Wire EDM Compensation and Offset Calculation: From Basic Formulas to Die Machining

Wire EDM shapes a workpiece by spark discharge through a molybdenum wire electrode. Because the wire has a finite radius (typically 0.16mm or 0.18mm) and the discharge leaves a gap, the wire path must be shifted a normal distance from the workpiece contour — the offset (compensation). Get it wrong and the part misses the drawing.

1. The Basic Offset Formula

Offset = wire radius + discharge gap (+ other influence factors)

Example: a 0.18mm wire with a 0.01mm discharge gap gives a basic offset of 0.18/2 + 0.01 = 0.10mm.

The discharge gap is looked up from standard reference tables according to workpiece thickness, wire radius and current. For high-precision work, secondary factors such as wire vibration should also be included.

2. Offset Direction: Internal Holes vs. External Profiles

Internal holes and external profiles use opposite compensation signs:

• Internal hole, cut tight: offset = (wire radius + discharge gap) + extra clearance wanted
• Internal hole, enlarge clearance: offset = (wire radius + discharge gap) − clearance required
• Out-of-range (negative) compensation is applied on the outside of the path
• External profiles: signs reversed — adding clearance means adding, reducing means subtracting, and out-of-range compensation goes inside the contour

3. Worked Examples for Die Machining

Given: wire radius 0.06mm, one-side gap 0.01mm, fit clearance 0.015mm; backing plate 0.01mm smaller, stripper plate 0.02mm larger than the punch.

Piercing die (part size governed by the punch; fit clearance taken off the die):

• Punch: f = 0.06 + 0.01 = 0.07
• Die: f = 0.06 + 0.01 − 0.015 = 0.055
• Backing plate: f = 0.06 + 0.01 + 0.01 = 0.08
• Stripper plate: f = 0.06 + 0.01 − 0.02 = 0.05

Blanking die (part size governed by the die; fit clearance taken off the punch):

• Die: f = 0.06 + 0.01 = 0.07
• Punch: f = 0.06 + 0.01 − 0.015 = 0.055
• Backing plate: f = 0.06 + 0.01 + 0.01 + 0.015 = 0.095
• Stripper plate: f = 0.06 + 0.01 + 0.015 − 0.02 = 0.065

Note: punches and mold ejector pins differ — a 5mm punch is a standard 5mm, but an ejector pin for the same hole is 4.975mm. The same hole cut with different compensation is a different part.

4. Offset Settings for Multi-Pass Cutting

Medium-speed wire EDM (MS-WEDM) commonly uses multi-pass cutting. Principles for setting each pass:

• The second pass decides final size; later skim passes barely affect it
• The final skim pass controls surface finish
• Each earlier pass leaves an allowance of wire radius + discharge gap + one-side skim allowance; the last pass takes none
• If finish improves but size accuracy worsens, re-check the allowances — thick parts and weak regimes are more sensitive to how allowances are distributed
• For critical or tall parts, verify offset data for every pass; do not rely on the manual alone

5. Manual Skim-Cut Compensation on HS-WEDM

High-speed wire EDM (HS-WEDM) has no automatic skim-cut function, so compensation is calculated by hand:

• Offsets depend mainly on workpiece thickness and vary with material (commonly 0.02–0.06mm)
• Parts over 50mm thick commonly use offset pairs of 0.055mm and 0.015mm
• Wrong first- and second-pass offsets stall the skim passes and produce a barrel-shaped part
• Trial-cut tight-tolerance parts first; measure the wire diameter regularly after prolonged use and recalculate from the actual diameter
• For parts that fit into an assembly, allow for tolerances, use a small R at small corners, and distinguish internal from external R

6. Broken Compensation Paths and Reverse Compensation

When two small arcs meet in a cavity and the software reports “path breaks after compensation”:

1. Offset the original geometry inward by one compensation value
2. Start an equidistant offset from the arc junction and draw a 90° auxiliary line from either arc center
3. Extend both arcs to the auxiliary line and connect the intersections
4. Apply reverse (outward) compensation to generate the correct program

Mastering the offset formula, the inside/outside sign convention and the die examples — plus “measure the wire often, trial-cut tight parts first” — is the foundation of accurate wire EDM programming.