Tapping - everything you need to know

Threads can be hand or machine drilled in various materials. Various taps are used for this. What else there is to know about tapping and how to drill a thread correctly by hand can be read in detail in the following instructions.

Important to note when tapping

  • Blind hole, core hole, through hole
  • Types of taps
  • pre-drill correctly
  • Long and short chipping materials
  • Also Read - Tapping And Cutting
  • Also read - pre-drilling a thread is precision work
  • Also Read - Regrinding a Tap

Blind hole, core hole and through hole

Threads can be cut in different ways. The terms blind hole, core hole and through hole have to be kept apart.

Blind hole

At the blind hole, the thread ends in the middle of the material. They are made with three-piece tap sets. The thread is first roughly pre-cut, then finely pre-cut and then finished.

Through hole

A through hole is created when the thread extends through the entire length of the material. The distinction between blind and through hole is important for choosing the right tap.

Core hole

The core hole is created by pre-drilling. It has to be a certain amount smaller than the finished thread. There are tables for reference for the diameter of the core hole. As an approximation, it can be assumed that the core hole must have about 80% of the thread diameter.

The core hole must be drilled exactly vertically - otherwise there is a risk of the tap breaking, especially with small diameters.

Types of taps

Threads can be cut by machine or by hand. When cutting by hand, three-part thread cutting sets are practically always used. In order to be able to differentiate them better, they are marked according to a certain standard.

The coarse pre-cutter always has a ring, the fine pre-cutter two rings and the finishing cutter does not have a ring. In some sentences, however, it is also marked with three rings.

In contrast to this, there are also single-cut taps that enable faster cutting. They can be used mechanically as well as manually. The use of blind holes is problematic because the chips are not conveyed cleanly to the outside. They also need a much higher torque.

Special taps are used for wood, mostly 10, 16 and 22 mm in diameter. Metal taps can also be used for plastics.

Pre-drill correctly

The drill hole must always be created exactly vertically. A pillar drill is therefore recommended. In the case of metals, drilling may only be carried out at low speed and while the drill is being cooled. Drilling oil is suitable for this.

The drill hole must then also be countersunk. The countersink should have a diameter around 10% larger than the thread.

Long and short chipping materials

Only with long-chipping materials does the tap have to be turned back a quarter turn again and again during drilling in order to break the chips. The same applies to threads with a larger diameter.

Tapping in metal - step by step instructions

  • Cutting oil
  • Tap set
  • Upright drilling machine with suitable drill
  • Countersink

1. Pre-drill and fix the workpiece

Pre-drill the core hole with the pillar drill exactly vertically. Work at slow speed and cool the drill. Then fix the workpiece immovably.

2. Remove chips and use countersink

Remove the chips from the core hole, otherwise the newly cut thread could be damaged. Countersink with the countersink.

3. Drill thread

Use the taps in the intended sequence of coarse spurs, fine spurs and finish cutters. Remove chips between tool changes. Always wet with cutting oil while tapping.

Tips & Tricks Not all metals allow tapping. A file test shows whether the tap can be used: if the file leaves traces even with light pressure, the material is suitable.

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