Failure Diagnosis

When Sinker EDM Produces a Wrong Part, the Cause Is Specific and Correctable

Oversized cavities, poor surface finish, positional errors, and in-service cracking from sinker EDM are not machine failures — they're process variable failures. Each symptom points to a specific cause.

• Internal corners rounded instead of sharp
  Cause: Electrode corner wear during roughing passes reducing the corner form, or orbital motion radius too large for the required corner geometry. Internal corners in sinker EDM cannot be sharper than the electrode corner after wear — the corner erodes first because it has the highest current density. For very sharp internal corners (radius ≤ 0.005"), plan for a dedicated finishing electrode with the correct corner geometry, apply minimal orbital motion on the corner pass, and account for electrode corner wear in the electrode form tolerance. Using the same electrode for roughing and corner finishing guarantees rounded corners — two-electrode sequence is required for tight internal corner radii.
• Cavity depth consistently short of target
  Cause: Electrode wear not compensated in the depth program, or Z-axis datum lost between electrode changes. Electrode wear removes material from the electrode tip as EDM progresses — if the program drives the electrode to a fixed Z depth without compensating for electrode shortening, the cavity will be shallower than intended by the wear amount. For graphite at 20% wear ratio on a ±0.0002" depth tolerance, this requires compensation on any cavity deeper than 0.001". Verify the electrode length before and after roughing to quantify wear, adjust the finishing electrode Z target accordingly, and re-verify the workpiece datum Z reference after each electrode change.
• Rough or pitted cavity surface finish
  Cause: Inadequate dielectric flushing allowing particle concentration to rise above the threshold for stable arc discharge. The symptom — a rough, randomly pitted surface with no regular pattern — is distinctive: it's not feed marks, it's re-arc damage from secondary discharges in contaminated dielectric. In deep cavities, the solution is pressure flushing (through-electrode or through-fixture), not simply increasing flow rate of the ambient bath. If the cavity geometry does not allow flushing holes in the electrode, consider reverse flushing (suction through the gap) or jet flushing aimed at the gap entry. Also check dielectric filter condition — a saturated filter recirculates rather than removes particles.
• Cavity position displaced from datum
  Cause: Datum reference not re-established after electrode change, or workpiece movement between electrode changes from fixture inadequacy. When the finishing electrode is installed and the datum is assumed rather than verified, any error in the electrode change setup — slightly different Z height in the toolholder, minor XY offset — displaces the finishing cavity from the roughing cavity position. On a cavity with positional tolerance of ±0.0005", an electrode change without datum re-verification can consume the entire positional budget in a single setup error. Verify XYZ position of every electrode at the datum reference point before beginning each EDM sequence.
• Workpiece cracking after EDM at or near the cavity
  Cause: Residual tensile stress in the recast layer combining with existing part residual stress from prior heat treatment, particularly in highly alloyed tool steels (D2, M2, H13) where residual stress after hardening is already elevated. The recast layer from sinker EDM carries residual tensile stress — in hardened steel, this can initiate cracking at stress concentrations immediately post-EDM or under light service load. The risk is higher in thin sections, at sharp corners, and in materials with limited ductility at the as-hardened condition. For fatigue-critical components, recast layer removal after EDM is required — confirm via acid etch (nital etch for steel reveals recast as a white, unetched layer) or metallographic cross-section to verify complete recast removal before releasing parts.
• Batch hardness or surface variation across production cavities
  Cause: Dielectric condition degrading across a production batch, or electrode wear changing parameters without corresponding program adjustment. As a dielectric bath accumulates eroded particles across a production run, the effective gap characteristics change — more frequent re-arcing occurs, material removal rate drops, and surface finish degrades. In production sinker EDM, dielectric condition must be monitored via conductivity measurement (target range 5–20 µS/cm for oil dielectric applications) and filtered or replaced at defined intervals. Also check electrode wear data at each part — if wear is accelerating across the batch, a consumable or dielectric problem is developing, not a machine problem.