The parts of an assembly are sometimes required to be joined together with an accuracy that is within a preselected tolerance. For example, in the aerospace industry, some parts may be required to be assembled together with less than a 0.005 inch gap between them. When the gap exceeds the preselected tolerance, a shim or similar filler may be inserted into the gap in order to assure a within tolerance fit between the parts. The process of assembling and fitting parts together with the required accuracy may become more challenging when the assembly process must be carried out within confined spaces.
Several known methods have been used for measuring and filling part gaps during the assembly process. According to one method, a set of feeler gauges is used in a progressive trial-and-error process to measure the gap between two interfacing part surfaces. This approach is both time consuming and its accuracy may be dependent on the skill of the technician making the measurements. Using the manual gap measurements, a custom shim is constructed either manually or using automated machine tool processes.
A second method of measuring and fitting gaps between parts relies on manual probing of the gap using an electronic feeler gauge. Electronic feeler gauges may be difficult to use and the measurement results may also be dependent on the skill of the technician who carries out the measurements.
A third method of measuring and filling gaps between parts involves filling the gap with a plastic slurry material that cures in place to form a solid filler object. This solution to the problem may have several disadvantages in some applications. For example, the plastic slurry material must remain frozen until just before use and must be bonded to one of the parts but not to the opposite part. The parts to which the slurry material is to be bonded must be coated with a release agent in advance of application. In addition, the slurry material may exert a hydraulic pressure on the parts during the application process, which may deform or displace the parts slightly, reducing assembly accuracy. Another disadvantage of the slurry material is that the material may shrink in a non-uniform manner during curing. Also, the application of the material is time critical, and material may require an extended period in which to cure during which further work on the assembly may not be performed.
Still another method of filling the gaps between mating parts, sometimes referred to as predictive shimming, involves scanning the interfacing part surfaces in an attempt to predict the exact shape of the gap or void between these surfaces. The parts of the assembly are virtually fitted together and a shim is fabricated based on the virtually predicted relationship between the parts. The problem with this approach, however, is that the parts of the assembly, especially large assemblies, may experience significant relative movement of the parts between the time the parts are initially scanned and the time of assembly, resulting in changes of the shape and/or dimensions of the gap. Another disadvantage of this method lies in its dependence on relatively high global accuracy of measurement and assembly.