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

As an 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.  

Let’s look at one variable: Hole size.

Generally, for 60 degree threads, the percentage of thread should be 75% as a rule-of-thumb.  That seems to be 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.

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 we show a range for the percentage of threads from 50% to 85%, with the recommended range between 65% and 75%.  The range to use is a reflection of many 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 even produce catastrophic failure with broken pieces of tap or galling in the thread form.

2. Lower percentages of thread still provide 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. 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.

At the end of the day, taps are made to very strict tolerances (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…

A. the tap is easily replaced with another one and

B. 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.