Sinker EDM inspection and measurement methods

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, medical is used as the example: validation evidence that matches the drawing exactly.

How it is measured

CMM and a comparator for the profile. The equipment is calibrated, the uncertainty is recorded, and the part is at room temperature before the first point is taken.

What the report contains

• Every feature called up on the drawing, with a measured value rather than a tick.
• The datum order used, so the report can be reproduced.
• The certificate reference for the material and the calibration dates for the equipment.

Common sources of dispute

Most disagreements are about method, not about the part: a warm part, a burr left on, or a datum taken in a different order. Allow an electrode for every level of detail, or accept a longer cycle

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

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: Wire EDM, Deburring, Duplex 2205 Stainless Steel. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.