An important feature of medium-speed wire EDM (WEDM) is its multi-pass cutting capability. Unlike conventional high-speed wire EDM, which usually completes the cutting process in a single pass, medium-speed wire EDM can gradually remove the machining allowance through multiple passes, including rough cutting, finish cutting, and even skim cutting, thereby improving dimensional accuracy and surface quality.
The basic principle is to use the first pass to remove material quickly at relatively high machining efficiency while leaving a certain amount of machining allowance. In subsequent passes, the discharge energy is reduced and the machining compensation is readjusted, allowing the electrode wire to gradually approach the final contour.
In this way, medium-speed wire EDM combines the machining efficiency and cost advantages of reciprocating wire travel with the improved machining quality achieved through multiple finishing passes.
High-speed wire EDM generally uses reusable molybdenum wire that travels back and forth at high speed. It offers relatively high machining efficiency, while equipment and consumable costs are comparatively low. However, the dimensional stability and surface quality after a single cutting pass are generally limited.
Low-speed wire EDM uses brass wire or other electrode wires that travel continuously in one direction. The electrode wire is generally not reused. Because the wire travel and discharge conditions are more stable, low-speed wire EDM can achieve higher dimensional accuracy and better surface quality, although equipment and consumable costs are also higher.
Medium-speed wire EDM adds multi-pass cutting and process compensation on the basis of reciprocating wire travel. During rough cutting, relatively high discharge energy is used to remove material quickly. During finishing, the discharge energy is reduced, while the wire-travel parameters and machining trajectory compensation are adjusted accordingly.
Therefore, medium-speed wire EDM is not simply a matter of reducing the wire speed of a high-speed wire EDM machine. Instead, it achieves a balance among efficiency, precision, and cost through the coordinated use of different machining passes and process parameters.
Comparison |
High-Speed Wire EDM |
Medium-Speed Wire EDM (Multi-Pass Cutting) |
Low-Speed Wire EDM |
Wire speed |
8–12 m/s (high-speed reciprocating) |
Rough cutting: 8–12 m/s / Finishing: 1–3 m/s |
<0.5 m/s (continuous one-way travel) |
Electrode wire |
Molybdenum wire (reusable) |
Molybdenum wire (reusable) |
Brass wire / zinc-coated wire (single-use) |
Dimensional accuracy |
±0.01–±0.02 mm |
±0.005–±0.01 mm |
±0.001–±0.005 mm |
Surface roughness |
Ra 1.6–2.5 μm |
Ra 0.8–1.6 μm (up to Ra 0.4 μm under optimum conditions) |
Ra 0.2–0.8 μm |
Cutting efficiency |
60–120 mm²/min |
40–80 mm²/min overall (including multiple passes) |
20–50 mm²/min |
Equipment cost |
Low (RMB 30,000–100,000) |
Medium (RMB 100,000–500,000) |
High (RMB 500,000–3,000,000) |
Consumable cost |
Low (reusable molybdenum wire) |
Low to medium (reusable molybdenum wire) |
High (single-use brass wire) |
The first pass is mainly used for rough cutting. Its primary objective is to remove material as quickly as possible rather than directly achieve the final dimensions. Relatively high discharge energy is normally used, with appropriate machining parameters selected according to the workpiece material, thickness, wire diameter, and machine performance. A certain amount of machining allowance is left for subsequent finishing passes.
The amount of material removed during the second and subsequent passes is significantly smaller. Therefore, the discharge energy can be reduced to decrease the size of the craters produced by individual discharges. The machining trajectory can also be adjusted to further correct the profile.
For workpieces with higher requirements for dimensional accuracy and surface quality, a third pass or additional finishing passes can be used.
It should be noted that there is no fixed wire speed or machining parameter applicable to all machines. In some typical medium-speed wire EDM processes, the wire speed during rough cutting can reach approximately 8–12 m/s, while it may be reduced to approximately 1–3 m/s during finishing. Actual machining parameters need to be adjusted according to the machine, workpiece material, workpiece thickness, and machining requirements.
During wire EDM, the electrode wire does not move directly along the final contour of the workpiece. The CNC system must compensate for factors such as the electrode-wire radius, discharge gap, and machining allowance, so the machining trajectory must be appropriately compensated.
The compensation value for the first pass is generally relatively large, because it needs to take into account the electrode-wire radius, discharge gap, and the machining allowance required for subsequent finishing passes.
During the second, third, and subsequent passes, the remaining machining allowance gradually decreases, and the trajectory compensation must therefore be adjusted accordingly. This allows the electrode wire to gradually approach the theoretical contour.
In addition, the molybdenum wire undergoes wear during repeated reciprocating use. The condition of the electrode wire, wire tension, and discharge gap can all affect the final dimensions.
Therefore, the machining accuracy of medium-speed wire EDM is not achieved simply by reducing the wire speed. Instead, it is determined by a combination of factors, including machine accuracy, wire-travel stability, wire-tension control, discharge parameters, electrode-wire condition, and CNC compensation.
The biggest advantage of multi-pass cutting is that it combines the high efficiency of rough machining with the quality of finish machining.
The first pass rapidly removes most of the material, reducing the overall machining time. Subsequent passes then use a smaller machining allowance and lower discharge energy to improve dimensional accuracy and surface quality.
At the same time, medium-speed wire EDM uses reusable molybdenum wire. Compared with low-speed wire EDM, which uses single-use electrode wire, it offers a consumable-cost advantage in certain machining applications.
Therefore, medium-speed wire EDM is well suited to applications such as general precision molds, stamping dies, and components that require a balance among machining efficiency, precision, and cost.
However, multi-pass cutting does not mean that medium-speed wire EDM can completely replace low-speed wire EDM.
For high-precision molds and components with extremely high requirements for profile accuracy, dimensional stability, and surface quality, low-speed wire EDM generally still has an advantage.
The essence of the multi-pass cutting principle of medium-speed wire EDM is to divide the machining process into roughing and finishing stages. By using different discharge parameters, wire-travel conditions, and trajectory compensation values, the machining allowance is gradually reduced and the electrode wire is progressively brought closer to the final contour.
Its advantage is not simply that the wire speed falls somewhere between that of high-speed and low-speed wire EDM. Rather, it improves the machining quality of reciprocating-wire EDM through multiple cutting passes while retaining the cost advantage of reusable molybdenum wire.
Therefore, medium-speed wire EDM can be better understood as a comprehensive machining solution that balances machining efficiency, dimensional accuracy, surface quality, and production cost.
The actual dimensional accuracy and surface roughness that can be achieved should be determined according to the machine model, workpiece material, workpiece thickness, wire diameter, machining parameters, and actual inspection results. A single set of parameters should not be used to represent the machining capability of all medium-speed wire EDM machines.
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