Die Design Fundamentals. Vukota Boljanovic
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