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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, the blanks are sawed out in a metal-cutting bandsaw. For contour sawing a number of blanks requiring greater accuracy, a short stack of square or rectangular blanks are clamped in a vise, then tack-welded together at several places around the edges. The outline of the blank is laid out on the upper sheet and the blanks are sawed directly to this outline. Thus, all the blanks are identical in contour.
c) Nibbling
The nibbling machine operates by reciprocating a punch up and down at about five strokes per second. The punch is provided with a pilot long enough so it is not raised above the material being cut. As the sheet is moved, the punch cuts a series of partial holes that overlap each other. A jagged edge is left around the edges of the blank and the sharp corners left by the punch must be die-filed after the nibbling operation. The nibbling process is used to produce blanks when only one, two, or a few are required.
d) Routing
A routing machine is provided with a long radial arm that can travel over a large area. Mounted at the outer end of the arm is a rapidly revolving cutting tool, similar to an end-mill cutter, that can cut its way through a stack of blanks. The router bit, as the cutting tool is called, rotates at about 15,000 revolutions per minute and is guided by a template to produce blanks identical to the template. Routing large aluminum blanks is common practice in the aircraft and missile industries.
e) Flame Cutting
Flame cutting or torch cutting means the cutting of thick blanks by the use of an acetylene torch. In operation, the torch heats the metal under its flame tip until it melts. Compressed air then blows the molten metal out, forming a narrow channel called the “kerf.” The width of the kerf ranges from 5/64 inch to 1/8 inch (2 mm to 3.2 mm) depending upon stock thickness and the speed of the torch. For producing thick blanks in quantity, a template guides the torch by means of a pantograph. Flame cutting is employed for cutting blanks ranging from 1/4 to 1 inch (6.35 to 25.4 mm) or more in thickness.
Holes in blanks can also be torch cut by the same method. Flame cutting leaves the edges somewhat rough and ridged. However, such edges are satisfactory for some parts for trucks, tanks, ships, and other similar applications.
f) Water-Jet Cutting
Water-jet cutting is a process used to cut materials using a jet of pressurized water. There are two main steps involved in the water-jet cutting process. First, the ultra-high pressure pump or intensifier pressurizes water to pressure levels above 60,000 PSI (400 MPa) to produce the energy required for cutting. Second, water is then focused through a small, precious stone orifice to form an intense cutting stream. The nozzle diameters used to achieve these pressures range from 0.002 inch (0.05 mm) to 0.04 inch (1 mm). The stream moves at a velocity of up to 2.5 times the speed of sound, depending on how the water pressure is exerted.
As in flame cutting, kerf is an important term in water-jet machining. The kerf is the width of the actual water-jet cutting beam. Depending on the nozzle, the kerf width for an abrasive jet ranges from 0.020 inch to 0.060 inch (0.5 mm to 1.5 mm). Plain water jets with no abrasives have a narrow kerf ranging from 0.005 inch to 0.014 inch (0.13 mm to 0.35 mm).
A water jet can cut both hard and soft materials. Soft materials are cut with water only, whereas hard materials require a stream of water mixed with fine grains of abrasive garnet. This method is used in cutting processes of materials including titanium, stainless steel, aluminum, exotic alloys, composites, stone, marble, floor tile, glass, automotive door panels, gaskets, foam, rubber, insulation, textiles, and many others.
Cutting speed is determined by several variable factors, including the edge quality desired. Variables such as amount of abrasive used, cutting pressure, size of orifice and focus tube, and pump horsepower can be adjusted to produce the desired results, whether your priority is speed or the finest cut.
Speed and accuracy also depend on material texture, material thickness, and the cut quality desired. In case of rubber and gasket cutting, water-jet motion capabilities would allow traversing at 0.1 to 200 inch/min (0.0025 to 5 m/min).
g) LASER Cutting
The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation. How does laser cutting work? Laser cutting can be compared to cutting with a computer-controlled miniature torch. Industrial laser cutting is designed to concentrate high amounts of energy into a small, well-defined spot. Typically the laser cutting beam is approximately 0.003 to 0.006 inch (0.07 to 0.15 mm) in diameter in short wavelength lasers. The distance between the nozzle and the material is approximately 0.2 inch (5 mm). The material thickness at which cutting or processing is economical is up to 0.4 inch (10 mm). The resulting heat energy created by the laser melts or vaporizes materials in this small-defined area, and a gas (or a mixture of gases), such as oxygen, CO2, nitrogen, or helium, is used to blow the vaporized material out of the kerf. The beam’s energy is applied directly where it is needed, minimizing the heat’s effect on the zone surrounding the area being cut.
There is almost no limit to the cutting path of a laser. The point can move in any direction. Small diameter holes that cannot be made with other machining processes can be done easily and quickly with a laser. The process is forceless. The part keeps its original shape from start to finish.
This method is ideal when production quantities or prototypes do not justify producing tooling for stamping or die cutting.
Lasers can cut at very high speeds. The speed at which materials can be processed is limited only by the power available from the laser. Laser cutting is a very cost-effective process with low operating and maintenance costs and maximum flexibility.
4.2.2 Square Shearing
Large, straight-sided blanks are produced in the shear by cutting sheets into strips, then cutting the strips to required lengths or widths. Blanks larger than 8 by 10 inches (203 by 254 mm) and composed primarily of straight sides are ordinarily produced by shearing because of the high cost of large dies.
Blanks cut in a modern shear can be held to an accuracy of 0.005 inch (0.125 mm). Four factors govern shearing accuracy:
•The shear must have sufficient rigidity to withstand the cutting load without deflection or spring.
•Knife clearance must be set correctly and proper rake selected to reduce twist, camber, or bow. Rake is the angle of the upper knife in relation to the horizontal lower knife of the shear. Twist is spiraling of the strip; it is more severe in soft, narrow, or thick strips than it is in hard, wide, or thin strips. Camber is curvature along the edge in the plane of the strip whereas bow is curvature perpendicular to the surface of the strip.
•Good gaging practice must be followed.
•The sheet must be held down securely while shearing occurs.
For producing square and rectangular blanks shown in the upper illustration of Figure 4.2, the sheet is first cut into strips to length A of the blanks. The strips are then run through the shear again and cut into blanks having width B. Here is the method of listing operations on the route sheet:
Figure 4.2 Rectangular blanks sheared from wide and narrow strips.
•Operation No. 1. Shear to length (A)
•Operation No. 2. Shear to width (B)
When the grain of the material must run lengthwise of the blank for extra stiffness, the sheet is cut into strips to width B of the blanks (Figure 4.2, lower illustration). The strips are then run through the shear again and cut into blanks having length A. On the route sheet, operations are listed as follows:
•Operation No. 1. Shear to width (B)
•Operation No. 2. Shear to length (A)
4.2.3 Triangular Blanks
Triangular blanks (Figure 4.3) are produced by shearing square or rectangular blanks, then splitting them to produce two blanks. In the upper illustration:
•Operation No. 1. Shear to length (A)
•Operation No. 2. Shear to double width (B)
•Operation No. 3. Split (C)
Although operation No. 2 states “Shear to double width,” this does not mean that the strip is to be cut twice the width of a single blank. It simply means that the square or rectangular blanks are to be made wide enough so splitting will produce two full blanks.
Figure 4.3 Triangular blanks made by shearing rectangular blanks sheared from wide and narrow strips.
For running strips the narrow way (Figure 4.3, lower illustration), operations are listed as follows:
•Operation No. 1. Shear to width (B)
•Operation No. 2. Shear to double length (A)
•Operation No. 3. Split (C)
4.2.4 Angular Edge Blanks
Wider blanks having one angular edge (Figure 4.4) can be produced by the same method employed for triangular blanks. For the upper illustration:
•Operation No. 1. Shear to length (A)
•Operation No. 2. Shear to double width (B)
•Operation No. 3. Split (C)
The function of some blanks renders it necessary to run them the narrow way (Figure 4.4, lower illustration). Operations are then as follows:
•Operation No. 1. Shear to width (B)
•Operation No. 2. Shear to double length (A)
•Operation No. 3. Split (C)
Figure 4.4 Angular edge blanks made by shearing rectangular blanks sheared from wide and narrow strips.
Added cuts
One or more extra cuts may be required to complete the blanks (Figure 4.5). Here is the order of operations for the blank in the upper inset:
•Operation No. 1. Shear to length (A)
•Operation No. 2. Shear to double width (B)
•Operation No. 3. Split (C)
•Operation No. 4. Trim (D)
In the lower illustration:
•Operation No. 1. Shear to width (B)
•Operation No. 2. Shear to double length (A)
•Operation No. 3. Split (C)
•Operation No. 4. Trim (D)
4.2.5 Parallelogram Blanks
Blanks in the shape of an angular parallelogram (Figure 4.6) are produced by shearing with the strip positioned at an angle to the shear blade. For the wide strips, upper illustration:
Figure 4.5 Blanks produced by added cutting of angular edge pieces.
Figure 4.6 Parallelogram-shaped blanks sheared from wide and narrow strips.
•Operation No. 1. Shear to length (A)
•Operation No. 2. Shear to width (B)
For narrow strips, lower illustration:
•Operation No. l. Shear to width (B)
•Operation No. 2. Shear to length (A)
4.2.6 Triangular Blanks
Triangular blanks with an acute angle at each of the three apexes (see Figure 4.7) are produced by splitting a parallelogram.
For wide strips (Figure 4.7, upper illustration):
•Operation No. 1. Shear to length (A)
•Operation No. 2. Shear to double width (B)
•Operation No. 3. Split (C)
For narrow strips (Figure 4.7, lower illustration):
•Operation No. 1. Shear to width (B)
•Operation No. 2. Shear to double length (A)
•Operation No. 3. Split (C)
Figure 4.7 Acute-angle triangular blanks made by shearing parallelogram-shaped blanks sheared from wide and narrow strips.
Figure 4.8 Trapezoidal blanks are sheared in a manner similar to acute angle triangular blanks.
4.2.7 Trapezoidal Blanks
Large trapezoidal blanks (Figure 4.8) are produced by employing the same method as triangular blanks.
For wide strips (Figure 4.8, upper illustration):
•Operation No. 1. Shear to length (A)
•Operation No. 2. Shear to double width (B)
•Operation No. 3. Split (C)
For narrow strips (Figure. 4.8, lower illustration):
•Operation No. 1. Shear to width (B)
•Operation No. 2. Shear to double length (A)
•Operation No. 3. Split (C)
Added cuts
