Original Date: 10/07/2002
Revision Date: 01/18/2007
Best Practice : Non-Conventional Inspection Methods
General Tool Company has demonstrated that non-conventional inspection methods can be efficient and cost effective for the small volume production of large parts with intermediate tolerance requirements. In many cases, the cost savings can approach 20% relative to conventional techniques such as functional gages, standard measurement instruments, and coordinate measuring machines. In some cases, the combination of part size, complexity and tolerance factors make non-conventional inspection techniques the only practical approach.
General Tool Company (GTC) is often chosen to supply one-of-a-kind, large parts with unique geometry. The tolerance requirement for these work pieces is typically around +/- 0.015 of an inch. This manufacturing environment requires efficient, economical, and short turn-around dimensional measurements for both process control and final product acceptance. In addition, while the measurements must be accurate, it is often inappropriate to move the parts to a remote station for in-process inspection operations due to the difficulty in realigning the work piece on the machine tool. In other cases, the lack of rigidity of the freestanding work piece makes off-machine certification a difficult and expensive challenge.
GTC developed non-conventional inspection methods that have proven to be particularly well suited to the intermediate-tolerance, cost-competitive, short delivery cycle job shop environment. These inspection methods include the use of a Laser Tracker; an articulated computerized, programmable inspection arm; and out-of-cycle dimensional measurements. The Laser Tracker is an optical inspection device that measures the position of a target element that is placed at various target points on a work piece. It provides dimensional inspection information on very large parts with measurement accuracy in the range of a few thousandths of an inch (in a relatively well- controlled environment) at a cost far below that of conventional large, multi-axis machine tools. In addition, the gage works directly with computer aided design files as well as supports other activities, such as layout work and assembly operations. The computerized inspection arm offers a flexible, efficient means of collecting dimensional inspection information on surfaces that can be accessed via a manually positioned articulating arm. The operator moves a tip located on the end of the arm to the desired inspection point of the work piece and presses a button to communicate with the system software. While this system’s accuracy is also limited to a few thousandths of an inch, it provides a great deal of flexibility since it can be easily mounted on surfaces such as a machine component (for on- machine measurements) or a surface plate for general use (see Figure 2-4). A similar out-of-cycle inspection approach is to mount a probe or other indicator on the machine tool holder and use it to measure the relative location between the work piece surface and an on-machine dimensional reference. This technique depends on the use of a machine-mounted master artifact that provides a positional reference for the measurement system and a machine calibration procedure that assures that the machine positional errors in the measurement zone are significantly smaller than the required tolerances.
Non-conventional inspection methods are efficient and cost effective for the small volume production of large parts with intermediate tolerance requirements. The methods provide an accurate means of inspecting without the expense of special gages or the time required of conventional methods.
Figure 2-4. Computerized Arm
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