Things I want or need to know (feel free to tell me the answers):\n\n* what are the typical machining pulse times and currents for thin wire EDM?\n* what wire can I buy and on what spool sizes?\n//I can get Berkenhoff wire in 5000M lengths, at about 200GBP err, too much wire and too much money :(//\n* Does carbide rust? I want to make guides from it perhaps.\n//no//\n* Why are most datasheets so badly written and why can't I read them even when they are not?\n

TheStart\n\n

\n''Control:''\n\nMultiMediaCard\nPcEndElectronics\nMachineElectronics\nThePic\n\n''Power''\n\nSparkGenerator\nGapControl

Gap control is a black art, mainly because you cannot measure the gap width directly (except for specific experiments).\n\nWhat are gap conditions?\n\nThe generator applies high voltage pulses between the electrode and the workpiece. If the gap is large then the voltage seen across the gap will be the same amplitude as the supply's and the current through the gap will be zero. If the gap is small enough then ionization will begin to occur and current will flow. This is the spark and this is what causes machining. However, if the gap is too small ''and/or'' the dielectric between the electrode and workpiece is not clean enough then the current can take a path of least resistance through the gap and a high current arc is seen. At even smaller gaps a short can occur. \n\nToo large a gap and unused pulses mean wasted time and slow machining. Too small a gap or inadequate flushing means arcing which damages the workpiece and electrode (causing spots of carbon and eventual break down of the machining process).\n\nThere are many ways to monitor the gap but most look at the voltage or current pulses seen. Some others send extra pulses to test the gap between the machining pulses but these can themselves affect the gap conditions and require longer off times for full de-ionization of the gap.\n\nMechanisms of monitoring the gap include:\n\n* Average voltage - If you low pass filter the voltage pulses these can be applied to comparators to give signals indicating too large a gap (high average v), arcs/shorts (too low a voltage) and correct machining (between the two)\n\n* Ionization time - There is normally a delay between applying the voltage and the breakdown of the dielectric in the gap. Small times indicated too small a gap, large times indicate too large a gap. This method is now very common but has been shown to be quite variable even with an optically measured constant gap. This also does not work well with current source supplies which raise the starting voltage until break down occurs if it can, these lead to constantly short times to break down.\n\n* Break down voltage- after the ionization there should be a nominal machining current. This should be within certain limits. Too low means too large a gap and too high means too small a gap and arcing. Of course the voltage across the gap is proportional to the current can be used for these measurements.\n\nThe indirect measurement of the gap conditions via the electrical signals is one cause of the "black art" nature of gap control but the other is the fact that the flushing cannot be relied upon to be constant and there are also other things such as the production of bubbles and material variations in both electrode and workpiece. Wire vibration and speed variation will also play a part.\n\nFor this machine gap control is a major issue as it will be quite important to have steady machining for good results on a small scale. Because of this although I may begin with simple averaging of the voltage I may move on to something that looks more specifically at the break down voltage. It may also be beneficial to be able to turn off pulses that become arcs half way through as has been shown to be possible.\n\nOnce you have these voltages or times the question becomes what to do with them. The time constant of the machine will be much higher than that of the generator and it may well be possible that discharges of several of these types have occurred between the points when the uP decids if it should move the drives or not. Some systems try to respond to the relative occurrence of each discharge type, keeping of course to a regime where there are more good pulses than open circuit or arcing ones.

This may seem a simple thing but it has been one of the hardest parts. Requirements to consider:\n\n* Low vibration and good stability for precise cutting.\n* Access to workpiece despite small tank.\n* threading through small holes in the workpiece.\n* Potential for 4-axis work (each end of the wire is moved independently for taper cutting)\n* No wet floors with submerged cutting\n\n''Move the tank:''\n\nBoth of these designs suffer from the fact that the lower guide is beneath the work, this makes hand threading very difficult.\n\n//Through the side//\nThere is a very common configuration where the tank sits on an x-y stage. The lower guide it mounted in an arm and this protrudes through the side of the tank. The lower arm is securely fixed to the machine base. The upper arm is often on an X,Y,X stage to provide control over the upper nozzle height and to allow for taper cutting.\n\nThe problem with this method is the seals that are required to stop the tank emptying on to the floor where the arm enters. This is normally done with a sliding plate seal and a bellow. On my tiny machine the increased rigidity would be offset by the vibration caused and the difficulty in building the thing. \n\n//Over the top//\nThis means two arms enter the tank over the top, one for the lower guide which does not mo