Blind detail that cannot be cut: mould and die cavities, deep ribs and slots, sharp internal corners, keyways in hardened parts, and fine surface texture. Wire EDM cuts a profile all the way through a workpiece, so anything with a floor rather than an opening through the part belongs to sinker EDM. It is a mainstay of toolrooms for exactly this reason.

Usually machined on a milling machine from graphite or copper. Graphite machines quickly, erodes slowly and suits large or complex electrodes; copper gives finer finishes and suits small detail and carbide work but is harder to machine. Because every sinker job needs an electrode made first, electrode machining is a significant part of the total cost and lead time of the process.

The small clearance between electrode and workpiece across which the discharge occurs, which means the cavity produced is always slightly larger than the electrode. The electrode must therefore be made undersize by the expected gap, which varies with the energy settings used. Roughing and finishing electrodes carry different offsets because they run at different energies, and getting this wrong puts the cavity out of size.

Moving the electrode in a small controlled path, usually circular, rather than straight down, so it sweeps out a cavity slightly larger than its own form. It improves flushing, refines the surface and allows a single electrode to produce a range of sizes by varying the orbit. It is standard practice for finishing, and is what allows one finishing electrode to be reused as it wears.

Because eroded debris must be cleared from a deep blind cavity where the dielectric has nowhere obvious to flow. Poor flushing causes unstable discharge, arcing, slow cutting and damage to both electrode and workpiece. Techniques include pressure flushing through holes in the electrode, suction, and periodic retraction of the electrode to let fluid in. Flushing strategy is designed alongside the electrode.

A specialist hydrocarbon or synthetic EDM oil rather than the deionised water used in wire machines. It provides a stable discharge, cools, and carries debris away, and it also affects surface finish and electrode wear. Because it is a flammable fluid heated by the process, fire protection, filtration and fluid condition monitoring are normal parts of running a sinker machine.

Commonly at least two, a rougher and a finisher, and often more where wear is significant or where several sizes or details are required. Electrode wear is a real consideration: as the electrode erodes it loses form, particularly on corners, so a worn rougher cannot produce an accurate finish. Planning the electrode set is part of planning the job rather than an afterthought.