According to tooth type, it can be divided into triangular, trapezoidal, rectangular, serrated and circular arc threads;
According to thread direction, it can be divided into left-hand and right-hand;
According to number of spiral lines, it can be divided into single line and multi line;
According to shape of thread body, it is divided into cylinder and cone.

 

Thread includes five elements: tooth type, nominal diameter, number of threads, pitch (or lead), and direction of rotation.

In cross-sectional area passing through thread axis, profile shape of thread is called tooth profile. There are triangular, trapezoidal, sawtooth, arc and rectangular tooth shapes.
Comparison of thread profile:

 

There are major diameters (d, D), intermediate diameters (d2, D2), and minor diameters (d1, D1) for the thread. Nominal diameter is used to indicate thread, nominal diameter is diameter that represents size of thread.
Nominal diameter of an ordinary thread is major diameter.

 

External thread (left) Internal thread (right)

Thread formed along a helix is called a single thread, thread formed by two or more helixes equally spaced along axial direction is called a multi-thread thread.

 

Single thread (left) Double thread (right)

Pitch (p) is axial distance between two adjacent points on pitch line.
Lead (ph) is axial distance between two adjacent teeth on same spiral line corresponding to two points on median diameter line.
For single thread thread, lead = pitch; for multi thread thread, lead = pitch * number of threads.

 

Thread screwed in when rotating clockwise is called right-hand thread;
Thread that is screwed in when rotating counterclockwise is called a left-hand thread.

 

Left-hand thread Right-hand thread

Ordinary threads are the most widely used. Threads on threaded fasteners (bolts, studs, screws, nuts, etc.) are generally ordinary threads.
Ordinary threads are divided into coarse ordinary threads and fine ordinary threads. Fine-tooth ordinary threads are mostly used for precision parts and thin-walled parts.
In marking of threads, pitch of fine-pitch ordinary threads must be noted, while pitch of coarse-pitch ordinary threads is generally not marked.
Mark of ordinary thread consists of five parts:

 

1) Feature code
M (represents ordinary thread)
2) Size code
Dimension code: Nominal diameter * pitch (lead and pitch of multi-threaded threads must be noted, pitch of single-thread coarse-threaded ordinary threads is not marked).
"M10" refers to a single-thread coarse general thread with a nominal diameter of 10 mm and a pitch of 1.5 mm.
"M10×1" means a single-thread fine-pitch ordinary thread with a nominal diameter of 10mm and a pitch of 1 mm.
2) Tolerance zone code of ordinary thread
It is composed of tolerance grade (number) and basic deviation (external thread is represented by lowercase letters, and internal thread is represented by uppercase letters), such as 5g6g, 6g, 6H, 7H.
When code of thread pitch tolerance zone and top diameter tolerance zone are different, they need to be noted separately, such as: M10-5g 6g.
When code of tolerance zone of middle diameter and top diameter is same, only one code shall be noted, such as: M10×1-7H
It is not marked when tolerance zone code is 6g or 6H (nominal diameter ≥ 1.6mm).
3) Screwing length of ordinary thread
There are three types of long, medium and short, which are represented by codes L, N, and S respectively.
M10-5g6g-S is thread with short screwing length
M10-7H-L is thread with long screwing length
When thread is of medium screwing length, code N is not marked.
When special needs, value of screwing length can be indicated.
M20×2-5g6g-40
4) Rotation code
Label LH when turning left, not when turning right.
M10-7H-L-LH is left-handed thread
M10-7H-L is right-hand thread

Pipe threads are generally used in connection of pipelines (water pipes, oil pipes, gas pipes, etc.). Markings of pipe threads are marked by guide method, guide line points to major diameter of thread.
Marking of pipe thread:
It is composed of thread feature code, size code and direction of rotation. Size code is not size of major diameter of thread, but size of pipe diameter (imperial). Rotation direction is not marked in mark is right-handed.

 

Marking example of 55° unsealed pipe thread:
G: Thread feature code of non-sealed pipe thread;
G3/4: Single-line right-handed cylindrical internal thread with size code 3/4;
G3/4A or G3/4B: Single-thread right-handed cylindrical external thread with dimension code 3/4, A and B in mark are tolerance grades of pitch diameter of thread;
LH in G3/4LH and G3/4A-LH means left-handed thread, thread pair formed by two is only marked with marking code of external thread.
Marking example of 55° sealing pipe thread:
Rp3/4LH: Single-thread left-handed cylindrical internal thread with size code 3/4;
Rc3/4: Single-line right-hand taper internal thread with size code 3/4;
Rp/R13/4 LH and Rc/R23/4: Internal thread and external thread are screwed together to form a thread pair.
Rp: thread feature code of sealed cylindrical internal thread;
Rc: thread feature code of sealed cone internal thread;
R1: feature code of conical external thread that matches with cylindrical internal thread;
R2: Characteristic code of tapered external thread that matches with tapered internal thread;

Trapezoidal threads and sawtooth threads are often used on lead screws that transmit motion and power. When trapezoidal thread is working, both sides of tooth are stressed, while serrated thread is stressed on one side when it is working.
Markings of trapezoidal and sawtooth threads are similar to those of ordinary threads.
Marking example of trapezoidal thread:
Tr40×7LH-7e, trapezoidal thread (thread feature code is Tr), nominal diameter φ40, single thread, pitch 7, left-handed, pitch diameter tolerance zone code 7e; medium screwing length. Note: Only code of pitch tolerance zone is marked, and there are only two types of screwing lengths (codes N and L). When screwing length is equal, N is omitted.
When thread is a multi-thread thread, it is marked as:
Tr40×14(P7)-7e, where "14" is lead, "7" is pitch, double thread.
Thread pair of trapezoidal thread is expressed as: Tr40×7-7H/7c, tolerance zone of internal thread is in the front, tolerance zone of external thread is in the back, and two are separated by "/".

Tapping:
Processing method of processing internal thread in hole with a tap is called tapping.

 

Calculation formula of bottom hole diameter: consider according to plastic size of material.
For steel parts and materials with large plasticity: D hole = D-P
D hole: threaded bottom hole drill bit diameter
D: Large diameter of internal thread
P: pitch
Example: What is diameter of bottom hole when we need to tap M10 threads on steel part?
According to 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 diameter of bottom hole when we want to tap M10 threads on cast iron?
According to formula D hole=D-(1.05-1.1)×1.5=8.35~8.42mm
Sleeve thread:
Method of using a die to cut an external thread on a round rod or pipe is called sleeve thread.

 

Calculation formula of round rod diameter: d rod=d-0.13P
d rod: diameter of round rod before threading, mm
d: Major diameter of thread, mm
p: pitch, mm
Example: We want to make an M10 screw. What is diameter of rod used?
Should be based on formula d rod = d-0.13P=10-0.13×1.5=9.8mm

 

Car external thread and car internal thread