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Manufacturing accuracy, especially position accuracy of fastener holes, directly affects service life and security of aircraft. The traditional modification has poor robustness, while the modification based on laser tracker costs too much. To improve the relative position accuracy of aircraft assembly drilling, and ensure the hole-edge distance requirement, a method was presented to modify the coordinates of drilling holes. Based on online inspecting two positions of pre-assembly holes and their theoretical coordinates, the spatial coordinate transformation matrix of modification could be calculated. Thus the straight drilling holes could be modified. The method improves relative position accuracy of drilling on simple structure effectively. And it reduces the requirement of absolute position accuracy and the cost of position modification. And the process technician also can use this method to decide the position accuracy of different pre-assembly holes based on the accuracy requirement of assembly holes.

According to investigations, almost seventy percent of fatigue related aircraft crashes result from fastener structures, while eighty percent of those fatigue cracks occurred at fastener holes. Since damaged system can be replaced after the accident, the whole life span of an airplane directly depends on each single unit of structure [

Under such circumstances, automatic assembly has been widely investigated and used in practice to increase the efficiency in massive production [

Recently, automatic assembly system has gained many benefits from offline programming. During the process of automatic drilling, model parameters like hole’s position and normal direction could be obtained from CAD modules of structures. However, chances are big that the CAD does not match the desired model of structures perfectly, which can further result in false positioning of fastener holes. As a result, post-modifications need to be made in practice. Instruments like laser tracking can produce accurate measurements but it is too expensive and complicated to use in the assembly of simple structures by comparison. For visual inspecting system, we have to move the system actively during measuring process, which could be highly unstable and inaccurate [

In practice, a great amount of airplane components is simple structural elements. They do not need high standard of position accuracy but as long as a fair position of intersecting point and reasonable hole margin. Given the limits of current measurements and assembly structures, we often measure the offset between positions of pre-assembly holes and their theoretical values. The average of those offsets is then used to modify the assembly. However, such modification is unstable and inaccurate in practice.

In this paper, we present a novel method for holes modification. Based on the online inspecting coordinates of the two pre-assembly holes in the drilling area and their ideal values from CAD modules, we calculate the transformation matrix between two systems. With the transformation matrix, the locations of holes along the line where two pre-assembly holes lie on could be obtained.

A common body of airplane consists of vertical elements (long truss and truss beam), horizontal units (the frame and rib) and coverings. We assemble the plane body or wing panel by riveting structural elements like coverings and long truss. The most typical scenario is straight drilling on single degree skin, as shown in

For the single curvature covering structure in

Suppose the coordinate system of automatic drilling and riveting machine is denoted S with origin O. T_{1} and T_{2} are the theoretical coordinates of the two pre-assembly holes: _{1} and A_{2} denote the realistic coordinates of two pre-assembly holes:

In practice, we aim to revise P_{kA} by P_{kT}_{.} so that the exchangeability, synchronization and structural strength of each element can be assured. The real coordinates of T_{1}, T_{2} and P_{kT} can be obtained by offline programming. We can also measure the locations of A_{1} and A_{2} through the executor by the end of automatic riveting machines. Thereafter, we can modify the drilling holes of single curvature covering as follows [

1) Compute the vector

2) Calculate the angles between ideal and real line of drilling holes from

3) Rotate

4) Compute the realistic coordinates of P_{kA} with the ideal locations P_{kT} _{ }

Provided a pair of ideal and real coordinates as T_{1} (1620, 2100, 800), T_{2} (1930, 2491.7908, 820), A_{1} (1621.8147, 2100.9084, 799.3500), A_{2} (1931.6375, 2491.5988, 832.9039). The distance between the two points is 500 mm.

Due to location errors during pre-assembly holes and inevitable measurement errors in instruments, we have to add a certain degree of perturbation errors to A_{1} and A_{2}. The location precision of most holes on the single curvature covering is simulated as 0.2 mm. In this case, the perturbation error is 0.3 mm accordingly.

The resulting vectors are

every holes along the line of two pre-assembly holes with interval 100 mm. Then we compute the coordinates of each drilling hole with 1000 iterations. The results with the 5 percent maximum eliminated are presented in

As in

Provided another pair of ideal and real coordinates as T_{1} (1620, 2100, 800), T_{2} (1930, 2491.7908, 820), A_{1} (1621.9832, 2100.9917, 799.4669), A_{2} (1931.9782, 2492.7892, 819.4439). The perturbation is 0.29 mm. The

resulting vectors are

0.5755, 0.3292). The rest settings are as same as in the last section. We also compute the coordinates of each drilling hole with 1000 iterations. The results with the 5 percent maximum eliminated are presented in

As in

Number | Error before modification/mm | Error after modification/mm | ||
---|---|---|---|---|

Maximum | Average | Maximum | Average | |

1 | 2.9176 | 2.6973 | 0.0405 | 0.0188 |

2 | 4.4863 | 4.4610 | 0.0810 | 0.0377 |

3 | 6.7897 | 6.5467 | 0.1214 | 0.0565 |

4 | 8.9826 | 8.7269 | 0.1619 | 0.0754 |

5 | 11.2180 | 10.9450 | 0.2024 | 0.0942 |

Number | Error before modification/mm | Error after modification/mm | ||
---|---|---|---|---|

Maximum | Average | Maximum | Average | |

1 | 2.4376 | 2.2160 | 0.0400 | 0.0181 |

2 | 2.4362 | 2.2074 | 0.0799 | 0.0362 |

3 | 2.4377 | 2.1988 | 0.1199 | 0.0543 |

4 | 2.4421 | 2.1905 | 0.1599 | 0.0724 |

5 | 2.4597 | 2.1824 | 0.1999 | 0.0905 |

We further validate the proposed algorithm upon the gantry type automatic drilling and riveting system at Nanjing University of Aeronautics and Astronautics, as shown in

The experimental scheme is shown in

1) Translate the coupon 3 mm with T_{1} and T_{4} as a reference, and get the holes in column A;

2) Modify the positions of the drilling holes with T_{2} and T_{3} as a reference, and get the holes in column C;

3) Rotate the coupon _{1} and T_{2} as a reference, and get the holes in column D;

4) Modify the positions of the drilling holes with T_{3} and T_{4} as a reference, and get the holes in column B;

The result of experiment is measured by the triple-coordinates instruments. Its model is Mistral 1070705, and its measurement accuracy can reach 3 μm. The measurement result and the simulation remain consistent, and the former is a little larger, which is acceptable. It turns out that, when the position accuracy of pre-assembly holes is 0.3 mm, the precision of drilling holes can get 0.2 mm after modified with the proposed modification method.

In this article, an online modification method is proposed for drilling holes based on double pre-assembly holes. The main conclusions are as follows:

1) Compared to traditional revisions, it is more robust in that it can guarantee the drilling accuracy along the line determined by the pre-assembly holes. With 0.29 mm precision of pre-assembly holes, the drilling accuracy is less than 0.2 mm with our modification algorithms.

2) The proposed method measures the local area of structural elements based on online programming, which avoids the reliance on integral precision of the whole structures. It also reduces the financial cost of modification

process by waiving the use of large instruments like laser tracking system.

3) With the proposed algorithm, the technician is able to optimize the arrangements of pre-assembly holes on structural units so that they can adapt accuracy of the pre-assembly holes to different manufacturing conditions.

Qiubai Yan,Wenliang Chen, (2015) Automatic Modification of Local Drilling Holes via Double Pre-Assembly Holes. World Journal of Engineering and Technology,03,191-196. doi: 10.4236/wjet.2015.33C028