Sinker EDM
Die-sinking EDM decisions for conductive cavities: electrode allowances, flushing, wear, surface integrity, and supplier acceptance.
Tags: processes, shaping, edm, tooling
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A hypothetical 20.000 mm cavity with a stationary finishing electrode and 0.025 mm overcut per side needs a 19.950 mm electrode. Subtracting 0.025 mm only once gives a 19.975 mm electrode and 20.025 mm cavity, exceeding a 20.010 mm upper limit by 0.015 mm. Qualify gap, orbit, wear and finishing passes for production.
A shaped electrode removes conductive material by discharge. Makino identifies small, deep features as candidates and notes that milling is generally faster. Makino: sinker EDM
Process and route selection
The electrode approaches the workpiece across a discharge gap; the dielectric supports the discharge process and carries eroded debris away. A custom electrode generates the cavity form. This differs from wire EDM's traveling wire and from dedicated EDM hole drilling. Sodick describes these as separate EDM methods; do not transfer one method's advertised capability to another. Sodick: EDM methods
Oil dielectric is used in commercial sinker platforms, including Sodick's AP series. Confirm the actual machine, fluid and work-material combination rather than selecting settings from a generic EDM description. Sodick: sinker platforms
Use the following questions to structure a supplier discussion:
| Decision | Information to resolve |
|---|---|
| Route | Which features need sinker EDM, and which can be economically pre-machined? |
| Geometry | Can electrodes approach, clear adjacent surfaces and flush the deepest features? |
| Material | What exact alloy, heat-treatment condition and conductivity are available at EDM? |
| Production | How many cavities and parts share electrodes, and what replacement allowance is included? |
| Completion | Which surfaces remain as EDM, and which receive polishing or another finishing operation? |
These questions are an RFQ framework, not a capability table. Require a proposed process for the actual drawing.
Electrode design, wear and flushing
Electrodes need an allowance for discharge clearance and, when used, orbit motion. Autodesk's electrode documentation explicitly distinguishes electrode undersize, spark gap and orbit; its terminology also warns that industry usage can blur these terms. Record whether an allowance is per side or across the full feature. Autodesk: Electrode Machining Wizard
MWI defines per-side overcut as half the diametral overcut. It describes staged roughing and finishing electrodes sized to leave enough material for the final cut. A roughing electrode's dimensions therefore cannot simply be reused as a finishing-electrode drawing. MWI: overcut and staged electrodes
Graphite is not one interchangeable electrode material. POCO's selection guide separates removal rate, wear resistance, finish, machinability and total job cost. It distinguishes volume, corner, end and side wear; corner erosion can change the reproduced contour. Its guide also links fine finish to electrode selection, flushing and generator settings. Evaluate the electrode/work-metal combination rather than purchasing only by graphite price or grain-size label. POCO: graphite selection guide, ©2013, p. 4
Makino identifies debris removal and adaptive electrode-jump flushing as important to its sinker process. Its reported improvements belong to the described equipment and comparison baseline, not to every sinker or cavity. Ask how the supplier will remove debris in the actual deep or narrow feature and what happens to cycle time when flushing becomes difficult. Makino: sinker EDM
Worked example: per-side allowance
Use the hypothetical stationary finishing electrode above. Exclude orbit, wear and other corrections from this calculation:
- Electrode width = 20.000 − 2 × 0.025 = 19.950 mm.
- An electrode mistakenly reduced by only 0.025 mm overall would be 19.975 mm wide.
- With the same assumed per-side overcut, the resulting cavity would be 20.025 mm wide: 0.025 mm larger than the target.
For a hypothetical drawing limit of 20.000 ± 0.010 mm, that second result exceeds the upper limit by 0.015 mm. The calculation illustrates the per-side convention only. Actual electrode sizing must incorporate the supplier's validated gap, orbit convention, electrode wear and finishing sequence; it does not establish process capability or a tolerance budget.
Surface finish and surface integrity
EDM is a thermal removal process. POCO's surface-integrity article distinguishes resolidified recast/white layer from underlying thermally affected material and describes microcracking concerns. It also distinguishes surface roughness from subsurface integrity and discusses the effects of pulse conditions, work material and finishing stages. A visually smooth cavity or a passing roughness result alone does not establish the condition of the underlying material. POCO: EDM Effect on Surface Integrity, pp. 1–3
For a fatigue-sensitive or otherwise integrity-critical feature, have the responsible design authority specify any recast or cracking limits, required finishing, sampling locations and examination method. Agree whether acceptance occurs before or after polishing and how geometry will be retained. This is a recommended acceptance discussion; this page supplies no universal recast-depth limit, removal allowance or approved inspection procedure.
RFQ and acceptance checklist
Provide the revision-controlled model and drawing, datums, cavity dimensions, corner radii, depths, material and heat-treatment condition, quantity, surface-texture requirements, and any applicable integrity specification. Ask the supplier to return:
- The division between pre-machining, rough EDM, finish EDM and subsequent operations.
- Electrode material/grade, allowance conventions, electrode count and replacement assumptions.
- An inspection plan for feature size, location, depth, radii and finish in the final delivery condition.
- Evidence for any claimed capability on comparable geometry and material, including access to the deepest inspection locations.
- A quote separating electrode fabrication, setup, machining, consumables, finishing and inspection, with assumptions that could change delivery time.
Treat catalog accuracy and best-finish figures as equipment claims. They are not simultaneous guarantees for every part, feature size or cavity depth. Obtain drawing-specific agreement before release.
Source versions and scope
Reviewed 2026-11-01. The cited POCO selection PDF carries ©2013 and document code EDM-104612-1113-UK. POCO's reference index lists the surface-integrity article on April 15, 2020; that is the index's publication listing, not proof of the PDF's original issue date. Verified issue dates were not established for the other linked web guidance. No numeric supplier capability is adopted as a general process limit. POCO reference index