I'm just getting going on my CNC-converted South Bend "Fourteen" 14x40 lathe. It's was factory converted to ballscrews and servomotors in the late 80's or early 90's, and recently the control was swapped out for a PMDx board and gecko drivers and I'm running Mach3. I'm having some strange issues with surface finish. It's not that the the finish isn't smooth, but there are regular waves in it. Basically the diameter of a finished part wanders in and out .001"-.002" every 1/4" of length or so. The strange thing is that changing the feed rate doesn't alter the location of these waves. You would think that a slower feed rate would make them closer together and a faster feed rate would make them farther apart, but that doesn't seem to be the case. It's most exaggerated on a high-rake cutter (like for aluminum). I would also note that it seems more inclined to do it when running a g-code program than when I'm just using the jog function. It seems like the x-axis servo wanders in and out about a thousandth depending on the z-axis position. I'm wondering if this is something that can be tuned out? I've included a picture of a pair of aluminum parts I turned (about 1" OD) with a nice CCGT high-rake aluminum insert. It left a smooth finish, but you can see the color striations which correspond to a diameter variance. The dark bands are lower diameter, about .001"-.002" smaller (diametral) than the light bands. Even another light pass yielded the same pattern. Any thoughts? I only recently acquired this machine and most of the converting and setup work was already done.