This is the amount of travel of the strip between stations. This dimension is used in selecting the proper length of sheet.
Figure 3.13 Complete strip layouts for blanks run either the wide (A) or narrow (B) way.
Figure 3.13 shows complete strip layouts for the typical representative blanks run either the wide (view A) or narrow (view B) way. Two views are applied, ordinarily. These are exactly the views of the strip that will be drawn on the die drawing except that an end view of the strip is added to the die drawing. The die is then actually designed around these views.
View A illustrates a strip layout for a blanking die in which the blank is run the wide way. View B shows a layout in which the blank is run the narrow way. For this particular job, more blanks per sheet are produced when the blanks are positioned the wide way and there is less waste. Therefore, all else being equal, this method of positioning the blanks would be selected.
Figure 3.14 Strip layouts for blanks run either the wide (A) or narrow (B) way.
Figure 3.14 shows strip layout for production of parts in cut-off die. The strip layout is prepared and copies are sent to the purchasing department and to the shear department. From the layout, sheets are ordered and, upon delivery, they are sheared to the strip width given on the layout. View A shows a representative strip layout for a blank for a cut-off die positioned the wide way, and view B shows a layout for a blank positioned the narrow way.
For this job we find that exactly the same number of blanks are produced with blanks positioned the narrow way as for wide-run positioning; there is no waste in either method. When blanks can be run either way, select the wide run method for three reasons:
•Fewer cuts will be necessary for producing the strips.
•The feed is shorter when running strips through the die, thus reducing the time required.
•More blanks are produced per strip and fewer strips have to be handled.
Figure 3.15 Strips ready for feeding either the wide (A) or narrow (B) way.
Figure 3.16 Strips ready for feeding either the wide (A) or narrow (B) way in a cut-off die.
3.4.4 Strips
Figure 3.15 shows a strip ready for feeding 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.
