DETERMINING STRIP
The first step in the actual production of stampings is to order standard-size sheets of the proper size and thickness from the mill. These are then sheared into strips, as described above. The widths of the strips into which the sheets are to be cut is specified by the die design department. Therefore, let us go over the steps taken in determining strip width so that your understanding will be complete.
Figure 3.5 Typical part drawing.
Figure 3.5 shows a part drawing of a typical representative stamping to be produced in a pierce and blank die.
3.4.1 Blank Layout
Figure 3.6 shows the two possible ways of running the strip of a typical representative stamping (Figure 3.5) through the die. The blanks may be positioned the wide way, necessitating a wide strip, or they may be run the narrow way, permitting the use of a narrower strip. These are called “blank layouts” and it is important that you understand exactly what is meant by the term. A blank layout shows the way in which the designer proposes to produce the blank. For both the wide-run and narrow-run layouts, two holes are to be pierced at the first station and the part is to be blanked out at the second station. It is customary to show small piercing punches in solid black. Section lines are applied through larger piercing punches and through blanking punches, as shown. The strip width and the feed are given directly on the blank layout.
Figure 3.6 Blank layouts necessitating either wide or narrow strips.
Now let us go through the first steps taken in the production of blanks in cut-off dies. Two of the sides of such blanks are originally sides of the material strip, and no scrap bridge is produced as in blanking dies. Figure 3.7 shows a representative stamping having the parallel sides typical of blanks suitable for production in cut-off dies.
Figure 3.8 shows two blank layouts for producing the stamping (Figure 3.7) in a cut-off die. At view A, the part is positioned the wide way in the strip. The edges of the strip are notched at the first station and a rectangular hole is punched. The blank is cut off from the strip at the second station. At view B, the part is positioned the narrow way in the strip. Observe how notching punches are sectioned. The heels C, which prevent deflection of the punches, are shown, but not sectioned. At D, short 45 degree lines and a long vertical line represent the “cut off” line.
Figure 3.7 Typical part for production in cut-off die.
Figure 3.8 Blank layout for part shown in Figure 3.7 run, either the wide (A) or narrow (B) way.
Blank layouts are drawn to explain the proposed operation of a die to others. When die designers are given a part print of a stamping for which a die is to be designed, they proceed to lay out a suitable scrap strip. Then they section significant punches and add cut-off lines to make the proposal layout clearer. This is the blank layout, and it must be approved by the group leader or chief engineer before design of the die is begun. When an outside engineering office is doing the work for a manufacturing company, the blank layout is submitted to the customer for approval.
3.4.2 Stripper Sheet
Sizes of sheets as they are manufactured by the mill are given in steel catalogs. Here is a representative list for #18 Gage (0.0478 in. or 1.2 mm) cold-rolled steel:
30 in. × 96 in. (762 mm × 2438 mm)
30 in. × 120 in. (762 mm × 3048 mm)
36 in. × 96 in. (914 mm × 2438 mm)
36 in. × 120 in. (914 mm × 3048 mm)
48 in. × 96 in. (1219 mm × 2438 mm)
48 in. × 120 in. (1219 mm × 3048 mm).
Figure 3.9 Number of strips obtainable with wide strip blank layout.
Figure 3.10 Strips per sheet with wide strip blank layout for production in a cut-off die.
The next step is to select the sheet that will be most economical, that is, the sheet from which a maximum number of strips can be cut, leaving a minimum amount of waste.
a) Wide Run
Strip width is taken from the blank layout. Divide the value given into the values for “width of sheet” in the steel catalog, and compare to determine which sheet leaves the smallest remainder. Figure 3.9 shows a sheet 48 by 120 inches (1219 mm × 3048 mm) divided into strips when the typical representative blank is run the wide way.
Figure 3.10 shows the sheet divided into strips for producing parts in a cut-off die when the blank is run the wide way.
Figure 3.11 Number of strips obtainable with narrow strip blank layout.
Figure 3.12 Strips per sheet with narrow strip blank layout for production in a cut-off die.
b) Narrow Run
Next, we must know how many blanks are produced per sheet with the blanks positioned the narrow way in the strip. With blanks arranged the narrow way, more strips are cut from the sheet, but fewer blanks are contained in each strip.
Figure 3.11 shows the same sheet divided into strips when the typical representative blanks are to be run the narrow way. More strips are produced from the same size of sheet.
Figure 3.12 shows the sheet divided into strips for producing a part in a cut-off die when the blank is run the narrow way.
3.4.3 Strip Layout
After it has been decided how the blanks are to be run (wide or narrow way), a stock layout is prepared complete with the following dimensions:
•Strip width. This dimension is used in selecting the proper width of sheet from which strips are to be cut.
•Feed.
