Sinker EDM Cost: Quoting Electrodes, Burn Time and Cavities

Sinker EDM is where a quote stops being about cutting metal and starts being about burning a shape into hardened steel one spark at a time — and where shops that price it like milling lose money on every job. Sinker EDM cost isn't driven by how fast you remove material; it's driven by the electrodes you have to make first, how long the burn takes to reach depth and finish, and how many cavities and electrode swaps the part forces before a single good feature exists.

This article builds a sinker (ram / die-sinking) EDM price the disciplined way — electrode design and manufacture, electrode count, burn time, setup and flushing, overhead and margin — but with the details that separate a one-cavity mold detail from a multi-cavity die so the number you quote survives the first burn and the production it leads to.

Why sinker EDM doesn't price like milling

On a mill the cost tracks how much material you remove and how fast. On a sinker EDM, almost none of that logic applies. You remove material with controlled sparks across a dielectric gap, so removal is slow and depends on the finish you need, not the metal you're cutting. The workpiece is usually already hardened — that's why it's on the EDM — so upstream value is already baked in and scrap is expensive.

Three things make sinker EDM its own cost category. First, you can't burn anything until you've made an electrode, so a real share of every quote is electrode design and machining that happens before the EDM ever switches on. Second, the burn rate collapses as you chase a finer surface finish, so a mirror finish can cost many times the burn time of a rough cavity of the same depth. Third, electrode wear means you often need more than one electrode per feature — a roughing electrode plus one or more finishers — so the electrode count, not the cavity count, frequently drives the bill. The build-up skeleton is the same as any CNC machined part; the line items underneath are what change.

The electrode is the job

Nothing burns until an electrode exists, and the electrode is a machined part in its own right — designed with spark gap offset, then milled or wire-cut, often to tighter tolerances than the cavity it produces. So the first cost in a sinker EDM quote isn't burn time at all; it's electrode engineering and manufacture.

The material choice sets the tone for the whole job:

Electrode material	Wear resistance	Machinability	Best for
Graphite	High	Fast, dusty	Deep cavities, large electrodes, fast burn
Copper	Very high	Slower, gummy	Fine detail, mirror finishes, small ribs
Copper-tungsten	Highest	Slow, expensive	Tiny features, carbide, sharp corners

Graphite mills fast and burns fast, which is why it dominates larger mold and die work; copper holds fine detail and produces the best finishes but machines more slowly; copper-tungsten survives the abuse of tiny, deep, sharp features in carbide at a material cost that can dwarf the burn. Every quote needs the electrode line itemized — design hours, machining time on its own burdened rate, and the blank material — because on a single-cavity job the electrodes can easily cost more than the burn. Treat electrode-making as its own operation in the work plan for the part, not as a rounding error inside the EDM rate.

Electrode count: roughing, finishing and wear

The number that surprises estimators is electrode count. A feature is rarely burned with one electrode. A roughing electrode opens the cavity fast at a coarse setting, accepting heavy wear; one or more finishing electrodes then refine size, corner sharpness and surface finish at low wear. Tight, deep, or sharp-cornered features may need two or three finishers because the corners erode as they work.

So the honest electrode count for a cavity is:

Electrodes per cavity = 1 roughing + (1 to 3 finishing, set by tolerance, depth and corner sharpness)

Multiply that by the number of identical cavities only when one set of electrodes can be re-burned across them — and only if wear allows. A four-cavity mold detail with one rough and two finish electrodes per cavity is twelve electrodes if each cavity is unique, but can be three electrodes re-used four times if the cavities are identical and the finishers survive the wear. Getting this assumption right is the difference between a quote that wins and one that bleeds; the wear allowance belongs in cost, not in hope.

Burn time: depth, area and the finish you actually need

Burn time is the EDM equivalent of cycle time, and it scales with three things: the volume to remove, the burn area, and — most of all — the surface finish required. Material removal rate (MRR) on the rough setting can be ten to thirty times faster than on the fine-finish setting that produces a Ra below ~0.4 µm. Quote a mirror finish at roughing speed and the burn time is wrong by an order of magnitude.

Walk the burn and add time for each stage:

1. Roughing burn — open the bulk of the cavity at high MRR and coarse Ra, with the roughing electrode.
2. Semi-finish — step the settings down, reduce the gap, switch electrodes.
3. Finishing burn — low MRR to hit final size, corner detail and the required Ra; this stage often dominates total hours.
4. Orbiting / planetary motion — for sidewall finish and corner detail, which adds time but reduces electrode count.
5. Electrode changes and re-referencing — every swap costs setup time on the machine.

The trap is pricing only the roughing burn because that's where the cavity "appears", and treating the finish stage as a quick pass. On a precision tool the finishing burn and the electrode swaps around it can be the majority of the EDM hours. Always cost the full burn against the fully burdened rate of the specific machine — and read the finish callout carefully, because a tighter Ra or a sharper corner radius changes the burn time far more than the depth does. The same callout-reading discipline that drives a milling quote applies here; the GD&T-for-estimators guide covers how tolerance and finish translate into process time.

Setup, flushing and the unattended-hours bonus

Sinker EDM setup is real and fixed per job: fixture and indicate the workpiece, set each electrode in the holder, establish reference and depth, dial in flushing, and burn a first feature to confirm gap and finish. On a multi-electrode job, re-referencing each electrode adds setup every swap, which is why electrode count drives setup as well as burn time.

Flushing — the dielectric flow that clears eroded particles from the gap — is the quiet variable. Poor flushing on a deep, blind cavity slows MRR, raises wear, and risks arcing and a scrapped (hardened, high-value) part, so deep features carry both more burn time and more risk allowance.

The offsetting good news is that EDM runs unattended. Once dialled in, a burn can run overnight without an operator, so the machine-hour and labour-hour are not the same number. A shop that bills EDM at a blended rate covering attended setup plus unattended burn — rather than a flat attended rate for the whole cycle — quotes competitively without giving away margin. Build that rate the same way you build any machine rate; the machine shop hourly rate calculation shows how to separate the labour and machine components.

A worked example: a single hardened cavity, batch of one

Take one cavity in a hardened H13 tool insert, 18 mm deep, with a 0.3 µm Ra finish and sharp internal corners. Graphite roughing electrode plus two copper finishing electrodes. Quantity one.

1. Electrode design + machining — 1 rough graphite + 2 copper finishers, ~6 h total at a €75/h burdened rate, plus €60 blanks = 6 × 75 + 60 = €510.
2. Setup — fixture, reference, flushing, first burn ≈ 1.5 h at €75/h = €112.
3. Burn time — rough + semi + finish + 2 electrode swaps ≈ 7 h at a €90/h burdened EDM rate = €630.
4. Scrap/rework allowance — ~5% on a hardened, high-value insert ≈ €63.
5. Cost = 510 + 112 + 630 + 63 = €1,315.
6. Price at 35% margin — using price = cost ÷ (1 − margin), 1,315 ÷ 0.65 = €2,023.

Now move to four identical cavities re-using the same three electrodes: electrode and setup cost barely move, only burn time multiplies, so the per-cavity number drops sharply — the quantity break a tooling buyer expects, defensible line by line rather than discounted by feel. The margin step uses the same arithmetic as any job; if dividing by 0.65 versus multiplying by 1.35 ever trips you up, the margin-versus-markup guide settles it. List every one of these lines on the quote so electrodes, burn, setup and risk sit in cost — the discipline a complete manufacturing quote template enforces on every job.

From a drawing to a sinker EDM price

The slow, error-prone part of sinker EDM cost estimation isn't the arithmetic — it's reading the tool drawing completely under deadline: which features must be EDM'd rather than milled, how deep and how sharp the corners are, the Ra each cavity demands, the electrode material and count the geometry forces, and the hardened state that sets both the burn and the scrap risk. Miss any of those and the price is wrong before a margin ever touches it.

QuoteBuddy reads the technical drawing and surfaces those features — cavity geometry, depth, corner radii, surface-finish callouts, and the operations the part implies — so the estimator builds the price from a complete picture instead of a hurried scan. The cost engine then assembles electrode work, burn time, setup, flushing and scrap allowance, and your target margin the same way every time, on every burned feature.

Start a 30-day trial and run a few real tool and mold drawings through it — from upload to a complete, itemized quote PDF — and see whether the sinker EDM price it builds matches what you'd have quoted by feel, with the margin landing exactly where you set it.