First, the type of thread
According to the tooth type, it can be divided into triangle, trapezoidal, rectangular, zigzag and arc thread;
According to the thread rotation direction, 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.
Second, the elements of the thread
Threads consist of five elements: profile, nominal diameter, number of wires, pitch (or lead), and direction of rotation.
1. Tooth shape
On the cross-sectional area through the thread axis, the profile shape of the thread is called the tooth profile. There are triangular, trapezoidal, zigzag, arc, and rectangular tooth shapes.
Comparison of thread profiles:
2. Diameter
Threads have large diameters (d, D), medium diameters (d2, D2), and small diameters (d1, D1), and the nominal diameter is used to represent the thread, and the nominal diameter is the diameter that represents the thread size.
The nominal diameter of an ordinary thread is the large diameter.
Male thread (left) Female thread (right)
3. Number of threads
Threads formed along one helix are called single-wire threads, and threads formed by two or more helix-lines distributed equidistantly along the axial direction are called multi-wire threads.
Single wire thread (left) Double wire thread (right)
4. Pitch and lead
Pitch (p) is the axial distance between two adjacent teeth at two points on the mid-diameter line.
Lead (pH) is the axial distance between two adjacent teeth on the same helix line corresponding to two points on the mid-radial line.
In the case of a single wire thread, lead = pitch; In the case of multiple threads, lead = pitch × number of threads.
5. Rotation direction
The thread that is screwed in when rotated clockwise is called a right-handed thread;
The thread that is screwed in when rotated counterclockwise is called a left-handed thread.
Left-hand thread Right-hand thread
3. Marking of threads
(1) Ordinary thread
Ordinary threads are the most widely used, and the threads on threaded fasteners (bolts, studs, screws, nuts, etc.) are generally ordinary threads.
Ordinary threads are divided into coarse threads, ordinary threads and fine threads. Fine tooth ordinary threads are mostly used on precision parts and thin-walled parts.
In the marking of threads, the pitch of the fine tooth ordinary thread must be injected, while the pitch of the coarse tooth common thread is generally not marked.
The marking of a common thread consists of five parts:
1) Feature code
M (for normal thread)
2) Size code
Size code: nominal diameter × pitch (the lead and pitch of multi-wire threads should be noted, and the pitch of ordinary threads of single-wire coarse teeth should not be marked).
"M10" indicates a single-wire coarse thread with a nominal diameter of 10 mm and a pitch of 1.5 mm.
"M10×1" indicates a single-wire fine-pitch thread with a nominal diameter of 10 mm and a pitch of 1 mm.
2) Tolerance band code for common threads
It consists of tolerance classes (numeric) and basic deviations (lowercase letters for external threads and uppercase letters for internal threads), e.g. 5g6g, 6g, 6H, 7H.
When the thread middle diameter tolerance zone and the top diameter tolerance zone code are different, they need to be injected separately, such as: M10-5g 6g
When the code of the tolerance zone of the middle diameter and the top diameter are the same, only one code is noted, such as: M10×1-7H
Tolerance zone codes of 6g or 6H (nominal diameter ≥ 1.6mm) are not indicated.
3) The length of the screw of the common thread
There are three types: long, medium, and short, which are represented by the code L, N, and S, respectively.
M10-5G6G-S is a thread with a short screw-length length
M10-7H-L为长旋合长度的螺纹
When the thread is of medium screw length, the code N is not marked.
When special needs are required, the value of the screwing length can be indicated.
M20×2-5g6g-40
4) Rotation substituency
LH is marked when left-handed, and not marked when right-handed.
M10-7H-L-LH为左旋螺纹
M10-7H-L为右旋螺纹
(2) Pipe thread
Pipe threads are generally used in the connection of pipelines (water pipes, oil pipes, gas pipes, etc.). The marking of the pipe thread is marked by the method of guidance, and the guide line points to the large diameter of the thread.
Marking of pipe threads:
It consists of a thread feature code, a size code, and a rotation direction. The size code is not the size of the thread diameter, but the size of the diameter (imperial) of the tube. All those that are not marked in the mark are right-handed.
Example of marking of 55° unsealed pipe threads:
G: Thread feature code of unsealed pipe thread;
G3/4: Single-wire right-handed cylindrical thread with size code 3/4;
G3/4A or G3/4B: Single-wire right-handed cylindrical male thread with size code 3/4, A and B in the mark are the tolerance class of the thread diameter;
The LH in G3/4LH and G3/4A-LH stands for left-handed thread, and the thread pair formed by the two is marked with the marking code of the external thread only.
Example of marking of 55° seal pipe threads:
Rp3/4LH: Single-wire left-handed cylindrical female thread with size code 3/4;
RC3/4: Single-wire right-handed cone female thread with size code 3/4;
Rp/R13/4 LH和Rc/R23/4：内螺纹与外螺纹旋合构成螺纹副。
RP: Thread feature code for sealing cylindrical internal threads;
RC : Thread feature code for sealing cone internal thread;
R1: The characteristic code of the conical external thread mated with the cylindrical internal thread;
R2 : The characteristic code of the conical external thread mated with the conical internal thread;
(3) Trapezoidal threads and zigzag threads
Trapezoidal and serrated threads are often used on lead screws that transmit motion and power. The trapezoidal thread is stressed on both sides of the tooth when working, whereas the zigzag thread is stressed on one side when working.
Trapezoidal and zigzag threads are marked in the same way as normal threads.
Example of marking for trapezoidal threads:
Tr40×7LH-7e, trapezoidal thread (thread characteristic code Tr), nominal diameter φ40, single wire, pitch 7, left-handed, middle diameter tolerance zone code 7e; Medium screw-length length. Note: Only the middle diameter tolerance zone code is marked, and there are only two screwing lengths (code N and L), and N is omitted when the medium screwing length is noted.
When the thread is multi-wire, it is marked as:
Tr40×14(P7)-7e, where "14" is the lead, "7" is the pitch, double wire thread.
The thread pair of trapezoidal thread is expressed as Tr40×7-7H/7c, with the tolerance zone of the internal thread in the front and the tolerance zone of the external thread in the back, and the two are separated by "/".
Fourth, thread processing
1. Tapping thread and sleeve thread
Tapping:
The machining method of machining an internal thread in a hole with a thread tap is called tapping.
The formula for calculating the diameter of the bottom hole: consider it according to the plasticity of the material.
For steel parts and materials with high plasticity: D hole = D-P
D-hole: Diameter of the threaded bottom hole drill bit
D: Large diameter of internal thread
P: Pitch
For example, what is the diameter of the bottom hole that we want to tap the M10 thread on the steel part?
According to the formula, hole D=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 the M10 thread on cast iron?
根据公式D孔=D-(1.05-1.1)×1.5=8.35~8.42mm
Sleeve thread:
The method of cutting out the external thread on the round rod or tube with the die is called the sleeve thread.
The formula for calculating the diameter of the round rod is: d rod = d-0.13P
D rod: diameter of the round rod before the thread, mm
d: Thread diameter, mm
p: pitch, mm
For example, if we want to make an M10 screw, what is the diameter of the round rod used?
应根据公式d杆=d-0.13P
=10-0.13×1.5=9.8mm
2. Threading
External and internal threads