Correct marking of thread in fastener

When engaged in mechanical design work, there are often thread features in the part drawing. In addition to drawing the thread in a standard way, it is also necessary to mark the thread correctly, so as to ensure that the processed parts are what you want and there is no error in assembly. In addition, threaded standard parts are often used in assembly drawings, and they should also be correctly marked in the list so that the purchasing department can purchase them correctly. When other personnel read mechanical drawings, they also need to understand the marking of thread.

Therefore, the thread marking knowledge is a necessary skill for the mechanical industry personnel. This paper will mainly introduce the marking of various threads.

1 common thread

Common thread is the most widely used thread in life and work. Most of the threads on threaded fasteners (bolts, studs, screws, nuts and other parts) are common threads.

Common thread is divided into coarse thread and fine thread. Fine thread is usually used in precision parts and thin-walled parts (for example, the common thread used in hydraulic pipe joint is fine thread).

In the marking of thread, the pitch of fine thread must be noted, while that of coarse thread is not marked (because fine thread of the same diameter may have multiple pitches, while the pitch of coarse thread is certain). The pitch of common thread is specified in national standard (GB / T 193 series of diameter and pitch of common thread). Hebei Shengxin fastener

The complete marking of common thread consists of five parts:

For example: M10 × 1-7h-l-lh

(1) Characteristic code and size code of common thread

Feature code: m (for common thread).

Dimension code: nominal diameter × pitch (the lead and pitch of multi thread thread shall be indicated, and the pitch of single coarse thread shall not be marked).

"M10" means single coarse thread with nominal diameter of 10 mm and pitch of 1.5 mm.

"M10 × 1" means single thread fine thread with nominal diameter of 10 mm and pitch of 1 mm.

(2) Tolerance zone code of common thread

The tolerance zone code of common thread is composed of tolerance grade (number) and basic deviation (external thread is represented by lowercase letter and internal thread is represented by capital letter), such as 5g6g, 6G, 6h and 7h.

When the codes of pitch diameter tolerance zone and top diameter tolerance zone of thread are different, they should be noted separately, such as: m10-5g6g (5g is pitch diameter tolerance zone code, 6G is top diameter tolerance zone code).

When the tolerance zone codes of the middle diameter and top diameter are the same, only one code should be noted, such as: M10 × 1-7h. When the tolerance zone code is 6G or 6h (nominal diameter ≥ 1.6mm), it is not marked (if the tolerance zone of ordinary thread is not marked, the default tolerance zone is 6G or 6h). The content of tolerance grade and basic deviation will be introduced in a later article.

(3) Thread length

There are three types of screw thread: long, medium and short, which are indicated by the codes L, N and s respectively.

For example, m10-5g6g ― s is the thread with short thread length, and the thread with long thread length is m107h ― L. When the thread is of medium thread length, code n is not marked.

In case of special needs, the value of the spinning length can be indicated. For example, M20 × 2-5g6g-40.

(4) Rotation code

Mark LH for left rotation and not for right rotation. For example: m10-7h-l-lh is left-handed thread, and m107h-l is right-hand thread.

2 pipe thread

Pipe thread is generally used in the connection of pipeline (water pipe, oil pipe, gas pipe, etc.). The marking of pipe thread shall be marked with the guide method, and the leader shall be pointed to the major diameter of the thread.

Marking of pipe thread: it is composed of thread feature code, dimension code and rotation direction. The size code (nominal diameter) of pipe thread is not the size of thread major diameter, but the size of pipe diameter (inch system).