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 is no scrap strip to handle.
After you have determined that a blank can be produced in a cut-off operation, consider three additional factors before making a final decision:
Accuracy in strip width. Sheared strips cannot be held to closer accuracy than ±0.010 inch (0.25 mm). If the width dimension between parallel sides of the blank must be held to closer limits, discard the idea of using a cut-off die.
Accuracy of the blank. If the blank must be held to close limits, it should be produced in a blanking die, regardless of the number of straight sides that it may have.
Flatness. If the part print contains the note “MUST BE FLAT,” you should plan to design a blanking die because it will produce considerably flatter parts. Cut-off dies produce blanks by a series of piercing, trimming, and cut-off operations. Uncut portions can become distorted, especially for heavier gages of strip. In blanking dies, the entire periphery is cut in one operation and distortion cannot occur.
Blanking dies produce flat, accurate parts. Whatever accuracy has been built into the die is duplicated in the blanks; each is identical to every other blank that the die produces. This is true because the entire blank contour is cut and none of the edges of the strip form any edge of the blank. Blanking is the most widely used method of producing blanks from sheet materials.
If the stamping is intricate and is to be produced complete in a progressive die, the contour of the blank may be formed by trimming away portions of the strip at one or more of the stations.
2. Size
Consider the size of the blank in relation to the number of parts required. This is especially important for large blanks because large dies are very costly to build. Determine if shearing and trimming would do the job, especially if production requirements are low.
3. Accuracy
Study the part print carefully to determine the degree of accuracy required in the blank. Very accurate blanks have to be produced in compound dies in which all operations are performed simultaneously at one station. Blanks requiring a lesser degree of accuracy may be produced in more economical two-station dies.
4. Number required
This information is taken from the design order and it often determines the type of die to be designed, as well as the class of die.
5. Burr side
The burr side must be known when blanks are to be shaved or burnished in a subsequent operation. The same applies for blanks that are to be assembled into other components and those that are to have components assembled into them. The presence of burrs at engaging edges can slow down assembly operations considerably.
4.2.1 Methods of Producing Blanks
Let us now gain an understanding of the various methods of producing blanks. We will begin by considering ways in which blanks may be shaped without the use of dies. These low-cost, but relatively slow methods are employed when only a few blanks are required and it would be uneconomical to design and build special dies for producing them.
a) Circle Shearing
Large, round blanks may be circle-sheared when quantities required are moderate. Square blanks are clamped in the center of a circle shearing machine. Two disk-shaped cutters are adjusted to the required radius. They apply rotation to the blank and, at the same time, cut it to a circular shape.
For larger quantities, it is less expensive to order round blanks precut to the required diameter. Steel companies stock various sizes of round blanks, or they can supply them cut to special sizes.
b) Contour Sawing
When only a few blanks are required, their contours may be laid out directly on sheet material. After lines have been scribed,
