Tapping Difficulties and Percent of Thread

Designing the assembly of an external and internal fastener so that the external member will break before either of the threaded part strips is a fairly common practice.  In industrial applications, the internal thread needs to be no stronger than a nut of the same size and pitch.  If a stronger assembly is needed, a tougher bolt or external threaded part will require:

  • Stronger material
  • Greater thread depth percentage
  • or longer engagement

All are conditions which make tapping more difficult.

Increasing Thread Depth

Increasing the thread depth from 60% to 72% in 1020 steel requires twice the torque, but with the same length of engagement, the strength of the thread only increases 5%. The relationship of various materials is seen when comparing the shearing strength. With 75% of thread in steel, the engagement of one half the diameter of the fastener will withstand a shearing stress of 150,000 pounds per square inch. Cast iron requires one diameter and brass one and a quarter times the diameter.

To learn more about these ratio's, click the thumbnail to the right for a large version of the chart. 

What does this mean in the practical application of taps?

When tapping a higher percentage of thread, more material is removed. If a longer length of engagement is necessary, higher torque is generated and more chips have to be removed from the threaded hole. When stronger material is required, tap design and tapping conditions have to be addressed.
If the job has prints with exact specifications as to percent of thread, depth of thread and material to be tapped - you are locked in. However, when you have some flexibility, use a drill giving you a lower percent of thread.  Tap to a depth which will give you adequate engagement, and select a material that can be machined efficiently.

Following these guidelines, when you can, will assist in trouble free tapping.

Have questions about this guide, or specific tapping difficulties?  Ask us and a Regal Representative will have an answer for you in no time!