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Abstract
crew threads are among the most important mechanical elements with wide and varied pplications. Generally, they have two functions to perform; they act as fasteners, and they ’8I1smit power ”and motion. Due to their presence in the vast majority of machinery and quipments, the accuracy of screw threads is mandatory for assuring adequate performance. ’hus, accurate and fast methods are needed for the measurement of screw thread dimensions. ’his increasing demand for accurate dimensions induced large developments in the leasurement and inspection techniques.
4any principles and methods are adopted in the measurement of screw thread dimensions. :crew thread measurement methods are generally divided into two types. Contact methods nd non contact methods. Contact methods depend on mechanical principles. Non contact ~ethods may, in turn, depend on optical, pneumatic or other non contact principles. Non ontact measurement methods have many advantages over contact methods.
:’he favorable properties of laser and its advanced performance make lasers play an important ole in optical measurements. Where, the vast majority of in-process and automated optical rleasurement applications depend on them during measurement.
n the present work a new computer-assisted non contact laser based screw thread rleasurement system is developed. The measurement technique is based on optical lbscuration. It is used to measure the simple effective diameter, pitch, tooth flank profile, and ncluded angle of the screw thread. The screw thread is fixed on a pneumatically driven table n order to achieve uniform speed during measurements. In the developed system, the screw bread is moved relative to a laser beam which is directed towards a photodetector. The !elector’s output signal is then used to evaluate the required dimensions.
)uring pitch measurement the screw is positioned on a vee-block which was previously digned with the laser beam then rotated by 90•. This insures that the laser beam is .dicular to the screw axis. During measurement of the effective diameter the screw is bed in a three-jaw vice, with its center line being perpendicular to the pneumatic table. This hsures that the direction of motion of the screw axis is perpendicular to the laser beam during ~urement.
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measurement setup used to evaluate the thread profile and included angle is the same as used for pitch measurement with the addition of a small bar being attached to the test
•.. . The bar is used to establish a reference position; from which the distances to various . ints on the thread flanks are evaluated. The thread profile is constructed through the urement of the tooth thickness and spacing at different thread heights. The constructed !tank profiles are then used to evaluate the included angle of the test thread.
computer program using MA TLAB 7.0 software was developed. The developed program !onsists of five different modules; the first module is used to condition the captured f
obscuration signal, while the four latter modules are used to evaluate the previously mentioned screw thread parameters. The first module is used to find the time duration that best corresponds to the measured dimensions. Where, in practice, the output signal obtained from obscuration measurements are usually not square signals. The laser beam spot diameter accounts for a gradual increase and decrease in voltage levels. The first module is thus used to convert the captured signal into a square signal that best represents the measured dimension.
The second, third, fourth and fifth modules are used to evaluate the effective diameter, pitch, flank profile, and included angle respectively. In practice, the obscuration signal used to evaluate each of the thread parameters signals requires conditioning. Therefore, the first module (obscuration signal conditioning module) is integrated with each of the four others.
To validate the results obtained from the developed measurement system, it was used to measure and evaluate the effective diameter ofa standard threaded plug gauge.
The measurement system was also used to measure and evaluate the pitch, effective diameter, flank profile, and included angle of a test screw. The results obtained from the developed system were compared to those obtained from a well established measurement technique. The results obtained from both measurement techniques are generally in good agreement. The differences between those values are mainly attributed to the method by which the measured parameter is evaluated in each case.
Furthermore, some known issues such as laser spot diameter, positioning and alignment methods, thread size, and the number of measured sections may have all contributed in obtaining results that are not identical to those obtained from the weIl established measurement technique. Discussion of those minor limitations is made and recommendations for overcoming them are also given.
The developed system is capable of measuring screw thread dimensions with different thread forms. It is non-contact, can be easily automated and is considerably faster than traditional measurement methods. The measurement time needed to measure and evaluate the various screw thread dimensions is in the order of a few seconds. The measurement time can be easily shortened but is maintained at this level to achieve the more favorable higher measurement resolution. The developed system has a measurement resolution of2 JllT1.