When there is more metal in a blank -- more mass than the die chamber allows (the die chamber is that area between two dies in the extended position and the collar) -- the metal must flow out somewhere. Pressmen hate this. Unless, of course, they are using open face dies (which do not use the restraing collar) and the excess metal flows out between the dies, as intended.

The excess metal is a burr or fin. In metalworking this is called flash, and it is easily removed by trimming. The flash formed on large round medals, as those struck with open face dies (called box dies in England), for example, is simply turned on a lathe and trimmed off.

For pieces struck in a coining press the only place for that excess metal to flow out is between a die and the collar. On a struck coin or medal this forms that burr or fin at the rim-edge juncture -- at the exact point where the edge meets the rim. That is a wire edge. It is also called a knife edge, or even a knife rim.

Coining press operators really hate this! They first try to readjust the amount of impression -- lower the pressure. They hope they can still get a fully struck up piece with surface metal filling every die cavity, even filling all the high points on that struck piece, with a single blow at the lower pressure.

If continued striking at the lower pressure still results in a wire edge, with any extraneous extrusion, they step up their swearing. It may require to replace the blanks of a thinner gauge. Continued use of oversize blanks or too strong an impression resulting in wire edge strikes will cause excessive wear to both the collar and the edges of dies. Dies are usually the first to break. Where? At the edge, creating what pressmen call a rim diebreak and what collectors call a cud on a struck piece.

Blanks are upset before being fed into a coining press. The upsetting process thickens the edge somewhat so it requires less metal that needs to be moved around when the piece is struck. For the usual business strikes, say coins intended for circulation, such a sharp point at that rim-edge juncture is not required. For striking proof surface coins and medals, however, it is intended to make that point at the rim-edge juncture as full as possible producing a 90-degree angle. Thus wire edges are more likely to be found on proof surface coins and medals than business strikes.

The private Canadian firm of Lombardo struck proof surface medals with wire edges so frequently it became a diagnostic of their production. This was probably because they were using old coining equipment (as with worn collars) and high pressure to get a fully struck piece. They ultimately solved that production anomaly by adding another step -- they trimmed off the wire edge by placing the proof piece in a lathe and milled off the wire edge down to the rim.