How to Tap-Thread Metal While Avoiding Tool Breakage in Blind Holes

Blind holes can become a problem when you need to thread with a tap, that tool that serves to create the useful profile to screw a bolt inside. Working on threading a hole that does not completely pass through the piece is a problem when working in the workshop: it is one of the most delicate operations because the tool operates in a tight space.

You are confined, in difficulty. And it is in these situations that disaster looms: the tool breaks and everything becomes more difficult because the tap has to be removed with complex extraction operations. How do you proceed to obtain a useful result without risking having to prolong the work to recover the broken tool? Technique comes to our aid.

Why is it Risky to Thread Blind Holes?

There are several reasons that make this operation particularly risky. The first is rather simple to imagine: if in the normal procedure, threading the chip falls through the hole and allows the cutting edge to proceed unimpeded, in a blind hole there are obvious problems. The chips tend to accumulate at the bottom.

If the space fills up, the tap gets stuck and risks breaking. Same if you miscalculate the depth of the hole and continue rotating even after hitting the limit. These problems occur especially when you make a hole in metal that has the same length as the required thread: having to proceed with the tap up to a certain length, if you push along the bottom or accumulated chips build up, there is a risk of breaking. With the resulting problem of having to pull the tap out of the hole.

How is the Break Risk Resolved?

This condition can be foreseen and resolved in different ways that are not mutually exclusive. First, for blind holes, taps with helical flutes are used. They are very similar to those of a drill tip for iron. This tool has a unique characteristic: it pushes the chips upward, expelling them through the specific groove instead of pushing them to the bottom.

This way you avoid ending up with a higher bottom made up of metal residue. Second point: you need to make a somewhat deeper hole. Example: if a 15 mm thread is needed, the hole will be at least 20 mm deep. Those extra 5 mm serve to accommodate the conical entrance of the tap.

The Method to Proceed the Right Way

To calculate the correct depth of the pilot hole in a blind hole, we must ensure that the screw can tighten the piece without touching the bottom, and that the tap has sufficient space to work. But above all, you must choose the right tools.

Such as, for example, machine taps made of rapid steel HSS, designed for ISO metric threads with coarse pitch according to DIN 371/C-15°. For specific applications, there are also taps with UNC threads in coarse pitch according to the DIN 376/C specification with a cutting angle of 35°.

In any case, in the specific section of our portal you will find all taps and dies useful both by hand and by machine. If you work with manual tools, you must proceed like this: half a turn forward and a quarter turn backward. This way you can break the chip and, if necessary, completely pull out the tap in the middle of the work to blow away debris with compressed air.