Thread is a fastening connection structure widely used in the industrial field. In military models and products, more than 80% of the structural connections are completed through threads. Whether the thread is qualified or not directly affects the assembly performance and safety of the equipment.

In the measurement of thread parameters, according to the different measuring equipment, the thread parameter measurement methods can be mainly divided into the following categories:

Thread pass gauge detection method

The traditional go-go and stop-gauge inspection methods according to the Taylor principle can only meet the minimum inspection requirements in thread inspection. The advantage of this test method is fast, economical and practical. For enterprises with a high level of production technology, the use of qualified thread forming tools and special thread processing equipment (such as thread taps, dies, thread rolling machines, thread rolling machines, etc.) can better control the quality of threads.

The main disadvantage of this inspection method is that it can only qualitatively check whether the pitch diameter of the thread is qualified, and can only know whether the pitch diameter of the thread is between the smallest single pitch diameter and the largest active pitch diameter, and cannot know the specific value of the thread size. Parameters such as half-angle error, pitch error and various shape errors cannot be quantitatively controlled separately.

Thread micrometer

The thread micrometer is a special screw micrometer measuring tool. The thread micrometer has a special measuring head. The shape of the measuring head is made to match the shape of the thread tooth. One is a conical measuring head, which matches the tooth groove. The micrometer has a set of interchangeable measuring heads, and each pair of measuring heads can only be used to measure threads within a certain pitch range.

According to the technical index of the thread micrometer, the maximum comprehensive error is ±0.028mm. Due to the certain angle error of the measuring head, there are also large errors in the pitch and flank angle of the external thread of the workpiece. Therefore, when measuring with the absolute method, among them The measurement uncertainty of diameter can reach 0.10mm. This method is used to measure the pitch diameter of the external thread of the workpiece that does not require high precision.

Three-needle method

The three-needle is a set of three measuring needles with the same determined diameter, and measures the pitch diameter of the thread by indirect method. By putting three measuring needles into the tooth grooves on both sides of the thread, and using a lever micrometer to measure the overall outer diameter, the single pitch diameter of the thread is calculated through the functional relationship between the pitch diameter of the thread and the diameter of the measuring needle, tooth angle and pitch. In order to simplify the calculation process, a measuring needle with an appropriate diameter should be selected.

Gauge type thread comprehensive measuring instrument

The high-precision thread comprehensive measuring instrument adopts the contact thread measurement method, which has great advantages and feasibility compared with the traditional measurement method and non-contact measurement method, and has the advantages of high accuracy, convenient use, and strong function. Measurement has great reference value.

Video method

The commonly used inspection equipment for thread measurement by image method includes projector and universal tool microscope. The projector measures the parameters of the stud by visual positioning and coordinate translation through the enlarged outline of the tested part. At the same time, the projection method has limitations in the measurement of the pitch diameter of the thread itself, and the accuracy of the projection method is generally very rough.

The universal tool microscope uses the optical system to project the thread profile in the field of view of the eyepiece, and uses the grating scale to translate, which can be used to test various thread parameters, especially the lens deflection can effectively solve the problem of the helix angle, and has a high measurement for the thread precision.

Laser triangulation

Laser triangulation, as another non-destructive testing method, has received a lot of attention. It is mainly based on the principle of traditional optical triangulation to obtain the data point set of the axial section profile of the thread surface, and then calculate the parameters of the thread.

This measurement method has the characteristics of non-contact, high precision, easy control, etc., and can also be applied to high-speed measurement of small sizes. However, this method is affected by the ambient temperature and the surface characteristics of the measured object, and the influencing factors must be taken into account in the later data image processing.

CMM measurement

Three-coordinate measuring machines are widely used in the measurement of boxes, racks, gears, cams, worm gears, worms, blades, curves, and curved surfaces in industries such as machinery, automobiles, aviation, military industry, and molds. Through the transformation of the three-coordinate measuring machine, there are many methods for measuring thread parameters.

However, the three-coordinate measuring machine is a widely used general-purpose equipment, lacks a probe for thread measurement, and requires a custom probe head for small-size threads. Software development is also limited by the software platform of the CMM itself, and it is difficult to customize the contour following control system for the thread geometry. And its cost is much higher than dedicated thread measuring equipment.

Stylus scanning thread profile measuring instrument

The stylus scanning thread profile measuring instrument continuously scans and measures the upper and lower profile surfaces of the axial section of the thread, and then calculates the pitch diameter, major diameter, minor diameter, pitch, tooth half angle, taper, etc. of the thread based on the obtained profile information. parameter.

Hope the above content is helpful to you.