CNC Knowledge

How to Use Slow Wire Cutting to Machine Perfectly Fitted Parts

Slow wire cutting removes material with a traveling wire and deionized water, so two mating parts can be cut to the same nominal path and still slide together by hand. This guide is for engineers and toolmakers who need a repeatable fit, not a one-off sample. Read it and you can set offsets, plan skim passes and judge whether wire EDM is the right process for a given joint.

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Slow wire cutting setup for a perfectly fitted mating part pair

On this page

1
What makes a wire-cut fit work
2
What slow wire cutting actually does to a mating surface
3
Turning a fit class into a wire offset number
4
Setup decisions that protect the fit
5
Why a wire-cut pair drifts out of fit
6
Seven steps to a perfectly fitted pair
7
When slow wire cutting fits, and when it does not

Key takeaways

What makes a wire-cut fit work

Cut both halves on one programUse the same wire path and the same offset value for male and female halves. Fit error then comes from machine drift, not from the drawing.

Offset controls clearanceWire radius plus spark gap defines the kerf. Trim the offset by 0.005–0.015 mm per side to move from a light press fit to a sliding fit.

Skim passes set the finishOne rough pass leaves Ra 1.6–3.2 μm and a recast layer. Two to four skim passes reach Ra 0.2–0.8 μm and improve straightness.

Flushing decides accuracyPoor water flow through a tall cut lets debris bridge the gap and shifts the wall. Set flush pressure before you chase offsets.

Measure the part, not the wireConfirm the first-off pair on a CMM or optical comparator, then lock the offset for the run.

Process basics

What slow wire cutting actually does to a mating surface

Slow wire cutting is wire EDM run at low travel speed. A brass or coated wire, typically 0.10–0.30 mm in diameter, moves through the workpiece while deionized water flushes the gap. The wire never touches the part. Material is removed by controlled electrical discharges, so hardness does not change the cutting strategy. A 60 HRC tool steel insert and a soft brass block cut with the same program.

That non-contact cut is why the process is used for fitted pairs. There is no cutter deflection, no tool wear on the workpiece, and no clamping force pushing the part out of position. The wall you get is the wall the machine commanded, within the machine's positioning accuracy. For a joint that must slide or press, that predictability matters more than raw removal speed.

The trade-off is speed and geometry. Slow wire cutting removes material at a fraction of the rate of a milling cutter, and it only cuts through-thickness profiles. Blind pockets, three-dimensional contoured surfaces and deep cavities are jobs for milling or sinking EDM. If your part is a two-dimensional profile with a tight fit requirement, wire EDM is usually the shortest path to a good result.

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Good fit candidatesPunches, dies, stripper plates, guide blocks, keyways, splines and thin-wall bushings.
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Poor fit candidatesBlind pockets, sculpted 3D surfaces, and parts where cycle time dominates cost.

Fit theory

Turning a fit class into a wire offset number

Every cut has a kerf wider than the wire. The kerf equals wire diameter plus twice the spark gap, and the control offsets the wire path by half that value. If the offset is wrong by 0.01 mm, the finished wall moves by 0.01 mm. On a mating pair, both halves move, so the clearance changes by 0.02 mm. That is the whole game in one sentence.

Start from the drawing. A nominal Ø20 mm bore and a Ø20 mm shaft need real clearance. For a hand-slide fit in steel, aim for 0.02–0.04 mm total diametral clearance, so cut the bore at +0.01 to +0.02 mm and the shaft at –0.01 to –0.02 mm from nominal. For a light press fit, go to zero or 0.005 mm interference and expect to need an arbor press.

Do not guess the offset. Cut a test coupon in the same material and thickness, measure the actual wall, and calculate the correction. The correction is simply the measured error subtracted from the current offset. One coupon is cheaper than scrapping a matched pair. On stainless and titanium, plan for a second coupon after the skim passes, because the recast layer can shift the effective wall by a few microns.

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Sliding fit0.02–0.04 mm total clearance; hand assembly, light lubrication.
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Light press fit0 to 0.005 mm interference; assembly with a press or mallet.
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Location fit0.005–0.015 mm clearance; dowel-like alignment without slop.

Machine setup

Setup decisions that protect the fit

Clamp the workpiece so the wire path is not fighting residual stress. Stress-relieve before the final cut when the stock is rolled or heavily machined. If a plate is clamped hard on one end and free on the other, the slot will close or open after unclamping. Rough the profile, let the part rest, then take the finishing passes with light clamping.

Set the wire perpendicular to the work surface. A wire that is out of square by 0.01 mm over a 50 mm height produces a taper of roughly 0.01 mm per side. For a fitted pair cut from one stack, taper shows up as a fit that changes with insertion depth. Use the machine's automatic wire alignment and verify with a squareness coupon before the production cut.

Keep the workpiece and the wire path clean. Rust, burrs and chips on the locating face push the part off the datum. Deburr the start hole, stone the clamping face, and confirm the datum edge with an indicator. On small parts, a 0.005 mm datum error is already half of your fit budget.

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Stress reliefDo it before finishing, especially on 4140, 4340 and cold-rolled plate.
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Squareness checkCut a test block and measure top and bottom widths; correct before production.
3
Clean datumsDeburr start holes and stone clamping faces; chips are a common fit killer.

Common faults

Why a wire-cut pair drifts out of fit

Taper is the first suspect. If the fit is tight at the top of the part and loose at the bottom, the wire is not square to the work. Re-align the wire and cut a squareness coupon. Do not compensate with offset, because offset changes the size evenly while taper changes it with height.

Wire wear and breakage leave a mark too. A wire that breaks mid-cut and is re-threaded can leave a step of several microns at the restart point. If the fit surface passes through that zone, the pair will feel rough. Restart on a scrap side or re-cut the affected pass.

Residual stress is the quiet one. A plate that was milled heavily before wire cutting can move after the first rough pass. The slot closes, the finishing pass removes less material, and the wall ends up oversize. Rough, rest, then finish. On 4140 and 4340, a stress-relief cycle before final wire cutting is worth the extra day.

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Tight top, loose bottomWire out of square; re-align and re-cut the finishing pass.
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Step at restartWire break or re-thread; move the restart off the fit surface.
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Size drifts over the runThermal growth in the tank or worn wire; check temperature and wire condition.
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Fit changes after deburringBurrs were carrying the load; measure after cleaning.

Step by step

Seven steps to a perfectly fitted pair

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1. Fix the fit class on paperWrite the target clearance in the setup sheet before programming. Sliding fit: 0.02–0.04 mm total. Location fit: 0.005–0.015 mm. Light press: 0 to 0.005 mm interference. If the drawing only says 'fit', ask for the number. Guessing here wastes the whole run.
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2. Choose wire and flush settingsUse 0.20–0.25 mm brass wire for general steel, 0.10–0.15 mm for small radii and fine features. Set flush pressure so water flows evenly on both sides of the cut. Low pressure on a 50 mm tall cut is the most common cause of a wall that drifts in the middle.
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3. Cut a test couponCut the same material at the same thickness with the planned offset. Measure the wall with a micrometer or CMM. Apply the correction to the offset. Do not skip this on titanium or on thickness above 40 mm.
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4. Program both halves from one pathUse the same nominal geometry for male and female. Apply the offset difference as the only variable: male at –0.01 mm, female at +0.01 mm, for example. This keeps the fit independent of drawing errors.
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5. Run rough plus skim passesOne rough pass, then two to four skim passes with decreasing energy. Rough leaves Ra 1.6–3.2 μm; skims reach Ra 0.8–1.6 μm and then Ra 0.2–0.8 μm. Skim passes also remove most of the recast layer and improve wall straightness.
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6. Measure the first-off pairCheck both halves on a CMM or optical comparator before running the batch. Record actual clearance. If it is outside target, correct the offset and cut one more pair. Only then release the run.
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7. Deburr, clean and re-checkRemove wire marks with a fine stone or bead blasting, then clean and re-measure. Burrs can add 0.01–0.03 mm and make a good pair feel tight. Re-check the fit after cleaning, not before.

Process selection

When slow wire cutting fits, and when it does not

Use this table to pick the process before you commit a drawing to wire EDM.

Requirement	Slow wire cutting	CNC milling	Sinker EDM
Through-profile with tight fit	First choice	Possible with care	Not typical
Blind pocket or cavity	Not possible	Good	First choice
Hardened steel above 50 HRC	Good, no re-hardening	Difficult	Good
3D contoured surface	Not possible	Good	Good with electrode
Typical wall tolerance	±0.005 mm	±0.01–0.025 mm	±0.005–0.01 mm
Metal removal rate	Low	High	Low
Sharp internal corners	Radius equals wire radius	Radius equals cutter radius	Radius equals electrode corner
Thin walls and delicate parts	Good, no cutting force	Risk of deflection	Good

FAQs

Questions engineers ask before cutting a fitted pair

Can slow wire cutting hold ±0.005 mm on a mating pair?

Yes, on a stable machine with a square wire and controlled flushing. The limit is usually the workpiece, not the process. Residual stress, clamping distortion and thermal drift move the wall more than the machine's positioning error does.

For the best result, stress-relieve the stock, cut a test coupon, and keep the finishing passes light. We inspect 100% of parts before shipment and can supply reports on request.

How much clearance should I design into a wire-cut sliding fit?

For steel on steel, 0.02–0.04 mm total diametral clearance gives a hand-slide fit that still locates well. Go tighter only if you accept press assembly.

For aluminium or brass, add 0.005–0.01 mm because these materials gall more easily. Lubricate before assembly and test one pair before releasing the batch.

How many skim passes do I need for a good fit surface?

Two skim passes after the rough cut are enough for most fits. That reaches roughly Ra 0.8–1.6 μm and removes most of the recast layer. Add a third or fourth pass when the joint slides under load or when the surface must reach Ra 0.2–0.8 μm.

More passes cost cycle time. Decide from the function, not from habit.

Does wire diameter change the fit?

It changes the minimum internal corner radius, not the achievable fit. A 0.25 mm wire leaves roughly a 0.13 mm corner radius; a 0.10 mm wire leaves about 0.05 mm.

If your fit depends on a sharp internal corner, pick the smaller wire and accept a slower cut. Offset still controls size in both cases.

Can you cut both halves in one setup?

Yes, and it is often the best approach for small pairs. Cutting male and female from the same plate, in the same setup, removes datum transfer error between two fixtures.

The trade-off is that a scrap in one half wastes the other. For expensive material, cut the halves separately and control each one against the same coupon.

What materials are suitable for wire-cut fitted parts?

Any conductive material works. We commonly cut 4140, 4340, tool steel, 17-4PH stainless, 304 and 316L stainless, titanium Ti-6Al-4V and copper alloys.

Non-conductive ceramics and most plastics cannot be wire cut. For those, use milling, or add a conductive coating only if the geometry allows it.

Send the drawing, get a wire EDM plan

We review your fit requirement, check whether slow wire cutting is the right process, and return a quotation with DFM notes within 12 hours. No minimum order quantity, from one pair to 10,000+ parts.

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