*3.3.2.1 Design process planning for machine center driven by FMA structural decomposition*

The design process of the machine center is optimized by using the FMA structural decomposition methodology [22]. There is a large amount of information coupling among each design unit; the basic planning of the design process is obtained based on the consideration of each unit's coupling, as shown in **Figure 7**, which can speed-up the design and the development of machining centers.

Firstly, the machine center is decomposed into sub-function design units, motion design units, and meta-action design units by FMA structural decomposition.

Secondly, the initial design sequence (IDS) of the function layer is obtained by considering the coupling among the design units of the sub-function layer. Next, the IDS of the meta-action layer is determined in the light of a sub-function, by taking its motion layer as a transition layer. The last step is to design the mechanical structures following an ascending order, i.e., from bottom to top (from the meta-action layer to the entire machine).

MU's reliability means the ability of the MU to remain functional. It can be also characterized by the degree of reliability. The reliability degree of the MU means the probability that the MU will perform its required function under given conditions for a stated time interval [21], namely, the probability that MU output characteristic parameters are within acceptable ranges in specified time periods. This is

where, *Y t*ð Þ means the output quality characteristic parameters (such as precision, accuracy life, performance stability, etc.), ½ � *Y*min*; Y*max defines the ranges of MU's output quality characteristic parameters under design requirements.

Taking the motion precision of MU, for example, and assuming that motion error values of MU follow the normal distribution, then the reliability of the MU can

In a practical situation, the CNC machine tool needs to accomplish multiple different missions by different MUs, so the system's mission reliability is actually a

The calculation of the machine system's mission reliability is shown in Eq. (2).

where, *R<sup>W</sup>* means the reliability of the *w*th mission, *α<sup>i</sup>* means the weight that the

Reliability design is a basic guarantee of the CNC machine tools' reliability. As everyone knows, design of the machine tool is a difficult system engineering

*<sup>R</sup><sup>W</sup>* <sup>¼</sup> <sup>∑</sup> *n i*¼1

*i*th MU relative to the *w*th mission, *RAi* means the reliability of the *i*th MU.

*R* ¼ *P e*ð *min* ≤*E* ≤*emax*Þ ¼ *P E*ð Þ� ≤*emax P E*ð Þ¼ ≤*emin* Φ

*3.3.2 Reliability design for CNC machine tool based on MU*

dynamic combination of each MU's reliability, shown in **Figure 6**.

*R* ¼ *P Y*½ � min ≤*Y t*ð Þ≤*Y*max (1)

*emax* � *μ σ* 

*αiRAi* (2)

� Φ

*emin* � *μ σ* 

shown in Eq. (1).

*The assembly model of a MU.*

*Reliability and Maintenance - An Overview of Cases*

**Figure 5.**

be described as below:

**54**

Firstly, the NC rotary table is decomposed into the meta-action layer and possible similar unit sets of each MU are obtained. Secondly, similar MUs are determined according to the similarity of possible similar units calculated by using interval number normal cloud model. Lastly, the failure data according to the similarity among units is modified, resulting in the reliability prediction, as

*Reliability Technology Based on Meta-Action for CNC Machine Tool*

FTA and FMEA for meta-action unit (MU-FTA and MU-FMEA) are more suitable for the CNC machine tools that showcase the main body of mechanical struc-

*Worm rotation meta-action unit: (1) slippery seat; (2) bearing cover; (3) bearing seat; (4) screw; (5) spacer sleeve; (6) bearing; (7) bushing; (8) worm; (9) spacer sleeve; (10) disk spring; (11) spacer sleeve; (12) tab*

*End-toothed disc indexing table schematic drawing: (1) pallet; (2) male tapper; (3) sealed shell; (4) gear shaft; (5) gear shaft bearing; (6) motor; (7) worm; (8) worm gear; (9) axisymmetric body bearing; (10) lift cylinder; (11) locked cylinder oil circuit; (12) lift cylinder oil circuit; (13) lower tooth disc; (14) upper tooth disc; (15) large spring; (16) pull stud; (17) claw; (18) generating cone; and (19) positioning nail.*

shown in **Figure 9**.

**Figure 10.**

**Figure 11.**

**57**

*3.3.2.3 FTA and FMEA for meta-action unit*

*DOI: http://dx.doi.org/10.5772/intechopen.85163*

ture rather than the traditional FTA and FMEA.

*washer; (13) round nut; (14) coupling; and (15) servo motor.*

#### **Figure 8.**

*Execution steps of the coupling strength calculation and coupling splitting.*

After the design process planning, the coupling strengths among the design units are calculated by using variability and sensitivity indices based on the information coupling among them (i.e., the design units). Then, the splitting method is been used for the coupling design structure matrix to optimize the IDS of each design unit. **Figure 8** illustrates the procedure.

The variable stands for the degree of information change transmitted from the top design structure units to the bottom design structure units. Sensitivity means the degree of the bottom design units' output information change caused by the top design units' output information change.
