What is an out-of-tolerance thread (undersized)?
An internal thread is undersized when its pitch diameter is below the specified tolerance, resulting in an excessively tight fit. The defect may be located in the first turns, along the entire thread or only in the final part of the hole.
This problem is particularly critical in components that require tight tolerances, high mechanical strength or repeated mating.
Signs that indicate an undersized thread
- screw enters with difficulty or locks after a few turns;
- GO/NO-GO plug gauges are non-compliant;
- thread visually appears “closed” or too tight;
- abnormal resistance during assembly;
- thread surface is irregular or compressed;
- tap seems to “tear” the material instead of cutting it.
Why the thread is out of tolerance: the main technical causes
An undersized thread is almost always the result of tap geometry that is not suitable for the material or cutting conditions. The most common causes include:
- Unsuitable cutting angle: if the angle is too small, the tap tends to compress the material instead of removing it correctly.
- Insufficient relief, which increases friction and narrows the thread profile.
- Tough or long-chipping materials that require specific geometries.
- Pre-hole too small, which forces the tap to remove too much material.
- Tap wear, which alters the cutting-edge profile.
UFS solutions to eliminate undersized threads
To obtain a compliant thread, it is essential to select the correct tap geometry and optimize the cutting parameters. The most effective UFS solutions include:
- Choose taps with a rake angle suitable for the material being machined, to ensure correct material removal and a stable profile.
- Use taps with greater relief when machining tough or adhesive materials.
- Check the pre-hole diameter to prevent it from being too small.
- Contact UFS technical support to choose the most suitable geometry for critical applications or difficult materials.
- Check tap wear and replace it when the cutting-edge profile is no longer regular.
Best practices to prevent out-of-tolerance threads
- always check the pre-hole before tapping;
- choose specific geometries for tough or abrasive materials;
- monitor chip shape as an indicator of process condition;
- avoid cutting speeds that are too high;
- regularly check tap wear;
- make sure the chuck ensures coaxiality and rigidity.