OKK's Soft Scale software compensates for thermal differences in two ways, said Bob Meier, an OKK spokesman. Soft Scale II adjusts the table based on the difference between temperature measurements from sensors in the spindle head and at the base of the machine. To correct for thermal expansion along the ballscrews, Soft Scale I tracks the activity of the servodrives. Compensation increases as the machine becomes more active and as the ballscrews' nuts travel farther from their anchors. Activity increases friction along each screw and the heat generated inside its motors.
Greater distances be-tween the anchors and nuts means more material is susceptible to expansion.
The software and an internal cooling system have allowed OKK's engineers to create a large machine that is accurate and repeatable to 10 microns along the Y axis. Meier said that this rating is good at all times, including warm-up periods at the beginning of the day. He said that repeatability improves once the machine reaches a steady state. Engineers in Japan created error maps for the machines and described them with multivariable empirical formulas. The algorithms account for both long, continuous cuts and interruptions from lunch breaks, tool changes and setups.