FIELD OF THE INVENTION

This invention relates to the roll-forming of metal strips or sheets, that is to say processes wherein a flat stock workpiece is passed between one or more stands of forming rolls which bend or curve one or more narrow zones extending from end to end of the workpiece about the longitudinal axis of the, or each, zone to produce the finished product.

In most, if not all, instances it is only those relatively narrow longitudinal zones which are plastically deformed during the roll-forming. For descriptive convenience those zones of the workpiece are referred to as the "fold zones" and the roll-formed bends or curves therein as "folds" hereinafter.

BACKGROUND OF THE INVENTION.

A frequently occurring defect in roll-formed sheet metal articles is that known as "oil-canning", which manifests itself as localised buckles in the substantially flat areas between the folds.

The present invention arose from investigations bringing forth an appreciation of the following facts.

The deformation of a fold zone during roll-forming is usually associated with a stretching of the metal in the fold zone causing extension thereof in the transverse direction which, if the fold zone were free of constraint, would cause a permanent contraction of the fold zone in the longitudinal direction.

The fold zone is not free to contract because of the remainder of the workpiece and the result after roll- forming is the generation of internal longitudinal tensile stresses in the fold zone and internal longitudinal

compressive stresses in the planar undeformed zone, or zones, adjacent to it.

Furthermore, those compressive stresses in the undeformed zone, or, more precisely, the longitudinal contraction in the fold zone, may be of sufficient magnitude to result in the localised buckling known as oil- canning in the undeformed zone in question, particularly if that zone is extensive in the transverse direction.

SUMMARY OF THE INVENTION. An object of the present invention is to eliminate, prevent or at least reduce the severity of oil-canning defects in roll -formed articles.

The invention achieves that object by plastically straining the metal of the, or each, fold zone of the article in a manner causing localised elongation thereof.

Ideally the magnitude of the fold zone elongation due to the plastic straining equates with and cancels the effect of the longitudinal contraction of the fold zone induced by the roll-forming, to leave the roll-formed zone, and the resultant work as a whole, stress-free in the longitudinal direction, and thus devoid of oil-canning deformation. Even if exact equivalence of the treatment induced elongation and roll-forming contraction of the fold zone is not achieved in practice, substantial reduction in the severity of oil-canning defects may readily be obtained.

Thus the invention consists in a method of roll- forming a workpiece to produce at least one longitudinally extending fold therein, characterised by the step of plastically straining a narrow elongated fold zone coinciding with the fold position in a manner causing localised elongation thereof.

The plastic straining of the fold zone may be effected by ironing the zone, that is to say by passing it between two ironing or pressure rolls whereby the thickness of the metal of the fold zone is reduced.

Preferably such ironing of the fold zone is effected by a stand of ironing rolls before the work piece is roll- formed, that is to say the ironing is effectively a pre- treatment of a substantially flat sheet which is subsequently roll-formed. However in other embodiments of the invention the plastic straining is effected simultaneously with the partial folding of the workpiece by one of the stands of the roll-forming rolls. Furthermore, in some instances, if the shape of the roll-formed work permits, its roll-formed fold zones may be post-treated to eliminate the oil canning induced by a previous roll- forming process.

^" When pre-treating a sheet as aforesaid, at least one of the ironing rolls is a narrow roll having an effective axial width equal to the transverse width of the relevant fold zone. That narrow ironing roll is positioned to coincide with that fold zone. Thus the rolls work on the metal in the fold zone only and have no direct effect on the remainder of the workpiece. They plastically strain and reduce the thickness of the metal of the fold zone. This results in its elongation in the direction of rolling.

The narrow ironing roll may be a plain cylindrical roll, in which event its effective width equals its actual width. For such a roll the width of the ironed zone is constant, and altering the pressure between the two ironing rolls alters the degree of plastic strain of the zone without changing its transverse width. Alternatively, the so-called narrow ironing roll may be a relatively wide, crowned roll, with only a central segment of the roll contacting the workpiece. In this event the effective width of the roll equals the width of that segment, and

altering the pressure between the ironing rolls alters both the degree of straining and the width of the zone.

Usually more than one fold is required to be made in a workpiece to produce the finished work and it is convenient to pre-treat all of the corresponding fold zones simultaneously by a single pass of the workpiece through the ironing rolls. In that event it is preferred to have a single wide ironing roll, at least as wide axially as the overall transverse width of the workpiece, co-acting with a plurality of appropriately axially spaced apart narrow ironing rolls. Each so-called narrow roll is then effectively a segment of a wider composite or profiled roll.

BRIEF DESCRIPTION OF THE DRAWINGS. By way of example two embodiments of the invention are described hereinafter with reference to the accompanying drawings.

Figure 1 is a diagrammatic, partly sectioned, side elevation of a bearing end portion of a stand of ironing rolls adapted to pre-treat a workpiece in accordance with the invention.

Figure 2 is a greatly enlarged diagrammatic view of that portion of figure 1 within the enclosure 2 in that figure. The figure is drawn not to scale to enhance its readability".

Figure 3 is a view similar to figure 2 of the operative portion of a pair of roll-forming rolls adapted to partly fold and simultaneously plastically strain a fold zone of a workpiece.

DESCRIPTION OF THE PREFERRED EMBODIMENTS.

The stand of ironing rolls illustrated by figures 1 and 2 may be positioned as the first stand of a multi-stand

roll-forming mill. It comprises a conventional pedestal structure 3 and four rolls , namely a plain wide ironing roll 4, a segmented ironing roll 5 and two back up rolls 6.

The segmented ironing roll 5 comprises a conventional arbor 7, a plurality of plain cylindrical, smaller diameter spacers 8 and a plurality of slightly crowned, larger diameter segments 9 each constituting a narrow ironing roll as that term is used herein. The axial positions of the larger diameter segments 9 coincide with the respective fold zones of a' workpiece 10 being pre-treated, which in turn coincide with the longitudinal fold lines about which the workpiece 10 will be folded by subsequent conventional roll stands of the mill.

The outstanding segments 9 thin, and therefore elongate, the metal of the respective fold zones of the workpiece which they contact as the workpiece travels between the ironing rolls, thereby to pre-treat the workpiece as described above.

The required degree of plastic working of the relevant fold zone or zones is related to the nature of the finished work, in particular, the thickness of the metal and the extent to which it is folded. Usually there is little difficulty in determining the appropriate parameters by a process of trial and error.

It may be that individual fold zones of a single workpiece require different degrees of straining. This may sometimes be effected conveniently by passing the workpiece sequentially" through two or more ironing roll stands. Alternatively, the pressure loading on the individual narrow rolls of a plurality of same in one stand may be separately adjustable.

By way of example, attention is directed to figure 2 in which the curvature of the crown is exaggerated to

enable other dimensions to be seen. The situation illustrated in that figure, wherein the narrow ironing roll has a nominal diameter of 100 mm., the effective width (dimension A) is about 10 mm., the maximum thickness reduction (dimension B) is about 0.03 mm., and the crown radius is about 00 mm., is satisfactory for pre-treating a steel workpiece of 0.55 mm. thickness when the future fold is to be through 90° with a corner radius of 6 mm.. In these circumstances the roll may be turned at say 100 rp . , giving a strip speed of about 30 m/min.

Figure 3 shows a pair of roll-forming rolls 11 and 12 respectively, being the first met stage of a roll-forming mill producing the first stage of what may ultimately be a 90° fold in a workpiece 13. The roll 11 has a protruberant peripheral edge margin 14. which locally thins and elongates the fold zone of the workpiece. The downstream roll- forming rolls may be quite conventional without such a protruberant margin. In the figure the extent to which the margin 1 projects has been exaggerated and the radius of curvature of its profile decreased relative to the scale of the figure as a whole.

It will be appreciated that the presence or otherwise of back up rolls for the ironing rolls and the nature of those rolls, when present, are matters of design choice not fundamental to the invention. Furthermore, if a segmented ironing roll such as roll 5 is used, it is not necessarily a multi-component roll; it may be a one piece profiled roll in which the segments constituting the individual narrow ironing rolls are integral with each other. Likewise the edge margin of a combined forming and zone straining roll, such as margin 14 _. may be a separate component from the main body of the forming roll with which it is associated.