The upper and lower diamond guides are usually accurate to , and can have a cutting path or "kerf" as small as using Ø wire, though the average cutting kerf that achieves the best economic cost and machining time is using Ø brass wire. The reason that the cutting width is greater than the width of the wire is because sparking occurs from the sides of the wire to the work piece, causing erosion. This "overcut" is necessary, for many applications it is adequately predictable and therefore can be compensated for (for instance in micro-EDM this is not often the case). Spools of wire are long — an 8 kg spool of 0.25 mm wire is just over 19 kilometers in length. Wire diameter can be as small as and the geometry precision is not far from ± .