ip and 200 on the long lip as shown in Fig. 4-12. This is the most visible difference between twist drills and deep hole drills. There is no chisel edge, but the drill has a very sharp, well-defined V-shaped cutting edge, which spins around the drill center and describes a w-shaped groove, which keeps the drill on center, even when the cutting edge is far inside the workpiece, away from the guide bushing. As the drill progresses into the work piece, the 3/4 cylindrical shape of the head and shank provides a better guide than the very narrow margin on the twist drills. This helps to prevent wandering of the drill 3. Positioning the drill This is done using a method similar to a drill jig. a drill bushing belongs to and is stored with the drill; this bushing guide is fastened to the machine, at the tip of the chip box, which receives the returning coolant and the chips and locates the drill accurately. The face of the guide is held tight against the surface of the work piece so that the coolant with the chips returning through the flute can pass into the chip box without leaking into the open as shown in Fig. 4-13 Drive Whip guide I Guide ece Fig. 4-13: positioning of the gun drill on the surface of the workpiece 4. Cooling of Cutting Edge The drill has a hole along its entire length through which coolant is pumped at high pressure he coolant exits right behind the cutting edge. It cools and lubricates the drill for minimum friction within the hole. It also washes the chips out through the open sector of the drill. The chi are then separated from the coolant, which is then filtered and pumped again through the work 5. Effects of Drill Wandering When drilling two deep holes from opposing sides to create an extra long hole, they may meet only partially. To prevent a flow restriction, the hole depths should be specified so that the holes overlap at least 10 mm at the meeting point. If the deep hole is too close to a surface, there may not be enough metal around it to act as an evenly distributed heat sink, and the coolant is insufficient to remove all the heat generated by drilling. The material will anneal at the side of drilling closer to the surface and cause the drill to wander off in the direction of the surface. Setup of drilling may prevent this from happening by providing an extra heat sink if the surface is flat e.g., by placing a suitable piece of steel on this surface. a deep hole may also wander off enough and break through into the open, or may weaken the material behind a molding surface or supporting surface. Experience has shown that it is safer to use plates of better grade steel, such as P20 or stainless steels, rather than the cheaper Aisi 4 140 to avoid catastrophic drilling errors caused by hard spots and resulting wandering drills 6. Design of Deep Holes Avoid interrupted cuts. Because the tungsten carbide head is fairly brittle, a gun drill should
lip and 200 on the long lip as shown in Fig. 4-12. This is the most visible difference between twist drills and deep hole drills. There is no chisel edge, but the drill has a very sharp, well-defined V-shaped cutting edge, which spins around the drill center and describes a W-shaped groove, which keeps the drill on center, even when the cutting edge is far inside the workpiece, away from the guide bushing. As the drill progresses into the work piece, the 3/4 cylindrical shape of the head and shank provides a better guide than the very narrow margin on the twist drills. This helps to prevent wandering of the drill. 3. Positioning the Drill This is done using a method similar to a drill jig. A drill bushing belongs to and is stored with the drill; this bushing guide is fastened to the machine, at the tip of the chip box, which receives the returning coolant and the chips and locates the drill accurately. The face of the guide is held tight against the surface of the work piece so that the coolant with the chips returning through the flute can pass into the chip box without leaking into the open as shown in Fig. 4-13. Fig. 4-13: positioning of the gun drill on the surface of the workpiece 4. Cooling of Cutting Edge The drill has a hole along its entire length through which coolant is pumped at high pressure. The coolant exits right behind the cutting edge. It cools and lubricates the drill for minimum friction within the hole. It also washes the chips out through the open sector of the drill. The chips are then separated from the coolant, which is then filtered and pumped again through the work piece. 5. Effects of Drill Wandering When drilling two deep holes from opposing sides to create an extra long hole, they may meet only partially. To prevent a flow restriction, the hole depths should be specified so that the holes overlap at least 10 mm at the meeting point. If the deep hole is too close to a surface, there may not be enough metal around it to act as an evenly distributed heat sink, and the coolant is insufficient to remove all the heat generated by drilling. The material will anneal at the side of drilling closer to the surface and cause the drill to wander off in the direction of the surface. Setup of drilling may prevent this from happening by providing an extra heat sink if the surface is flat, e.g., by placing a suitable piece of steel on this surface. A deep hole may also wander off enough and break through into the open, or may weaken the material behind a molding surface or supporting surface. Experience has shown that it is safer to use plates of better grade steel, such as P20 or stainless steels, rather than the cheaper AISI 4140 to avoid catastrophic drilling errors caused by hard spots and resulting wandering drills. 6. Design of Deep Holes Avoid interrupted cuts. Because the tungsten carbide head is fairly brittle, a gun drill should
always cut into solid material to avoid breaking of the cutting edge. This is not always possible because the channels used in molds do frequently intersect. However, the designer and the machine operator can take certain measures to minimize the risk of damage to the drill. The larger the drill, the easier it is to drill, within reason. Particularly for air lines, if the final hole is small e.g.,4 mm diameter, the approach holes should be 8 or 10 mm if possible, rather than drilling a very long hole with a 4 mm drill. Avoid offsetting of centerlines of channels. Where the holes meet, there is an interrupted cut; also, the cutting edge finds more resistance away from the centerline of the already existent hole and makes the drill wander off in the direction of least resistance as shown in Fig. 4-14. In a way, this is similar to the drill finding a"soft spot"in the material. For intersecting channels having a large difference in diameter, the small hole must always be drilled first, before drilling the larger hole. Otherwise, the small drill would lose its guidance as it passes through the large hole, as shown in Fig. 4-15. Finally, always locate the entrance of a deep hole so that it can be drilled with standard bushings as shown in Fig. 4-16 Fig. 4-14: intersecting channels with offset centerlines cause wandering-off of drill Drill shank ead of drill Fig 4-15: small drill loses guidance as it passes through larger hole nterferes with chip box Bad a)poor placement of hole causes interference with drill b) good placement of hole Fig. 4-16: entrances to deep holes should not prevent use of standard bushings 4.2 Electric-Discharge Machining(EDM)
always cut into solid material to avoid breaking of the cutting edge. This is not always possible because the channels used in molds do frequently intersect. However, the designer and the machine operator can take certain measures to minimize the risk of damage to the drill. The larger the drill, the easier it is to drill, within reason. Particularly for air lines, if the final hole is small, e.g., 4 mm diameter, the approach holes should be 8 or 10 mm if possible, rather than drilling a very long hole with a 4 mm drill. Avoid offsetting of centerlines of channels. Where the holes meet, there is an interrupted cut; also, the cutting edge finds more resistance away from the centerline of the already existent hole and makes the drill wander off in the direction of least resistance as shown in Fig. 4-14. In a way, this is similar to the drill finding a "soft spot" in the material. For intersecting channels having a large difference in diameter, the small hole must always be drilled first, before drilling the larger hole. Otherwise, the small drill would lose its guidance as it passes through the large hole, as shown in Fig. 4-15. Finally, always locate the entrance of a deep hole so that it can be drilled with standard bushings as shown in Fig. 4-16. Fig. 4-14: intersecting channels with offset centerlines cause wandering-off of drill Fig. 4-15: small drill loses guidance as it passes through larger hole a) poor placement of hole causes interference with drill b) good placement of hole Fig. 4-16: entrances to deep holes should not prevent use of standard bushings 4.2 Electric-Discharge Machining (EDM)
Principle of process Dielectric fluid Tool Serve control Dielectric fluid 电介质流体 电介质流体 DC电源 发生器 D. C Generator Electric spark ork D.C. supply Fig 4-17: principle of electrical discharge machining Electric-discharge machining is a reproducing forming process, uses the material removing effect of short, successive electric discharges in a dielectric fluid. Hydrocarbons are the standard dielectric, although water-based media containing dissolved organic compounds may used. The tool electrode is generally produced as the shaping electrode and is hobbed into the workpiece, to reproduce the contour as shown in Fig. 4-17 With each consecutive impulse, a low volume of material of the workpiece and the electrode is heated up to the melting or evaporation temperature and blasted from the working area by electrical and mechanical forces. Through judicious selection of the process parameters, fa greater removal can be made to occur at the workpiece than at the tool, allowing the process to be economically viable. The relative abrasion, i.e., removal at the tool in relation to removal at the workpiece, can be reduced to values below 0. 1% This creates craters in both electrodes, the size of which are related to the energy of the spark Thus, a distinction is drawn between roughing with high impulse energy and planning. The multitude of discharge craters gives the surface a distinctive structure, a certain roughness and a characteristic mat appearance without directed marks from machining. The debris is flushed out of the spark gap and deposited in the container. Flushing can be designed as purely movement-related operation. This type of flushing is very easy to realize since only the tool electrode, together with the sleeve, has to lift up a short distance. The lifting movement causes the dielectric in the gap to be changed. Admittedly, this variant is only realy adequate for flat cavities For complex contours, pressure or suction flushing by the workpiece or tool electrodes would need to be superimposed In plain vertical eroding, the eroded configuration is already dimensionally determined by the shape and dimensions of the electrode. Machining of undercuts is not feasible. The introduction of planetary electric discharge machining has now extended the possibilities of the erosion technique It is a machining technique featuring a relative motion between wokpiece and electrode that is achieved by a combination of three movements, vertical, eccentric and orbital. The planetary electric-discharge machining is also known as the three-dimensional or multi-space technique. Fig 4-18 shows the process schematically
Fig. 4-17: principle of electrical discharge machining Electric-discharge machining is a reproducing forming process, which uses the material removing effect of short, successive electric discharges in a dielectric fluid. Hydrocarbons are the standard dielectric, although water-based media containing dissolved organic compounds may be used. The tool electrode is generally produced as the shaping electrode and is hobbed into the workpiece, to reproduce the contour as shown in Fig. 4-17. With each consecutive impulse, a low volume of material of the workpiece and the electrode is heated up to the melting or evaporation temperature and blasted from the working area by electrical and mechanical forces. Through judicious selection of the process parameters, far greater removal can be made to occur at the workpiece than at the tool, allowing the process to be economically viable. The relative abrasion, i.e., removal at the tool in relation to removal at the workpiece, can be reduced to values below 0.1%. This creates craters in both electrodes, the size of which are related to the energy of the spark. Thus, a distinction is drawn between roughing with high impulse energy and planning. The multitude of discharge craters gives the surface a distinctive structure, a certain roughness and a characteristic mat appearance without directed marks from machining. The debris is flushed out of the spark gap and deposited in the container. Flushing can be designed as purely movement-related operation. This type of flushing is very easy to realize since only the tool electrode, together with the sleeve, has to lift up a short distance. The lifting movement causes the dielectrice in the gap to be changed. Admittedly, this variant is only realy adequate for flat cavities. For complex contours, pressure or suction flushing by the workpiece or tool electrodes would need to be superimposed. In plain vertical eroding, the eroded configuration is already dimensionally determined by the shape and dimensions of the electrode. Machining of undercuts is not feasible. The introduction of planetary electric discharge machining has now extended the possibilities of the erosion technique. It is a machining technique featuring a relative motion between wokpiece and electrode that is achieved by a combination of three movements, vertical, eccentric and orbital. The planetary electric-discharge machining is also known as the three-dimensional or multi-space technique. Fig. 4-18 shows the process schematically. Principle of process Dielectric fluid Dielectric fluid Tool Serve control D.C.Generator Workpiece Electric spark D.C. supply
As function of Z axis automatically controlled Direction of controlled Z axis Z axis tion r z Lateral velocity of rotation Constantly adjustable Process depender Process dependent Combinations of motions R-controlled motion Fig 4-18: basic movements during planetary erosion This technique now allows undercuts to be formed in a cavity. A further, major advantage is that, through compensation of the undersized electrode, it is possible to completely machine mold with just one electrode The electrodes are made by turning, milling or grinding, the mode of fabrication depending on the configuration, required accuracy, and material. High-speed cutting can be used to optimize fabrication of graphite or copper Because of the high demands on the surface quality of injection molds and the wear on electrodes, several electrodes are used for roughing and finishing a cavity, especially for vertical eroding. Thus, microerosion permits a reproducing accuracy of lum and less, with roughness heights ofoluum. A mold made by this technique usually only needs a final polishing. In some cases, this is not sufficient, however, e.g. for the production of optical parts or for cavities whose surface must be textured by etching In spark erosion, the structure of the surface is inevitably changed by heat. The high spark temperature melts the steel surface and, at the same time, decomposes the high molecular hydrocarbons of the dielectric fluid into their components. The released carbon diffuses into the steel surface and produces very hard layers with carbide-forming elements. Their thickness depends on the enery of the spark. Moreover, a concentration of the electrode material can be detected in the melted region. Between the hardened top layer and the basic structure there is transition layer. The consequences of this change in structure are high residual tensile stresses in the outer layers that can result in cracking and may sometimes impede necessary posttreatment, e.g. photochemical etching Nevertheless, the eDM process has founded a permanent place in mold making nowadays Some molds could not be made without it. Crucial advantages of it are that materials of any hardness can be processed and that it lends itself to the fabrication of complex, filigree contours
Eccentricity Manual As function of Z axis Automatically controlled Direction of controlled motion R Z axis Z axis Lateral axis dependent on Z Z axis Lateral axis dependent on Z Velocity of rotation Constantly adjustable Process dependent Process dependent Combinations of motions R-controlled motion Fig. 4-18: basic movements during planetary erosion This technique now allows undercuts to be formed in a cavity. A further, major advantage is that, through compensation of the undersized electrode, it is possible to completely machine a mold with just one electrode. The electrodes are made by turning, milling or grinding, the mode of fabrication depending on the configuration, required accuracy, and material. High-speed cutting can be used to optimize fabrication of graphite or copper. Because of the high demands on the surface quality of injection molds and the wear on the electrodes, several electrodes are used for roughing and finishing a cavity, especially for vertical eroding. Thus, microerosion permits a reproducing accuracy of 1μmand less, with roughness heights of 0.1μm . A mold made by this technique usually only needs a final polishing. In some cases, this is not sufficient, however, e.g. for the production of optical parts or for cavities whose surface must be textured by etching. In spark erosion, the structure of the surface is inevitably changed by heat. The high spark temperature melts the steel surface and, at the same time, decomposes the high molecular hydrocarbons of the dielectric fluid into their components. The released carbon diffuses into the steel surface and produces very hard layers with carbide-forming elements. Their thickness depends on the enery of the spark. Moreover, a concentration of the electrode material can be detected in the melted region. Between the hardened top layer and the basic structure there is transition layer. The consequences of this change in structure are high residual tensile stresses in the outer layers that can result in cracking and may sometimes impede necessary posttreatment, e.g. photochemical etching. Nevertheless, the EDM process has founded a permanent place in mold making nowadays. Some molds could not be made without it. Crucial advantages of it are that materials of any hardness can be processed and that it lends itself to the fabrication of complex, filigree contours
4.3 Cutting by Spark Erosion with Traveling-Wire Electrodes This is a very economical process for cutting through-hole of arbitrary geometry in workpieces. The walls of the openings may be inclined to the plate surface. Thanks to the considerable efficiency of this process, some cavities are increasingly being cut directly into mold plates as shown in Fig. 4-19 The metal is removed electrical discharge without contact or mecha ca action between the workpiece and a thin wire electrode. The electrode is numerically controlled and moved through the metal like a jig or band saw. Deionized water is the dielectric fluid, and is fed to the cutting area through coaxial nozzles. It is subsequently cleaned and regenerated in separate equipment. Modern equipment has 5-axis CNC controls with high-precision positioning Deionized water has several advantages over hydrocarbons. It creates a wider spark gap, which improves flushing and the whole process. The debris is lower. There are no solid decomposition products and no arc is generated that would inevitably result in a wire break DCGenerator F-Dcztxyl 喷流冲 Flushing Control system 找电极 Wire electrode Serve control 步进马 Numeral control Step motor Fig 4-19: principle of machine control for electric-discharge band sawing with wire electrodes Standard equipment can handle complicted openings and difficult contours with cutting heights up to 600 mm. The width of the gap depends on the diameter of the wire a diameter of 0.03 to 0.3 mm. The wire is constantly replaced by winding from a reel. Abrasion and tension would otherwise cause the wire to break Furthermore the cuts would not be accurate as the wire diameter would become progressively shorter. 4. 4 Mold Surface Finishing and polishing 4.4.1 Molding Surface Finishing and Symbols All mold part drawings must show the required finish specifications and any additional
4.3 Cutting by Spark Erosion with Traveling-Wire Electrodes This is a very economical process for cutting through-hole of arbitrary geometry in workpieces. The walls of the openings may be inclined to the plate surface. Thanks to the considerable efficiency of this process, some cavities are increasingly being cut directly into mold plates as shown in Fig. 4-19. The metal is removed by an electrical discharge without contact or mechanical action between the workpiece and a thin wire electrode. The electrode is numerically controlled and moved through the metal like a jig or band saw. Deionized water is the dielectric fluid, and is fed to the cutting area through coaxial nozzles. It is subsequently eleaned and regenerated in separate equipment. Modern equipment has 5-axis CNC controls with high-precision positioning systems. Deionized water has several advantages over hydrocarbons. It creates a wider spark gap, which improves flushing and the whole process. The debris is lower. There are no solid decomposition products and no arc is generated that would inevitably result in a wire break. Fig. 4-19: principle of machine control for electric-discharge band sawing with wire electrodes Standard equipment can handle complicted openings and difficult contours with cutting heights up to 600 mm. The width of the gap depends on the diameter of the wire a diameter of 0.03 to 0.3 mm. The wire is constantly replaced by winding from a reel. Abrasion and tension would otherwise cause the wire to break. Furthermore, the cuts would not be accurate as the wire diameter would become progressively shorter. 4.4 Mold Surface Finishing and Polishing 4.4.1 Molding Surface Finishing and Symbols All mold part drawings must show the required finish specifications and any additional D.C. Generator Control system Serve control Numeral control Step motor Wire electrode Flushing