Basic Knowledge of Thread Gauge

A thread gauge is a gauge used to check whether threads meet specifications. Thread plug gauges are used to check internal threads and thread ring gauges are used to check external threads.

Thread is an important and commonly used structural element. Threads are mainly used for structural connection, sealing connection, transmission, reading, and bearing. From general use conditions to harsh conditions (high temperature, high pressure, severe corrosion), from rough levels to very quiet, in short, a wide range of applications.

NPSM-American Standard Straight Pipe Thread for Mechanical Joints: These internal and external threads are used for free mechanical connection without internal pressure, and the product is inspected with a straight pipe through a stop ring plug gauge.

NPSL-American Standard Straight pipe threads for lock nuts: These internal and external threads are used for mechanical fit to prevent thread feed.

NH-American Standard Fire Hydrant Thread: These internal and external threads are used in fire hydrants, garden hoses, chemicals, and elevators.

NPSH– American Standard Thread for hose connection: These internal and external threads are used for steam, air, water, and other standard pipe thread connections.

NPSC– American Standard Straight pipe threads for pipe joints: The pipe joint has the same tooth shape as the inner straight pipe thread. When the outer cone thread NPT is tightened by a wrench when assembling the sealing packing, it can usually form a sealing connection and is used for a low-pressure pipeline system.

NPSF-American Standard Straight pipe internal thread for dry seal standard fuel oil: These internal threads are used on soft materials or ductile iron parts without sealing with NPTF external threads.

NPSI-American Standard Dry seal intermediate thread: These internal threads are used for assembly of hard or brittle materials with short PTF-SAE external threads, but can also be used for full-length assembly of male NPTF threads.

Thread gauges are classified according to their performance: Work gauge, acceptance gauge, proofreading rules, and benchmark rules.

Work Gauge: Thread gauges used in the manufacture and inspection of workpiece threads.

Acceptance Gauge: The thread gauge used by the inspection department or the user’s representative to accept the thread of the workpiece.

Check Gauge: Thread gauges for manufacturing and testing work gauges. For cylindrical threads, there are usually only proofing plug gauges, which are used for the inspection of working ring gauges.

Benchmark Gauge: Certain tapered threads (such as oil drill pipe joint threads and NPT threads) require a datum gauge for checking calibration gauges.

When checking the thread with a cylindrical thread gauge, the thread gauge at the through end should be screwed in completely, and the thread gauge at the end should not be screwed in or be allowed to be partially screwed in, then the thread to be inspected is qualified.

When using tapered thread gauges to inspect threads, the following conditions are met: Use a limit gauge to check whether the position of the measuring surface of the tested thread (internal thread is the big end, and the external thread is the pipe end plane) is within the specified upper and lower limits; Use a standard gauge (such as a special thread gauge for petroleum) to check whether the measuring surface of the thread to be inspected is within the specified range (whether the tight distance of the special thread for petroleum is qualified). For petroleum-specific threads, it is also necessary to test whether the single element of the thread is qualified.

Structural Elements of Thread

1. Thread form: On the section through the thread axis, the thread profile is called thread profile.

Common thread teeth are: Common screw threads, pipe threads, trapezoid thread, saw tooth thread, rectangular thread.

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

2. Major Diameter, Mean Diameter,  Minor Diameter

Name: Major Diameter

Representing Method: d, D

Definition: The diameter of an imaginary cylinder overlaps the crest of an external thread or the base of an internal thread(i.e. the maximum diameter of the thread).

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

Name: Mean Diameter

Representing Method: d2, D2

Definition: There is an imaginary cylinder between the major diameter and the minor diameter, and its generatrix passes through the place where the width of the groove and the convex width on the tooth form are equal. This imaginary cylinder is called a middle diameter cylinder, its generatrix is called a middle diameter line, and its diameter is called a thread diameter.

Code name: In (d, D), (d1, D1), (d2, D2), the lowercase letters represent the diameter of the external thread, and the uppercase letters represent the diameter of the internal thread.

Name: Minor Diameter

Representing Method: d1, D1

Definition: The imaginary cylinder diameter that coincides with the root of the external thread or the crest of the internal thread(i.e. the smallest diameter of the thread).

3. Lines: The number of threads on the cylindrical end face, denoted by n.

A thread formed along a helix is a single thread; A thread formed by two or more helical threads distributed at equal distances in the axial direction is a multi-thread thread, as shown in Figure 3.

                                                                                   Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge              Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

Figure 3.

(a) Single thread

(b) Double thread

4. Pitch and lead: as shown in figure 3.

Pitch (code: P): The axial distance between two points corresponding to the middle diameter line of two adjacent teeth.

Lead (Code: Ph): The two adjacent teeth on the thread formed by the helix correspond to the axial distance between the two points on the center diameter line.

For a single thread, pitch = lead,

For multiple threads, pitch =lead/ lines.

5. Direction of rotation: as shown in Figure 4 (b).

Clockwise rotation of the screw, called the right-handed thread.

Counterclockwise rotation of the screw, called the left-handed thread.

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

Figure 4.

Tooth type, major diameter, pitch, number of threads, and direction of rotation are the five elements that determine the geometric dimensions of the thread. Only the external thread and the internal thread with the same five elements can be screwed together.

Type of Thread

Thread refers to the continuous convex grooves with the same section formed along the spiral line on the surface of a cylinder or cone, as shown in Figure 1.

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

Figure 1.

There are many ways to process thread in production practice. Figure 2 shows the process of turning thread on a cylindrical surface. The thread formed on the outer surface of the workpiece is called an external thread, and the thread formed on the inner surface of the workpiece is called an internal thread.

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

Figure 2.

1. Sort by Standard

Standard Thread:

Tooth type, major diameter, and pitch all meet national standards.

Special Screw Threads:

The tooth type conforms to the national standard, and the major diameter or pitch does not conform to the national standard.

Non-Standard Thread:

The tooth shape does not meet the national standard.

2. Sort by Use

Tooth Form:

Connection thread, regular screw threads, pipe thread, transmission thread, trapezoidal thread, sawtooth thread, rectangular thread. 

3. Sort by the Thread Pitch

Coarse Thread:

The common thread of the same diameter, which is the largest pitch of a thread.

Fine Thread:

All threads except the one with the largest pitch.

Specified Drawing of Thread

1. External Thread

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

①The crest of the thread is represented by a thick solid line, and the root of the thread is represented by a thin solid line.

②The minor diameter of the thread can be drawn approximately as 0.85 times the major diameter (ie 0.85d).

③In the view of the projection plane perpendicular to the thread axis, the thin solid line circle representing the tooth base is only drawn about 3/4 of the time; The chamfered circles of thread in this view are omitted.

④The stop line of the thread is indicated by a thick solid line. When the stop line of the external thread is cut, only a short segment of the thread stop line is drawn to indicate the height of the tooth profile.

⑤All lines with invisible threads are dotted.

⑥In the section view, the section lines of the internal and external threads should be drawn too thick solid lines.

⑦Drawing method when connecting internal and external threads: When represented in section view, the screwing part is drawn in accordance with the external thread drawing method, and the best part is still represented in accordance with their own drawing method.

⑧When the thread shape needs to be represented, it can be represented by partial section view, partially enlarged view, or directly represented in section view.

2. Internal Thread

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

3. Drawing Method of External Thread Connection

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

Thread Gauge Types

Cylindrical Thread Gauge

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

Ordinary Thread Gauge

New standard common thread mark middle diameter tolerance zone code and top diameter tolerance zone code (if the two are the same, only the former is marked). But the common thread gauge only marks the mean diameter tolerance zone code of the checked thread. This is because the diameter of the thread gauge is calculated from the mean diameter tolerance zone of the inspected thread, independent of the top diameter tolerance zone of the inspected thread. See Table 1-1-1 and Table 1-1-2 for thread tolerance zone codes and corresponding marks for thread gauges.

Table 1-1-1 

Thread tolerance zone and Plug gauge mark

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

Table 1-1-2

Thread tolerance zone and ring gauge mark

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

The principle of judging the qualification of the pitch diameter of the thread: The effective pitch diameter of the actual thread cannot exceed the pitch diameter of the largest solid tooth profile, and the single pitch diameter of any part of the actual thread cannot exceed the pitch diameter of the smallest solid tooth profile.

Use a full-tooth type through-end thread plug gauge or ring gauge (marked with the letter T) with a large number of thread turns to check the effective pitch diameter of the workpiece thread. Check the single pitch diameter of the workpiece thread with the end-stop thread plug gauge or ring gauge (marked with the letter Z) of the cut-off profile with fewer thread turns.

American Standard Unified Thread Gauge

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

Commonly used American standard unified thread gauge, its standard number is H28/6 or ANSI B1.2.

American standard thread ring gauge is adjustable.

According to user requirements, our factory will move the pitch diameter tolerance zone of the through-end thread plug gauge and ring gauge by one gauge manufacturing tolerance value to the pitch diameter tolerance zone of the tested thread. In this way, the through-end thread plug gauge has a wear margin, and the through-end thread ring gauge can better ensure the size of the external thread of the workpiece.

British uniform thread gauge standard B.S 919:I. In theory, this standard is the most rigorous, but it is more difficult to manufacture proofreading and ring gauges.

Gauges for Metric Trapezoidal Screw Threads

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

It is important to ensure the gap between the large diameter and small diameter when the trapezoidal thread is matched. Therefore, it is appropriate to ensure that the top diameter of the through-end thread plug gauge and the ring gauge is designed.

Taper Thread Gauge

Pipe Thread Gauge with Thread Seal

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

New Standard ZB J42 037-89. The Old Standard rOCT 7157-54.

The old standard was a flat top gauge. The new standard requires dome and round bottom gauges, allowing the manufacture of flat top gauges. The flat top is formed by cutting off the threaded dome.

The old standard gauge has three steps: upper limit, lower limit, and the base surface. The new standard work gauge has only two-step surfaces, the upper limit, and the lower limit, and the proofing plug gauge has only the base surface step (standard type).

The old standard plug gauge and the ring gauge base design have the same pitch diameter, and the new standard plug gauge base design has a larger pitch diameter than the ring gauge.

Gauge for 60° Taper Pipe Thread

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

Old Standard rOCT 6485-53. American Standard H28/7.

The old standard only had limit gauges. The United States is marked with standard and limit gauges. The limit type generally has three shoulder surfaces: upper limit, lower limit, and the base surface.

NPT Gauge Pass: Benchmark (plug, ring) gauge → proofreading (plug, ring) gauge → work gauge → workpiece thread. At the same time, the single element of the gauge thread is required to be qualified.

Metric Taper Thread Gauge

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

Trapezoidal threads are mainly used in transmission (feed and lift) and position adjustment devices and are widely used in the mechanical industry. The tolerance of metric trapezoidal thread for general use adopts the tolerance system of metric common thread, and there is no separate tolerance value for individual parameters such as thread lead (pitch) and sub-measurement angle. Therefore, this trapezoidal thread is not suitable for precision transmission threads that have high requirements for transmission accuracy. The precision transmission trapezoidal thread needs to supplement the specified thread single parameter tolerance on the basis of the general trapezoidal thread standard.

Petroleum Special Thread Gauge

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

Drill pipe joint thread gauge standard GB 4749. There are thread gauges for digital, flat, through-hole, and regular joint threads. Close distance transfer: original gauge→regional gauge→check gauge→work gauge→Workpiece thread.

Sleeve round thread gauge standard GB 9253.5. The gauge is flat on top. Close distance transfer: check gauge→work gauge→Workpiece thread.

Standard GB 9253.5 for casing buttress thread gauge. Close distance transfer: check gauge→work gauge→Workpiece thread.

Tubing round thread gauge standard GB 9253.6. Gauge is a flat top. There are thread gauges for two thread size series TBG and UP TBG. Close distance transfer: check gauge→work gauge→Workpiece thread.

Line pipe thread gauge standard GB 9253.7. Close distance transmission: proofing gauge → work gauge → workpiece thread.

Petroleum special thread gauges (plug, ring gauge) require the detection of individual elements of its conical thread.

 Gas Cylinder Special Tapered Thread Gauge

The special taper thread for gas cylinders is used for the connection between the cylinder body and the valve of various steel cylinders (such as oxygen cylinders, gas cylinders, acetylene cylinders, etc.). The reliability of the locking and sealing of the threaded connection is the main factor to ensure safety in the process of production and use. Available for PZ19.2PZ19.8PZ27.8PZ39 taper thread ring gauge, plug gauge, tap.

Code for Thread and Thread Gauge

The thread and thread gauge codes are shown in Table 1-1-3.

Thread Basics, Thread Gauge, Thread Pitch Gauge,Thread Plug Gauge

  1. If the general gauge is flat, change T to TB
  2. Listed as tolerance zone code
  3. When the gauge is a flat top, mark Rb
  4. GB/T 12716-1991 only NPT

American Inspection System for Fastening Threads (UN, UNR, UNJ, M and MJ)

As there are many misunderstandings, certain risks, and economic requirements in the field of thread inspection, it brings many troubles to the acceptance of thread products and lays many hidden dangers to the quality of mechanical products. In order to fundamentally reverse this passive situation, the United States has carried out a lot of technical research on thread detection and proposed the tightening thread detection system standard (ASME standard) and 60º Uncertainty data for thread gauge measurements (ASME Technical Report). The United States leads the world in thread processing and testing technology. In the future, other countries in the world will learn from the experience of the United States and formulate their own national thread testing system standards to improve the product quality of their own threads. In the future, other countries in the world will learn from the experience of the United States and formulate their own national thread testing system standards to improve the product quality of their own threads. If the majority of technicians in our country can learn and master this thread detection system technology as soon as possible, the quality of thread products in our country will be rapidly improved, and the situation of rough thread production will be eliminated.

From the United States, the thread detection systems can also learn some advanced thread processing technology. For example, the adjustment accuracy of machine tools and cutters can be improved by using the differential indicator gauge detection technique, and the thread size can be machined close to the correct theoretical size. At the same time, the tool life will be increased.

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