In using a fly cutter to machine potassium dihydrogen phosphate (KDP) crystals, rippling in machined surfaces will remain that will have a significant impact on the optical performance. An analysis of these lowspatial frequency ripples is presented and its influence on the root-mean-squared gradient (GRMS) of the wavefront discussed. A frequency analysis of the machined KDP crystal surfaces is performed using wavelet transform and power spectral density methods. Based on a classification of the time frequencies for these macroripples, the multimode vibration of the machine tool is found to be the main reason surface ripples are produced. Improvements in the machine design parameters are proposed to limit such effects on the wavefront performance of the KDP crystal. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)