The importance of direct inspection methods for machine tool accuracy

Machine tool performance is essentially determined by the dynamic and static accuracy of machine movement. 

For precision machining, it is important to measure and compensate motional deviations. Guidelines and standards for inspecting machine tools stipulate a number of measuring methods for determining dynamic and static deviations. 

Conventional inspection and acceptance testing of machine tools has been limited to static measurement of the geometrical machine structure without load, and on controlled machines to measuring positioning accuracy. Since the results of machining depend increasingly on dynamic deviations from the nominal contour and on high acceleration in the machine tool, the finished parts are also inspected for dimensional accuracy to draw conclusions about the dynamic behaviour of the machine. 

The Heidenhain measuring systems KGM and VM 182 can be used to directly measure both dynamic and static components of deviation. The advantage of this direct inspection method over inspecting only the results of the machining lies in its separation of technological influences from machine influences, and in its capability of distinguishing individual factors of influence.  

Dynamic measurements — especially at high traversing speeds — provide information on contouring behaviour that permit conclusions about both the condition of the machine tool, as well as the parameter settings of the control loop consisting of the CNC control, drives and position feedback systems. 

Static measurements — such as the measurement of position deviations in the linear axes using a comparator system — permit conclusions exclusively about the geometric accuracy of the machine. 

Machine tool builders use the results of machine accuracy inspection for the purpose of making design improvements to increase accuracy. Such measurements also help them optimize the commissioning parameters of the control loop wherever they influence the accuracy of a CNC machine. 

Machine tool users can use these measuring systems for acceptance testing and regular inspection of their machine tools.  

For more information, contact Heidenhain. 

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