An Investigation Into The Relative Accuracy OfBall-screws And Linear Encoders Over A BroadRange Of Application Configurations And UsageConditions

Abstract

It has been found that ball-screw expansion is the single largest thermal error source within a broad range of CNC machine tool configurations and over a wide range of running conditions when rotary encoder position measurement is used. This paper describes thermally related axis positioning errors exhibited by ballscrew with rotary encoder and linear encoder feedback systems when used within CNC machine tools. Indicative data resulting from many thermal tests has been used to support the analyses. The paper details the thermal error components that make up the total thermal error in both types of positioning system. It is known that the position of the measuring system on an axis can affect the accuracy achieved at the component through the Abbe error. It is accepted that machining often occurs in uncontrolled environments and that thermal errors from external sources can combine to cause additional thermal errors on the work-piece. The study proposes that an axis using a ball-screw and rotary encoder for position measurement exhibits offset, scale and reversal errors that change quickly according to the thermal state of the ball-screw. Linear encoder measurement systems exhibit the same error categories, but generally with smaller maximum values, and slower rates of change. The thermal errors exhibited by ball-screws are primarily due to an inability to decouple the measuring system from the heat in the ball-screw and mounting system that sets the position of the tool relative to the work-piece. Transactions on Engineering Sciences vol 23, © 1999 WIT Press, www.witpress.com, ISSN 1743-3533