Electrical Discharge Machining
EDM erodes conductive metal with controlled electrical sparks across a dielectric gap, cutting hardened material and sharp internal corners.
- Part
- Cutting
- Variants
- 2
- Revised
- 2026-08-11
At a glance
- Family
- Thermal
- Typical tolerances
- Wire EDM ±0.0005 in (±0.013 mm) commercial, ±0.0001 in (±0.0025 mm) achievable with multiple skim passes; sinker EDM ±0.001 in (±0.025 mm) typical, ±0.0005 in (±0.013 mm) achievable
- Surface finish
- 250 µin Ra (6.3 µm) rough cut; 16 µin (0.4 µm) after skim passes; 4–8 µin (0.1–0.2 µm) with fine-finish circuits
- Typical volumes
- 1–1,000 parts; standard for tooling, dies and low-volume precision components
- Lead time
- 1–3 weeks; sinker work adds electrode fabrication time
- Materials
- Metal
What it is
Electrical discharge machining removes metal with thousands of controlled electrical sparks per second across a gap flooded with dielectric fluid. There is no cutting force and no mechanical contact, so hardness is irrelevant — fully hardened D2 at 62 HRC erodes at the same rate as annealed stock, and tungsten carbide, Inconel and titanium cut as readily as tool steel.
Two forms dominate. Wire EDM pulls a brass or coated wire, typically 0.010 in (0.25 mm) diameter, through the part like a bandsaw blade, cutting any 2D or tapered profile to ±0.0001 in (±0.0025 mm) on well-controlled work. Sinker (die-sink) EDM plunges a shaped graphite or copper electrode to burn its mirror image into the workpiece, producing blind cavities and internal corners far sharper than any milling cutter.
The one hard requirement is electrical conductivity: EDM cuts metals and graphite, and cannot touch ceramics, glass or plastics.
How it works
- Setup and dielectric. The work is submerged in deionized water (wire) or hydrocarbon oil (sinker). The dielectric insulates the gap until breakdown voltage is reached, then flushes the eroded debris away — flushing quality, not spark energy, is what usually limits speed.
- Start hole (wire only). An internal profile needs a hole to thread the wire through, typically 0.020–0.080 in (0.5–2 mm), drilled conventionally or by small-hole EDM. Profiles that break out to the edge do not need one.
- Pulse generation. The generator applies pulses across a gap of a few thousandths of an inch. Each discharge melts and vaporizes a microscopic crater; pulse on-time and current set both removal rate and finish, trading one directly against the other.
- Rough cut. The first pass uses high energy and removes most of the material, leaving a rough surface near 250 µin Ra (6.3 µm) and a recast layer of roughly 0.0002–0.001 in (5–25 µm).
- Skim passes. Wire EDM then makes successive low-energy passes over the same profile, each removing a few ten-thousandths of an inch. Skims are what deliver both the final tolerance and a finish down to 16 µin Ra (0.4 µm) or better; each one is another full lap of the profile and another slice of the bill.
- Sinker electrode sequence. Sinker work typically uses a roughing electrode and one or more finishing electrodes, each made undersized by the spark gap it will burn — 0.0005–0.002 in (0.013–0.05 mm) per side on finishing settings.
- Post-processing. For fatigue-critical parts, the recast and heat-affected layer is removed by additional skims, polishing or chemical etching, and the drawing should say so explicitly.
Design guidelines
Wire diameter sets the internal corner radius
A wire EDM internal corner cannot be sharper than the wire radius plus the spark gap. Standard 0.010 in (0.25 mm) wire gives about a 0.006 in (0.15 mm) corner radius; fine 0.004 in (0.10 mm) wire gets to roughly 0.003 in (0.076 mm) at a much slower cutting rate. Draw the radius; do not specify a sharp internal corner on a wire-cut profile.
Sinker EDM is how you get a genuinely sharp corner
A sinker electrode reproduces its own shape, so the practical internal corner radius drops to about 0.002 in (0.05 mm) — the reason mold and die cavities with square internal corners are burned rather than milled.
Allow for overburn
The cut is always larger than the tool by the spark gap. Wire kerf is roughly wire diameter plus 0.002 in (0.05 mm) total; sinker cavities finish 0.0005–0.002 in (0.013–0.05 mm) per side larger than the electrode. Machine offsets handle this, but it drives electrode design and dictates the minimum slot width you can ask for.
Specify surface integrity, not just roughness
Every EDM surface carries a recast layer. A rough-cut-only part keeps 0.0002–0.001 in (5–25 µm) of recast plus a heat-affected zone beneath it, which lowers fatigue strength. If the part is cyclically loaded, call out the number of skim passes or a post-process rather than leaving it to the shop.
Thickness, taper and stock preparation
Wire EDM cuts material up to about 12–16 in (300–400 mm) thick on standard machines, and most machines can cut tapers up to about 30°. Stress-relieve hardened stock before wire cutting — releasing residual stress mid-cut is the most common cause of a part that closes on the wire or walks out of tolerance.
Finish expectations
Rough cut 250 µin Ra (6.3 µm); after skims 16 µin (0.4 µm); with fine-finish circuits 4–8 µin (0.1–0.2 µm). The texture is matte and non-directional, which is why EDM finishes are specified deliberately on mold surfaces (surface finish chart).
| Material | EDM behavior | Notes |
|---|---|---|
| Hardened tool steel (A2, D2, H13, S7) | Excellent | Cut after heat treat; hardness does not slow it |
| Tungsten carbide | Good | Slower and harder on wire; standard for punches and dies |
| Inconel, Hastelloy | Good | Cuts at a similar rate to steel regardless of strength |
| Titanium alloys | Good | Common for aerospace and medical profiles |
| Copper, brass, aluminum | Fast | High conductivity gives high removal rates |
| Graphite | Cuts readily | Also the standard sinker electrode material |
| Ceramics, glass, plastics | Not possible | Non-conductive — use water jet |
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Internal corner radius (wire) | 0.006 in (0.15 mm) | 0.003 in (0.076 mm) | Wire radius plus spark gap |
| Internal corner radius (sinker) | 0.005 in (0.13 mm) | 0.002 in (0.05 mm) | Electrode fabrication plus gap |
| Wire start hole | 0.040 in (1 mm) | 0.020 in (0.5 mm) | Wire must thread through |
| Material thickness | ≤ 8 in (200 mm) | 12–16 in (300–400 mm) | Flushing and wire guidance |
| Taper angle | ≤ 15° | ~30° | Wire guide geometry |
| Tolerance | ±0.0005 in (±0.013 mm) | ±0.0001 in (±0.0025 mm) | Each tighter band adds skim passes |
Cost drivers
EDM is priced almost entirely by machine hours, and it is slow — removal rates are far below milling. What it buys is geometry and hardness that milling cannot reach.
Cut area, not part complexity. Wire EDM cost tracks profile length multiplied by stock thickness. A complicated outline in thin plate is cheap; a simple outline in 4 in (100 mm) plate is not. Complexity itself is free.
Skim passes. Each pass is another complete lap of the profile. A part needing ±0.0001 in (±0.0025 mm) and a 16 µin (0.4 µm) finish may take three or four skims after the rough cut, several times the cost of a separation cut.
Electrode fabrication (sinker). Electrodes are milled from graphite or copper and are frequently the dominant cost. A cavity needing rough and finish electrodes carries two complete machining jobs before the burn starts.
Hardness is not a cost driver. This is the key economic fact: cutting hardened 62 HRC tool steel costs the same as cutting annealed stock, so parts can be heat treated first and finished afterward with no distortion allowance.
Four ways to take cost out:
- Rough the part by milling and use EDM only for the features that need it.
- Specify the loosest tolerance and finish the function allows — every skim pass is a full lap.
- Draw corner radii of 0.006 in (0.15 mm) or larger so standard 0.010 in wire can be used at full speed.
- Stress-relieve hardened stock before wire cutting to avoid scrap from parts that move mid-cut.
Variants
2 named
Die Sink EDM
Wire EDM
Questions
6 questionsWhat tolerance can wire EDM hold?
±0.0005 in (±0.013 mm) is normal commercial work and ±0.0001 in (±0.0025 mm) is achievable with multiple skim passes and a temperature-controlled machine. Each tighter band costs another full lap of the profile, so the tolerance you specify translates directly into passes and therefore into price.
Can EDM cut any material?
Any electrically conductive material, including fully hardened tool steel, tungsten carbide, Inconel, titanium, copper and graphite. Hardness does not slow the process, which is why parts are heat treated first and EDM'd afterward. Ceramics, glass and plastics are non-conductive and cannot be EDM'd — those go to water jet cutting.
What is the smallest internal corner radius wire EDM can cut?
About 0.006 in (0.15 mm) with standard 0.010 in (0.25 mm) wire, since the corner radius equals the wire radius plus the spark gap. Fine 0.004 in (0.10 mm) wire reaches roughly 0.003 in (0.076 mm) but cuts much more slowly. For sharper internal corners, sinker EDM reaches about 0.002 in (0.05 mm).
What is the recast layer in EDM and does it matter?
Each spark melts a microscopic crater and some of that melted metal resolidifies on the surface as a recast layer, roughly 0.0002–0.001 in (5–25 µm) thick after a rough cut, with a heat-affected zone beneath it. It reduces fatigue strength, so for cyclically loaded parts you should specify skim passes, polishing or chemical etching to remove it rather than leaving the decision to the shop.
How thick a part can wire EDM cut?
Standard machines cut up to about 12–16 in (300–400 mm) of stock thickness, and most can cut tapers to roughly 30°. Cost scales with profile length times thickness, so a thick part with a simple outline can easily cost more than a thin part with an intricate one.
Why does EDM stock need to be stress relieved first?
Hardened and heavily machined stock carries residual stress. As the wire cuts through it, that stress redistributes and the part moves — closing on the wire, bowing, or walking out of tolerance mid-cut. Stress relieving before wire cutting is the standard preventive step and the most common cause of scrap when skipped.