The KGM grid encoders dynamically tests the contouring accuracy of CNC-controlled machine tools. It can perform, for example, circular interpolation tests with radii ranging from 115 mm down to 1 µm at feed rates up to 80 m/min. The KGM can also perform free-form tests in two axes. The advantages of the KGM include its contact-free measurement, which eliminates the influence of ball bearings such as with the DBB. Also, the error resulting from the machine's geometry has no influence on the results of circular interpolation tests with very small radii.
Measuring assembly and signal evaluation
The KGM consists of a grid plate with a waffle-type graduation, which is embedded in a mounting base, and a scanning head. During measurement, the scanning head moves over the grid plate without making mechanical contact. For setup, the mounting base is fixed onto the workpiece-holding element (such as the table of a machining center) and aligned. The scanning head is mounted in the locked toolholding element (for example, the spindle of a machining center) and is also approximately aligned. A spacer foil is used to set the scanning gap to 0.5 ±0.05 mm. A finer setting is then attained by turning a set of screws on the scanning head to optimize the measuring signals displayed with the aid of the ACCOM evaluation software.
For each of the two perpendicular axis directions the KGM outputs two 90°-phase-shifted signals with a signal period of 4 µm. The measuring signals are sent to a PC for further processing. The EIB 741 interface box (Ethernet) and the IK 220 counter card (PCI bus) are available from HEIDENHAIN to interface the KGM to the PC. Both devices can subdivide the sinusoidal measuring signals 4096-fold to provide measuring steps down to approx. 1 nm in each axis. The HEIDENHAIN software ACCOM evaluates the measured values in accordance with the DIN ISO 230-4 standard. ACCOM makes it very easy to program circular interpolation tests and free-form tests for the KGM. The ACCOM evaluation software asks for all the necessary parameter information and generates the corresponding NC program, which can be immediately loaded into the CNC control through its communications interfaces (serial or Ethernet TCP/IP).