Spindle error motion

Assessment of the sources of error affecting the accuracy of the machine tool (MT) can be divided into quasi-static errors and dynamic errors. Quasi-static errors are errors in the machine that occur relatively slowly. This category is formed by geometric and kinematic error, thermal errors, etc [1]. However, dynamic errors are caused by structural vibration, spindle error motion, controller errors, etc. Unlike the quasi-static errors, these errors are more dependent on the working conditions of the machine. Overall, quasi-static errors account for about 70 percent of the total errors of a machine [2]. 

A slot end-milling test is carried out to investigate the machine tool spindle error motion in radial direction. Table 29.3 shows the tool and cutting conditions. The cutting time for one operation should be short enough to avoid the thermal disturbances on displacement signals. 

For machine tools, we group errors in error motions of spindles and rotary axes (e.g., axial error motion, radial error motion), error motions of linear axes (e.g., linear positioning error motion, straightness error motion, pitch error motion), contouring errors (e.g., circular error) (ISO 230-1 2012). Errors of machine tools may be also grouped according to the error source geometric errors, errors of the numerical control, dynamic errors, thermal errors, load errors, etc. 

For example, the compliances of Q = 0.1 irm/kgf in radius direction, Ca = 0.03 xm/kgf in axial direction, and Q = 0.1 jirad/kg m in rotational direction are sufficient for the main spindle whereas the error motions of 50 nm in both radius and axial directions and 0.2 firad in rotational direction or better are expected. As for X-Y table, the dynamic rigidity should be fcx 0.1 kgf/ xm (traverse direction) and Ay 0.05 kgf/ xm whereas the thermal rigidity should be as good as that of the main spindle. The temperature of the environment is normally maintained at room temperature +0.1°C. 

High-Precision Machining by Measuring and Compensating the Error Motion of Spindle s Axis of Rotation in Radial Direction... 

Measurement of the Spindle Displacement to Investigate the Spindle s Axis Error Motions... 

As can be seen in Fig. 29.2, before cutting, all the displacement profiles involve periodical type of fluctuation. These periodic fluctuations are related to the error motion of a spindle s axis of rotation in radial direction and roundness errors of the target surface. During cutting, it is observed that the displacement profile shifts but it returns to its original position after the end of cutting. 

Tu JF, Bossmanns B, Hung Spring C C (1997) Modeling and error analysis for assessing spindle radial error motions. Precis Eng 21(2-3) 90-101... 

Shinno H, Mitsui K, Tatsue Y, Tanaka N, Omino T, Tabata T, Nakayama K (1987) A new method for evaluating error motion of ultra precision spindle. CIRP Ann Manuf Technol 36(l) 381-384... 

Based on the design concept, an innovative ultraprecision machine tool of perfect noncontact type can be realized, as shown in Fig. 3. The overall machine tool structure is symmetrically designed with respect to vertical nanomotion axis. All the moving structural modules, i.e., an air turbine-driven aerostatic spindle system, a planer nano-motion table system, and a vertical nano-motion system with gravity compensator, are levitated by aerostatic bearings and driven by voice coil motors in a perfect noncontact condition. In consequence, various nonlinear phenomena can be removed from the machine structure and then the error factors caused in the overall machine structure can be successfully minimized. 

The lathe spindle was assembled in the following manner with the adhesive. The bearings were first installed on the spindle and the sleeves bonded. The spindle nut was then tightened to apply appropriate pressure on the bearings. Then the whole spindle unit was inserted into the spindle box with the adhesive on its sleeves. The spindle location was adjusted during the pot-life of the adhesive, so that assembly errors in the parallelism between the spindle and the carriage motion, and difference between the heights of the head and tail stocl, were reduced to tolerable levels. 

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