barcode scanner integration in asp.net MACHINING AND ROUTING in Software

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MACHINING AND ROUTING
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35.1 INTRODUCTION
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Laminate machining consists of the mechanical processes by which circuit boards are prepared for the vital chemical processes of image transfer, plating, and etching. Such processes as cutting to size, drilling holes, and shaping have major effects on the final quality of the printed board. This chapter will discuss the basic mechanical processes that are essential to producing the finished board.
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35.2 PUNCHING HOLES (PIERCING)
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35.2.1 Design of the Die It is possible to pierce holes down to one-half the thickness of XXXPC and FR-2 laminates and one-third that of FR-3 (Fig. 35.1). Many die designers lose sight of the fact that the force required to withdraw piercing punches is of the same magnitude as that required to push the punches through the material. For that reason, the question of how much stripperspring pressure to design into a die is answered by most toolmakers: as much as possible. When space on the dies cannot accommodate enough mechanical springs to do the job, a hydraulic mechanism can be used. Springs should be so located that the part is stripped evenly. If the board is ejected from the die unevenly, cracks around holes are almost certain to occur. Best-quality holes are produced when the stripper compresses the board an instant before the perforators start to penetrate. If the stripper pressure can be made to approach the compressive strength of the material, less force will be required and the holes will be cleaner. If excessive breakage of small punches occurs, determine whether the punch breaks on the perforating stroke or on withdrawal. If the retainer lock is breaking, the cause is almost certain to be withdrawal strain. The remedy is to grind a small taper on the punch, no more than 11.2 in. and to a distance no greater than the thickness of the material being punched. If the grinding is kept within those limits, it will have no measurable effect on hole quality or size. The other two causes of punch breakage are poor alignment, which is easily detected by close examination of the tool, and poor design, which usually means that the punch is too small to do the job required.
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Shrinkage of Paper-Based Laminates
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When paper-based laminates are to be punched, it must be remembered that the materials are resilient and that their tendency to spring back will result in a hole slightly smaller than the punch that produced the hole. The difference in size will depend on the thickness of the material. Table 35.1 shows the amount by which the punch should exceed the print size in order to make the holes within tolerance. The values listed should not be used for the design of tools for glass-epoxy laminates, the shrinkage of which is only about one-third that of paper-based materials.
FIGURE 35.1 Illustration of the proper sizing and locating of pierced holes with respect to one another and to the edge of paper laminates. Minimum dimensions are given as multiples of laminate thickness t.
TABLE 35.1 Shrinkage in Punched Hole Diameters, Paper-Base Laminates Material at thickness 1.64 1.32 3.64 1.16 3.32 1.80 Material at room temp. 0.001 0.002 0.003 0.004 0.006 0.010 Material at 90 F or above 0.002 0.003 0.005 0.007 0.010 0.013
Tolerance of Punched Holes
If precise hole size tolerance is required, the clearance between punch and die should be very close; the die hole should be only 0.002 to 0.004 in. larger than the punch for paper-based materials (Fig. 35.2 and Table 35.2). Glass-based laminates generally require about one-half that tolerance. Dies have, however, been constructed with as much as 0.010 in. all-around clearance between punch and die. They are for use where inspection standards permit rough-quality holes. A die with sloppy clearances is less expensive than one built for precision work, and wide clearance between punch and die causes correspondingly more break and less shear than a tight die will cause.The result is a hole with a slight funnel shape that makes insertion of components easier. FIGURE 35.2 Example of proper tolerance of a Always pierce with the copper side up. Do not use piercpunch and die. ing on designs with circuitry on both sides of the board, because lifting of pads would probably occur. 35.2.4 Hole Location and Size Designs having holes whose distance from the edge of the board or from other holes approaches the thickness of the material are apt to be troublesome. Such designs should be avoided; but when distances between holes must be small, build the best die possible. Use tight clearance between punch and die and punch and stripper, and have the stripper apply plenty of pressure to the work before the punch starts to enter. If the distance between
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