English size is 1/2 across flutes. 5/8 is diameter of steel. 
Pro-PM turning tools are the "elite" of all turning chisels. 
Made from an advanced Particle Metal, the molten steel has been compressed and sintered giving our Pro-PM turning tools a cutting edge which lasts up to four/five times that of a conventional High Speed Steel tool. 
The chisels are hardened to between 66 and 68 Rockwell, enabling the tools to have a vastly superior wear resistance, excellent grindability, superior toughness, a high working hardness and a high retention of hardness. 
The ergonomically designed beech handles fitted to these tools make the finished chisels unbeatable.