A thread is a shape with uniform helical protrusions on the cross-section of a solid outer or inner surface. According to its structural characteristics and uses, it can be divided into three categories: 1. Ordinary threads: the tooth shape is a triangle, used for connecting or fastening parts. Ordinary threads are divided into two types based on pitch: coarse thread and fine thread, with fine thread having higher connection strength. 2. Transmission thread: The tooth shape includes trapezoidal, rectangular, saw shaped, and triangular shapes. 3. Sealing thread: used for sealing connections, mainly for pipe threads, taper threads, and taper pipe threads.
1、 Thread type
According to the tooth shape, it can be divided into triangular, trapezoidal, rectangular, serrated, and circular arc threads;
According to the direction of thread rotation, it can be divided into left-handed and right-handed;
According to the number of spiral lines, it can be divided into single line and multi line;
According to the shape of the thread matrix, it is divided into cylindrical and conical shapes.
2、 Elements of Thread
Thread includes five elements: tooth shape, nominal diameter, number of threads, pitch (or lead), and direction of rotation.
1. Tooth shape
On the cross-sectional area passing through the thread axis, the contour shape of the thread is called the tooth profile. There are tooth shapes such as triangles, trapezoids, serrations, arcs, and rectangles.
2. Diameter
Threads have major diameter (d, D), medium diameter (d2, D2), and minor diameter (d1, D1). When representing threads, nominal diameter is used, which represents the diameter of the thread size.
The nominal diameter of a regular thread is the major diameter.
3. Number of lines
A thread formed along a spiral is called a single thread, and a thread formed by two or more equally spaced spiral lines along the axis is called a multiple thread.
4. Pitch and lead
The pitch (p) is the axial distance between two adjacent teeth on the corresponding two points on the centerline.
The lead (ph) is the axial distance between two adjacent teeth on the same helix and their corresponding two points on the median diameter line.
When using a single thread, lead=pitch; When threading multiple threads, lead=pitch × Number of lines.
5. Rotation direction
The thread inserted when rotating clockwise is called a right-handed thread;
The thread inserted when rotating counterclockwise is called a left-hand thread.
3、 Marking of threads
(1) Regular thread
Ordinary threads are the most widely used, and the threads on threaded fasteners (such as bolts, studs, screws, nuts, etc.) are generally ordinary threads.
Ordinary threads are divided into coarse thread ordinary threads and fine thread ordinary threads. Fine tooth ordinary threads are commonly used on precision parts and thin-walled parts.
In the marking of threads, the pitch of fine pitch ordinary threads must be indicated, while the pitch of coarse pitch ordinary threads is generally not indicated.
2) Size code
Size code: nominal diameter × Thread pitch (both the lead and pitch of multi thread threads must be indicated, and the pitch of single thread coarse thread ordinary threads is not indicated).
"M10" represents a single thread coarse thread with a nominal diameter of 10 mm and a pitch of 1.5 mm.
M10 × 1 represents a single thread fine thread with a nominal diameter of 10mm and a pitch of 1mm.
2) Code for tolerance zones of ordinary threads
Composed of tolerance levels (numbers) and basic deviations (lowercase letters for external threads and uppercase letters for internal threads), such as 5g6g, 6g, 6H, 7H.
When the codes for the pitch diameter tolerance zone and the top diameter tolerance zone of the thread are different, they need to be marked separately, such as M10-5g 6g
When the tolerance zone codes for the middle diameter and top diameter are the same, only one code is indicated, such as M10 × 1-7H
When the tolerance zone code is 6g or 6H (nominal diameter ≥ 1.6mm), it is not indicated.
3) The turning length of regular threads
There are three types: long, medium, and short, represented by the codes L, N, and S.
M10-5g6g-S is a thread with a short turning length
M10-7H-L is a thread with a long turning length
When the thread is of medium thread length, the code N is not indicated.
When there is a special need, the numerical value of the rotation length can be indicated.
M20 × 2-5g6g-40
4) Rotation code
Mark LH when turning left and not when turning right.
M10-7H-L-LH is a left-hand thread
M10-7H-L is a right-hand thread
(2) Pipe threads
Pipe threads are generally used in the connection of pipelines (water pipes, oil pipes, gas pipes, etc.). The marking of pipe threads is indicated using a guiding method, with the leader pointing to the major diameter of the thread.
Marking of pipe threads:
Composed of thread feature code, size code, and rotation direction. The size code is not the size of the major diameter of the thread, but the diameter (in English) of the pipe. The unmarked rotation direction is right-handed.
Example of marking for 55 ° non sealed pipe threads:
G: Thread feature codes for non sealed pipe threads;
G3/4: Single line right-hand cylindrical internal thread with size code 3/4;
G3/4A or G3/4B: Single line right-hand cylindrical external thread with size code 3/4, where A and B in the marking are the tolerance levels of the thread pitch diameter;
The LH in G3/4LH and G3/4A-LH represent left-hand threads, and the threaded pair formed by the two only indicates the marking code of the external thread.
Example of marking for 55 ° sealing pipe threads:
Rp3/4LH: Single thread left-hand cylindrical internal thread with size code 3/4;
Rc3/4: Single thread right-hand cone internal thread with size code 3/4;
Rp/R13/4 LH and Rc/R23/4: Internal and external threads are screwed together to form a threaded pair.
Rp: Thread feature code for sealing cylindrical internal threads;
Rc: Thread feature code for sealing conical internal threads;
R1: Feature code for conical external threads that match cylindrical internal threads;
R2: Feature code for conical external threads that match with conical internal threads;
(3) Trapezoidal and serrated threads
Trapezoidal and serrated threads are commonly used on screw threads for transmitting motion and power. When trapezoidal threads work, both sides of the tooth are subjected to force, while serrated threads are subjected to force on one side during operation.
The marking of trapezoidal and serrated threads is similar to that of regular threads.
Example of marking for trapezoidal threads:
Tr40 × 7LH-7e, trapezoidal thread (thread feature code Tr), nominal diameter φ 40, single line, pitch 7, left-hand rotation, pitch tolerance zone code 7e; Medium twist length. Note: Only the diameter tolerance zone code is indicated, and there are only two types of screw lengths (code N and L). When the screw length is equal, N is omitted and not indicated.
When the thread is a multi thread, it is marked as:
Tr40 × 14 (P7) -7e, where "14" is the lead and "7" is the pitch, with double thread.
The thread pair of trapezoidal thread is represented as Tr40 × 7-7H/7c, the tolerance zone for internal threads is at the front and the tolerance zone for external threads is at the back, separated by a "/".
4、 Thread processing
1. Tapping and threading
Thread tapping:
The machining method of using a tap to process internal threads in a hole is called tapping.
The formula for calculating the diameter of the bottom hole is based on the plasticity of the material.
For steel parts and materials with high plasticity: D-hole=D-P
D-hole: diameter of threaded bottom hole drill bit
D: Internal thread major diameter
P: Pitch
Example: What is the diameter of the bottom hole when we want to tap M10 threads on steel parts?
According to the formula D hole=D-P=10-1.5=8.5mm
For cast iron and materials with low plasticity: D-hole=D - (1.05-1.1) P
Example: What is the diameter of the bottom hole when we want to tap M10 threads on cast iron?
According to the formula D hole=D - (1.05-1.1) × 1.5=8.35~8.42mm
Thread fitting:
The method of cutting external threads on a round rod or pipe using a die is called threading.
The calculation formula for the diameter of a circular rod: d-rod=d-0.13P
D-rod: diameter of circular rod before threading, mm
D Major diameter of thread, mm
p: Thread pitch, mm
Example: What is the diameter of the round rod used to make M10 screws?
According to the formula d pole=d-0.13P
=10-0.13 × 1.5=9.8mm
2. Car threads
External and internal threads of the vehicle