Die Design Fundamentals. Vukota Boljanovic
either the wide or narrow run. View A shows one of the strips ready to be fed through the die with blanks to be removed from it positioned the wide way. At view B, the blanks are positioned the narrow way in the strip. Five of the parts have been blanked out of each strip. After the strip is run completely through the die, only a narrow scrap bridge is left.
Figure 3.16 shows a strip ready for feeding either the wide or narrow run in a cut-off die. View A shows one of the strips ready to be fed through a die, with blanks to be removed from it positioned the wide way. At view B, the blanks are positioned the narrow way. Five blanks are shown in each strip. Because they are run in cutoff dies, no scrap bridge is produced.
3.5 METHODS FOR PRODUCING STRIPS
3.5.1 Shearing
The oldest and simplest method of producing metal strips is by shearing. In the steel mill, metal is formed into large sheets by rolling and trimming. A sheet that is to be cut into strips is introduced under the blade of a shear. Gages register the edges of the sheet for cutting correct widths of strips. Descent of the shear blade causes each strip to be parted from the sheet. Advancing the sheet against the gages brings it into position for cutting the next strip and this process is repeated until the sheet has been cut entirely into strips. Figure 3.17 at A shows a sheet in position under the shear blade C ready to be cut. At B, the blade has descended and the strip has been cut from the sheet.
Figure 3.17 Producing metal strips from sheet by shearing.
The power shear can cut material in any direction—lengthwise of the sheet, across the sheet, or at any angle.
3.5.2 Slitting
Slitting machines (Figure 3.18) are also used for producing material strips. In slitting operations, the sheet is fed though rotating cutting rolls, and all strips are cut simultaneously. In the illustration, cutting rolls A are mounted the proper distances apart on arbors B. The cutting edges of the rolls are separated by the required amount of clearance to effect cutting of the material as shown in inset C. Turning the rolls under power causes the sheet to advance, and it is cut into strips. As many as 20 or more strands can be cut at one time. In other types of slitters, the sheets are pulled through the rolls instead, and the rolls are free to turn.
Figure 3.18 Cutting rolls for slitting strips from sheet.
Figure 3.19 The various edge contours shown are the result of different production processes.
Slit strips are very accurate in width, flatness, and parallelism of sides because accuracy is built into the machine instead of depending upon the operator. Unlike the shear, which can cut strips only as long as the blade, the slitter will cut continuously to any length, without limit.
3.5.3 Edge Contour (Contour of the Edge of a Strip)
The contour of the edge of a strip (Figure 3.19) depends upon the process by which the strip is produced. Five contours may be recognized:
1.Strips produced in a shear have the burnished bands along the edges on opposite sides of the strip. If burrs are produced because of dull cutting edges, they will also occur on opposite sides of the strip. In addition, sheared strips often become spiraled or curved because the upper blade of the shear is at an angle to the lower blade. This makes the strips difficult to feed through the die unless they are first straightened.
2.Strips produced in the slitter have the burnished bands on the same side of the strip. Blanks produced from these strips in cut-off dies have a better appearance and they are fed more easily because they are straighter. Sheared strips or slit strips may be produced in the shear department of the plant, or they may be ordered directly from the mill.
3.Mill-edge strips have a radius at each corner. They are produced by rolling sheared or slit strips at the mill. Mill-edge strips are used for long stampings, such as for handles, shelf brackets, and other parts where sharp edges would be objectionable.
4.Rolled-edge strips have a full radius at each side, rolled at the mill. They are used for parts where appearance is a deciding factor, such as in ornamental grills, gratings, and the like.
5.Square-edge strips are ordered from the mill when the sides of the strips must be square and smooth. The widths of these strips are held very accurately. Square-edge strips are also specified when blanks are to be bent or formed edgewise. The square edges prevent cracking or splitting in the bending or forming operation.
THE BLANK
4.1 Definition and Types of Blanks
4.3 Blanking Force
4.1 DEFINITION AND TYPES OF BLANKS
A blank is a piece of flat steel or other material cut to any outside contour. The thickness of a blank may range between 0.001 and 0.500 inch (0.025 mm and 12.7 mm) or more depending on its function. However, most stampings are between 0.025 inch and 0.125 inch (0.6 and 3.2 mm) in thickness.
Some blanks have simple round, square, or rectangular contours. Others may be very irregular in shape. Many blanks are subsequently bent, formed, or drawn. It is important to realize, however, that when we refer to a blank, what is meant is the flat part before any deformation has been applied.
There are only two basic types of blanks (see Figure 4.1):
1.Blanks having straight, parallel sides, two of which are originally sides of the material strip (see view 1). Small blanks of this type are produced in cut-off dies. Large blanks are produced by square-shearing and trimming.
Figure 4.1 Two basic types of blanks.
2.Blanks having irregular contours cut entirely out of the material strip (see view 2). When they are required in quantity, such blanks are produced in blanking dies. When only a few blanks are required, they may be shaped by contour sawing, nibbling, routing, or other machining operations.
To select the best method of producing a particular blank, consider five factors:
1. Contour
If the blank has two parallel sides, determine if it can be produced in a cut-off operation. The width between the parallel sides would then become the width of the strip. Four advantages are realized when cut-off dies are used:
•There is a minimum waste of material.
•Cut-off dies cost less to build.
•Faster press speeds are possible.
•There