Freeform optics facilitate a revolutionary approach to optical design. Ultra-precision diamond machining is an enabling technology for the manufacture of freeform optics. However, optical designs require tight tolerances across spatial wavelengths ranging from micro-roughness to overall form. To manufacture freeform optics to the necessary tolerances, a novel artifact-based error reduction methodology applicable to multi-axis diamond milling was developed. As demonstrated on a test sphere, the methodology reduces dominant error sources on the test sphere surface from 194 to 40 nm RMS. The approach is then successfully applied to an example freeform optical design where comparable error levels are achieved.