An incomplete thread is one of the most common defects in roll tapping. Unlike cutting tapping, where the thread may be incomplete due to insufficient material removal, in roll tapping the problem is caused by insufficient deformation of the material. The tap enters the hole and starts forming the thread, but the material does not flow completely into the tool geometry, leaving a partial or non-compliant thread.

In this article, we analyze the technical causes of the defect, the signs that make it possible to recognize it and the most effective UFS solutions to obtain complete and stable threading.

Incomplete thread in roll tapping: troubleshooting UFS

What is an incomplete thread in roll tapping?

In roll tapping, the thread is generated through plastic deformation. The material must flow and completely fill the tap geometry. When this does not happen, the thread is:

  • partial in the first turns;
  • incomplete along the entire thread;
  • non-compliant in depth or profile.

The defect may emerge during assembly or during GO/NO-GO plug gauge inspection.

Signs that indicate an incomplete thread

  • screw enters but does not reach the required depth;
  • thread is visibly poorly defined or “flat”;
  • GO/NO-GO plug gauges are non-compliant;
  • thread surface is irregular or compressed;
  • abnormal tapping torque, either too low or too high;
  • material does not completely fill the tap profile.

Technical causes of incomplete threads in roll tapping

The main causes are related to pre-hole quality, material ductility and tap geometry. The most common include:

  • Pre-hole too large, which reduces the amount of material available for deformation.
  • Material with poor ductility, which does not flow correctly into the tap geometry.
  • Rough or non-cylindrical pre-hole, which hinders uniform deformation.
  • Insufficient lubrication, which increases friction and prevents proper flow.
  • Tap geometry unsuitable for the material or type of hole.
  • Non-rigid chuck, which generates vibrations and irregular deformation.

UFS solutions to obtain a complete thread

To ensure complete and compliant threading, it is essential to work on the pre-hole, lubrication and tool. The most effective UFS solutions include:

  • Check the pre-hole diameter according to the UFS tables specific to roll tapping.
  • Use taps with anti-friction coatings to improve smoothness.
  • Prefer taps with internal lubrication (FOR/FORY versions) in blind or deep holes.
  • Optimize lubrication with fluids with high friction-reducing capacity.
  • Check pre-hole quality (roughness, concentricity, cylindricity).
  • Choose taps specifically designed for materials with poor ductility or difficult to deform.

Best practices to prevent incomplete threads

  • use high-quality drills to obtain smooth and regular pre-holes;
  • avoid pre-holes that are too large or out of tolerance;
  • ensure constant and adequate lubrication;
  • monitor tapping torque as a process indicator;
  • choose taps specifically designed for roll tapping and for the material being machined;
  • regularly check the surface of the thread produced.

Incomplete threads in roll tapping are an avoidable defect: with a correct pre-hole, adequate lubrication and the choice of the most suitable UFS tap, it is possible to obtain complete, stable threading that complies with the required specifications.