Machine Tool 3D Volumetric Positioning Error Measurement
Under Various Thermal Conditions

O. Svoboda and P. Bach
Research Center of Manufacturing Technology
Czech Technical University in Prague, Czech Republic
And
G. Liotto and C. Wang*
Optodyne, Inc.,
Compton, CA 90220, USA
Email: optodyne@aol.com

ABSTRACT

To manufacture good quality or accurate parts, the measurement and compensation of three dimensional volumetric positioning errors of a machine tool are very important. Using a conventional laser interferometer to measure the straightness and squareness errors is very difficult and time consuming. Recently, Optodyne has developed a laser vector technique for the measurement of 3D volumetric positioning errors, including 3 linear displacement errors, 6 straightness errors and 3 squareness errors in a very short time. Using this laser vector technique combine with the data obtained from a set of thermocouples placed at key locations of the machine tool structure, the relations between the machine temperature distribution and the 3D positioning errors can be measured and modeled. The results can be used to compensate the 3D volumetric positioning errors under various thermal conditions.

Reported here are the definition of the 3D volumetric positioning errors; the basic theory and description of the laser vector technique; the temperature sensors and the laser vector technique measurement results obtained on a vertical CNC machining center under different spindle load, machine temperature and environmental temperature.

Keywords: Positioning Accuracy, Thermal effect, Squareness & Straightness, laser measurement, and compensation.

*Corresponding author: Charles Wang, Optodyne, 1180 Mahalo Place, Compton, CA
90220, USA, Tel. 310-635-7481, Fax 310-635-6301, e-mail optodyne@aol.com.

1. Introduction
Twenty years ago, the largest machine tool positioning errors are lead screw pitch error and thermal expansion error. Now, most of the above errors have been reduced by better ball-screw or linear encoder and pitch error compensation. Hence, the largest machine tool positioning errors become squareness errors and straightness errors. Furthermore, due to the machine temperature changes with the environment, heating of the lead screw, high speed machining, cutting force and coolant, etc., it is important to calibrate and compensate the 3 dimensional volumetric positioning errors at various machine temperatures.

Using conventional laser interferometers to measure the straightness and squareness errors are very difficult with complex optics, expensive equipment and time consuming. To solve this difficulty, Optodyne has developed a new laser vector measurement technique[1, 2, 3] for the measurement of straightness and
squareness errors. The setup and operation is simple. It can measure the volumetric errors in 2 hours for a machine working volume of 1 cubic meter. Using the measured volumetric positioning errors, a lookup correction table can be generated for the controller to compensate the machine positioning errors volumetrically[4]. Furthermore, several volumetric positioning error tables can be generated at various machine temperatures to compensate the errors caused by material thermal growth and achieving higher accuracy on a repeatable machine. To achieve this, it is important to understand the 3D positioning errors
caused by machine structure and thermal distortion.