1. Thread types
According to the tooth shape, it can be divided into triangle, trapezoid, rectangle, zigzag and arc thread;
According to the thread direction, it can be divided into left-hand and right-hand;
According to the number of spiral lines, it can be divided into single line and multi-line;
According to the shape of the thread body, it is divided into cylinder and cone.
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2. Elements of thread
Threads include five elements: tooth profile, nominal diameter, number of threads, pitch (or lead) and direction of rotation.
1. Tooth shape
In the cross-sectional area passing through the thread axis, the outline shape of the thread is called the profile. There are tooth shapes such as triangle, trapezoid, zigzag, arc and rectangle.
Thread profile comparison:
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2. Diameter
Threads have major diameters (d, D), medium diameters (d2, D2), and minor diameters (d1, D1). When expressing threads, the nominal diameter is used, and the nominal diameter is the diameter that represents the thread size.
The nominal diameter of an ordinary thread is the major diameter.
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External thread (left) Internal thread (right)
3. Number of threads
Threads formed along one spiral are called single-thread threads, and threads formed by two or more spirals equidistantly distributed along the axial direction are called multi-threads.
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Single thread (left) Double thread (right)
4. Pitch and lead
Pitch (p) is the axial distance between two corresponding points on the mid-diameter line of two adjacent teeth.
Lead (ph) is the axial distance between two corresponding points on the mid-diameter line of two adjacent teeth on the same spiral line.
For single-thread threads, lead = pitch; for multi-thread threads, lead = pitch × number of threads.
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5. Direction of rotation
Threads that are screwed in when turned clockwise are called right-hand threads;
Threads that are screwed in when turned counterclockwise are called left-hand threads.
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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 and fine threads. Fine-thread ordinary threads are mostly used on precision parts and thin-walled parts.
In the marking of threads, the pitch of a fine-thread ordinary thread must be marked, while the pitch of a coarse-thread ordinary thread is generally not marked.
The marking of ordinary threads consists of five parts:
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1) Feature code
M (meaning ordinary thread)
2) Size code
Dimension code: nominal diameter × pitch (the lead and pitch of multi-thread threads must be noted, and the pitch of single-thread coarse-thread ordinary threads is not marked).
"M10" means a single-thread ordinary thread with a nominal diameter of 10mm and a pitch of 1.5mm.
"M10×1" means a single-line fine-thread ordinary thread with a nominal diameter of 10mm and a pitch of 1mm.
3) Tolerance zone code for ordinary threads
It consists of the tolerance level (number) and the basic deviation (external threads are represented by lowercase letters and internal threads are represented by uppercase letters), such as 5g6g, 6g, 6H, 7H.
When the thread pitch diameter tolerance zone and the top diameter tolerance zone have different codes, they need to be noted separately, such as: M10-5g 6g
When the tolerance zone codes for pitch diameter and top diameter are the same, only one code should be noted, such as: M10×1-7H
It is not marked when the tolerance zone code is 6g or 6H (nominal diameter ≥ 1.6mm).
4) The screwing length of ordinary threads
There are three types: long, medium and short, which are represented by codes L, N and S respectively.
M10-5g6g-S is a thread with short engagement length
M10-7H-L is a thread with long engagement length
When the thread is of medium screwing 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
5) Rotation code
LH is marked for left-hand rotation and not marked for right-hand rotation.
M10-7H-L-LH is left-hand thread
M10-7H-L is right-hand thread
(2) Pipe thread
Pipe threads are generally used for the connection of pipelines (water pipes, oil pipes, gas pipes, etc.). Pipe threads are marked using a guide, with the guide line pointing to the major diameter of the thread.
Pipe thread markings:
It consists of thread feature code, size code and direction of rotation. The size code is not the major diameter of the thread, but the diameter (imperial) of the pipe. If the direction of rotation is not noted in the mark, it is right-handed.
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Examples of markings for 55° unsealed pipe threads:
G: Thread characteristic code of non-sealed pipe thread;
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, A and B in the mark are the tolerance levels of the thread pitch diameter;
The LH in G3/4LH and G3/4A-LH means left-hand thread, and the thread pair formed by the two is only marked with the marking code of the external thread.
Examples of markings for 55° sealing pipe threads:
Rp3/4LH: single-line left-hand cylindrical internal thread with size code 3/4;
Rc3/4: single-line right-hand conical internal thread with size code 3/4;
Rp/R13/4 LH and Rc/R23/4: The internal thread and the external thread are screwed together to form a thread pair.
Rp: Thread characteristic code of sealed cylindrical internal thread;
Rc: The thread characteristic code of the sealed conical internal thread;
R1: The characteristic code of the conical external thread that matches the cylindrical internal thread;
R2: The characteristic code of the conical external thread that matches the conical internal thread.
(3) Trapezoidal thread and zigzag thread
Trapezoidal threads and zigzag threads are commonly used on screws that transmit motion and power. When the trapezoidal thread is working, both sides of the thread are stressed, while the zigzag thread is stressed on one side.
Trapezoidal and zigzag threads are marked the same as ordinary threads.
Examples of markings for trapezoidal threads:
Tr40×7LH-7e, trapezoidal thread (thread characteristic code is Tr), nominal diameter φ40, single thread, pitch 7, left-hand, pitch diameter tolerance zone code 7e; medium screwing length. Note: Only the pitch diameter tolerance zone code is marked, and there are only two screwing lengths (codes N and L). When the screwing length is medium, N is omitted.
When the thread is a multi-thread, the marking is:
Tr40×14 (P7)-7e, where "14" is the lead, "7" is the pitch, double thread.
The thread pair of the trapezoidal thread is expressed as: Tr40×7-7H/7c. The tolerance zone of the internal thread is in the front and the tolerance zone of the external thread is in the back. The two are separated by "/".
There are many types of threads, and there are also many drawing methods. Novice friends may be confused. Here is a summary of thread drawing methods.
1. External thread
The national standard stipulates that the crest (major diameter) of the thread and the termination line of the thread are represented by thick solid lines, and the crest (minor diameter) of the thread is represented by thin solid lines. In the view of the projection plane parallel to the axis of the screw, the chamfer of the screw or The chamfered part should also be drawn; in the view of the projection plane perpendicular to the thread axis, only about 3/4 of the thin solid line circle representing the tooth bottom is drawn, and the chamfering of the thread is omitted at this time, as shown in Figure 1 .
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2. Internal thread
Figure 2 shows how to draw internal threads. When shown in section, the tooth base (large diameter) is a thin solid line, the tooth crest (small diameter) and the thread termination line are thick solid lines. When expressed without sectioning, the tooth base, tooth crest and thread termination line are all dotted lines. In the view perpendicular to the axis of the thread, the root of the tooth is still drawn as a thin solid line of about 3/4 turn, and it is specified that the chamfer of the thread is also omitted.
3. Thread blind hole drawing method
When drawing a non-penetrating screw hole, the depth of the drilled hole and the depth of the threaded part should generally be drawn separately, as shown in Figure 3(a). When it is necessary to indicate the end of the thread, the root of the thread tail is represented by a thin solid line at 30° to the axis, as shown in Figure 3(b). Figure 3(c) shows how to draw intersecting lines in threaded holes.
4. How to draw internal and external threaded connections
Figure 4 shows how the internal and external threaded connections are assembled together. The national standard stipulates that when a threaded connection is represented in a cross-sectional view, the screwed part should be represented by the drawing method of the external thread, and the non-screwing part should still be represented by their respective drawing methods.
5. How to draw non-standard threads
When drawing threads with non-standard thread forms, the thread thread form should be drawn and the required dimensions and related requirements should be marked, as shown in Figure 5.
Precautions
When various threads are drawn in the same way, they must be marked on the graphics to facilitate distinction.
