Slow Thread Processing: What Level Should You Actually Buy?

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  Key takeaways
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  Slow thread processing levels: what each one can and cannot do
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  What slow thread processing actually means on the shop floor
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  Machine and setup checks that decide the achievable level of slow thread processing
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  When slow thread processing is the wrong choice
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  How to read a quote for slow thread processing
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  Step by step: vetting a supplier for the level you need

Slow thread processing is wire electrical discharge machining run at low feed speed with the workpiece under deionized water. The wire never touches the part. A spark gap removes material, so hardness does not limit the cut. A 60 HRC punch and a soft aluminum plate cut the same way.

The word slow refers to the traverse rate, not the cycle time you should expect from a quote. A rough pass may run 2–6 mm/min on thick stock. Trim passes run faster but remove only 0.01–0.03 mm per side. The reason to slow down is accuracy: lower energy per spark means a smaller recast layer and a tighter kerf.

Best level is a misleading phrase. There is no single best level. There is a level that matches your tolerance, your surface callout, and your budget. A shop that quotes one number for every part is not giving you a process plan. Ask which passes are included before you compare prices.

The practical ceiling for most job shops sits near ±0.003 mm with Ra 0.4 μm on a 50 mm tall cut. Go taller than 100 mm and wire deflection starts to push the achievable tolerance looser, usually toward ±0.008 mm.

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  Submerged cuttingWater cools the gap and flushes debris. Dry or flushed-only cuts lose finish control.
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  Multi-pass trimmingEach trim pass removes a thin skin left by the previous pass and improves straightness.
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  Thermal stabilityA room held at 20 ±1 °C keeps the part and the machine frame at the same length.
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  Wire diameter range0.25 mm wire reaches internal radii that 0.33 mm wire cannot enter.

Two shops can own the same machine model and deliver different results. The gap usually comes from setup and housekeeping, not the brand on the door. Start with the wire system. A machine that only accepts 0.25 mm and 0.30 mm wire is a different tool from one that runs 0.10 mm through 0.33 mm. Small wire costs more per meter and cuts slower, but it is the only way to hit a 0.15 mm internal corner.

Next, look at water quality. Conductivity should sit in a narrow band, typically 5–20 μS/cm for finishing passes. If the resin bed is exhausted, conductivity climbs and the trim pass leaves a dull, pitted surface. You cannot see this on a quote. You can ask what the shop logs each shift.

Then check thermal control. Wire EDM holds tolerance because the machine, the part, and the water stay at one temperature. A shop that runs its EDM room at ambient temperature will hold ±0.005 mm on a cool morning and drift past ±0.010 mm by mid-afternoon. Ask whether the room is climate controlled and whether parts soak before the final pass.

Finally, look at the fixture. Thin plates and long shafts need support, or the cut releases residual stress and the part bows after the wire passes. Good shops stress-relieve before cutting, or leave tabs and take a light finishing pass after the part relaxes.

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  Wire range0.10–0.33 mm covers fine slots through thick roughing.
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  Conductivity logFinishing passes need stable water chemistry, not just clean water.
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  Room temperature20 ±1 °C is the working target for ±0.005 mm work.
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  Stress reliefPre-cut annealing prevents post-cut bowing on thin sections.

Wire EDM is slow per unit volume and expensive per hour. If your part is a simple through-pocket in aluminum with a ±0.05 mm tolerance, a 3-axis mill will finish it in a fraction of the time. Choosing slow thread processing there adds cost with no functional gain.

Blind pockets are another limit. The wire is a straight line between two guides. It cannot cut a blind floor unless you use a specialized sinker or a rotating head. If your drawing shows a pocket with a flat bottom and sharp internal corners, wire EDM alone will not produce it. Plan a milling step for the floor and EDM for the walls, or switch to sinker EDM.

Very large parts run into travel limits. A machine with 4,000 mm of X travel can take long extrusions, but only if the shop also has the tank and the handling gear to load them. Confirm the actual work envelope, not the catalog number.

High-volume runs are also a poor fit. Above a few thousand identical small parts, stamping or die casting wins on piece price. Wire EDM is strongest from one prototype to a few thousand parts, where tooling cost would otherwise dominate.

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  Loose tolerance + simple shapeMilling is faster and cheaper.
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  Blind floorsWire cannot reach a flat pocket floor.
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  Very high volumeTooling-based processes beat EDM piece price.

A useful quote separates setup, cut time, and inspection. If everything is bundled into one line, you cannot tell whether the shop planned two trim passes or five. Ask for the pass count in writing. That single number predicts both the tolerance and the price.

Check what the quote says about material. Wire EDM cuts any conductive metal, but the starting stock still has to be machined, ground, or sawn to size. Some shops quote only the EDM cut and leave the prep to you. Others include it. Compare the scope, not the total.

Look at the inspection line. For ±0.005 mm work, a first-article report with CMM data is normal. If the quote offers only a caliper check, the shop is not set up for the tighter levels. On a 15-year-old machine with fresh wire and good water, a careful operator can still hit ±0.005 mm. On a new machine with tired water, nobody can.

Finally, confirm the minimum order quantity and the confidentiality terms. GreatLight runs no minimum order quantity, from one prototype to 10,000+ part runs, and signs an NDA on request. That matters when your drawing is the only copy of a design you have not yet protected.

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  Pass count in writingTwo trim passes and five trim passes are different products.
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  Scope of cutDoes the price include stock prep and stress relief?
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  Inspection dataAsk for a CMM report, not a verbal assurance.
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  NDA and MOQBoth should be settled before you release drawings.