Threads are widely used in industrial fastening connections, and in military products and models, over 80% of structural connections are made using threads. Whether a thread is qualified directly impacts the assembly performance and safety of the equipment.
The Chinese standard GB/T 197-1981 "General Thread Tolerances and Fits" specifies the criteria for judging the pitch diameter of a thread as follows:" The effective pitch diameter of the actual thread must not exceed the maximum pitch diameter of the thread, and the pitch diameter at any point on the thread must not exceed the minimum pitch diameter of the thread".
Thus, using the maximum pitch diameter as the boundary value to determine whether the thread can be assembled is the sole basis for inspection. The Go Thread Gauge is designed according to this boundary value. In other words, using the Go Thread Gauge to check the effective pitch diameter ensures that the thread can properly engage.
When measuring the parameters of a threaded screw, the methods can be categorized based on the measurement equipment used:
Go/No-Go Thread Gauge Inspection Method
Following Taylor's principle, the traditional Go/No-Go gauge inspection method only meets the minimum inspection requirements for threads. The advantage of this method is that it is quick, economical, and practical. For companies with advanced production processes, using qualified thread-forming tools and specialized thread processing equipment (such as taps, dies, thread rolling machines, and thread-cutting machines) can help control thread quality effectively.
The main disadvantage of this method is that it only qualitatively checks whether the pitch diameter of the threaded screw is qualified. It can only determine if the thread pitch diameter falls between the minimum single pitch diameter and the maximum effective pitch diameter, without providing the exact value of the thread dimension. Parameters such as thread angle errors, pitch errors, and various shape errors cannot be quantitatively controlled independently.
Micrometer Thread Gauge
A thread micrometer is a specialized measuring tool for thread pitch diameters. It features a unique measurement head shaped to match the thread form—one side has a V-shaped measuring head that fits the thread crest, and the other side has a cone-shaped head that fits the thread groove. The micrometer comes with interchangeable measuring heads, each designed for a specific pitch range of threads.
According to the technical specifications of the thread micrometer, the maximum combined error is ±0.028 mm. However, due to angular errors in the measuring heads and significant pitch and flank angle errors in the external threads, the uncertainty of pitch diameter measurement can reach up to 0.10 mm when using this method. This method is used for measuring the pitch diameter of external threads where high precision is not required.
Three-Needle Method
The three-needle method uses a set of three needles of the same diameter to indirectly measure the pitch diameter of the threaded screw. By placing the three needles in the thread grooves on both sides and using a lever micrometer to measure the overall external diameter, the pitch diameter can be calculated using the functional relationship between the thread pitch diameter, needle diameter, thread angle, and pitch. To simplify the calculation process, needles of appropriate diameters should be selected.
Thread Profile Measuring Instrument
High-precision thread profile measuring instruments employ contact-based methods for measuring threads, offering significant advantages over traditional and non-contact methods. These instruments are highly accurate, easy to use, and multifunctional, making them highly valuable for thread measurement.
Image Measurement Method
Common equipment used for image-based thread inspection includes projectors and universal tool microscopes. The projector magnifies the outline of the part being measured, and by visual alignment and coordinate translation, various parameters of the thread can be measured. However, because visual boundaries are used for alignment and the projection method has inherent limitations in measuring pitch diameters, the accuracy is typically coarse.
Laser Triangulation Method
Laser triangulation measurement, as a non-destructive testing method, has gained significant attention. This method is based on the traditional optical triangulation principle to obtain the surface profile data points of the threaded screw, from which various thread parameters are derived.
This method offers advantages such as non-contact measurement, high precision, and ease of control, making it suitable for high-speed measurements of small dimensions. However, it is affected by environmental temperature and the surface characteristics of the measured object, and these factors must be accounted for during data processing.
Coordinate Measuring Machine (CMM)
Coordinate measuring machines are widely used in industries such as machinery, automotive, aerospace, military, and mold making to measure components like boxes, frames, gears, cams, worm gears, blades, curves, and surfaces. Modifications to CMMs allow for thread parameter measurements, and several methods are available for measuring threads using this equipment.
Touch Probe Scanning Thread Profile Measuring Instrument
A touch probe scanning thread profile measuring instrument continuously scans the upper and lower profile surfaces of the threaded screw's axial cross-section. The profile information gathered is then used to calculate parameters such as pitch diameter, major diameter, minor diameter, pitch, thread angle, and taper.
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