# Automatic Thread Defect Examination System
## 1 Introduction  
Threaded joints account for approximately half of all joints produced in mechanical systems. They are prevalent across all fields of industry. Internal threads have a wide variety of sizes, depths, and shapes, including thru-bores, such as a nut, and blind-end internal threads. Threaded joints offer easy assembly and disassembly of systems, and are important for maintenance and recycling purposes. Both external threads and their internal counterparts have been comprehensively studied over many years. The examination methods used for this purpose can be divided into contactless examination and contact-based examination. Contactless examination commonly utilizes eddy currents, lasers, or cameras. In [1, 2], the authors utilized laser sensors for non-contact inspection of threads. In [3, 4], the authors conducted optical examination of screw threads utilizing cameras. The authors of [5] addressed an Optical Thread Plug Gauge (OTPG) for internal thread examination utilizing cameras. A sequence of partial wall images of an internal thread were retrieved and reconstructed into a 2D unwrapped image. Then, a digital image processing and classification procedure was utilized to normalize, segment, and determine the quality of the internal thread. The authors of [6] experimented with 3D scanners and their application to measurement of large threads. A laser-optical internal thread measurement system was discussed in [7]. The authors of [8] addressed tubing-based thread testing equipment based on magnetic flux leakage theory. In order to examine tiny surface fatigue cracks, ref. [9] utilized eddy current testing, which is an electromagnetic method. For detection of steel surface defects, refs. [10, 11, 12] utilized camera images and then applied deep learning approaches to the images. Contactless examination can detect various faults in a time-efficient manner. Despite its speed, contactless examination has a major disadvantage in that it cannot ensure that the mating parts will assemble together correctly [13]. Recently, refs. [14, 15] tackled examination of external threads using a contact scanning probe. In [15], a model was established for a special probe of a 3D thread-measuring machine. In [14], the influence of the geometric error of a contact probe on the accuracy of external thread measurement was investigated in order to improve the measurement accuracy of the thread measuring machines. This paper considers only the examination of internal threads, not external threads. We consider examination of a Thread Hole (TH) via contact with the TH. In the process ofmaking THs, faulty holes may be generated. The proposed Inspection Device (ID) has a screw shape, enabling it to smoothly assemble with a TH without faults. If the ID and a TH have distinct threads, then they cannot be joined accurately. For instance, an examined TH can have cracks or delamination in its composites, and can thus have threads that are different from those of the ID. A TH fault can be detected by observing the torque generated while tightening the ID through the TH. Under contact-based examination methods, successful assembly of the gauged part (with its mating part) is assured [13]. Hence, many companies, such as BEHCO [16] and Newwistacorp [13], have developed commercial contact-based examination systems. However, these companies do not make the detailed structure of their products public. In this article, we propose a contact-based examination system using simple hardware. We present details of the developed examination system and develop a simple hardware design to generate the ID's axial motion from the motor's rotational motion. A simple design such as the one proposed here is desirable in light of its positive effects on both system maintenance and costs. As the ID must be used many times, the arrangement error between its axis and the TH can increase with time and use, which decreases examination accuracy.