Principles And Methods of Thread Inspection

Thread inspection disputes and inspection rule characteristics

Thread inspection is one of the most controversial events in trade. Due to the inconsistency of thread inspection methods and rules, thread gauges sometimes "fight".

Different coatings on the surface of threaded fasteners, thread gauge disputes caused by surface coatings (after electroplating, Dacromet, hot-dip galvanizing), will directly affect the interchangeability of threads! Sometimes there will be disputes over thread inspection after coating, how to inspect the thread end and surface bumps, and thread deformation after heat treatment?

Purpose of thread inspection

If the internal and external threads of the fasteners used are not interchangeable, the parts cannot be assembled; if the assembled threads cannot withstand a certain load, or are destroyed early after assembly, the thread tightening function will be lost.

The factors that affect the interchangeability and precision of threads mainly include:

——Major diameter of thread

——Medium diameter (single medium diameter)

——Minor diameter

——Flank angle

——Lead angle

The interaction of various thread parameters such as pitch is directly reflected in the effective medium diameter of the thread, that is, the matching size.

Difference between effective mean diameter and single mean diameter of thread

1. Thread mean diameter

Refers to the diameter of an imaginary cylinder or cone, the generatrix of which passes through the place where the width of the groove and the protrusion on the thread tooth profile are equal. This imaginary cylinder is called the thread mean diameter cylinder.

2. Effective mean diameter

The effective mean diameter is the mean diameter that actually works in the screwing, and is an imaginary thread mean diameter.

The effective mean diameter is due to the tooth side angle deviation to the mean diameter equivalent △dα + the pitch cumulative error mean diameter equivalent △dp, and the effective mean diameter is the fundamental factor affecting the assembly and interchangeability of the thread. The effective mean diameter of the external thread is actually larger than the single mean diameter. The effective mean diameter of the internal thread is actually smaller than the single mean diameter.

3. Single mean diameter of thread

The single mean diameter excludes the influence of pitch and half angle on the mean diameter.

Measure the single mean diameter of the thread (three-needle method).

Thread inspection principle

The influence on the interchangeability and assembly of threads: the interaction of various thread parameters such as thread major diameter, pitch diameter, tooth side angle, lead angle, pitch, etc. is directly reflected in the effective pitch diameter of the thread (accumulated error), that is, the thread matching size.

Taylor principle: within the standard gauge length, the through-end thread gauge (standard tooth profile thread) is used to control the effective pitch diameter of the major diameter and tooth profile, which does not exceed the maximum physical limit; the stop-end thread gauge only controls the single pitch diameter of the thread, which does not exceed the minimum physical tooth profile pitch diameter limit.

Three methods of thread inspection

For a long time, the judgment of thread qualification in thread inspection and measurement has been controversial, and the standards of various countries are not unified. In order to standardize and unify the inspection methods, the United States has proposed a set of more scientific thread inspection methods.

Three thread measurement methods: "21" inspection method; "22" inspection method; "23" inspection method.

"21" method: within the standard gauge length, the through-end thread gauge is used to control the effective pitch diameter of the thread within the maximum physical limit; the single pitch diameter is controlled by the stop (NOT GO). In short, it is to use a standard thread gauge to inspect the internal and external threads, and the thread pass and stop should meet the requirements. Ensure assembly. Control the size of the external thread major diameter and the internal thread minor diameter.

"22" method: within the standard gauge length, control the effective size of the thread within the maximum physical limit; measure whether the effective mean diameter and the single mean diameter are within the tolerance range. On the basis of the qualified inspection of the "21" method, further measure the geometric dimensions of the thread.

"23" method: within the standard gauge length, control the effective mean diameter of the thread within the maximum physical limit; measure whether the effective mean diameter and the single mean diameter are within the tolerance range, and perform single measurement on the thread elements with specified limit deviation values.

Applicability of three thread inspection methods

Method 21 is applicable to products for thread inspection: general engineering, machinery and medium-strength thread matching occasions. Thread inspection of commercial fasteners (including high-strength steel structures) specified in the American IFI-9 Fastener Compendium.

Method 22 is applicable to products for thread inspection: special high-strength fasteners with external threads that require tighter matching, or 3A grade external thread fasteners.

Method 23 is applicable to products for thread inspection: UNJ, aviation fasteners or instrumentation fasteners.