Common problems with Sinker EDM and how to avoid them
Sinker EDM is a spark erosion process that forms a cavity with a shaped electrode in dielectric fluid. It is used for producing blind cavities, deep ribs and fine detail in hardened material. This entry explains what it is, what the numbers on a data sheet mean, and what to put on a drawing.
It is written from the point of view of the buyer and the drawing office: the values that are realistic in production, the ones that cost money, and the way the result is verified. Where an industry sets the requirement, semiconductor is used as the example: cleanliness and surface condition that matter more than the nominal size.
The problems that actually occur
These are the failures that show up on a real job, listed in the order in which they are usually discovered.
- Electrode wear ignored, so the cavity ends up shallow. The cause is usually upstream: a datum that is not the functional one, a fixture that does not repeat, or a specification that cannot be measured the way it is written.
- The recast layer left on a fatigue loaded part. The cause is usually upstream: a datum that is not the functional one, a fixture that does not repeat, or a specification that cannot be measured the way it is written.
What causes them
In most cases the cause is upstream of the operation, not in it. Electrode wear ignored, so the cavity ends up shallow is usually a symptom of one of the three causes above rather than a problem in its own right.
How they are prevented
Prevention is duller than diagnosis. Allow an electrode for every level of detail, or accept a longer cycle, and the result is confirmed by CMM and a comparator for the profile before the batch is released.
| Item | Note |
|---|---|
| Failure | electrode wear ignored, so the cavity ends up shallow |
| Prevention | allow an electrode for every level of detail, or accept a longer cycle |
| Tolerance to hold | ±0.01 mm |
| Finish to check | Ra 0.8 µm standard |
Applications
Sinker EDM is used for producing blind cavities, deep ribs and fine detail in hardened material, and in practice it appears most often in mould cavities, forging dies, turbine components, deep ribbed housings.
How it fits into a machining route
Sinker EDM is one operation in a sequence: roughing to leave even stock, any stress relieving the geometry needs, semi finishing, then the finishing operation that holds the tolerance. The order is what keeps the result stable as the machine warms up and the batch runs.The route is fixed before the first part is cut, and it is the route rather than the individual setting that holds the result: electrode manufacture and the number of electrodes. Where a drawing calls for something the route cannot hold economically, that is raised at quotation rather than after the parts are made.
Standards, documentation and what is issued
The documentation requirement is in-house approval with dimensional reports. Material is supplied against a certificate where the application calls for it, inspection results are recorded with the job, and the records are kept so that a repeat order can be compared with the first instead of being measured from scratch.
CMM and a comparator for the profile is the verification step. Where a customer format or a standard report is required, it is agreed before the batch starts, because a report written after the event is a report nobody can use.
Frequently asked questions
What tolerance should be specified for Sinker EDM?
±0.01 mm is realistic in production. Tightening it beyond the functional requirement increases cost without improving the part.
Which material is usually used?
H13 tool steel is the usual choice, supplied against in-house approval with dimensional reports.
How is the result verified?
CMM and a comparator for the profile, with the results recorded so a repeat order can be compared with the first.
What is the most common mistake?
The recast layer left on a fatigue loaded part.
How long does it take and what does it cost?
A realistic cycle is 4 to 20 hours for a cavity, and the cost driver is electrode manufacture and the number of electrodes.
Related capability
If you are specifying Sinker EDM on a drawing and want a second opinion on the route, send the model and the quantity. The first response is a manufacturability review with the likely risk points and a quotation, so the decision can be made on facts rather than on a price alone.
Machining, inspection and documentation are handled in one place, which is why the dimensional report and the material certificate arrive with the parts rather than after them.
Related entries
These entries sit next to Sinker EDM in the encyclopedia: Deep Hole Drilling, Reaming and Boring, 17-4PH Stainless Steel. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
