The retarder shafts are produced for the end customer and gear box manufacturer. The production process of these shafts comprises an oil tempering as well as a blasting with steel balls after the metal cutting. The tempering oil hardens while the shaft is cooling down and blasting material adheres to it – particularly in the internal thread of the hollow cone. This resulted in very expensive claims and return shipments of complete trailer trucks full of goods. With the introduction of our technique this problem is completely solved.