Ultra-precision turning method efficient for optical freeform surfaces with a hybrid slow–fast tool servo

The machining of freeform surfaces is a current research hotspot: A slow tool servo (STS) has limitations in machining accuracy and efficiency for large steep freeform surfaces. Most fast tool servo (FTS) tools are limited by their stroke and cannot manufacture freeform surfaces with a large sag. We propose a hybrid slow–fast tool servo method that combines STS and FTS to machine large steep freeform surfaces by decomposing the freeform surfaces and simultaneously turning efficiently with STS and FTS. Experimental studies were undertaken to fabricate a saddle surface. Meanwhile, a variable feedrates tool path was designed and applied to further improve the machining efficiency. The results show that this method can improve the processing efficiency by 47.5%. The arithmetic mean of surface roughness (Ra) is 2–4 nm, and the peak-to-valley (PV) value is 0.4780 µm at the hollows and 0.3884 µm at the swells.