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I noticed that the ball screws on my machine get quite hot after being run for a few hours but repeatability doesn't appear to be affected. If the ball screws expand and grow in length when hot, how come I can still hold size?
When in a production environment all ball screws generate heat, preloaded ball screws even more. After the start of a production run or shortly after the beginning of a shift, ball screws reach operating temperature. For most machine operators the first 60 minutes is a frenzy of offset and insert changing, then when reaching operating temperature the machine settles down to consistently hold size without artificial controls on heat expansion.
There are machine manufacturers that feed liquid coolant into the core of the ballscrew to stabilize and control expansion. Still others incorporate electronic compensation calculated to override the disadvantages of heat expansion and pitch errors. Incredible as it may seem, on CNC internal and external grinders it is now possible to regularly hold a diameter tolerance of 1 or 2 micron on production runs of many thousands. CNC machine tools are remarkable devices but without the consistent reliability of the modern ball screw they would be nothing.