# Measurement of Static Stiffness after Motion on a Three-Axis CNC Milling Table
## 6. Conclusions  
Measurement of static stiffness is performed under load during positioning of the working axis. Measurement was performed on a tri axis CNC milling machine tool. Since the modified measurement of static stiffness takes place immediately after the start of the programmed position, the stiffness measurement results reflect the actual circumstances occurring during machining more faithfully.  
The classic stiffness measurement returned stiffness values up to three times greater than those yielded by the modified method using identical instruments and conditions. The reasons for such a considerable difference in stiffness stem from the arrangement of the structure and the influence of non-linearities on the contact surfaces of the sliding table components. The measurement is performed immediately after the movement. It can be stated that the measurement is carried out in the static load area. During the measurement, all parts of the machine were stationary.  
The SSaS stiffness concept is introduced to make a difference between static stiffness and stiffness in motion, or immediately upon the completion of the machine's sliding. This is not a dynamic stiffness measurement, so it is appropriate to introduce a new concept of distinction.  
Measurement of stiffness by laser interferometer brings new possibilities of evaluation, which are not feasible in case of classic measurement by dial micrometer indicator. As it follows from the  
Figure 15. The principle of preloading in the +X direction.  
measured results, the static stiffness depends on both, the previous manner of load application and the direction in which the measured position is assumed. The proposed SSaS stiffness measurement methodology allows for a better assessment of the machine's working ability. It also allows for detection of faults caused by, for example, incorrect assembly of machine components. The method also points to the machine's design deficiencies, which would not demonstrate in classic stiffness measurement.  
Due to a considerable difference between the SSaS and static stiffness, a conceptual design of new types of high-precision machine structure has been executed and introduced.  
This research work was supported by projects: APVV-15-0149 \"Research of new measuring methods of machine condition\", VEGA 1/0437/17 \"Research and development of rotary module with an unlimited degree rotation\", KEGA 039 TUKE-4/2016 \"Creating of virtual laboratories based on WEB technologies to support the educational process in the field of Manufacturing Machinery\", VEGA 1/0224/18 Research and development of testing and measuring methods in coordinate metrology and VEGA 1/124/15 \"Research and development of advanced methods for virtual prototyping of manufacturing machines\".  
[PERSON] suggested the concept of rigidity after shifts; [PERSON] and [PERSON] performed experimental measurements; [PERSON] and [PERSON] analyzed the measured data; [PERSON] and [PERSON] proposed a concept for the use of high-precision machinery; [PERSON] and [PERSON] wrote the paper.  
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.