Standard Drawing

In mechanical drawing, the view drawing of thread and thread fastener is rather tedious. In order to improve the efficiency of drawing, the prescribed drawing method is usually used (see figure). International Standard ISO and Chinese Machinery Drawing Standard stipulate that the top of threaded teeth is denoted by thick solid line and the bottom is denoted by fine line. In the view of projection plane perpendicular to the thread axis, only 3/4 circles are drawn to denote the solid line of the bottom of teeth, and the termination limit of threads is denoted by thick solid line. Tooth shape, diameter, pitch of thread... Others are marked with thread code in dimension marking. In the sectional view of assembly drawings, it is stipulated that all threaded fasteners are drawn according to the uncut drawing, and a simplified drawing method is provided for common threaded fasteners, such as a group of fasteners of hexagonal head bolts, hexagonal nuts and gaskets, which can be drawn according to the drawing method.

Annotation Notes

In the standard thread marking method stipulated in the national standard, the first letter represents the screw code, such as M for ordinary thread, G for non-threaded sealing pipe thread, R for threaded sealing pipe thread, Tr for trapezoidal thread, etc. The second number represents the nominal diameter of the thread, that is, the diameter of the thread. It represents the maximum diameter of the thread in millimeters. Later symbols are pitch, lead, rotation, mid-diameter tolerance code, top-diameter tolerance code, screw length code.

M6 means a common thread with a nominal diameter of 6 mm.

Inspection and measurement

detecting system

The hardware system is based on PC architecture. It consists of lighting system, CCD camera and optical imaging system, image acquisition card, image processing system, mechanical detection system and electronic control system.

When measuring, after illuminating by illumination system, the screw image of fastener is collected by CCD camera and optical imaging system, and then the digital image is transmitted to the computer through image acquisition card. After processing the digital image with corresponding algorithm, the parameters of tooth profile angle, pitch and diameter of screw thread are calculated and compared with the standard. Compare and calculate the deviation.

(2) The software system of automatic detection of thread parameters in software system mainly includes image processing module of system control module.

System control module: initialization; complete the initialization configuration of system image interface card and standard parameters. Image operation; capture and save 8-bit gray image, capture image to memory.

Image processing module: image processing module includes image pre-processing algorithm to reduce noise, highlight thread contour edge information, edge detection and extraction algorithm to detect and extract contour edge information, system calibration algorithm to measure the actual size of digital image, and algorithm to calculate thread parameters.

measuring method

(1) Measurement of screw thread angle

Thread angle is also called tooth angle. The measurement of thread angle can be realized by measuring the side angle, which is the angle between the side of thread and the vertical plane of thread axis. The approximate contour of the threaded teeth is sampled in the straight line section on both sides of the thread, and the sampling points are fitted by the linear least square method.

(2) Measurement of pitch

The pitch refers to the distance between a point on the thread and the corresponding point on the adjacent thread teeth. The measurement must be parallel to the thread axis.

(3) Measurement of thread diameter

The thread diameter is the distance between the mid-diameter line and the axis, and the mid-diameter line is an imaginary line.

measurement result

Using machine vision technology, an automatic measuring system for thread parameters was designed, and the parameters of thread angle, pitch and diameter of standard thread samples were measured. The measurement results show that the thread parameters are within the prescribed error range. Because visual inspection has the characteristics of high efficiency and reliability, it is considered that it can be used in production practice after further investigation.