Common methods for thread inspection in fastener testing
Thread Content
1) The inspection method using thread go and no-go gauges follows the traditional go-and-no-go gauge approach based on Taylor’s principles, and can only meet the minimum requirements for thread inspection. The advantages of this testing method are speed, cost-effectiveness, and practicality. For enterprises with a high level of manufacturing technology, using qualified thread-forming tools as well as specialized thread processing equipment (such as tap machines, die sets, thread rolling machines, thread shaping machines, etc.) can help to maintain a good quality level of the threads. The main drawback of this inspection method is that it can only qualitatively determine whether the pitch diameter of the thread is acceptable; it allows one to know only whether the pitch diameter falls within the range between the minimum possible diameter and the maximum effective diameter, but it does not provide the exact value of the thread dimension. Parameters such as half-angle error, pitch error, and various shape errors cannot be quantitatively controlled individually.
2) Thread micrometer: The thread micrometer is a specialized type of screw gauge. It features a special measuring head whose shape is designed to match that of the thread profile – one type of measuring head is V-shaped and fits against the protruding parts of the thread profile, while the other is conical in shape and fits against the grooves in the thread profile. The micrometer comes with a set of interchangeable measuring heads, and each pair of measuring heads can only be used to measure threads within a specific pitch range. According to the technical specifications of the thread micrometer, its maximum combined error is ±0.028 mm. Due to the angular error in its probe, significant errors also occur in the pitch and flank angle of the workpiece’s external threads; as a result, when using the absolute method for measurement, the uncertainty in the diameter measurement can reach 0.10 mm. This method is used for measuring the pitch diameter of external threads on workpieces where high precision is not required.
3) Three-needle method: The three needles are a set of three measuring needles with a fixed identical diameter, used to measure the pitch diameter of threads in an indirect manner. By inserting three measuring pins into the 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 based on the functional relationships between the thread’s pitch diameter, the diameter of the measuring pins, the thread profile angle, and the pitch. To simplify the calculation process, a gauge needle of appropriate diameter should be selected.
4) Gauge-type comprehensive thread measuring instrument: The high-precision comprehensive thread measuring instrument uses a contact-based thread measurement method, and it offers significant advantages and feasibility compared to traditional and non-contact measurement methods. It features high accuracy, ease of use, and strong functionality, making it highly valuable as a reference for thread measurement.