Ultraprecision turning with binderless CBN

Ultraprecision machining of steel with geometrically defined diamond cutting edge is not yet a stable industrial process. Therefore the optical surface quality corresponding to the optical function on the final product is achieved by polishing, classically. Unfortunately, the form accuracy of the work piece is affected by polishing processes. Successful ultraprecision facing experiments of fully heat-treated steel 40CrMnMoS8.6 has been made with binderless cBN as cutting material. Results of roughness values Ra < 35 nm and peak to valley of P-V < 200 nm were achieved. Removal rate is Q > 0,1 mm(exp 3)/s and the tool total path is about W(ind Ra) = 80 m. The generated surfaces show a homogeneous structure. Tools can be evaluated regarding geometrical characteristics and abrasion. Experiments show that the cutting material binderless cBN is of high potential regarding the machining of steel for moulds and dies. Despite the achieved roughness, the obtained surfaces are not of optical quality. This is documented by a Ra/PV ratio of about 1/7. And despite the homogeneity, the surface is refractive and therefore appears rough. In further research activities, particularly the preparation of the cutting edge has to be scrutinized.