Guides / Process

Wire EDM and Invar: When to Use It Instead of Milling

September 20, 2026

Most Invar parts are milled and turned, but some features do not machine well with a cutting tool no matter how the parameters are tuned: very fine internal corners, narrow slots, or geometry with no room for a milling cutter to approach. Wire and sinker EDM handle these cases through a fundamentally different process, and knowing when to reach for it keeps a difficult feature from turning into a difficult, expensive milling job.

How EDM differs from cutting

Electrical discharge machining removes material through controlled electrical sparks between an electrode, either a moving wire or a shaped tool, and the workpiece, submerged in a dielectric fluid. Material is removed by localized melting and vaporization rather than mechanical cutting force. Because there is no cutting tool pressing against the part, EDM does not introduce the mechanical work hardening described in why Invar work hardens, which is one of its practical advantages on this alloy.

EDM does leave a thin recast layer, a very thin zone of resolidified material on the cut surface, which is a different phenomenon from work hardening. On most parts this layer is thin enough not to matter functionally. On parts with a critical surface finish or fatigue consideration, the recast layer is sometimes removed with a light finishing pass or a secondary process.

Where wire EDM fits

Wire EDM cuts a profile using a thin traveling wire electrode, similar in concept to a very precise, non-contact bandsaw. It excels at:

• Fine internal corners that a milling cutter’s radius cannot reach, since the wire itself can be a fraction of a millimeter in diameter
• Narrow slots where a milling cutter would be too fragile or too slow to run at a reasonable feed
• Complex profiles cut through the full thickness of a plate, particularly where the part geometry would otherwise need multiple milling setups
• Very tight tolerance profiles, since wire EDM can achieve fine positional accuracy on cut edges

Where sinker EDM fits

Sinker EDM uses a shaped electrode that erodes a cavity into the workpiece, useful for:

• Blind cavities and complex 3D pockets that do not go all the way through the part
• Features with sharp internal corners in a blind pocket, where wire EDM cannot reach since it requires a through-cut path
• Fine detail work on small, intricate features where a milling cutter would be too fragile

How EDM fits into the overall sequence

EDM is typically used for specific difficult features within a part that is otherwise milled or turned conventionally, rather than as the primary process for an entire part. A typical sequence might rough and stress relieve the part conventionally, then use EDM for the one or two features that milling cannot produce well, before final inspection. Because EDM does not introduce mechanical stress the way cutting does, it does not usually require its own separate stress relief step, though the part’s overall stress relief sequence, covered in stress relief for Invar machined parts, still applies to the conventionally machined portions.

Cost and lead time considerations

EDM is generally slower than milling for material removal rate on straightforward geometry, and it requires programming and setup specific to the process, separate from a milling program. It earns its cost on features where milling genuinely cannot produce the geometry, or where it could only do so with a fragile, slow approach that risks tool breakage or poor finish. For a part with only one or two features that need EDM, the added cost is usually modest relative to the whole part. For a part designed with many EDM-only features, the cumulative cost can be significant, which is worth considering at the design stage if there is flexibility in the geometry.

What to flag on your drawing

• Internal corner radii smaller than a standard milling cutter can produce
• Narrow slots or features with a width to depth ratio that would be difficult to mill reliably
• Any surface where the EDM recast layer needs to be removed for a functional reason

Getting a quote

Not every Invar shop has in-house EDM capability, and some subcontract it for parts that need it occasionally. Flagging difficult features early lets a shop plan the right combination of processes rather than discovering mid-job that a feature cannot be milled as drawn.

Send your drawing through our RFQ form with any hard-to-machine features noted, and we match you with shops equipped to handle the combination your part needs.

Frequently asked questions

Keep reading

• Invar 36 Machining Parameters: Speeds, Feeds and Tool Wear
• Invar Drilling and Tapping: Hole Quality and Thread Practices