Manufacturing Technology By Martijn Pakkert & Eriano Troenokarso Table of content 2 Introduction 3 Description of the studied topics 4 Manufacturing techniques @ GTD 14 Product Analysis Mass production 20 Single piece production 25 Literature list 29 Introduction This report is about the assignment Manufacturing Technology. During this assignment we needed to discover what kind of techniques there were possible for manufacturing products. We had two guided tours at the GTD on the TU/e campus. There we had seen a lot of different manufacturing processes. After that we had to do research on several topics that came from the book Fundamentals of Modern Manufacturing from M.P. Groover. Our topic was Material Removal Processes, so we dived into the techniques of removing material. At last we had to study mass and single piece production products. We chose to do research on the manufacturing of nuts & bolts and on a custom made acoustic guitar. In this report you can find an eloboration of all these topics, Enjoy! 3 Description of the studied topics Material Removal Processes Theory of Metal Machining The advantages of metal machining are the variety of part shapes and geometric features, variety of work materials, the dimensional accuracy and good surface finishes. But there are also some disadvantages like the wasteful of material and that it’s time consuming. There a lot of different ways to remove material. In this report there is an explanation given of turning, drilling, milling, shaping, planing, broaching, sawing, and all the things that are related to those ways of material removal. Turning A work tool (single point cutting tool), which is parallel to the surface of the work part, cuts the outside or inside (boring) of a cylinder. That work piece rotates but is stationary, the work tool has two axes of motion. Those motions can be handled manually, but nowadays CNC (computer numerical control) is more common. This manufacturing process is carried out on a lathe. There are many types of turning, which are done best on different types of lathes. Types of turning: - Facing; tool is fed radially to create nice surface - Tapert turning; tool is fed at angle to create tapered cylinder - Contour turning; tool follows a contour - Form turning; tool has shape that is imparted to the work - Chamering; tool is used to cut an angle on the corner of the workpiece - Cutoff; fed radially in to the rotating work to cut off the end - Threading; pointed tool fed linearly across outside of rotating workpart - Boring; same but parallel to the axis of rotation on the inside of workpart - Drilling or reaming; on a lathe by feeding the drill into the rotating work along it axis - Knurling; not a machining operation, doesn’t involve cutting material; its metal forming operation Types of lathes: - Toolsroom lathe; high accuracy; small; lot of speeds and feeds - Speed lathe; high speed; no cross slide assembly; no leadscrew - Turret lathe; six cutting tools; high production; quick change - Chucking machine; chuck to hold workpart (in dutch it is a klauwplaat); small/light parts - Barmachine; almost the same; collet instead of chuck (collet is a huls) - Single spingle barmachine - Multiple spindle barmachine (just has more axels) 4 Drilling Drilling has a stationary work part that can move along two axes. The third axis can be moved along by the work tool. The work tool is a multipoint end cutting tool. This work tool rotates so it can drill round holes in the work part. This process is carried out on a drill press. There are lots of different types of drilling, carried out on different types of drill presses. Types of drilling: - Reaming; slightly enlarge a hole - Tapping; provide internal screw threads on an existing hole - Counterboring; big diameter follows small diameter to create bigger hole - Countersinking; same except the step in the hole is coneshaped - Centering; drills a starting hole to establish its location - Spotfacing; similar to milling; provide flat surface Types of drill presses: - Upright drill - Radial drill - Bench drill - Gang drill - Multiple spindle drill - Numerical control drill presses Milling Milling is a process in which a rotating multiple cutting edge tool cuts a stationary work part. The axes are not set, there are a lot of different types of milling that don’t look like each other. This also means that there are a lot of different machines. Types of milling: - Peripheral milling/plain milling; axis of tool parallel to axis workpiece o Slab milling; tool extends beyond the workpiece o Stot milling; workpiece extends beyond the tool o Saw milling; • Side milling; cuts side of workpiece • Straddle milling; both sides of workpiece o Up milling; milling against the feed o Down milling; milling with the feed - Face milling; axis of tool is nonparallel to axis workpiece o Conventional milling; tool extends beyond the workpiece o Partial milling; tool cuts part of workpiece o End milling; workpiece extends beyond the tool o Profile milling; tool cuts outside of workpiece to make profile o Pocket milling; create pockets in flat parts o Surface contouring; cuts top of workpiece to make a contour 5 Milling Machines types: -Horizontal - Vertical knee-and-column - Bedtype milling oSimplex oDuplex - Planer type - Tracer/profiling mill - CNC (which stands for Computer Numerical Control) Single point picture 1 Tool Geonometry - Single Point - Chipbreaker - Multiple cutting edge tools Chipbreaker picture 2 - Milling cutter - Broach - Saw Multiple cutting edge tools picture 3 Broach picture 5 Milling cutter 6 picture 4 Saw picture 6 High Speed Machining (HSM) The speed in HSM is significantly higher than conventional machining ways. This can be proven by a calculation of the DN-ratio. In this calculation the bore diameter (in meters) gets multiplied with the maximum spindle speed of that machine (rev/min). So even large bores can reach a high DN-ratio and are part of HSM. Regular DN-ratios for HSM are 500.000 – 1.000.000. It also has a average spindle velocity of 8.000 – 35.000. Another measurement is the hp/ rpm. With HSM this number is between 0,005 and 0,010. With other machining types this number is normally 0,0005 hp/rpm. The conditions of cutting can be calculated with a formula. This formula look like this: R_MR=v∙f∙d It’s called the material removal rate. The v stand for the cutting speed. The f for the feed. The feed is the secondary motion of the work tool. And d stands for the depth of the cut. Every cutting process has its own material removal rate. Cutting fluid Cutting fluid is a very important part of cutting. The fluid improves the cutting performances and provides the tools to become worn down. Cutting fluid is directly put on the machining operation. There are coolant fluids and lubricant fluids. But many fluid take care of more than one problem. Types of Cutting Fluids - Cutting oil; there are more types, to reach maximum lubricity some are combined - Emulsified oil; oil droplets suspended in water, typical ratio water to oil is 30:1 - Chemical fluid; chemicals in a water solution; good cooling; less lubricant - Semi chemical fluid; adding small amount of emulsified oil to make the lubricity better The fluid can be appliquéd in different ways. Coolant fluids can be used flooding. This means directly at the work tool. Mist application is also a coolant method. With this method the fluid is directed as high speed mist. It is not as effective as flooding, but is coolant every part of the work tool and work piece. And at last, manual application, this is for coolant and lubricant. It can be directed with a squirtcan or paintbrush. It’s generally not preferred on manufacturing processes. 7 Economic and Product Design Considerations in Machining Machinability The machinability stands for the relative ease with which a material can be machined using appropriate tooling and cutting conditions. Most important evaluation criteria of machinability are: - Tool life - Forces and power - Surface finish - Ease of chip disposal Machinability Rating The evaluation of machinability is hard, because some material have a pro in one criteria, but a con in another criteria, so it is hard to judge. Machinability is tested by a comparison of work materials. The test material is relative compared to a base material upon a few characteristics. And the outcome (the relative performance) is given as an index number, called the machinability rating (MR). The base material always has an MR of 1,00. - Materials that are easier to machine than the base have bigger than 1,00 rating. - Materials that are harder to machine have a smaller machinability rating than 1,00. Chemistries Chemistries are of a big essence to the machinability of metals. For example adding alloying elements, to enhance the properties of the metal, will increase the tool wear and thus reduce the machinability of the material. On the other hand you can add certain elements such as lead, sulfur and phosphorous which will reduce the friction between the tool and chip, forces and temperature and thus increase the machinability of the material. Steels that are alloyed like this, are called Free Machining Steels. Tolerance of the Machines An important aspect to machining is the tolerance of the machines. Machining is often picked over hand-made because of the closer tolerance you can achieve by machining a material. Within machining there are various machining operations that all have their capability of tolerance. The closer you want to tolerance to be, the higher the price will be. Though if you look at the long term, a higher tolerance (although you pay a higher price) will save you money. A higher tolerance will often lead to a fewer problems in the assembly and for that lead to better control in the manufacturing process. 8 Surface of material The roughness of a surface is mostly determined by machining the material. There are a lot of factors to this that can be categorized in: - Geometric factors These factors include the machining parameters that determine the surface geometry Such as the type of machining operation, geometry of the cutting tool and the feed. The surface that would result from these factors is called the ideal/theoretical surface roughness, which would be obtained if the other factors were absent. Geometric factors picture 7 - Work material factors The material itself won’t react the optimal way while it is being manufactured. It will prevent the surface from being ideal because of (1) Damage to the surface caused by chips curling back into the surface, (2) accidentally tearing of the work surface, (3) cracks in the surface caused by a discontinuous chip formation, (4) friction between the tool flank and the new surface. - Vibration and machine tool factors As it says these factors are caused by a chatter or vibration in the machine tool or cutting tool. A vibration will cause waviness in the work surface. This can be eliminated by (1) improving the setup of the machining, (2) operating at speeds that are natural to the machine and (3) reducing feeds and depths to reduce the forces in cutting. Cutting Conditions Each operation has its own proper cutting conditions. These conditions consist of speed, feed, depth of cut and cutting fluid. Depth of cut is predetermined by workpiece geometry and operation sequence. When the material is in the roughing operation stage the depth is made as large as possible. And in the finishing stage the depth is set to the dimensions that desired. For the selection of feed you have 4 factors to deal with. - Tooling: From what kind of material is your tool made. Tougher material can withstand a higher feed. - Are you busy roughing or finishing: Roughing operations involve high feeds, while finishing involves low ones. The selection of the cutting speed is based on making the best use of the particular cutting tool. Which means that you need to find the balance between high speed removal and long lasting tool life. For this selection formulas are derived, particularly for 2 objectives, namely maximum production rate or minimum unit cost. 9 Production Rate Maximum production rate is used for cases where you need to produce as quickly as possible. The formula is: Tc = Tn + Tm + (Tt/np). Tc = Production cycle time per piece in minutes Th= Part handling time; the time the operator spends loading the part in to the machining tool when beginning and unloading when finished. Tm= Machining time; time the tool is engaged. Tt= Tool change time; The time it takes to change the tool (when it is run down) Np= The number of pieces cut in one tool life There is also a formula for the selection of speed in which you can minimize the cost per unit. This formula looks like: Cc = CoTn + CoTm + (CoTt/np) + Ct/ np. Cc= Total costs per piece Co= Cost rate of … Ct/np= Tool cost per unit (added because it not only costs time) Abrasive Machining Abrasive machining removes material by friction with hard abrasive particles. The most important/well-known abrasive process is grinding, others are honing, lapping, superfinishing, polishing and buffing. Abrasive processes are used as finishing process in common. Importance: - They can be used on all types of materials ranging from soft metals to hardened steels and hard nonmetallic materials - Some of the abrasive processes can produce extremely fine surfaces Grinding There are similarities between milling and grinding. Both have a periphery and face way of use. And in both processes the work is fed to the machine for removing material. Though there are a few differences, namely (1) the particles are smaller and more plural than the teeth on a milling machine, (2) the speed is higher, (3) the particles are randomly oriented and (4) a grinding wheel is selfsharpening. A grinding wheel has 5 basic parameters to deal with: - Abrasive material - Grain size - Bonding materials - Wheel structure (spacing of grains in the wheel) and - Wheel grade (bonding strength of the wheel) Within grinding there are 4 types: - Surface grinding - Cylindrical grinding - Centerless grinding - Creep feed grinding 10 Honing Honing is an abrasive process performed by a set of bonded abrasive sticks (mostly stones) and it is used to obtain precise dimensions and surfaces in cylindrical shapes (around the 0.1-0.8 µm). The sticks are rubbed in a circular motion over the surface of the workpiece to smoothen it. Honing tool picture 8 Superfinishing Superfinishing is an abrasive process that is similar to honing. It only differs in: (1) shorter strokes, (2) higher frequencies in the strokes, (3) lower pressure, (4) workpiece speeds are lower and (5) grit sizes are smaller, so this all leads to a smoother surface (0.013-0.2µm). And it is mostly not used for internal, but external cylinders. Lapping Then there is lapping, which is an abrasive process that is more accurate than honing (around the 0.025-0,4 µm) roughness. Lapping uses a fluid suspension of very small abrasive particles, which look like a chalky paste. And this process is used for i.e. optical lenses. Lapping picture 9 Polishing At last there is polishing, which uses a flexible (mostly leather or other fabric) rotating wheel with very miniscule grains that rotate at a very high speed. And as an extreme there is buffing, that uses the same technique, but is even more accurate than polishing. 11 Nontraditional Mechanical Energy Processes: - Ultrasonic machining: In this process abrasive particles are flowing through a fluid past the work. And a tool vibrating at a high frequency (approx. 20,000 Hz) makes the particles cut the work. - Water jet cutting: In this process a high pressure and high velocity steam of water is used to cut the work surface. - Abrasive water jet cutting: The same technique is used as with Water Jet Cutting, but there are abrasive particles added to the steam to ease the cutting. - Abrasive jet machining: This goes by the same principle as Abrasive water jet cutting, but instead of using steam, gas is used. The difference is that it is much more appropriate as a finishing process and not so much of a cutting process. Ultrasonic machining picture 10 Water jet cutting picture 11 12 Electrical Discharge Machining (EDM) picture 12 Electrochemical Machining Processes - Basic process: Material from the workpiece is removed through an electrochemical process. The tool that reacts as an cathode shoots electrolytes to the work that reacts as an anode and removes material. And through precisely the same manner there is also electrochemical deburring and grinding. Thermal Energy Processes - Electrical discharge machining and Electric discharge wire cutting: This form of nontraditional machining works through a lightning that goes through the work piece and thereby removes material. This must be done in dielectric fluid, because the lightning must be guided to the workpiece. With the wire cutting, it is the same principle, but then it is done through a wire, thus the removal will be through in one time, instead of piece by piece. - Electron beam machining: This process removes material through an electron beam, created by electrons that go from a high voltage cable through a cathode grid to an anode where they are collected and after that shoot to the workpiece while the beam is guided by lenses. - Laser beam machining: This type of machining is similar to electron beam machining. But this beam is provoked through light that is being reflected and concentrated, so that the beam contains a lot of energy, to remove material. 13 Manufacturing techniques at GTD This machine is called a turner. This example (picture) is the conventional version of this machine, which is used as a material removal machine. The material is removed by a cutting blade, such as a chisel. The chisel removes the material from the workpiece which is spinning around in a lathe. Conventional turner Besides the conventional turner, there are also computer controlled lathes in this modern world. The computer controlled turner work through programs scripts that are written for them. In these scripts the programmer puts all the information that is needed to create his desired shape. Then it is put into the turner and this will execute as it is commanded. By manufacturing through computer controlled turner you can achieve a higher accuracy, this means smoother surface, smoother edges, almost no chance for flaws etc. As a result your product will be better compared to the conventional way. Computer controlled turner This machine at the left is also a computer controlled lathe. But it is a special machine, because of the extreme precision and also because it is manufactured by the GTD themselves. As you can see a bit on the picture, the created surface of the workpiece is just like a mirror. This extreme accuracy is achieved by a few features. A few of those are mirror ruler that is able to measure within the nanometer. And the mechanics within the turner that need to shift are provided with air pressure to prevent friction. 14 Drill This device is called a drill. And similar to the turner this machine is also used for removing material from the workpiece. This machine removes material through drill that penetrates the work with a circular motion and cutting material away. In opposition to the turner with the drill not the workpiece spins around, but the tool does. So this machine is better for manufacturing the inner parts of a piece and the turner is better to use for the This machine is called a grinder and it removes material as well. It removes material by a motion that we know as rubbing. A paper with abrasive particles on it is attached to a wheel which rotates at a high speed. When pushing your workpiece against the wheel, it will grind the material off. This way of removing material is very useful when you want to create a very smooth surface, as it is possible to grind with very small abrasive particles to make the grinding more gen- Grinder 15 Electric Discharge Machining Thisway wayofof manufacturing a product calledDischarge Electric Machining Discharge(EDM). MaThis manufacturing a product is calledisElectric Itchining is a non (EDM). traditional way of manufacturing and it uses thermal energy for removing It is a non traditional way of manufacturing and it uses material. thermal is provided via electricity. Electric discharges will cause thermalThis energy forenergy removing material. This thermal energy is provided a high amount of energy existing of electrons to be shot at the workpiece, which will via electricity. Electric discharges will cause a high amount of energy cause removal of material. For the EDM to work properly, it is also needed to do this existing of electrons to be shot at needs the workpiece, which will cause within a dielectric fluid. Because a path to be created after each discharge removal of material. For the EDM to work properly, it is also needed and this is done by ionizing the dielectric fluid. to do this within a dielectric fluid. Because a path needs to be created after each discharge and this is done by ionizing the dielectric fluid. A variation of EDM is Electrical Discharg Wire Cutting (EDWC). At this type of machining the electrons are lead through a wire. This is put through a hole of the work as a starting point. From there on it will cut its way through the workpiece. The wire is also being advanced through spools, because this will cause cutting with fresh electrons. And this leads for a constant cutting diameter. This variation is better for accurately removing material. And the material can only be a metal, because the electrons can only be guided through metals. 16 Electric Discharge Wire Cutting A whole different type of manufacturing is glassblowing. This does not work through removing the material, but now you are deforming the material. First you need to heat up the glass so that it gets soft and it is deformable. After that you create the shape that you like. You can for example flatten it with pliers made of ceramics. Or you can create a bulb. This is done by first heating and seal one side of a glass tube. Then you heat the seal and from the other side you blow into the tube so that the soft seal will get filled with air and a glass bulb will arise. Glass Blowing This machine (left) is used for cutting material. It is one of the processes that is used as a preperation of shaping a material, getting the right size of a plate of metal. The blade of the machine is just like scissors angled (from down to up) to lighten the needed cutting power. Cutting The cutting machine on the right is in principle exact the same machine is the other one, except for the angled blade. This cutting machine is appropriate to cut out angled corners out of the material. 17 The punch press is a machine that is used for making holes into your material. A punch with the desired shape is put into the press. Then the material is fixed underneath the press. And next the punch is pressed through the material which will create a desired hole in the workpiece. Punch press Resistance welder 18 Another part of manufacturing is the assembly. Welding is one of the techniques to assembly parts. On the left side there is a resistance welding machine. Two metal plates are put in between the two metal welders. Then a current is sent through the welders which causes a resistance (=heat). By this heat the both of the metals will melt, and when they are soft enough, a bit pressure will be applied so that the plates will be welded. Besides resistance welding there also is arc welding. This kind of welding is done with addition of a filler material. The filler material as a solid is put on top of the gap between the two parts you want assembled. Then you heat the filler material so that it melts and flows into the gap. And then let it cool down so it will assemble the parts. Arc welding Air bender Bending iron At last there were also a lot of bending techniques shown for manufacturing at the GTD. The top picture is an example of an air bender, where a plate is put in between. Next a hydraulic v-formed punch will press the plate into the underlaying die and manufacture a bend. Another bending technique is used for pipes. A bending iron is used there. The pipe is guided through a wheel with a bender sticking out at the end. When you turn the wheel, the bender will prevent the pipe from rotating and that will cause a bend. And the last bending technique is the roll bender, which uses the principle of a rolling pin. A plate is put in between two rollers and by pulling the plate through the rollers a bend will be created. Roll bender 19 Product analysis Mass Production Nuts and Bolts Nuts and bolts are not as simple as you might think. Making nuts and bolts requires a complicated manufacturing process. How are nuts and bolts made? And which manufacturing techniques are used? On these questions will be answers in the following text. Preparing Nuts and bolts consists of steel. The steel is delivered to the factory as wire rods. Before those rods can be unrolled, they have to bath 30 hours in a furnace to make it soften enough to be worked. After that the steel wire rod are baptized in a bath of sulfuric acid. This provides that rust particles are getting removed. Then it’s rinsed in water to remove all the rust particles. At last it’s coated with phosphate, so it doesn’t get rusty in the manufacturing process. It also makes the steel lubricated to make it easier to form. Forming Cold forging at room temperature by forcing the steel trough various dies. The dies straighten the wire rod and a then cuts it in to pieces. Those pieces are a bit longer then the final bolt. Cutting the wire is done with the method called shearing. In this process it’s done on a machine called a power shear. Steel wire rods picture 13 Power shearing picture 14 20 The tip becomes the bolts head. Each piece goes through a die that makes it perfectly round. The manufacturing techniques that are used here are part of the basic bulk deformation processes. Here is rolling the process that the various dies generate. A form of drawing, which also is part of the basic bulk deformation, takes place when the steel wire rod is pulled to give it a smaller diameter. Rolling picture 15 Drawing picture 16 The head of the bolt A series of dies progressively shape the head of the bolt on one end. Bolts head forging picture 17 The first die form a slight collar at the head of the bolt. That looks like this. The second turns it into a round head. The last one turns the round head into a hexagonal head, which is the most common head to a bolt. Bolt heads picture 18 21 The same basic manufacturing technique is used to shape the head of the bolt as forming the steel rods. But this one is called forging: the steel is formed with high pressure into the right shape. Because the steel is still very hot, this takes a lot less pressure. Forging picture 19 The tail of the bolt; the chamfer Another machine forms the tail of the bolt. First a chamfer has to made. This is for the nut to catch on to. This is how a chamfer looks like Chamfer picture 20 & 21 Here is a turning technique used. The specific technique is called chamfering. The cutting edge of the tool is used to cut an angle on the corning of the cylinder. 22 The tail of the bolt; the threads: Again the cold forging method is used to press in the threads. It is done under high pressure at a very high speed. Thread rolling picture 22 The threads are literally pushed in the steel. This method is called thread rolling, which is part of the bulk deformation processes family. Thread rolling picture 23 The nuts The nuts are made with a hot forging method. A steel bar is heathen up to a very high temperature and gets cut, formed and drilled at the same time. The cutter takes care of the good thickness of the nut. The former from the part into a hexagonal form, and a die drills a hole in the nut. After this process the nut will look like this. Nut thread tapping picture 24 23 A general way of drilling is used here to make the hole in to the nut. Because the hole is through, this method is called through hole. After that a tapper drives into the hole of the nut to cut the threads of the nut. Tapping come from the drilling family. The tap provides internal screw threads on an existing hole. A lot of lubricant oil is used to minimize the wear and tear on the tappers. This oil consists of special oil and an additive zinc dithiophosphate. This isv an additive that prevents wear. Tapping picture 25 Final process: Next the nuts and bolts go in to a oven of 870 °C for an hour to give them the required strength. This follows by a rapid cooling in oil to solidify them. This means making it hard and non fluid. Only now the nuts and bolts are not only hard but also brittle. So they go for another time in the oven. This time the heat removes the brittleness but keeps the strength. When the nuts and bolts are cooled down slowly, they’re finished and ready for packaging. 24 Product analysis Single Piece Production Cusom guitar building A product that can be manufactured as a single piece product is a guitar. Many people want to build their own guitar, because they are fans of music-making and also because of the exclusivity. For these people there are building kits, such as this one, which is meant to create an acoustic guitar from the brand Martin. Acoustic guitar kit picture 26 Front plate The front plate is made by cutting the shape out of an wooden plate. And after the outer shape is created, the inner hole is created by drilling and cutting. The size of the hole is determine for the sound that it makes. Back plate The back plate is made by a method called bookmatching. This means that you take one piece of wood, which you slice into slices you want. Then take two slices which have similar grain pattern. And then mirror these, so that you get a symmetrical back plate. Bookmatching principle picture 27 25 Side strips The side strips are made by heating the strips so that they are manufacturable and then place them in a press, which has the curved mold. Press them and let them cool down. The neck The neck of the guitar is made by milling a wooden block into the right shape. There are a few features that influence the sound of the guitar. For example the thickness of the neck. And also the angle that the top of the neck has is determine for the guitar’s sound. Assembling First you start with gluing the sides together. Next you reinforce the front and back plate by bracing them. This will cause sturdiness, so that the plate wont bend or such. Gluing the side strips picture 28 Bracing the plates picture 29 Next you attach the back and top to the sides. Gluing the plates 26 picture 30 Next you fit the neck, to ensure that everything is glued in the correct dimensions. Fitting the neck picture 31 Next you fit the neck, to ensure that everything is glued in the correct dimensions. Applying fingerboard and frets picture 33 Next the fingerboard is put on the neck and on top of that the frets are put. Reinforce the neck picture 32 27 Next the finishing sanding is applied, and after that the body and the neck are being enameled. Enamel the body and neck picture 34 Now the neck needs to be glued on top of the body. This has also consequences for the sound that the guitar makes. Because a neck that is bolted on will cause a different sound At last the bridge and the strings are being assemblied and the guitar is finished. Gluing the neck picture 35 Finished guitar. 28 picture 36 Literature list Pictures 1-12, 14-16, 19, 21, 23, 25: Groover, M.P. Fundamentals of Modern Manufacturing : Materials, Processes, and Systems, 3rd ed. New York City: John Wiley & Sons, Inc., 2007. ISBN 978-0471-74485-6 Pictures 13, 17-18, 20, 22, 24: http://dsc.discovery.com/ Pictures 26, 28-36: http://www.angelfire.com/il3/jkumorek/Page33.html Picture 27: http://forums.klipsch.com/forums/storage/6/1098339/IMG_0275a.jpg All GTD-pictures made by ourself. 29
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