A sinker EDM electrode has to be smaller than the finished cavity, because every spark removes metal one gap width beyond the electrode's outline. The rule is simple: finished electrode width equals cavity width minus twice the single-side overcut, using the overcut from the burn setting that will cut the final surface. If you orbit, subtract twice the orbit radius as well.

The hard part is not the arithmetic. It is knowing which overcut number to use, because the gap changes with current, pulse timing, voltage, flushing and the material pair. Once you understand what moves the gap, you can plan a roughing, semi-finishing and finishing electrode set that hits size, and you can tell when an oversize cavity is a setting problem rather than a bad electrode.

Gap, Overcut and Why They Differ

Shops use these terms loosely, so define them on your process sheets:

• Spark gap: the working distance between electrode and workpiece while sparks are firing
• Single-side overcut: how much larger the cavity is than the electrode on one side
• Total overcut: the growth across a full width or diameter, which is twice the single-side value

Single-side overcut runs slightly larger than the measured gap, because the craters left on the wall add a little extra growth. If the electrode touches the workpiece, the gap drops to zero and no spark forms, which is why the servo constantly adjusts electrode position as the burn progresses.

Walls and floors grow differently

The floor faces the electrode head-on, and the servo controls that distance directly. The walls see side sparks plus debris rising past them as it flushes out, and that debris can trigger secondary sparks. In deep cavities this often makes wall overcut larger near the top than at the bottom. Electrode wear also shortens the electrode, which affects depth more than width. Plan wall and floor allowances separately.

What Moves the Gap

Spark energy sets the gap, and four things change it.

Peak current has the largest effect. More current creates a wider spark channel, larger craters, faster removal and more overcut.

Pulse on-time works in the same direction. Longer on-times melt deeper, wider craters, leaving a rougher surface, a thicker recast layer and more overcut.

Off-time behaves differently. With current and on-time fixed, changing off-time mainly changes removal rate. Crater size stays about the same, so overcut barely moves.

Gap voltage sets how far the electrode sits from the work. Higher voltage widens the gap, which helps flushing and stability but adds size.

Beyond the settings, the material pair matters. Graphite on tool steel, copper on hardened steel and either one on carbide will each leave a different overcut at the same current, so never copy a chart value from one pair to another. Dirty or hot dielectric also changes how the gap breaks down, and a slow servo lets the gap swing between too wide and too tight, which shows up as size scatter. The power supply's pulse design affects how steady all of this stays, which is one reason EDM Zap engineers its Advantage series EDM power supplies for stable discharge output across the duty cycle.

If your older sinker holds size on one job and drifts on the next, the cause may be servo response, power supply condition or calibration. Our engineers can help you sort it out before you start remaking electrodes. Talk to an Engineer.

The Undersize Formula

For any width or diameter without orbit:

Electrode size = cavity size minus (2 x single-side overcut)

For depth there is only one side, so subtract the floor overcut once and add expected end wear to the electrode length.

With orbit, the electrode must also be smaller by the orbit radius on each side:

Electrode size = cavity size minus 2 x (single-side overcut + orbit radius)

Apply the undersize to every feature, including corner radii. An inside corner cannot be sharper than the overcut, because the spark rounds it. If the drawing calls for a small inside radius, confirm that your finishing overcut is smaller than that radius.

One allowance, one owner

The most common sizing mistake is counting an allowance twice. Some shops build orbit into the electrode undersize and then add an offset in the control program. Others let the control calculate orbit from its overcut table and undersize the electrode by the full amount anyway. Either way the cavity comes out oversize. Note on every electrode drawing whether its undersize includes orbit, and make the program match that note.

Planning a Multi-Stage Electrode Set

Each stage runs a different setting, so each electrode needs its own undersize.

• Roughing: high current and large overcut. Leave enough stock for semi-finishing to remove the roughing craters and recast.
• Semi-finishing: medium current. Removes roughing damage and brings the cavity close to size.
• Finishing: low current and small overcut. Sets final size and surface finish.

A practical starting rule is to leave at least the next stage's crater depth plus its recast as stock. Too little leaves rough marks on the final wall. Too much adds long, slow finishing time. Our guide to sinker EDM electrodes covers material choice for each stage.

Worked example (illustrative numbers only)

The values below are for teaching, not machine data. Use overcut values from your own trial burns.

Target cavity: 1.000 in square, 0.500 in deep.

• Roughing: illustrative overcut of 0.006 in per side, leaving 0.004 in per side of stock. Roughing electrode = 1.000 minus 2 x (0.006 + 0.004) = 0.980 in.
• Finishing: illustrative overcut of 0.0015 in per side with a 0.003 in orbit radius. Finishing electrode = 1.000 minus 2 x (0.0015 + 0.003) = 0.991 in.

The roughing burn leaves the cavity at 0.992 in. The finishing electrode starts cutting at 0.994 in (0.991 plus 0.003 total overcut), then orbits out to 1.000 in, so every wall gets cleaned up.

Proving the Number Before Production

Charts and calculators give you a first estimate, and they assume a specific material pair, clean dielectric, good flushing and a well-tuned machine. Treat the chart value as a starting guess and confirm it with a trial burn at the exact production settings.

Measure the electrode before and after, and measure the cavity at the top, middle and bottom of the wall:

• Cavity minus original electrode, divided by two, is your real single-side overcut
• Original electrode minus worn electrode shows wear at edges, corners and end face
• The top-to-bottom wall difference shows taper from debris and flushing

If overcut is larger than expected, check flushing and gap voltage before resizing the electrode, because resizing only hides a process problem. Record the electrode material and polarity, current, on-time, off-time, gap voltage, flushing method, dielectric temperature, orbit pattern and radius, measured overcut, wear and the machine's calibration date. Axis error looks exactly like overcut error on the part, so regular EDM calibration services keep your measured overcut tied to the burn rather than to machine drift.

Accurate electrode sizing depends on measured overcut from your own machine, material pair and settings. If cavities keep coming out oversize or tapered, our engineers can review your burn settings and machine condition and recommend the right fix, whether that is calibration, parts or a power supply upgrade. EDM Zap has supported sinker EDM shops since 1971. Talk to an Engineer, or call (630) 852 1699.

Frequently Asked Questions

What is a typical EDM spark gap?

It ranges from a few microns in fine finishing to a fraction of a millimeter in heavy roughing, depending on current, on-time, voltage and the material pair. Measure it on your own machine rather than relying on a general figure.

Is electrode undersize always twice the single-side overcut?

For a width or diameter without orbit, yes. With orbit, add twice the orbit radius. For depth, subtract the floor overcut once and allow for end wear.

How reliable is an EDM spark gap calculator?

It is fine for sizing a first trial electrode. Replace its value with measured overcut from a trial burn before you cut production electrodes.

How does wire EDM handle the gap differently?

Wire EDM compensates for the gap with a wire offset in the program, so no undersized tool is needed. On a sinker, the electrode itself has to account for the gap.