Tap Drill Hole Size: Why it Matters

Recently we had an application question at North American Tool.   A customer had opened up the drilled hole to 55 percent of thread in hopes of reducing wear and increasing tool life.  Instead, it lead to the cut tap breaking at the first full thread of engagement.  That prompted a engineering discussion and is the basis of this technical article to address this very common misperception.

If a tap doesn’t create the hole, why is the hole size so important?

Any experienced machinist will tell you, threading holes is the last thing you master in this craft.  Why is that so? 

1. Drilling and milling are usually easier.   
2. Threading the hole is usually the last operation on a part.  

There are a number of things that can impact creating good quality threads but one of the first things to consider is the drill hole size.

As a rule-of-thumb for 60 degree threads a percentage of thread of 75% is considered the norm.  That is a good guideline, as it falls within both 3B and 2B minor diameter limits. 

However, when we look up a size on a tap/drill chart or access the recommended drill for a given tap in North American Tool’s Thread Tap App, the drill size required for 75% thread depth doesn’t exist. 

The red line depicts the chamfer angle on a (A) Taper Tap, with a 7-10 thread chamfer, B Plug Tap, with a 3-5 thread chamfer and a (C) Bottoming Tap with a 1-2 thread chamfer.

This leaves the good machinist to select a drill that is more or less than 75%.  If 75% is good, then a higher number is better, right?  WRONG (in many cases).  On page 115 of the current North American Tool catalog a range for percentage of threads from 50% to 85%, with the recommended range between 65% and 75%.  This guideline range to use is a reflection of several factors:

1. The higher the percentage of thread, the greater the stress factors on the tap and the greater the likelihood of failure.  Excessive torque can lead to catastrophic failure, with broken pieces of tap or galling in the thread form.
2. A lower percentage of thread still provides the same pitch diameter, which is the only point the flanks of a fastener contact with the threads that have been tapped. They still provide the same pull out torque.
3. Too low of a percentage of thread can lead to cross threading when engaged with the fastener.

In higher tensile steels, larger hole sizes (less percentage of thread) may work better to reduce torque in tapping. When using oversize hole diameters, the tap should also have an oversize chamfer point diameter so all threads in chamfer are cutting and not just dropping in a hole.

Such was the case with the application porblem. As was stated above the operator thought that by increasing the drill diameter size would lead to better tool life. Instead, because the diameter was of the hole was so large the cut tap did not engage the threads along the chamfer length and instead engaged at the first full thread at the end of the chamfer.  The caused premature wear and stress on the that thread in the flutes and eventually to catastrophic failure.  The solution was a modified tap from North American Tool.

Summary: You've got to use the correct tap for the application!

At the end of the day, taps are made to very strict tolerances, often much more so than the piece parts they are called on to machine, and the first place one might want to find blame is with the tap. 

Why?
Because;

1. the tap is easily replaced with another one and 
2. it is the least expensive component of possible issues with an application.

What if one manufacturer makes a tap that works and another one made by a different company doesn’t work at all? 

In all likelihood, the “general purpose” of one is closer to the correct geometry for the application.  One might get a completely different result if another material were being machined. Therefore it is always best to order a tap for the specific application.

North American Tool