±0.0006" (0.015 mm); roundness ±0.00015" (0.004 mm); straightness ±0.00015" (0.004 mm); bearing area 90% deep 0.00008" (0.002 mm).
In conventional turning, which generates a continuous groove, surface finish is largely determined by insert nose radius and feed rates, and, to some extent, also cutting speed and depth of cut. In contrast, the surface quality of plunge- turned components is mainly dependent on the quality of the cutting edge. As a result, plunge-turned components display low residual stress in the axial direction, and sealing proper- ties (as with a bearing surface) are equal to or better than ground surfaces.
Using solid PCBN inserts, these stress patterns remain constant, even when large batches of parts are machined. In another test, in which Seco examined the physical properties of 250 plunge-turned parts, the residual stress was found to be significantly lower than the stresses caused by grinding. Part quality is also affected by the distribution of residual
stress in the tangential direction. Still in another test of 250 parts, both grinding and plunging produced tensile residual stress at the surface, but the thickness of the affected zone was significantly thinner on plunge-turned components. Plunge turning also was found to reduce the occurrence of white layers, because it reduces flank wear on the tool edge due to shorter cutting time per machined surface. This also generates lower cutting forces and less friction, which results in less heat exposure to the surface of the workpiece. The problem of white layer generation exists in both grind- ing and hard turning. Although they are commonly associated with residual tensile stress at the surface, white layers may also indicate residual compressive stresses. Either way, the cause of white layers and the effects they have on the finished workpiece aren’t entirely understood, although tool wear could be a contributing factor. ME For more information from Seco Tools Inc., go to www.secotools.com, or phone 248-528-5200.