Wire EDM: Number of Passes, Surface Finish and Accuracy

Wire EDM cuts punches, die openings, inserts and stripper plates from hardened steel with a thin wire and no cutting force. The accuracy and surface it delivers depend mostly on one decision: how many passes are made. One rough cut is fast and adequate for clearance openings. Precision punches and dies need the rough cut plus several skim cuts, and each skim adds time. This article explains what each pass does, what else affects accuracy, and how to specify the work so you pay only for what the part needs.

Rough cut and skim cuts

The rough cut (main cut) separates the part from the block with high spark energy. It leaves a surface with a relatively thick recast layer, a slight taper or barrel shape through the thickness, and a contour that has moved a little as internal stresses were released.

Skim cuts (trim cuts) follow the same contour with a smaller offset and lower energy. Each one removes a few hundredths of a millimetre or less, corrects the shape, improves the finish and thins the recast layer.

The values are typical for tool steel 30–60 mm thick on a modern machine in a temperature-controlled room. Older machines, tall workpieces and poor flushing all shift them toward the coarser side.

What a pass costs

Rough cutting speed is quoted as area per minute: cut length × workpiece thickness ÷ time. On tool steel with 0.25 mm brass wire, 100–250 mm² per minute is a normal range.

Example. A punch with a 200 mm perimeter cut from a 50 mm thick block:

The EDM Wirecut Cutting Speed/Time Calculator on this site estimates the time from your own values. The point for the designer: asking for Ra 0.3 where Ra 0.8 would work adds 30–40 % to the wire time of every part.

Wire diameter and the smallest inside radius

The wire cannot cut an internal corner sharper than its own radius plus the spark gap.

On drawings for parts cut with standard wire, specify inside radii of 0.2 mm or larger wherever function allows. Matching punch corners should carry the corresponding outside radius so the clearance stays constant around the corner.

What affects accuracy besides the number of passes

1. Internal stress in the workpiece. When the wire opens a block, stresses from heat treatment and grinding are released and the material moves. This is the most common cause of out-of-tolerance wire-cut parts.

• Temper the steel properly: two or three tempers, and for D2-type steels the high tempering range, which leaves less stress and less retained austenite.
• Start from a hole inside the block, not from the edge, so the frame stays closed.
• Make the rough cut around all openings first, let the part relax, then make the skim cuts.
• For punches, leave holding tabs and cut them last. For large openings, remove most of the material with a relief cut or by drilling before the final contour.

2. Flushing. The dielectric must remove particles from the gap. Nozzles should sit close to the top and bottom faces. On stepped or hollow workpieces the nozzles cannot seal; cutting speed must be reduced, and accuracy and wire breakage both suffer. Where possible, wire cut before the block is stepped or pocketed.

3. Workpiece height. In tall cuts the wire bows backward and the wall can become slightly barrel-shaped or hollow. Skim cuts correct it. Expect more skims for the same tolerance above about 80–100 mm.

4. Temperature. Dielectric, machine and workpiece must be at a steady temperature. A block that comes warm from grinding and cools in the tank changes size during the cut.

5. Corners. The wire lags in direction changes and rounds or undercuts corners. Machines reduce speed and energy in corners automatically if the corner strategy is switched on. Sharp outside corners can be improved with a small loop outside the contour.

6. Taper cutting. Die relief angles and tapered inserts are cut by tilting the wire. Accuracy is lower than in straight cuts, and it falls as the angle and the height increase. Keep the precise part of a die opening as a straight land and put the taper below it.

Surface integrity

The recast layer is hard and brittle and carries tensile stress. In ordinary tool steel components, two or more skim cuts reduce it to a few micrometres, which is acceptable for most uses. For highly loaded tools:

• use at least two or three skims on cutting edges;
• polish or lightly hone the cutting edge;
• stress temper after wire cutting, about 25 °C below the last tempering temperature.

Carbide needs extra care. The cobalt binder can be dissolved by electrolysis in the water dielectric, leaving a weak surface. Use a generator mode designed to prevent electrolysis, keep the exposure time short, and make enough skim cuts to remove the affected layer.

Matching passes to components

Specifying wire EDM work

Put the following on the drawing or the order, and let the operator choose the technology from it:

1. Tolerance on the wire-cut contour, and which features it applies to.
2. Surface roughness required, as Ra.
3. Minimum inside corner radius allowed.
4. Land length and taper angle for die openings.
5. Material and hardness, and whether the block has been stress relieved.
6. Any requirement for recast layer removal or stress tempering afterwards.

Do not write "four cuts" on a drawing. Write the result you need. The same result may need three passes on one machine and five on another.

Common mistakes