7.  How to select a laser calibration system?

A. Specifications

1. Do I need more than 10E-7 laser frequency stability?
   No, since the laser system accuracy is 10E-6, a laser frequency stability of 10E-7 is more than enough.
2. Do I need more than 10E-6 system accuracy?
   No, in a shop environment, the error due to air turbulence and the uncertainty of the material temperature, the achievable measurement error is much larger than 10E-6 Also, the machine positioning errors are much larger than 10E-6. Hence a laser system accuracy of 10E-6 is more than enough.
3. Do I need more than 0.001 mm resolution?
   No, for a typical machine with working volume of 1 cubic meter, the positioning accuracy is more than a few µm. Hence a resolution of 0.001 mm is more than enough.
4. What is a realistic error budget in a shop environment?
   For a shop environment even with a temperature control of +/- 1 degree C, the measurement uncertainty is already much larger than 10E-6. Hence a laser system accuracy of 10E-6 is more than enough.
   
   

1. Do I need to measure the linear displacement errors?
   Yes, usually the largest position error is the linear displacement error and the thermal expansion.
2. Do I need to measure the squareness errors?
   Yes, usually the squareness error becomes large due to shipping and new installation and leveling. Hence it is important to measure and to compensate the squareness errors.
3. Do I need to measure the straightness errors?
   Yes, since for most of the CNC machines, the backlash, reversal spike and pitch errors can be compensated, the next largest errors are the vertical and horizontal straightness errors due to the non-straightness of the guide way. ASME B5.54[2] and Boeing ATA recommend measuring the volumetric positioning errors by body diagonal displacement measurement. The vector technique or sequential step diagonal technique [1] measures all the above (items 1, 2, and 3). The measured volumetric positioning errors can be used to generate a volumetric error compensation file for the controller to compensate these errors.
4. Do I need to measure the angular errors?
   For most CNC machine tools, the volumetric positioning errors already included the effect of angular error motions. Hence it is not necessary to measure the angular error. Furthermore, the CNC controllers do not have the capability to compensate the angular errors. For some CMM, the angular errors can be compensated [5].
5. Do I need to measure the dynamic contouring errors?
   For high speed cutting and mold making, the dynamic contouring accuracy is very important. It is important to optimize the servo parameters and to tune the controller properly to achieve the best performance.
6. Do I need to quick check the volumetric positioning accuracy and circular contouring accuracy?
   It is recommended by the ASME B5.54, Boeing Aircraft, and many large corporations, that a quick check to be performed regularly or in need [4].
7. How I compensate these errors?
   Linear displacement errors: for most CNC machines, the linear displacement errors can be compensated. After the linear calibration, the compensation files can be generated automatically by the software for most of the controllers.
8. Volumetric positioning errors: for some CNC controllers, the volumetric positioning errors, including 3 displacement errors, 6 straightness errors and 3 squareness errors, can be compensated. After the volumetric calibration, the compensation file can be generated automatically by the software for most of the controllers. The software can also generate a 3 D error table for some controllers