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
