Metal deforming – With cleaning – descaling – or lubrication of work or product – Mechanical cleaning
Reexamination Certificate
2000-02-24
2001-03-27
Crane, Daniel C. (Department: 3725)
Metal deforming
With cleaning, descaling, or lubrication of work or product
Mechanical cleaning
C072S161000, C072S302000
Reexamination Certificate
active
06205830
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to flat rolled metal and sheet metal processing. More particularly, the present invention relates to a method and apparatus for leveling and conditioning sheet metal using a stretcher-leveling machine in combination with a surface conditioning system.
BACKGROUND OF THE INVENTION
A wide variety of manufactured goods contain processed sheet metal. For example, aircraft, automobiles, file cabinets and household appliances, to name only a few, contain sheet metal. The sheet metal is typically purchased directly from steel mills and/or steel service centers, but may be passed through intermediate processors (sometimes referred to as “toll” processors) before it is received by an original equipment manufacturer.
Various methods exist for flattening sheet metal and for conditioning the surfaces thereof. Flatness of sheet metal is important because virtually all stamping and blanking operations require a flat sheet. Also, in certain applications, such as in the aerospace industry, residual stress free material is critical. Good surface conditions are also important, especially in applications where the top and/or bottom surfaces of the metal sheet will be painted.
There are a number of common defects that effect sheet metal flatness. For example, when sheet metal is rolled into coil form for convenient storage and transportation, the strip takes on a coiled shape. This curvature is commonly referred to as “coil set.” Coil set occurs because the sheet metal has been bent past its yield point. More specifically, when sheet metal is coiled, the metal fibers near the inside surface of the curved sheet are compressed past their yield point, and the metal fibers near the outside surface of the curved sheet are stretched past their yield point. Another type of shape defect known as “edge wave” occurs if the edge portions of the sheet are longer than the center portion of the sheet, resulting in undulations in one or both of the edge portions of the sheet. A similar type of shape defect known as “center buckle” results if the center portion of the sheet is longer than one or both of the edge portions, which results in bulging or undulating of the central portion of the sheet.
One method of removing coil set in sheet metal is “straightening.” A conventional straightener is shown schematically in FIG.
1
. In a straightener process, a strip of sheet metal S is advanced through a series of large diameter upper rollers U and lower rollers L, which are positioned relative to one another to put deep upward and downward bends in the sheet sufficient to reverse the coil set. However, straightening can only remove coil set and some cross bow. It is a rather crude and imprecise method that is typically used only as a first pass.
Another conventional method of flattening sheet metal is “roller leveling.” A conventional roller leveler, shown schematically in
FIG. 2
, comprises a top set of small diameter rollers T and a bottom set of small diameter rollers B mounted in a frame (not shown) so that top and bottom sets of rollers are offset from one another. A series of larger diameter “back-up” rollers R engage the small diameter rollers T and B and can be adjusted as needed to flatten the material moving through the top and bottom rollers T and B. A strip of sheet metal S is advanced between the top and bottom sets of small diameter rollers T and B and is alternately flexed upwardly and downwardly between the top and bottom rollers such that the amount of flexing decreases as the sheet travels toward the exit end E of the roller leveler. The small diameter rollers T and B work the sheet S by bending the metal fibers near the inside surface of the curve and the metal fibers near the outside of the curve past their yield point (i.e., beyond their elastic limit). A roller leveler produces a reasonably flat metal sheet, but is extremely difficult to operate and requires a highly skilled operator. Moreover, the roller leveling process itself is less than ideal because there still exists a neutral axis in the sheet metal where the yield point of the metal has not been exceeded by the small diameter rollers. Metal fibers lying at or near this neutral axis may be in a stressed condition (and tend to spring back toward their original shape) because they have not been deformed past their elastic limit. Therefore, even after roller leveling, the material at or near the neutral axis will possess internal residual stresses because the grain structure is not uniform. Also, roller leveling alone does nothing to remove scale and corrosion from the surface of the sheet metal.
Another method of flattening sheet metal is “temper passing.” A conventional “2-high” temper mill cut-to-length line is shown schematically in
FIG. 3
, along with a roller leveler L and a shearing machine M. The “2-high” temper mill comprises two large diameter rollers D that significantly compress the metal fibers at the top and bottom of the metal sheet S into uniformity. This results in a substantial reduction of internal residual stresses at the top and bottom surfaces of the metal sheet, but typically does not work the fibers near the neutral axis of the metal sheet past their yield point. Therefore, even after a 2-high temper passing process, the material at or near the neutral axis may still possess internal residual stresses and the material may not be sufficiently flat. A 2-high temper passing process does little to remove scale and corrosion from the surface of the sheet metal. In fact, because of the substantial compressive forces applied to the top and bottom surfaces of the sheet metal by the temper passing rolls, surface scale tends to become embedded in the metal surface, which can increase the likelihood of point source corrosion and consequent rusting.
“Temper passing” can also be accomplished with “4-high” temper mill (not shown), which comprises two upper rolls (an upper sheet engaging roll and a back up roll therefor) and two lower rolls (a lower sheet engaging roll and a back up roll therefor) all generally aligned in a vertical plane. The two sheet engaging rolls are much smaller in diameter than the rollers D of a “2-high” temper mill. As such, the two sheet engaging rolls of the “4-high” temper mill apply a more concentrated force at the point of contact. Like the “2-high” temper mill, the “4-high” temper mill significantly compress the metal fibers at the top and bottom of the metal sheet into uniformity, resulting in a substantial reduction of internal residual stresses at the top and bottom surfaces of the metal sheet. However, like the “2-high” temper mill, the “4-high” temper mill also fails to work the fibers near the neutral axis of the metal sheet past their yield point. “4-high” temper mills tend to do a better job of removing scale and corrosion than “2-high” temper mills.
In some applications, a certain amount of crown (i.e., thicker gauge in the center than at the ends) may be desired. A problem with both “2-high” and “4-high” temper mills is crown reduction (known as “feathering”) in which the crown in the metal sheet is compressed out by the temper mill's rollers.
Another method of flattening sheet metal is “stretcher leveling.” A conventional C-frame stretcher leveler is shown schematically in FIG.
4
. Stretcher leveling is generally considered to be a superior flattening process because, unlike roller leveling and temper processing, it rectifies the problem of internal residual stresses and produces a flatter product without crown reduction. As shown in
FIG. 4
, a typical C-frame stretcher leveler includes a pair of generally C-shaped grippers or jaws G that securely grip the opposing ends of the sheet S to be stretched. The surface portions of the grippers that engage or grip the sheet metal to hold the sheet against movement during stretching are typically grooved, knurled or serrated to provide a secure grip. In operation, the grippers G are hydraulically or pneumatically controlled to engage the opposed ends of the sheet S and, once a firm conta
Crane Daniel C.
Howell & Haferkamp LC
The Material Works, Ltd.
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