In Wire Electrical Discharge Machining (WEDMing), or Wire-Cut EDM, a thin single-strand metal wire is fed through the work piece, typically occurring while in a submerged tank of dielectric fluid. This process is used to cut plates and to make punches, tools, and dies from any conductive material, including hard metals that are too difficult to machine with other methods.
The wire is held between upper and lower guides. The guides move in the (X-Y) plane controlled by a CNC, the upper guide can also move independently in the (Z-U-V) axis, giving rise to the ability to cut tapered and transitioning shapes and can control axes movements. This gives the Wire-Cut EDM the ability to be programmed to cut very intricate and delicate shapes.
Wire EDMing is commonly used when low residual stresses are desired. Wire EDM has no added residual stress because it has no cutting forces. There is little change in the mechanical properties of a material in Wire EDMing due to its low residual stresses.
- Copper wire: good electrical conductor; able to handle low average machining currents and slow machining speeds; shortcomings are flush ability and low tensile strength;
- Brass wire: copper alloy with zinc (63/37); lower melting point and improved flush ability; becomes brittle and unsuitable for fine wire diameters at higher zinc contents (more than 40%);
- Molybdenum wire: high tensile strength; available in close tolerances; shortcomings are expense, high melting temperature, low vapor pressure, and poor flush ability;
- Zinc-coated copper wire: increased cutting speed; shortcomings include low tensile strength, variations in machining accuracy, increased wire breakage, and higher manufacturing costs;
- Zinc-coated brass wire: higher tensile strength; thin oxide layer helps wire slide through guides without flaking; oxide “masks” certain servo system sensitivities to electrical circuit properties (e.g. shorts); surface micro cracking helps in flushing accumulated debris; suitable for high taper cuts because of high elongation, and for cutting exotic composite and man-made materials;
Diffusion annealed wires: thin surface coating with an increase in zinc content, over a solid copper or copper/zinc core, for improved flush ability and greater speed; represents the leading edge of development for high-speed EDM.
Like the term artificial intelligence, the term fuzzy logic is bound to intrigue as well as confuse. Its application to EDM is especially interesting. Fuzzy logic means that the control unit does not follow strict rules to reach conclusions or arrive at decisions, but weighs options according to desired results. This approach is appropriate in situations where questions may have more than one correct answer. "Correctness" is determined by the priorities of the user, which may vary from case to case.
Fuzzy logic is such a promising development for EDM because it encourages strategic planning while enhancing automatic operation. For example, the cutting conditions to produce high accuracy and fine surface finish are unlike those to produce fast cutting speed. Choosing the desired results is a strategic decision that implies various machining parameters and power settings, but selecting the right parameters and settings is complex. It is the nature of EDM that the variables are both numerous and highly interrelated. Managing these variables is the key to successful and efficient EDM, and fuzzy logic supposedly lets any user manage them with the expertise of an old pro.
Novick’s control, which is based on 32/64-bit computer processing, queries the user about the given workpiece material, its size, and the electrode. The user is then asked to indicate desired cutting speed, surface finish, and accuracy (in order of priority), along with how many passes or cuts the user plans for. The unit responds, based on fuzzy logic, by presenting a series of "results", based on the priorities of the user.
At this point, the user can look over the series and "think again", perhaps choosing an alternative to the top-listed recommendation. For example, if an extremely fine surface finish and very high accuracy are given top priority, cutting times are likely to be very long. The user may decide that they are too long. By scanning the list, the user can review the alternatives and see at a glance what finishes and accuracies are possible at faster speeds. The appropriate power settings for the final selection are established automatically.