A thread is a shape with a uniform spiral line projection on the outer or inner surface of a solid cross-section. According to its structural characteristics and uses, it can be divided into three major categories: 1. Common thread: The threads have triangular-shaped teeth and are used to connect or fasten parts. The common threads are divided into two types according to pitch: coarse threads and fine threads, and the fine threads have higher connection strength. 2. transmission threads: the threads have trapezoidal, rectangular, saw-shaped and triangular shapes, etc. 3. sealing threads: used for sealing connections, mainly pipe threads, tapered threads and tapered pipe threads.
I. Thread type
By thread type, it can be divided into triangular, trapezoidal, rectangular, serrated and circular threads.
By thread rotation can be divided into left-hand and right-hand.
The number of spiral threads can be divided into single thread and multi-thread.
According to the shape of the thread parent, it can be divided into cylindrical and conical, etc.
Second, the elements of threads
The thread consists of five elements: tooth type, nominal diameter, thread number, pitch (or lead), and direction of rotation.
1. Thread type
The shape of the thread profile in the area of the profile through the thread axis is called the thread profile. There are triangular, trapezoidal, serrated, circular, and rectangular shapes.
2. Diameter
There are large (d, D), medium (d2, D2), and small (d1, D1) diameters for threads. The nominal diameter is the diameter that represents the thread size when expressing threads.
The nominal diameter of a common thread is the major diameter.
3. Number of threads
A thread formed along one helix is called a single thread, and a thread formed by two or more helixes equally spaced along the axial direction is called a multi-thread.
4. Pitch and lead
Pitch (p) is the axial distance between two points on the center diameter line of two adjacent teeth.
The pitch (ph) is the axial distance between the two points on the mid-diameter line between two adjacent teeth on the same helix.
In the case of single thread, the lead = pitch; in the case of multi-thread, the lead = pitch x number of threads.
5. Rotation direction
A thread that is screwed in when rotating clockwise is called a right-hand thread.
A thread that is screwed in when rotating counterclockwise is called a left-hand thread.
3. Marking of threads
(1) Common thread
The threads on threaded fasteners (bolts, studs, screws, nuts and other parts) are generally common threads.
The common thread is divided into coarse thread and fine thread. Fine threads are used for precision parts and thin-walled parts.
In the thread marking, the pitch of fine threads must be noted, while the pitch of coarse threads is generally not marked.
(2) Size code
Size code: nominal diameter × pitch (both the lead and pitch of multi-threaded threads must be indicated, while the pitch of single-threaded threads is not indicated).
“M10” indicates a single thread with a nominal diameter of 10 mm and a pitch of 1.5 mm.
“M10×1” denotes a single thread with a nominal diameter of 10 mm and a pitch of 1 mm.
(2) Tolerance band code of common thread
It consists of the tolerance level (number) and the basic deviation (lowercase letters for external threads and capital letters for internal threads), such as 5g6g, 6g, 6H, 7H.
When the middle diameter tolerance zone and the top diameter tolerance zone are different, they should be noted separately, for example: M10-5g 6g
When the middle diameter and the top diameter tolerance zone code are the same, only one code is noted, such as: M10×1-7H
When the tolerance band code is 6g or 6H (nominal diameter ≥ 1.6mm), it is not marked.
3）The length of common thread
There are three kinds of threads: long, medium and short, which are indicated by code L, N and S respectively.
M10-5g6g-S is the thread with a short length of rotation
M10-7H-L is the thread with a long length.
When the threads are of medium screwing length, code N is not indicated.
When special requirements are met, the value of the length can be specified.
M20×2-5g6g-40
4）Spin code
LH is marked for left-hand rotation, but not for right-hand rotation.
M10-7H-L-LH is for left-hand thread
M10-7H-L is right-hand thread
(2) Pipe thread
Pipe threads are generally used in the connection of pipelines (water, oil, gas, etc.). The pipe threads are marked with a guideline that points to the large diameter of the thread.
Marking of pipe threads.
It consists of a thread feature code, a size code, and a direction of rotation. The size designation is not the size of the thread diameter, but rather the size of the pipe through-hole (inch). Any markings that do not specify the direction of rotation are right-handed.
Examples of markings for 55° non-sealed pipe threads.
G: Thread feature designator for non-sealed pipe threads.
G3/4: Single-line, right-handed, cylindrical female thread with a size designation of 3/4.
G3/4A or G3/4B: single threaded right-handed cylindrical male thread with a size designation of 3/4, with A and B in the marking being the tolerance class of the thread median.
LH in G3/4LH and G3/4A-LH indicates a left-hand thread, and the thread pair consisting of both is marked with the male thread designation only.
Examples of markings for 55° sealed pipe threads.
Rp3/4LH: A single-line, left-handed, cylindrical female thread with a size designation of 3/4.
Rc3/4: single-line, right-hand, tapered female thread with a size designation of 3/4.
Rp/R13/4 LH and Rc/R23/4: female and male threads screwed together to form a threaded pair.
Rp: thread characteristic designation for sealed cylindrical female threads.
Rc : Thread feature designator for sealing conical female threads.
R1: Characteristic designator for conical male threads mating with cylindrical female threads.
R2: feature code for conical male threads with conical female threads.
(3) Trapezoidal and serrated threads
Trapezoidal threads and serrated threads are commonly used on screws that transmit motion and power. Trapezoidal threads are subjected to forces on both sides of the thread during operation, while serrated threads are subjected to forces on one side during operation.
The markings for trapezoidal and serrated threads are similar to those for common threads.
Example of marking for trapezoidal threads.
Tr40×7LH-7e, trapezoidal thread (thread feature code Tr), nominal diameter φ40, single thread, pitch 7, left-hand rotation, mid-diameter tolerance band code 7e; medium rotation length. Note: Only the middle diameter tolerance zone code is marked, and there are only two types of thread lengths (code N and L).
When the thread is multi-threaded, the markings are
Tr40×14 (P7)-7e, where “14” is the lead and “7” is the pitch, and the thread is double threaded.
In the case of trapezoidal threads, the sub-threads are marked as follows: Tr40×7-7H/7c, with the tolerance zone of the inner thread in the front and the tolerance zone of the outer thread in the back, separated by a “/”.
IV. Thread processing
1、Tapping thread and sleeve thread
Tapping.
The processing method of using a tap to create internal threads in a hole is called tapping.
The formula for calculating the diameter of the bottom hole: It is considered according to 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: large diameter of the internal thread
P: Pitch
Example: We want to tap M10 thread on a steel part to calculate what is the diameter of the bottom hole?
According to the formula D hole = D-P = 10-1.5 = 8.5 mm
For cast iron and less plastic materials: D hole = D-(1.05~1.1)P
Example: What is the diameter of the bottom hole when we want to tap M10 thread on cast iron?
According to the formula D hole = D-(1.05-1.1)×1.5=8.35~8.42mm
Bushing threads.
The method of cutting external threads on a round bar or pipe with a plate is called sleeve threading.
The formula for calculating the diameter of a round rod: d rod = d – 0.13P
d rod: diameter of the round rod before threading, mm
d thread diameter, mm
p: Pitch, mm
Example: We want to make a M10 screw, what is the diameter of the round rod used?
Should be based on the formula d rod = d – 0.13P
=10-0.13×1.5=9.8mm
2. Turning threads
Turning external threads and turning internal threads