Precise measurements of fitting threads

Precise measurements of fitting threads are crucial for avoiding expensive leaks and system failures. In our blog, you will learn what measuring tools are truly important and which frequent errors you should absolutely avoid when measuring threads. Read more and get valuable tips for error-free joints in your industrial applications.

What are fitting threads?

Fitting threads are of great importance in industrial systems that require a secure and tight joints between pipes, valves and other components. Fitting threads are manufactured according to various standards, including NPT threads (National Pipe Thread), BSP threads (British Standard Pipe) and metric ISO threads. Each type of thread is specifically selected and precisely cut to ensure a secure and tight connection in applications such as pipelines, hydraulic and pneumatic systems.

The importance of precise measurement of fitting threads

The precise measurement of fitting threads is a very critical aspect in industrial systems that transport liquids and gases under high pressure. Even minor errors can lead to serious consequences such as leaks, wasted material, and malfunctions. Leaks not only put safety at risk, but also cause high costs due to loss of resources and maintenance. Leaky joints can also lead to pressure drops that affect the production process.

Precisely sized thread sizes and tolerances prevent unnecessary material waste and ensure that components are not damaged or become unusable. Precise measurements ensure smooth interaction of all system components, thus avoiding malfunctions and optimizing system performance.

Selecting the right measuring tool

In industrial applications that place a premium on precision, the right measuring tool can make all the difference between smooth operation and costly downtime. Each thread profile, dimension, and tolerance requires specific tools to ensure that the measurements are accurate and reproducible.

Selecting the appropriate tool depends on several factors, including the type of thread (e.g. metric or imperial), the application (e.g. high pressure or low pressure systems) and the requirements for measurement accuracy. Analog calipers, digital calipers, and specialized thread gauges offer different advantages and precision levels that must be adapted to the respective requirements. An informed decision on the selection of the measuring tool is therefore essential to ensure the integrity of joints and to ensure optimized performance in industrial systems.

Overview of various measuring tools for fitting threads

The correct measurement of the thread diameters, pitches and angles is key to ensuring the compatibility and functionality of components. Various measuring tools that are specifically adapted to thread measurement requirements are available for this purpose. These include:

• Calipers: A caliper is a versatile measuring instrument that can measure the inner and outer diameter of a thread. Although analog versions are still used in many industrial applications, digital calipers provide particularly high accuracy and easy readability.
• Angle gauges: An angle gauge is used to measure the angle of sealing surfaces on threaded connections. This is particularly important for conical threads or fittings that require an exact fit to ensure a tight joint.
• Thread gauges: Thread gauges are used for measuring the thread pitch, i.e. the distance between the individual threads. They consist of a set of gauges that each correspond to a specific thread pitch. Thread gauges are used for external threads and internal threads.

At MISUMI you will find a comprehensive range of measuring tools and equipment to accurately determine the required measurands. This allows you to ensure that your components fit together optimally and function smoothly.

Measuring fitting threads in 3 steps

The 3 steps for measuring fitting threads are described below:

Measure thread diameter

In a first step, the thread diameter is measured with a caliper. The outer diameter is determined on external threads and the inner diameter is determined on internal threads. It is important to ensure that wear and tear or deformations can influence the measurement, especially on used fittings. These deviations should be taken into account when evaluating the measurement.

Measure thread pitch

In a second step, the thread pitch is measured with a thread gauge. The number of threads per inch (TPI) is measured on threads with inch dimensions, whereas the thread pitch is determined in millimeters on metric threads,. The gauge is placed on the threads until they fit accurately, and the measurands are compared against a thread table to identify the exact thread type.

Determine cone angle

If the fitting has an inclined or conical interface, the cone angle is measured using a special gauge. This gauge is placed on the sealing surface of the fitting, while making sure that the center line of the fitting and the gauge run parallel to one another. This measurement is intended to ensure that the angle is correct, which in turn is important for the tightness of the joint.

Verify measurement accuracy

The measurement accuracy of fitting threads depends significantly on the correct preparation and use of the measuring tools. Even small alignment errors of the workpiece can result in inaccurate results and jeopardize the quality of the joint. Precise measurements ensure secure joints in industrial systems, but measurement errors often occur that should be avoided.

Measuring worn or damaged threads

When threads have already been used, they may be worn or damaged, resulting in inaccurate readings. Deformations or wear can change the diameter or the thread pitch, so that the measurement no longer corresponds to the original values. Before the measurement, the threads should be thoroughly checked for wear or damage. If significant wear is evident, the fitting should be replaced. It is also important to work carefully while measuring to avoid further damage.

Incorrect use of the measuring tool

A common error is the improper handling of the measuring tools, such as incorrect alignment of the caliper or thread gauge. This can lead to incorrect measurands. It is important to familiarize yourself with the correct use of the measuring tools. The caliper should always be placed at right angles to the threaded surface, and the thread gauge should be placed on the threads so that it is aligned without clearance.

Incorrect alignment of the workpiece

Incorrect alignment of the workpiece during the measurement can also lead to measurement errors because the measuring tool in such a case does not accurately measure the desired locations of the thread. The workpiece should be securely held in a fixed position such that the measuring tool can be positioned straight and perpendicular to the thread. Vibrations or movements during the measurement process should be avoided to ensure a precise measurement.

Contaminants on the workpiece

Dirt, grease, corrosion or chips can influence the measurement by falsifying the contact between the measuring tool and the workpiece. The workpiece should be thoroughly cleaned before the measurement. Brushes, compressed air or special solvents are useful for removing contamination. Cleaning ensures clean contact between the measuring tools and the thread.

Using uncalibrated measuring tools

Any use of uncalibrated or poorly maintained measuring tools can lead to inaccuracies. Tools that are used regularly can become worn and lose their accuracy. Caliper calibration in particular is crucial for ensuring that the measurands are precise. Measuring tools should therefore be regularly calibrated and maintained to ensure their reliability.

Incorrect interpretation of the measurement results

Even if the measurements are carried out correctly, errors in interpreting the results can occur, e.g. due to incorrect reading of the scale or misunderstanding of the thread table. It is important to read the measuring tools correctly and to carefully compare the measurands against the corresponding tables and standards.

Interpreting the measurement data

The precise measurement of threads provides essential data that must be correctly interpreted and used to ensure the integrity of components and systems. Measurement data, such as the diameter, the thread pitch and the cone angle, provide information on whether the measured threads meet the required specifications. Interpreting this data correctly ensures that the components are compatible with each other and function reliably.

After the measurement, the measurands are generally compared against standards and tables to confirm that they meet specifications. This data is important not only for assembly, but also for quality assurance and compliance with industrial safety requirements. If the measurands deviate from the specified tolerances, either corrective measures must be taken or the faulty parts must be replaced.