Patent Application
20150047178
TECHNICAL FIELD
This disclosure relates to sheet metal hemming tools and methods of hemming a flange of one panel over another panel to form a sharp hem flange.
This disclosure is directed to hemming processes for forming a flange into a hem that joins together two sheet metal panels. Sheet metal panels formed from aluminum alloys and AHSS have a reduced ability to be bent over another panel to form a hem having a sharp radius.
Several techniques have been developed to address this issue. In one known press hemming process, the hemming area is compressed in a die while forming the hem. A roller hemming process has been proposed in which local pressure is applied to the blank to better control the hemming process than in a press hemming process. However, the roller hemming process is substantially slower and is feasible only for low volume or mid-volume applications. The roller hemming process requires several roll hemmers and possibly several sets of tools to meet the production demands that are comparable to the production rates of conventional hemming processes. Three passes of roller hemming tool may be required in some instances to form a hem with some materials.
An alternative technique is to form a sharp flanging radius during the flanging step with the inner radius being controlled by the inner radius of the flanging die.
One problem that is not resolved is the problem of the outer panel creeping during the hemming process. “Creep” occurs due to the tendency of the flange to continue to bend during the subsequent hemming process. The flanging radius may roll-in during the hemming process. Creep causes material in the flange to move from the outer side of the flange towards inner area of the hem. Creep can change the overall radius of the hem and strain the previously formed corner at the base of the flange.
This disclosure is directed to solving the above problems and other problems as summarized below.
This disclosure is of a method and tool for sharp hemming in a press hemming operation that uses tools having selected areas with different coefficients of friction to create a frictional counteraction force to minimize creep and roll-in during the hemming process. Avoiding creep and roll-in preserves the sharp flange radius formed in the sharp flanging operation. The frictional counteraction force is applied at certain stages of the hemming operation using friction to limit creep. The counteraction force is applied through the use of a high friction material or a high friction coating on a localized part of the surface of the tool that engages the flange.
One example of a tool that can supply a frictional counteraction force includes a strip of polyurethane set in a groove on the final hemming tool. The surface of the polyurethane strip is flush with the surface of the hemming tool. Increased frictional force is provided by the polyurethane strip. Alternatively, the frictional counteraction force can be provided by applying a coating of a material, or a surface treatment to a selected limited portion of the hemming die that has a higher coefficient of friction than the steel surface of the die.
A limited amount of friction is present at the interface between the tip of the sheet metal flange and the steel hemming tool. The frictional force applied by the tool changes when the tip of the sheet metal flange reaches a predetermined point on the hemming tool where the polyurethane strip or high friction coating is located. The coefficient of friction of the material used to make the insert is selected to balance the other forces applied to the flange during hemming. The strip or coating with a higher coefficient of friction must be disposed at a predetermined location to contact the tip of the flange at a selected time during the hemming process.
The hemming tool may be used to hem either closed flanges or open flanges. A closed flange is a flange with an inner angle of less than 90 degrees. The hemming tool may be used to form a hem on an open flange having an inner angle greater than 90 degrees. A hemming tool for forming an open flange may include angled outboard sections that pre-hem the open flange to bend the flange to an orientation that is inside of 90 degrees and in a position for final hemming. The tip of the flange comes into contact with an area having an increased coefficient of friction (eg. a polyurethane insert) in the hem tool once the flange reaches a critical point in the bending and hemming process. The polyurethane strip applies a frictional force to the tip of the flange to apply pressure through the flange toward the corner of the outer panel. The frictional force counteracts the bending moment that causes creep and roll-in during the hemming processes.
According to one aspect of this disclosure, a hemming tool is provided for forming a hem to secure an outer panel to an inner panel. The hemming tool comprises an anvil die that supports the inner panel on the outer panel inside a flange formed on the outer panel that extends away from the anvil die in an initial position. A hemming die is spaced from the flange in the first position. The hemming die includes a first portion of a forming surface that has a first coefficient of friction and a second portion that has a second coefficient of friction that is greater than the first portion. The first portion engages the flange to begin forming the flange over the inner panel, and the second portion subsequently engages the flange to continue forming the flange towards the inner panel.
According to other aspects of this disclosure, the flange may be oriented at an oblique angle relative to the outer panel, and a beveled surface may be provided that is outboard of the first portion of the forming surface. The beveled surface may contact the flange before the first portion of the forming surface to reduce the angle of the flange from an oblique angle to an acute angle. A flange finishing tool may be inserted between the forming surface and the flange after the second portion engages the flange to complete forming the flange.
According to additional aspects of this disclosure, the flange has a distal end that slides across the first portion of the forming surface with less friction than when the distal end slides across the second portion. The increased friction force of the second portion prevents the flange from bending at a larger radius outside the initial flange radius. The hemming die may define a recess in the second portion of the forming surface and an insert may be received in the recess and attached to the hemming die. The insert may provide the surface that has second coefficient of friction or the area may be provided by coating, machining or otherwise treating the surface to create a surface with a higher coefficient of friction that the first portion of the surface.
The anvil die may include an end wall that is engaged by the hem flange as the hem flange is formed over the inner panel. The hem flange may be driven into the end wall by the hemming tool when the second portion of the forming surface is in engagement with the hem flange.
According to another aspect of this disclosure, a method of hemming a sheet metal assembly is disclosed that provides a sharper radius and reduced creep and roll-in. The method includes loading an inner panel and an outer panel that includes a flange into a hemming tool. The hemming tool includes a hemming die that has a first hemming surface and a second hemming surface that has a higher coefficient of friction than the first hemming surface. The flange engages the first hemming surface to begin forming the flange towards the inner panel. The flange then engages the second hemming surface to continue forming the flange towards the inner panel until the hem flange is formed against the inner panel.
According to other aspects of the method, a finish hem tool may be inserted between the hemming die and the flange after the second hemming surface forms the flange towards the inner panel. The method may also include the step of retracting the hemming die away from the inner panel and the flange after the second hemming surface forms the flange towards the inner panel and before the step of inserting the finish hem tool between the hemming die and the flange.
The above aspects of this disclosure and other aspects will be described in greater detail below with reference to the attached drawings.
A detailed description of the illustrated embodiments of the present invention is provided below. The disclosed embodiments are examples of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale. Some features may be exaggerated or minimized to show details of particular components. The specific structural and functional details disclosed in this application are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art how to practice the invention.
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The flange engaging surface 12 is divided into a first portion 22 and a second portion 24. The second portion 24 comprises the surface of the polyurethane insert 18 in the illustrated embodiment. The first portion 22 has a coefficient of friction that is less than the coefficient of friction of the second portion 24. Instead of providing a polyurethane insert 18, an insert made of another material having a different coefficient of friction than the first portion 22 may be used.
In another alternative, the second portion 24 may be provided with a higher coefficient of friction by machining, grinding, etching or otherwise treating the second portion 24 of the flange engaging surface 12.
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A clamp 40 may be provided to hold the inner panel 34 against the outer panel 32 during a hemming operation. The clamp 40 engages a flange 42 on the inner panel 34 and holds the flange 42 against the outer panel 32.
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While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the disclosed apparatus and method. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the disclosure as claimed. The features of various implementing embodiments may be combined to form further embodiments of the disclosed concepts.
