Sandeep Sawant has programmed wire-cut EDM at Accurate Industries since 1980. The wire never rides on the steel. A spark crosses a gap, and that gap has to be in the program or the part comes out small or large. This note is how that allowance is thought about on the shop floor. The technology table on the machine still sets the number that is cut.
The wire does not cut on its centre line
The path you draw is the finished profile. The wire has thickness, and the spark removes a little more than the wire itself. Program the centre of the wire on the finished line and the opening is oversize by the wire radius plus the spark gap, on every side.
A 0.25 mm brass wire is the usual toolroom wire here. Its radius is 0.125 mm. Add the spark gap from the power setting you are actually using. That sum is the offset: outside a punch, inside a die opening. A 0.20 mm or 0.30 mm wire changes the radius, so the offset changes with it. Do not reuse yesterday's offset after a wire change.
Rough cut and skim
The first pass is there to drop the slug and leave stock. The skim passes take that stock, improve the finish, and bring the size onto the drawing. How much stock the main cut leaves depends on the height, the material, the flush, and the technology. On a short steel die opening it is often on the order of 0.05 mm to 0.15 mm a side. Tall work, carbide, and a weak flush need a different power and more stock. Copying one forum number onto every job is how a punch comes out undersize.
| Item | What the program uses |
|---|---|
| Wire | The diameter on the machine. 0.25 mm brass is the common toolroom wire. |
| Offset | Wire radius plus the spark gap from that technology setting. |
| Skim stock | Left by the rough pass. The technology sets it. A short steel opening is often a few hundredths up to about 0.15 mm a side. |
| Finished size | The drawing. The last skim is aimed at that band, not at a nominal. |
Punch and die are opposite offsets
A die opening is an inside path. The wire centre sits inside the finished profile by the offset. A punch is an outside path. The wire centre sits outside the finished profile by the same kind of offset. Mixing those two up cuts a punch that will not enter, or a die that the punch falls through.
Land, taper, and a relief behind a cutting edge are separate passes. The land height on the drawing is not the skim allowance. One is a length of the profile. The other is stock left for the next spark.
The drawing still wins
If the opening is toleranced, look up that code at that diameter. A 20 mm H7 hole is not “20 mm and a bit.” The limits for that size are on the 20mm H7 page, and any other class is on the H7 tolerance calculator. The last skim is set so the measured size lands in that band. A nominal program with no allowance for the wire will not.
Measure the land after the cut. A spark gap also depends on how dirty the water is and how well the nozzle seals. When the measured size walks, change the offset from the part you just cut. Do not keep a single shop constant for every height.
What this note is not
There is no wire-EDM calculator on AlwaysReach, and this page is not one. It does not replace the machine manual or the technology table. Job work — a die, a punch, a mould insert — goes to the shop, not into a browser tool. Drawings can be sent from the contact page. How the other charts on this site are checked is how the charts are kept.