Spiral Fluted Taps: adjusting the bored hole size

When tapping different materials using spiral fluted taps, some issues such as the chipping of the tap, tap breakage, tap welding may take place occasionally. The adoption of some precautions helps overcoming these problems.

Selecting a different diameter of the bored hole is a simple and feasible trick. Making the bored hole as large as possible may avoid the problems mentioned above. Unfortunately, the choice of the bored hole diameter is still a low-emphasized topic: a better understanding of its role within the threading applications might fix once and for most of the tapping problems.

It may be argued that there is only one recommended bored hole diameter for each tap size. For example, the tapping of M8x1.25 threads requires a bored hole of 6.8 mm. However, this statement arises from common practice and it is not strictly correct. Indeed, in ISO 2 6H class internal threads, the smallest diameter is specified to be within the 6.912~6.647 mm range.
When tapping problems occur, the first solution to solve them is to enlarge the bored hole diameter as much as possible and use the maximum value among those within the specified range, (therefore a diameter of 6.912 mm in this case).

Take a look at the example in the picture below:

In a M8x1.25 thread, by adjusting the bored hole diameter from 6.8 mm to 6.9 mm (overall variation about 1%) reduces the volume of the chip by 15%. As a consequence, the tapping load reduces by 13%, overcoming both the problems of breakage of the tool and that of welding.

The following table summarizes the bored hole diameter (in mm) for main metric threads. Yamawa suggests considering the bored hole diameter closest to maximum minor diameter (D1) of the internal thread.