Thread marking refers to the method of marking with screws by using specific symbols. Threads can be divided into two types of coupling threads and transmission threads according to their uses. Common markings include ordinary threads and pipe threads.
Thread callout
Types of
Feature code
Uses and instructions
Ordinary thread
Coarse teeth
M
The most commonly used connection thread. When the diameter is the same, the pitch of the fine thread is smaller than the pitch of the coarse thread.
Fine teeth
M
Pipe thread
G
The commonly used threads in pipe connections, the pitch and the tooth profile are small, and their size code is in units, which is approximately equal to the diameter of the pipe. The large diameter of the thread should be found from the relevant standards. The code R represents a conical external thread, Rc represents a conical internal thread, and Rp represents a cylindrical internal thread
55 ° sealed pipe thread
Rc
The sealing thread can keep the inside and outside of the pipe joint sealed under a certain pressure
On the drawing
When the thread accuracy is required, in addition to the thread code, the thread tolerance zone code and the thread turn-on length should also be marked. The marking format of the thread mark is: thread code + thread tolerance zone code (middle diameter, top diameter) + twist length.
Notes on labeling content
1. Tolerance zone code is represented by numbers and letters (uppercase letters for internal threads and lowercase letters for external threads), such as 7H, 6g, etc., it should be noted that 7H, 6g, etc. represent thread tolerances, and H7, g6 represent cylinder tolerance Code.
2. The turn-on length is specified as short (represented by S), medium (represented by N), and long (represented by L). In general, the thread turn-on length is not marked, and its thread tolerance zone is determined by the medium turn length (N). If necessary, the spiral length code S or L can be added, such as "M20-5g6g-L". For special needs, the value of the turn-on length can be specified, such as "M20-5g6g-30".
Annotation method
Except for the pipe thread, the marking of the thread mark on the view is the same as the linear dimensioning method; and the pipe thread is in the form of a guide line, which should be drawn from the large diameter and should not be parallel to the section line.
1. M 16-5g6g means coarse thread common thread, nominal diameter 16, right-handed, thread tolerance zone with middle diameter 5g, large diameter 6g, screwing length is considered as medium length.
2. M16 × 1 LH-6G represents fine-tooth ordinary thread, nominal diameter is 16, pitch is 1, left-handed, the thread tolerance zone is 6G, and the diameter is 6G. The length of the screw is considered as medium length.
3. G1 represents inch non-sealed pipe thread, size code 1 in, right-handed.
4. Rc 1/2 means inch thread sealing tapered pipe thread, size code 1/2 in, right-handed.
6, B20 × 2LH means zigzag thread, nominal diameter 20, single wire, pitch 2, left-handed.
The commonly used internal and external thread tolerance zones are specified as follows:
i, G and H positions are specified for internal threads; e, f, g, and h positions are specified for external threads.
ii, the small and medium diameters of internal threads are specified as five tolerance levels of 4, 5, 6, 7, and 8; the large diameters of external threads are specified as three tolerance levels of 4, 6, and 8; , 5, 6, 7, 8, 9 seven tolerance levels.
When the internal and external threads are screwed together, their tolerance zone codes are separated by oblique lines, such as 6H / 6g, 6H / 5g6g, and so on.
Example of labeling ordinary threads
Label legend
Description
Measurement of the included angle of the thread
The angle of the thread is also called the profile angle. The measurement of the thread angle can be achieved by measuring the side angle. The thread side angle is the angle between the side of the thread and the vertical plane of the thread axis. The approximate profile of the threaded teeth is sampled at straight line segments on both sides of the thread, and a straight least square fit is performed on the sampling points.
Pitch measurement
The pitch is the distance from a point on a thread to a corresponding point on an adjacent thread. The measurement must be parallel to the thread axis.
Measurement of thread diameter
The middle diameter of the thread is the distance along the middle diameter line perpendicular to the axis, and the middle diameter line is an imaginary line.
Using machine vision technology, an automatic detection system for thread parameter measurement is designed, and the parameters such as the angle, pitch and diameter of the thread of the standard thread sample are measured. The measurement results show that the thread parameters are within the specified error range. Because of the high efficiency and reliability of visual inspection, it is considered that it can be used in production practice after further investigation.
The hardware system automatic parameter parameter detection system is based on the PC architecture and is mainly composed of an illumination system, a CCD camera and a light path imaging system, an image acquisition card, an image processing system, a mechanical detection system, and an electronic control system.
During the measurement, after the lighting system is illuminated, the thread image of the fastener is collected by the CCD camera and the optical path imaging system, and then the image acquisition card is used to transfer the digital image to the computer. The computer processes the digital image according to the corresponding algorithm and calculates The parameters such as the profile angle, pitch and middle diameter of the thread are compared with the standard to calculate the deviation.
Software system The software system for automatic detection of thread parameters mainly includes a system control module and an image processing module.
System control module: initialization; complete the initialization configuration of the system image interface card and standard parameters. Image operation; capture and save 8-bit gray image, capture image to memory.
Image processing module: The image processing module includes an image preprocessing algorithm that reduces noise and highlights the contour edge information of the thread, an edge detection and extraction algorithm that detects and extracts the contour edge information, a system calibration algorithm that measures the actual size of the digital image, and is used to calculate the thread Arithmetic of parameters, etc. [1]
