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Encoder Provides Static and Dynamic Contouring Accuracy Inspection of CNC Machine Tools

Machine tool performance from the point of view of compliance to tolerances, surface definition, etc., is determined essentially by the accuracy of machine movement. For precision machining it is therefore important to measure and, if necessary, compensate for deviations in motion. Standards and directives for inspecting machine tools stipulate a number of measuring methods for determining static and dynamic deviations.

These can be measured exactly with grid encoders for static and dynamic testing of the positioning and contouring behavior of CNC machine tools. In conjunction with the powerful evaluation software, HEIDENHAIN measuring devices for the inspection and acceptance of machine tools permit meaningful machine measurements with minimal mounting and adjustment effort

The Heidenhain KGM 200 grid encoders series provides for static and dynamic contouring accuracy inspection of CNC machine tools per ISO 230-2, ISO 230-3, ISO 230-4, and VDI/DGQ 3441. The encoder provides a direct inspection method for static and dynamic behavior with zero-contact and zero-wear scanning allowing measurement of any circular diameter and freeform shape in the working plane.

The acceptance testing and inspection of machine tools essentially includes the static verification of the geometrical machine structure in the unloaded state and, for NC controlled machines, on the verification of positioning accuracy. The final results of machining are increasingly influenced by dynamic deviations from the nominal contour and by high acceleration rates in the machine tool. Test workpieces are therefore produced and inspected for dimensional accuracy in order to draw conclusions about the dynamic behavior of the machine.

HEIDENHAIN offers measuring devices for direct capture of dynamic and static deviations. 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, particularly at high traverse speeds, provide information on contouring behavior from which conclusions can be drawn 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. This information (e.g., kv factor, reversal spikes) can be used to optimize the machine’s behavior.

Static measurements, such as the measurement of position error in linear and rotary axes using a comparator system, permit conclusions about the geometric accuracy and thermal behavior of the machine.

Circular interpolation tests with very small radii and free-form tests provide information on the dynamic behavior of the control, and circular
interpolation tests with large radii provide information on the machine geometry.

Position accuracy and repeatability, as well as guideway errors of linear machine axes, are determined with a comparator system. The position accuracy and repeatability of rotary axes, rotary tables, and tilting tables can also be determined.

The HEIDENHAIN KGM grid encoder consist of a scanning head and a grid plate with a waffle-type graduation, which is embedded in a mounting base. The plate and the base are decoupled mechanically, so that tensions resulting from mounting do not influence the measurement accuracy. During measurement, the scanning head moves over the grid plate without making contact. The KGM encoders capture any motions in a plane and separately transmit the values measured for the two axes.

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