EDM is a precision non-traditional machining technique. It is widely used to manufacture mechanical parts. It has unique advantages over conventional cutting methods for machining certain parts. Under certain special conditions, parts with good surface finish can be obtained by machining parts using EDM. It also meets the requirements of design drawings well and achieves a certain machining accuracy. Therefore, EDM is becoming more and more common in various industries. The next part of this paper will first discuss what EDM surface finish is and the factors that affect it. After that, we will introduce the methods of measuring surface finish. Finally, we will discuss how to improve EDM surface finish and the application areas of EDM machining.
What is EDM surface finish?
EDM surface finish refers to the measurable roughness left by spark erosion on a conductive workpiece. It is typically defined using roughness parameters like Ra and Rz, along with measurement standards and settings (e.g., cutoff and evaluation length).
- Ra: The arithmetic average roughness, measured in micrometers (μm), is the most common parameter on prints.
- Rz: Represents peak-to-valley behavior over defined sampling lengths and is more sensitive to isolated pits or spikes.
A “good” EDM surface finish isn’t just about appearance. It must meet functional requirements such as sealing, sliding, fatigue life, and coating performance.
How EDM Creates Surface Texture
EDM generates surface texture through repeated discharges that melt and vaporize tiny volumes of material, leaving overlapping micro-craters. Unlike milling or grinding, EDM does not create a directional “lay” because there is no cutting edge moving across the surface.
After each discharge, some molten material resolidifies on the surface, forming a recast (white) layer. This layer, along with the crater geometry, makes the surface appear uniform but still measure “rough” under a profilometer.
If energy levels are too high or flushing is unstable, discharges can turn into arcing. This often results in random pits, an “orange peel” texture, and inconsistent roughness across zones.
Key Parameters That Control EDM Surface Finish
The quality of an EDM surface finish depends on the energy delivered by each spark and the stability of the gap. Lower discharge energy and greater process stability generally improve surface roughness, even if cutting speed decreases. Below are the key parameters that influence Ra and Rz:
- Peak Current (Ip):Peak current determines discharge energy and crater size. Higher Ip increases material removal rates but leaves a rougher surface. Lower Ip is better for finishing as it reduces crater depth, though it slows cutting and requires better flushing.
- Pulse On-Time (Ton) : Pulse on-time controls how long energy is applied during each discharge. Longer Ton increases crater size and roughness, especially with high current. Shorter Ton supports fine finishing by limiting melt volume and reducing thermal damage.
- Pulse Off-Time (Toff) : Pulse off-time allows deionization and debris evacuation. If Toff is too short, ionization persists, causing instability and higher roughness. A longer Toff improves stability and surface consistency but reduces productivity.
- Gap/Servo Control: Gap control ensures stable sparking instead of arcing or shorting. A stable servo response creates uniform craters, improving Ra repeatability. Poor gap control often results in burnt marks, pits, or banding, which increase Rz.
- Flushing and Dielectric Condition: Flushing removes debris and cools the cut zone, while dielectric fluid controls ionization. Poor flushing leads to re-deposition, secondary discharges, and random pitting. Stable flushing is often the quickest way to improve surface finish.
- Number of Passes (Rough + Skim) : Multiple passes are the most effective way to improve surface finish without compromising stability. A rough cut removes bulk material, while skim passes use lower energy to refine the surface. More skim passes reduce Ra, but returns diminish after a point.
| Strategy | What changes | Typical finish outcome |
|---|---|---|
| Rough cut only | Higher energy, faster removal | Higher Ra, more texture variation |
| Rough + 1–2 skim cuts | Lower energy refinement | Lower Ra, more uniform surface |
| Rough + 3+ skim cuts | Very low energy, stable gap | Best repeatability, smaller incremental gains |
How to Measure and Specify EDM Surface Finish?
To specify EDM surface finish correctly, pair Ra/Rz with the standard and the measurement settings. If cutoff and evaluation length are not aligned between parties, the same surface can produce different reported values.
Common standards include ISO 4287, ASME B46.1, and JIS B0601. Pick one, write it on the drawing or inspection plan, and keep it consistent through incoming, in-process, and final inspection.
- Stylus Profilometer: A stylus profilometer traces the surface and converts vertical motion into roughness values. It is fast and works well for most EDM metals.Record cutoff length, evaluation length, and filter settings. Avoid contact measurement on soft surfaces or delicate features if damage is a concern.
- Optical / Interferometry: Optical methods measure surface topography without contact. They are useful when finish targets are very fine or the surface cannot be touched.They usually cover a smaller area and can be sensitive to vibration and reflectivity. Use them when stylus access is limited or when a non-contact method is required.
- Comparison Specimens: Comparison specimens provide a quick visual/tactile check. They are useful for non-critical parts or informal process checks.They are subjective and should not be the acceptance method for tight finish requirements.
- Reporting Basics: EDM surfaces often show little directional lay, but geometry and flushing can still create localized patterns. Document where the measurement is taken and the path used.When pits or burn marks are possible, report both Ra and Rz. Ra alone can hide isolated defects that matter in sealing or wear.
Practical Ways to Improve EDM Surface Finish
Improving EDM surface finish involves lowering discharge energy, stabilizing the gap, and using finishing passes to reduce recast and peak height. Here are step-by-step strategies:
- Use Skim Cuts: Skim cuts re-machine the surface left by roughing, reducing recast layer thickness and making Ra more consistent. Add skim passes for sealing or sliding surfaces, but stop when measurements show diminishing returns.
- Lower Discharge Energy: Reduce Ip, shorten Ton, and stabilize Toff to shrink crater size. Make one change at a time and confirm improvements with measurements. Avoid ultra-low energy if it destabilizes the gap.
- Stabilize Flushing: Improve nozzle position, flow, and debris evacuation. Many roughness issues, like pits or burn marks, stem from poor flushing. Fixing this often stabilizes Ra and lowers Rz.
- Control Wire/Electrode Condition: Worn wire, poor tension, or unstable feed can cause vibration and spark instability, leading to inconsistent finishes. For sinker EDM, ensure stable electrode wear and quality.
- Post-Finish Options: If EDM alone cannot meet Ra targets, use secondary processes like polishing or grinding. Manage recast and microcracks carefully for fatigue- or sealing-critical applications.
Typical Finish Targets by Application
“Smooth enough” means the finish that meets function without unnecessary time and cost. Set the roughness target from the job requirement, not from appearance alone.
Mold Cavities
Mold surfaces often need controlled texture transfer and reliable demolding. Too rough can print defects; too smooth can raise sticking risk in some materials.
EDM finishing plus controlled polish is common when the cavity finish directly affects part appearance and release.
Sealing / Sliding Surfaces
Sealing and sliding surfaces are sensitive to peaks and defects. Rz can be as important as Ra when leakage or wear is driven by isolated pits or spikes.
Skim cuts plus stable flushing are usually the most cost-effective finish improvements. Confirm with functional tests when possible.
Aerospace / Medical
These applications often emphasize repeatability and control of thermally affected layers. Local defects can become crack initiators or wear accelerators.
Use stable finishing settings, control the measurement method, and lock the process window so finish stays consistent batch to batch.
Conclusion
Achieving the best surface finish in EDM is a pursuit of great significance. By optimizing the EDM parameters and post-processing techniques, the surface finish of the parts will be better. With the continuous development of technology, wire EDM machine shops are playing a crucial role in delivering precision machining solutions, making EDM more widely used across various industries. If you still have questions about EDM technology, you can contact us for answers. Also, if you have EDM projects and needs, please contact us. We will be happy to give you top-notch Wire EDM Machining Services. We look forward to hearing from you!
FAQ
Can EDM produce a mirror-like surface?
Yes, but it usually needs low-energy finishing and multiple skim passes. Stable sparking, clean dielectric, and good flushing are the foundation.
Why does Ra look OK but the part still leaks or wears fast?
Ra is an average and can miss isolated defects. Check Rz and inspect for pits, burn marks, and arcing damage.
What is the quickest shop-floor change to improve EDM surface finish?
Add skim cuts and correct flushing first. Those two changes often improve both Ra stability and defect rate quickly.
Does dielectric condition affect EDM surface finish?
Yes. Contaminated or unstable dielectric increases erratic discharges and raises the chance of pits and rough zones.
How do I reduce burn marks and random pits?
Improve debris evacuation and stabilize the gap (servo behavior and adequate off-time), then reduce discharge energy. Burn marks are commonly linked to arcing triggered by trapped debris or unstable gap conditions.
