Multi-Pass Cutting in Wire EDM: Finish-Cut Essentials

Cutting efficiency, molybdenum wire wear and workpiece surface roughness pull in opposite directions, and no single pass can maximize all three: high efficiency means poorer finish and heavier wire wear; a fine finish demands low cutting efficiency. Multi-pass cutting (finish cutting) is the process that resolves this conflict.

1. The Cutting Sequence

The core upgrade from HS-WEDM to MS-WEDM is finish-cut capability (multi-pass cutting). The usual sequence:

• First pass: cutting, at high efficiency.
• Subsequent passes: refining surface finish.

This is why MS-WEDM achieves a higher surface finish than HS-WEDM.

2. Controlling Molybdenum Wire Wear (Key to Accuracy)

The molybdenum wire discharges repeatedly during multi-pass work. As the cut area grows, the wire stretches and wears, growing thinner, which erodes accuracy and weakens discharge quality in later passes. Therefore:

1. Measure the actual wire diameter before each cutting cycle and set compensation from the measured value.
2. Re-tension new wire after cutting the lead-in (guide) path and before the formal cut.
3. After long reuse, measure diameter and roundness, and replace the wire when wear becomes significant.

3. Checks Before and During Finish Cuts

1. Dry run: run the wire with no discharge and check for vibration and correct tension — the sapphire guide eyes fitted as standard on MS-WEDM keep the wire stable.
2. Conductive blocks: check the contact surface for discharge marks and make sure the wire is not cutting into the block.
3. Dielectric flow: verify normal flow rate.
4. Program simulation: simulate the sequence before cutting to confirm the generated program is correct.
5. Remaining stock: for the final cut-off part, leave a generous remaining stock based on the material to reduce distortion effects.

4. Handling Surface Issues After Cutting

1. Surface line marks: possible causes include wire vibration, material distortion and improper offset settings — check each one.
2. Severe dark staining at both ends: usually a dielectric problem; switch to a suitable working fluid.
3. Cleaning: finished parts can be cleaned with oxalic acid, concentrate or kerosene, wiped dry with a clean white cloth, then coated with rust-prevention oil.

Summary: multi-pass cutting is about “separating efficiency and quality across passes.” Follow the first-pass-cuts, later-passes-refine sequence, control wire wear, and complete the pre-cut checks, and high-finish, high-accuracy results become routine.