I tried to search the forums for answer to these questions but I really didn't find anything. I understand the process of calculating the steps/mm value and the calibration process for the steps/mm value. I think I'm in pretty good shape with my machine. My two questions are: 1.) What are the primary variables that lead to the need to calibrate the steps/mm values. In essence if all variables were completely predictable then the steps/mm calculation would not need to be tweaked. I'm using the 8mm Acme threaded rod. I'm assuming the manufacturing process for the acme rod is adequate to provide a consistent product. Am I wrong here? Is the current variable a factor? The anti-backlash nuts? 2.) Based on the variables in play, how often does the steps/mm calibration need to be checked? It would be great if it was one and done but something tells me that might not be the case. Any input would be appreciated. Thanks.