Wire EDM Cutting Speed & Kerf Offset Calculator
Calculate total spark kerf offsets, volumetric cutting speeds, multi-pass skim cut step-overs, wire lag deflection, and minimum internal corner radius limits.
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Wire Electrical Discharge Machining (WEDM) Physics
Wire EDM erodes conductive metals through millions of microscopic electrical spark discharges occurring between a continuously moving brass or zinc-coated wire and the workpiece submerged in deionized water.
1. Total Programmed Kerf Offset Formula
CNC wire EDM paths must be offset from the finished drawing contour by the total radial kerf allowance (S):
$$S = \frac{d_w}{2} + g + d_{\text{skim}}$$
Where (d_w) is wire diameter, (g) is the electrical spark discharge gap (typically 20 to 35 μm on roughing, decreasing to 5 μm on ultra-fine finishing), and (d_{\text{skim}}) is stock left for subsequent skim passes.
2. Material Removal Rate (MRR) & Feed Velocity
WEDM speed is rated in square millimeters per minute of cut area ((\text{mm}^2/\text{min})):
$$V_{\text{feed}} = \frac{\text{MRR}}{H}$$
For example, cutting 50 mm thick tool steel at an MRR of 120 mm²/min yields a linear table feed of (120 / 50 = 2.4\text{ mm/min}).
3. Wire Lag & Inside Corner Radii Limits
Electromagnetic spark pushback and dielectric flushing pressure deflect the wire backward mid-span relative to the upper and lower guide heads (wire lag (\delta_{\text{lag}})):
- Inside Corners: The minimum achievable inside corner radius equals the total roughing offset (S) ((R_{\text{min}} = r_{\text{wire}} + g \approx 0.155\text{ mm}) with 0.25 mm wire).
- Corner Washout: Unless the CNC decelerates or pauses at vertices to let wire lag catch up, corners will barrel or wash out by 5 to 15 μm.
Frequently Asked Questions
Why does wire EDM require multiple skim passes to achieve high precision?
The initial roughing pass generates significant thermal stress, a recasting white layer (5-10 μm thick), and wire deflection. Skim passes reduce discharge energy to progressively vaporize the recast layer without deflecting the wire, yielding sub-micron tolerances and mirror Ra < 0.2 μm finishes.
Why is submerged machining vastly superior to spray nozzles?
Submerged cutting completely floods the kerf, eliminating air bubbles that cause catastrophic wire breaks, stabilizing workpiece temperature to prevent thermal growth, and doubling achievable cutting speeds in thick plates.