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External thread inspection in screws and bolts is crucial for ensuring product quality and reducing false rejections, thereby lowering selection costs. Various technologies are employed for this purpose, each with specific characteristics suited to different situations.
THE TRADITIONAL METHOD FOR THREAD INSPECTION IS AN OPTICAL PROFILE CONTROL USING ROTATING STATION AND DIMENSIONAL CAMERA.
The control station puts in rotation the piece while the dimensional camera captures multiple images. The profile analysis solution with a rotating station has two main limitations: the piece is not inspected over its entire surface despite the increased number of captured images, and the rotation entails high machine costs and long selection times.
TO ADDRESS THESE NEEDS, DIMAC HAS DEVELOPED THREE ADVANCED INSPECTION STATIONS.
The first solution uses a line surface scanning camera that grabs a line of pixels for each trigger while the piece turns 360°, the grabbed surface lines are joined together to obtain the unrolled image of the cylindrical side surface of the thread.
The surface line scanning camera is triggered by the 360° rotation station’s encoder to cover the entire piece surface, the resolution of the trigger pulses can be set to improve the accuracy of the surface inspection.
The vision software analyses the thread using DIMAC AI TOOLS, identifying irregularities like “z-thread” or foreign hits and dents or excess surface coating between crests. The high quality of the acquired side surface image allows the detection of minimal defects, including those invisible to the human eye, such as head-fine cracks.
This method is very accurate but does not perform dimensional measurements as well as its scanning cycle time limits the output rates.
IT’S A METHOD RECOMMENDED WHEN HIGH-QUALITY CONTROLS ARE REQUIRED, SUCH AS IN THE AEROSPACE APPLICATIONS.
The second solution is a multi-side camera station equipped with three or four side surface cameras positioned 90° or 120° to one another to look at the threaded shank surface, all around.
Although the image quality is lower than the above-mentioned line scanning version, the multi-camera station doesn’t require the piece rotation, allowing high productivity (up to 700 pieces per minute for M6 screws).
DIMAC AI TOOLS are used with this station to detect macroscopic defects such as dents, scratches, open cracks and radial holes.
THIS IS AN EXCELLENT SOLUTION TO EXAMINE THE ENTIRE EXTERNAL SURFACE OF THE PIECE WITH HIGH PRODUCTION RATES.
The third technology, patented by Dimac in 2023, is electro-mechanical inspection using a gauging roller station, where a threaded master roller gauges the thread’s compliance.
A group of two gripping rollers keeps the piece in rotation while the third threaded roller gauges the threaded shank simultaneously. The roller gauge sliding axis movement is checked to stay in a threshold envelope by a special sensor.
Any defect affecting the roller gauging effort to spike out of the envelope is immediately identified, to define the thread quality.
Thanks to a special control algorithm, the system is insensitive to core ovalization, reducing the rate of pseudo-reject. The output rate is lower than the above-mentioned solutions, but this station ensures the screwability of the conformed parts.
THIS SOLUTION IS RELIABLE FOR FASTENERS USED IN AUTOMATIC ASSEMBLY LINES BECAUSE IT HELPS TO REMOVE SMALL BURRS OR DIRT IN THE THREAD.
