface 22 of the inner panel 12. The inner surface 34 of the peripheral edge 16 at the distal portion 26 of the outer panel 14 abuts the inboard surface 20 of the perimeter flange 18 of the inner panel 12. The beveled surface 22 is located between the end surface 24 and the inboard surface 20 of the perimeter flange 18. The end surface 24 generally lies in a plane perpendicular to the inboard surface 20. The beveled surface 22, as illustrated, is generally flat and is located in a third plane with respect to the end surface 24 and the inboard surface 20. Alternatively, the beveled surface 22 could be configured as a curved or partially conical shape. A marriage gap 36 is defined between the end surface 24 and the inner surface of the peripheral edge 16 at the bend portion 32.
Referring to FIG. 3, a thick inner panel 12 made according to the present invention is shown. The top corner of the inner panel 12 has been removed to form the beveled surface 22. The top corner may be ground off during a deburring operation prior to assembly. Other methods may be used to form the beveled surface 22, such as chamfering, molding, or stamping. In the illustrated embodiment, the beveled surface is oriented at approximately a 45° angle relative to the inboard surface 20.
The invention may be practiced on all or only a portion of a hem assembly 10. For example, a door may have straight hem areas that are simple to form and cut lines or curved areas that are difficult to form. The beveled surface 22 may be formed on the perimeter flange 18 of the inner panel 12 only where the difficult to form cut lines and curved surface areas exist to eliminate hemming problems. The beveled surface 22 could also be formed in straight hem areas to improve overall craftsmanship of the body panel.
Referring to FIG. 4, a method for forming a hem according to the present invention is described. A roller 40 applies a force to the peripheral edge 16 to achieve approximately a 60° angle in a pre-hemming step. In this pass, the radius of the hem assembly is set before contact is made between the inner surface 34 and the inner panel 12 at the beveled surface 22.
Referring to FIG. 5, in the second pass of the roller 40 the peripheral edge 16 is formed to approximately a 30° angle and does not come into contact with the beveled surface 22.
Referring to FIG. 6, in the final hemming pass the roller 40 compresses the distal portion 26 of the outer panel 14 to contact the inner panel 12 inboard surface 20. The method creates a hem assembly 10 having a sharp radius with adequate peripheral edge coverage.
While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.

## Claims

1. A method of hemming an outer metal panel having a peripheral edge to an inner metal panel having a perimeter flange, the method comprising:
removing a top corner of the perimeter flange of the inner panel to form a beveled surface across a portion of the perimeter flange;
assembling the inner panel and the outer panel together;
forming the peripheral edge in a pre-hem pass with a hemming tool to bend the peripheral edge adjacent to the perimeter flange of the inner panel in a spaced relationship relative to the beveled surface; and
forming the peripheral edge of the outer panel into engagement with a portion of an inboard surface of the perimeter flange of the inner panel.

2. The method of claim 1, wherein the removing of a top corner of the perimeter flange of the inner panel comprises a deburring process.

3. The method of claim 1, wherein the removing of a top corner of the perimeter flange of the inner panel comprises a grinding process.

4. The method of claim 1, wherein the removing of a top corner of the perimeter flange of the inner panel to form a beveled surface across a portion of the perimeter flange is achieved by molding.

5. The method of claim 1, wherein the hemming tool is a roller.

6. A method of claim 1, wherein the beveled surface is oriented at approximately a 45° angle relative to the inboard surface.

7. A method of hemming an outer metal panel having a peripheral edge extending generally perpendicularly relative to the body of the outer panel and an inner metal panel having a perimeter flange together, the method comprising:
removing a top corner of the perimeter flange of the inner panel whereby a beveled surface is formed;
placing the inner panel and outer panel together;
rolling the peripheral edge in a first pre-hem pass to bend the peripheral edge to an acute angle relative to the body of the outer panel and spaced relative to the beveled surface;
rolling the peripheral edge in a second pre-hem pass to bend the peripheral edge to a second acute angle relative to the body of the outer panel; and
rolling the peripheral edge of the outer panel in a final pass over and into engagement with the perimeter flange of the inner panel.

8. A method of hemming an outer metal panel having a peripheral edge extending generally perpendicularly relative to the body of the outer panel and an inner metal panel having a perimeter flange together, the method comprising:
providing the perimeter flange of the inner panel with a beveled surface at an outboard edge of the perimeter flange, wherein the perimeter flange has a nominal thickness and the outboard edge defines an end surface that is shorter than the nominal thickness;
assembling the inner panel and outer panel together with the outer panel receiving the inner panel with the perimeter flange being spaced inwardly from where the peripheral edge of the outer metal panel extends perpendicular from the body of the outer panel; and
rolling the peripheral edge of the outer panel into engagement with the perimeter flange of the inner panel wherein the peripheral edge of the outer panel is formed to the beveled surface, and wherein a marriage gap of reduced height is defined between the end wall, the body of the outer panel, and the peripheral edge after the peripheral edge is rolled into engagement with the perimeter flange of the inner panel.