Geometric errors measurement for coordinate measuring machines

Error compensation is a good choice to improve Coordinate Measuring Machines’ (CMM) accuracy. In order to achieve the above goal, the basic research is done. Firstly, analyzing the error source which finds out 21 geometric errors affecting CMM’s precision seriously; secondly, presenting the measurement method and elaborating the principle. By the experiment, the feasibility is validated. Therefore, it lays a foundation for further compensation which is better for CMM’s accuracy.


Introduction
With the development of society and economic, the need for higher quality products appears urgently, which calls for high accuracy manufacturing and measuring equipment. Coordinate Measuring Machines (CMMs) are famous measuring equipment, which are widely used in much field, such as machinery, automobile, aviation and military industry. The precision plays a key role in CMM, so how to improve it is a hot topic among all the researchers Generally speaking, two methods can be adopted to enhance the accuracy, one is error avoiding, which costs much time and money to design, manufacture and assemble CMM carefully; the other one is error compensation, which absorbs new errors to counteract the CMM's original errors, so it is quick and economical[1].

The steps to enhance the CMMs' precision
In order to increase the CMMs' precision by error compensation, there are 3 basic steps shown as Fig.1, which are source of errors' analysis, errors modeling and measurement, respectively. The source of errors is analyzed to find out the errors having a strong impact on CMMS' accuracy. The error is modeled to establish the relationship between each error. The measurements are down for acquiring errors which should be comp ensated. The last step-measurement is very important, so the paper focuses on the topic.  Therefore, the other two translational axes of CMMs have similar errors, given as follows. X-axis: The well-known Michelson configuration is shown as Fig.2[5].A single incoming beam of light from source will be split into two identical beams by a beam splitter (a partially reflecting mirror). Each of these beams travels a different route, called a path, and they are recombined before arriving at a detector. The path difference, the difference in the distance traveled by each beam, creates a phase difference between them. If the phase difference is integer multiples of coherent light's wave length, 3

MSETEE 2017
IOP Publishing IOP Conf. Series: Earth and Environmental Science 81 (2017) 012117 doi :10.1088/1755-1315/81/1/012117 constructive interference will happen; if the phase difference is half of the wave length(except the integer multiples ), he destructive interference will occur. When the mobile mirror moves from M1 to M2', the constructive interference and destructive interference will take in turns, and at the detector, the bright fringe and dark fringe will appear alternately, just as Fig.3.

Experiments
In order to avoid redundancy, only one geometric error of CMM is selected to elaborate, which is the angular error (the pitch error ) z y ( ).
The brand of CMM for experiment is Zeiss Opton, which model is UMC550S, the work volume is 5501200450mm. The measurement accuracy is 1.  error occurs at 55mm , which is 1.87 arc-sec; the largest negative direction error occurs at 605mm, which is 5.63arc-sec. The result of backhaul is similar with that of forward trip. The largest positive direction error also occurs at 55mm , which is 0.95 arc-sec; the largest negative direction error occurs at 655mm, which is 4.78 arc-sec.

Conclusion
In the paper, the error source of CMM is analyzed and 21 geometric errors are found out which affect CMM's precision seriously. In order to gather the errors' information, the measurement method is presented and its principle is introduced. By the experiment, the feasibility is validated. Therefore, it is better for later research on error compensation, which might lay a foundation for improving CMM's accuracy.