This invention relates to metal deformation and particularly but not exclusively to a method of forming a cylindrical, internally threaded opening in an end wall of a metal, preferably mild steel, container drum.
It is known to provide cylindrical openings in the end walls of container drums by fitting a pre-fabricated insert into the end wall. This is disadvantageous because of the additional costs involved in pre-fabricating and fitting the insert.
It is also known from US-A-2493933 to provide a method of forming a cylindrical opening in a metal sheet, comprising piercing, pressing and drawing the metal sheet to provide a raised cylindrical section having an axis and partly closed at one end by an annular flange, and bending the flange into the cylindrical section while rolling the flange so as to permit the requisite enlargement thereof. Such subject matter is also known from U.S.-A-2460721.
According to the present invention there is provided a method of forming a cylindrical opening in a metal sheet, comprising piercing, pressing and drawing the metal sheet to provide a raised cylindrical section having an axis and partly closed at one end by an annular flange, and bending the flange into the cylindrical section while rolling the flange so as to permit the requisite enlargement thereof, characterised in that said bending and enlargement of the flange is achieved by orbiting a tool in engagement with said flange about said cylindrical section axis and rolling the tool along the flange about a second axis offset from said cylindrical section axis.
The invention will now be further described by way of example only with reference to the accompanying drawings in which:-
Referring now to the drawings, Fig. 1 shows part of a cylindrical container drum made of mild steel having an end wall provided with a closure opening further illustrated in Fig. 2 as comprising a raised cylindrical section 1 of double thickness with a screw thread 2 on the cylindrical inner surface of the inner metal skin. The outer end of the cylindrical section 1 is thickened to provide a sealing surface 3 which is engaged by a sealing ring 4 of a conventional externally threaded bung 5 having an internal handle 6.
The closure opening illustrated in Fig. 2 is made by first piercing, pressing and drawing the metal sheet constituted by the drum end wall (prior to fixing to the drum end in conventional manner) to form a cylindrical section 7 raised out of the plane of the end wall by a step 8 and terminating at its outer end in an annular flange 9 which partly closes the cylindrical section leaving a circular opening 10 around a central axis 11 (Fig. 3).
The raised cylindrical section 7 is then surrounded by clamping supports 12 and a tool 13 is progressively introduced into the opening 10 so as to bend the flange 9 inwardly. Such deformation of the flange 9 requires a corresponding enlargement of the opening 10 which cannot be achieved without extension of the flange 9 along its circumferential dimension. Such extension is achieved by orbiting the tool 13 about the axis 11 while simultaneously rolling the frusto-conical tool surface on the flange 9 to achieve the necessary thinning of the metal in this region. When the flange 9 has been bent inwardly to an extent conforming to the shape of the tool 13, the tool 13 is replaced by a tool 14 which has a cylindrical working surface which, using the same rolling action as the tool 13, completes the inward deformation of the flange 9 and the attendant enlargement of the hole 10. The tool 14 also creates the shoulder 3 by pressing metal into a circular recess 15 in the clamping segments 12.
The cylindrical section 1 is then completed by using a thread forming tool 16 which has an orbiting and rolling action like tool 14 but in this case is provided with a thread formation on its outer cylindrical periphery, the action of the tool 16 serving to impress said thread formation onto the internal surface of the cylindrical section.
When internal thread forming, it is necessary to adjust for pitch error originating from the difference in peripheral length between the outer tube diameter being formed and the thread rolling tool diameter.
This difference, when the tool rolls on the component without slipping, causes a relative angular positional displacement of the two thread forms (forming and generated).
Compensation for this difference may be accomplished by axial displacement of the tool amounting to the pitch of the thread times the ratio of the two diameters.
As the depth of thread form increases during the rolling process this ratio varies with the instantaneous root diameter of the generated thread form.
The axial displacement of the forming tool may be achieved by allowing the tool to float freely or by providing an incremental movement of the tool spindle relative to tool rotation.
Alternatively, the accumulative effects may be avoided by alternating the direction of rotation of the forming tool after each full revolution of the tool spindle.