GlossaryKind of term: Manufacturing termSinker EDM
What is sinker EDM?
Sinker EDM forms a cavity in a conductive workpiece with a shaped electrode that erodes its own negative into the material by electrical discharges.
Sinker EDM, also called die-sinking, is a spark erosion process in which a shaped electrode, usually graphite or copper, is fed into the workpiece in a dielectric fluid. Discharges across the narrow gap between electrode and workpiece melt and vaporise a little material at a time, until the cavity has the electrode's shape enlarged by the spark gap. No cutting force acts on the part. The material must conduct electricity; its hardness does not limit the process. Typical uses are mould cavities, small internal radii, deep narrow ribs and splines in hardened tool steel.
What decides size and surface in sinker EDM?
The cavity comes out larger than the electrode by the spark gap all round. Electrodes are therefore made undersize, often as separate roughing and finishing electrodes; an orbiting motion can reduce the number needed. Removal rate and surface depend on discharge energy, flushing, electrode material and the thermal properties of the workpiece. A thin recast and heat-affected layer remains at the surface, which on highly loaded parts may need removing. Higher energy removes volume faster but gives a rougher surface, a larger spark gap and a thicker recast layer. Electrode wear depends on electrode material, polarity and pulse settings, and is greatest at corners and edges. An internal corner is therefore never perfectly sharp: the smallest radius follows from the electrode corner, the spark gap and wear.
What does the standard say about sinker EDM?
EDM surfaces are often specified on the scale of the VDI 3400 guideline, which is also used for mould textures. A VDI value is linked logarithmically to Ra: VDI = 20 × log10(Ra / 0.1 µm), so VDI 18 is about Ra 0.8 µm and VDI 30 about Ra 3.2 µm. It is a guideline from a German engineering association, not an ISO standard. Comparison plates give a visual reference, not a measured value; what is measured is Ra, to the standard the drawing refers to.
How does sinker EDM work in practice?
An estimate for sinker EDM has two items: the electrodes, with material, milling time, programming and sometimes a drawing of their own, and the erosion time. A deep or fine shape can need several electrodes because of wear. Erosion time depends on the volume to remove, the required roughness and the flushing conditions; a finer surface needs more finishing stages. Where the internal radius a small cutter leaves is acceptable for the function, milling is often cheaper, because no electrodes have to be made. In hardened steel, sinker EDM can still form features after heat treatment. The alternatives are machining soft, with the risk of distortion during hardening, or hard milling, which often falls short on deep, narrow shapes.
What is sinker EDM often confused with?
Sinker EDM is not wire EDM, where a travelling wire electrode cuts a through profile; here a shaped electrode forms a cavity that need not go through. Hole EDM uses a rotating tube electrode for small, deep, round holes.
Frequently asked questions
Why is the electrode so expensive in sinker EDM?
It has to be made first: graphite or copper milled to shape, undersize for the spark gap, with its own program and sometimes its own drawing. Often a roughing and a finishing electrode are needed, and more where a deep or fine shape wears them.
When do you choose sinker EDM over milling?
For a small internal radius, a deep narrow rib or a cavity in hardened steel that a cutter cannot reach or cannot cut stably. If the radius a small cutter leaves does the job, milling is often the cheaper route, with no electrodes to make.
What does VDI 18 mean on a drawing?
A surface class from the VDI 3400 guideline, linked to Ra: VDI 18 corresponds to roughly Ra 0.8 µm. The lower the number, the smoother the surface and the more finishing stages it takes.