Medium Wire CNC Wire Cutting Machine: How It Cuts and When to Use It
A medium wire CNC wire cutting machine sits between slow wire and fast wire EDM, and that middle position decides which jobs it wins. This page explains the discharge mechanism, the flushing and wire-tension limits that set accuracy, and the part features where it beats milling or slow wire.
How a medium wire CNC wire cutting machine removes metal
Material is removed by electrical discharge, not by a cutting edge. The electrode never touches the workpiece. A servo moves X and Y so the gap between electrode and part stays at a controlled width, usually 0.02–0.05 mm. When that gap closes, the dielectric breaks down and a discharge vaporises a small volume of steel. Flushing carries debris away before the next pulse.
The electrode is brass or zinc-coated brass, most often Ø0.15–0.25 mm. It runs from a supply spool through the work zone to a take-up spool and is used once. Tension, typically 8–15 N, keeps the strand straight and limits vibration. Higher tension cuts straighter but breaks more often.
Each pulse lasts microseconds and removes a crater a few micrometres deep. Because the process is thermal, hardness does not matter. A 60 HRC die steel cuts at the same feed rate as a soft low-carbon plate. That property is the main reason shops keep a wire machine next to the mills.
The dielectric is deionised water, not oil. Its conductivity is held near 5–15 μS/cm. Too low and the gap will not break down; too high and stray sparks widen the kerf. The machine measures conductivity continuously and tops up the resin tank when needed.
Where a medium wire CNC wire cutting machine sits
EDM wire machines split into three lanes by wire diameter and generator power. Fast wire uses recut molybdenum wire around Ø0.18 mm and trades accuracy for speed. Slow wire uses single-use brass down to Ø0.10 mm, runs multiple skim passes, and can hold ±0.002 mm. Medium wire sits between them.
A typical medium wire machine uses single-use coated brass of Ø0.15–0.25 mm, one rough pass and one or two skim passes, and a generator that cuts roughly two to three times faster than a slow wire of the same class. It reaches ±0.005 mm on a well-supported part and Ra 0.8–1.6 μm on the finished wall.
The trade is straightforward. You give up the last two or three micrometres of accuracy and the mirror finish below Ra 0.4 μm. In return you get a cut that costs less per hour and finishes a 50 mm tall die block in a fraction of the time a slow wire would take.
What shapes this process can and cannot cut
The electrode is a straight line, so every cut is a ruled surface. If you can draw the top profile and the bottom profile, the machine can produce the surface between them. That covers dies, punches, extrusion tooling, gears, splines, and any through-feature with a constant or tapered wall.
A taper of ±15° is routine on a medium wire machine. Past that the flushing nozzle cannot follow the angle and debris starts to short the gap. Thin, tall walls also bend. A 0.5 mm rib standing 40 mm tall will deflect under flushing pressure, and no amount of skim passes will bring it back.
Blind pockets are impossible. The electrode has to pass all the way through the part, or the machine has to start from a drilled hole. A Ø0.3–1.0 mm start hole is normal, drilled by EDM or a small drill before the wire job runs. Hole position sets the entry point, so it has to be placed with the same care as the contour itself.
Internal corners come out with a radius equal to the electrode radius plus the gap, roughly 0.10–0.15 mm on a medium wire setup. If the drawing calls for a sharp internal corner, either the corner gets a radius or the feature moves to milling with a small cutter.
Choosing between wire EDM and milling
Milling wins when the part is soft, has pockets with depth, or needs a 3D curved surface. A 5-axis machining center cuts those features in one setup and leaves a finish you can anodise. Wire EDM cannot produce a curved floor at all, because the electrode is a line.
Wire EDM wins when the material is already hardened, when the wall is thin, or when the feature is a tight through-profile. Hardened 4140 at 50 HRC machines the same as annealed stock. There is no tool wear, no deflection from cutting force, and no burr to remove.
A practical split on a die insert: mill the pockets, the bolt holes, and the relief before heat treatment, then wire the main profile after hardening. This keeps the tight tolerance features on the wire machine where the heat treat distortion has already happened, and the soft features on the mill where material removal is cheap.
If the profile tolerance is looser than ±0.05 mm and the part is soft, skip wire EDM entirely. A mill will hit it faster and cheaper. Wire only earns its place when hardness, thin walls, or a tight through-profile make milling unreliable.
Fast wire vs medium wire vs slow wire
Same 50 mm tall hardened steel block, single contour
| Item | Fast wire | Medium wire | Slow wire |
|---|---|---|---|
| Wire diameter | Ø0.18 mm recut | Ø0.15–0.25 mm single use | Ø0.10–0.25 mm single use |
| Passes | 1 rough, no skim | 1 rough + 1–2 skim | 1 rough + 3–4 skim |
| Achievable tolerance | ±0.02 mm | ±0.005 mm | ±0.002 mm |
| Wall finish | Ra 2.5–3.2 μm | Ra 0.8–1.6 μm | Ra 0.2–0.4 μm |
| Cutting speed | Fastest | 2–3× slow wire | Baseline |
| Taper capability | Limited | ±15° typical | ±30° typical |
| Best for | Blank prep | Dies, fixtures, prototypes | Fine cores, medical |
| Hourly cost | Lowest | Middle | Highest |
The practical verdict
Pick a medium wire CNC wire cutting machine for hardened steel profiles at ±0.005 mm with a Ra 0.8–1.6 μm wall. Go slow wire only when the tolerance is tighter than ±0.003 mm or the finish must beat Ra 0.4 μm. Stay on the mill when the part is soft and the corners are sharp.
Common questions
Does medium wire EDM need a start hole?
Yes, for any closed internal profile. The electrode has to thread through the part before the cut begins. A Ø0.3–1.0 mm hole is drilled or EDM-sunk at a position you specify on the drawing, and that hole becomes the wire entry point.
For an open profile that starts at the outer edge of the blank, no start hole is needed. The machine threads the electrode at the edge and cuts inward. If your design allows it, an open contour saves one operation and one setup.
Can it cut aluminium and copper?
It can, but the cut is less stable than on steel. Aluminium oxide forms quickly in the gap and shorts the discharge, so feed rates drop. Copper and brass behave better but wear the electrode faster and leave a rougher wall.
For most aluminium parts, milling is faster and cheaper. Wire EDM on aluminium only makes sense when the feature is a thin, tight through-slot that a cutter cannot reach without chatter.
How thick a part can a medium wire machine cut?
Flushing sets the limit more than the generator does. Above roughly 100 mm tall, water pressure at the top of the cut falls off and debris collects in the kerf. Feed rate drops, wire breaks increase, and the wall tapers even with compensation.
On tall parts we run lower tension and slower feed, and we check the wall with a micrometer at the top, middle, and bottom of the cut. If the drawing needs a straight wall over 150 mm, the job usually goes to a slow wire machine with better flushing control.
Why does the wall come out tapered?
Taper has three sources: the machine's own compensation error, wire deflection from flushing force, and workpiece distortion from residual stress released during the cut. The first is corrected by the control. The second is reduced by lowering tension and feed. The third is fixed before the cut starts.
Stress relief before hardening, or a rough pass followed by a skim, removes most distortion. On a part that has been properly stress relieved, a medium wire machine holds the wall within ±0.005 mm over the full height.
How is a wire EDM job priced?
Price follows cut length and part height, not part weight. A tall part takes longer because the electrode must travel through more material, so the hourly rate is applied over more time. Start holes, skim passes, and taper add operations and add cost.
The most useful thing you can send is a 2D drawing with the profile tolerance and the surface finish called out. Those two numbers decide how many passes the job needs, and pass count drives most of the price difference.
What finish can the wall reach without extra work?
One rough pass and one skim pass land around Ra 1.6–3.2 μm. Adding a second skim brings it to Ra 0.8–1.6 μm, which is where most die and fixture work sits. Going below Ra 0.4 μm needs three or four skim passes and belongs on a slow wire machine.
If the wall will be painted, anodised, or used as a sliding surface, tell us the finish requirement up front. Adding a pass later means re-clamping the part and re-referencing the datum.
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