1 Classification of fastening threads

The fastening threads can be divided into three categories: fastening threads, pipe threads and drive threads. Ordinary threads can be divided into three types of threads according to the unit of measurement and the country of invention, namely metric, American, and inch threads.

1.1 metric ordinary thread

The basic thread of the metric thread is the truncated contour of the equilateral triangle, the tooth angle is 60°, and the thread is marked with the letter M (representing the metric common thread). The size is expressed by the nominal diameter and the pitch, and the unit of measurement is mm. That is, the metric system, the main criteria are:

1) International standards ISO68-1, ISO261, ISO965-1, -2, -3 "Metric Threads";

2) Chinese standard GB/T192～193, 196～197, 9145 “Ordinary thread”;

3) German standard DINISO68-1, 261, 965-1, -2, -3 "metric metric ordinary thread";

4) Japanese standards JISB0205-1~-4, B0209-1~-5 "Metal Threads";

5) Australian Standard AS1275 "Metric Threads";

6) American metric standard ASMEB1.1M "rice ordinary thread";

7) American metric standard ANSI B1.3M-1983 (R2001) "Metric Thread Gauge and Measurement".

1.2 American ordinary thread

The threaded basic profile is a truncated profile of the equilateral triangle crest, with a 60° angle (same as a metric thread), and the thread size is marked by the nominal diameter and the number of teeth per inch, measured in inches. .

The thread code is:

UNC/UNRC (unified coarse thread / external thread root is a circular arc thread);

UNF/UNRF (uniform fine thread / external thread bottom is a fine thread of the arc);

UNEF/UNREF (unified superfine thread / external thread root is an ultra-fine thread of the arc);

UN/UNR (unified constant pitch thread / external thread root is a constant pitch thread of the arc);

UNS (unified thread of special series), etc. Among them, the commodity fasteners are commonly used for coarse and fine teeth (UNC/UNF, UNRC/UNRF) thread series and 4UN, 6UN, 8UN, 12UN, 28UN, 32UN and other constant pitch series threads.

The thread and thread gauge standards mainly include:

1) American ASMEB1.1-2002 "Unified inch thread";

2) American ASMEB 1.2-1983 (R2001) "Unified inch thread gauge and measurement";

3) American ASMEB1.3M-1992 (R2001) "Unified inch thread size acceptance inspection method".

1.3 inch ordinary thread

The basic thread type of the inch thread is the contour of the isosceles triangle crest and the bottom arc transition. The angle of the tooth is 55°. The thread is marked with the nominal diameter and the number of teeth per inch, measured in inches. Thread codes include BSW (Wyeth coarse thread), BSF (Wyeth fine thread), Whit (non-standard series Wyeth thread) and Whit.S (selected series of Wyeth threads). Among them, commodity fasteners are commonly used in BSW and BSF series. The main criteria for this thread are:

1) British BS84-1966 "Cylinder Wyeth Thread";

2) British BS919-2-1966 "Whit Thread and BA Thread Gauge Limits and Tolerances".

2 thread inspection

1) Due to many misunderstandings, certain risks and economic requirements in the field of thread inspection, many troubles have been brought to the acceptance of threaded products, which has buried many hidden dangers to the quality of mechanical products.

2) Due to the inconsistency of thread inspection, many quality disputes have been brought about by the inspection of both sides of the export trade.

3) In order to fundamentally reverse this passive situation, the United States has conducted a large number of technical studies on thread testing and proposed a standard for tightening thread testing systems. This is a relatively advanced and complete set of thread inspection methods in the world.

2.1 Thread inspection purposes

The US ANSI/ASMEB 1.3-1979 "Thread Size Acceptance Inspection Method" standard states:

There are two problems with any thread fit.

1) Can the internal and external threads be assembled smoothly? (Interchangeability or adaptability)

2) Can the assembled thread withstand a certain load? (Mechanical strength)

Except for the above two points, others are secondary. If the inner and outer threads of the fastener used cannot be matched, the parts cannot be assembled; if the assembled thread cannot withstand a certain load or is destroyed early after assembly, the screw fastening effect is lost.

2.2 Three methods for specifying thread inspection in the United States

The influence of the thread assembly is the mutual action of various thread parameters such as the large diameter, the medium diameter, the small diameter, the flank angle, the lead angle and the pitch of the thread, which are directly reflected on the working diameter of the thread, that is, the matching size.

The inspection and measurement of various elements of the thread have been used to determine the eligibility of the thread. The controversy is relatively large, and the national standards are not uniform. In order to standardize the inspection method for uniform threads, the United States first issued the ANSI/ASMEB1.3-1979 (confirmed 2001) "Thread Size Acceptance Inspection Method" standard in 1979, which better solved this problem. The standard specifies three measurement methods for the thread, namely the “21” method, the “22” method and the “23” method. The specific content is:

1) Method "21": In order to ensure the interchangeability of the thread within the specified range of the screw length, the maximum physical size of the thread is checked by the gauge, and the diameter of the thread is checked by the stop LO or HI.

2) Method "22": In order to ensure the interchangeability of the thread within the specified range of the screw length, the minimum physical size of the thread length is also controlled when the maximum physical size of the thread is checked. .

3) Method "23": In order to ensure the interchangeability of the thread within the specified range of screw lengths, in addition to the general physical dimensions of the thread, the minimum physical size of the full length of the thread and all other thread elements are controlled: Such as lead angle, half angle, ellipticity, taper and so on.

The thread inspection of commodity fasteners specified in the US IFI-7 fastener assembly generally adopts the "21" method.

External thread inspection mark method:

1/420UNC2A-(21)

That is to use the 2A thread ring gauge GO Tong gauge to comprehensively test the maximum physical size of each element of 1/4-20-UNC thread; check the thread diameter diameter with 2ALO gauge, and check the thread diameter to make a judgment of pass or fail.

Internal thread inspection mark method:

1/420UNC2B - (21)

That is, the 2BGO general gauge of the thread plug gauge comprehensively checks the maximum physical size of each element of the 1/4-20-UNC internal thread; 2BHI stops checking the thread diameter, and also checks the thread diameter to judge whether it is qualified or not.

2.3 Principles of use of gauges

2.3.1 Work gauges and acceptance gauges

In the daily inspection of threads, there is often a controversy over the use of gauges. That is to say, when testing the thread, different inspection results appear on the thread of the same workpiece with the new gauge and the old gauge. This is a frequent occurrence in the factory inspection and daily export trade, and is the most controversial inspection event.

In the international standard ISO3269-2000 "Container Acceptance Inspection" (GB/T90.1-2002, IDT) and other foreign standards, the requirements for the inspection of threads and other methods of inspection are different. That is to say, the use of thread gauges to inspect the fastener threads of the commodity is the only measurement method that can finally determine the acceptance or not.

Article 4.4 of the Standard stipulates: “When the gauges and measuring instruments are used in inspection, if the size and performance of the fasteners are within the specified limits, no fasteners shall be rejected. In case of dispute, direct use shall be used. Measurement. But not for thread inspection. It is decisive for the gauge to check the thread."

In order to solve the controversy caused by the different specifications of the old and new gauges, the actual limit of the gauge thread (new and old wear), this is stipulated in GB/T3934--1983 "Ordinary Thread Gauge" (ISO1502) Acceptance thread gauge: refers to the thread gauge used by the inspection department or user representative when accepting the thread of the workpiece. At the same time, it also stipulates in Article 1.6: “When a dispute arises during the inspection, if the thread of the workpiece is judged to be a qualified thread gauge If the requirements of this standard are met, the workpiece thread shall be treated as qualified.". In the American Standard ANSI/ASME B1.2 "Unified UN/UNR Inch Thread Series Gauge and Measurement" standard, two gauges (X deviation) of the gauge and the acceptance gauge are also proposed. In actual use, the following method is recommended.

Work gauges are generally used for gauges for thread inspection in production processes.

Use new or newer regulations, old or near-wear limit stops as work gauges.

The acceptance gauge is generally used for the arbitrage gauge of the enterprise inspection department or the user representative to determine whether the thread is qualified.

Use old or near-wear limit gauges, new or newer gauges as acceptance gauges.

2.3.2 Inspection method

1) When using the general inspection thread, it is stipulated that no tool should be borrowed, and the general rule "GO" can be freely screwed into the full length of the thread without excessive force by hand. So how much force is applied to the gauge? When the American IFI Fastener Association proposed the thread inspection, the maximum torque allowed on the gauge is 12 times the nominal diameter of the British pound.

2) When using the stop gauge to check the thread, do not use excessive force by hand, screw the stop gauge "NOGO" or "HI" into the thread and stop it to exit the thread assessment.

3) The basis of national standards for allowing thread stop gauges to enter the number of threads and standards:

The inspection standard for 1 metre threaded pass and stop gauge is based on:

International Standard ISO 1502 "Ordinary Thread Gauge and Inspection" (GB/T3934-1983) standard "General Provisions":

Internal thread stop is not allowed to enter 2 teeth. The effective thread for the product is less than 3 teeth, and the stop gauge is not allowed to enter the thread.

External thread stop is not allowed to enter 2 teeth. The effective thread for the product is less than 3 teeth, and the stop gauge is not allowed to enter the thread.

2 Inspection standards for US threaded and stop gauges:

The American Standard ANSI/ASMEB 1.2 "Uniform Thread Gauge" standard states:

Internal threads are specified in Section 4.2 of the standard:

The HI thread stop is not allowed to enter 3 teeth.

External threads are specified in Section 5.2 of the standard:

The LO thread stop is not allowed to enter 3 teeth.

3 Japanese thread pass, stop gauge inspection standard basis

Japanese standard JISB052 "Metric Thread Thread Gauge Limit", Section 9 "Usage Purposes and Uses of Thread Gauge" stipulates:

The internal thread stop is not allowed to enter 2 teeth, the thread of the product is less than 3 teeth, and the stop gauge is not allowed to enter the thread.

The external thread stop is not allowed to enter 2 teeth, the thread of the product is less than 3 teeth, and the stop gauge is not allowed to enter the thread.

4 Smooth gauges are free to pass through the full length of the thread

Smooth stop allows entry: (Z1+Z2) ≤ 1/2 H (or m)

Where: Z1, Z2 D stop entry amount;

H (or m) D refers to the height of the nut.

5 Thread gauges are to pass freely through the full length of the product.

references:

[1]ASME B1.2 Gages and Gaging for Unified Inch Screw Threads.

[2]ANSI B1.3MScrew Thread Gaging Systems for Dimensional Acceptability - Inch and Metric Screw Threads.

[3]ISO 3269Fasteners -- Acceptance inspection.

[4]ISO 1502 general-purpose metric screw threads -- Gauges and gauging.

[5] GB/T 3934 "Ordinary thread gauge".

[6] GB/T 90.1 "Container Acceptance Inspection".

[7] JIS B052 "Metric Thread Gauge Limit".

[8] BS 84 Parallel screw threads of Whitworth form.

[9] BS 919-2 Screw gauge limits and tolerances. Specification for gauges for screw threads of Whitworth and BA forms.

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