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How to cut an internal thread in 3 steps

May 24, 2018

The most common connection is still the screw connection. To screw a screw into a workpiece, you need an internal thread. In this article we show you how to cut an internal thread in steel, cast iron, aluminium etc. in only three steps.

An internal thread can be tapped by hand (with hand taps) or with a machine (with machine taps).

You can find an overview of which internal tap you need for your application in our article 6 features of internal taps at a glance.

Required tools

To cut an internal thread you need the following tools:

If available:

  • Try square
  • Center punch
  • Hammer
  • Cutting spray, for optimum surface quality


Diameter of the core hole

If you know which tap you need, the next question is what diameter is required to drill the core hole.

Internal taps are designated according to nominal size. For example, an M 8 tap means "metric with a diameter of 8 mm".

The diameter of the twist drill for tapping is calculated as follows:

Core hole diameter = Tap diameter - Thread pitch

Example using the M8:
Core hole diameter = 8 mm - 1.25 mm = 6.8 mm

The core hole diameter is rounded up to one decimal point. In our table (PDF) you will find all common thread sizes and the corresponding diameters for the twist drill.


The working procedure:

1. Drill core hole (twist drill)

First you need to drill a core hole with the twist drill.

If you want, you can use the center punch, by punching with the hammer a centering in the right spot. This simplifies centering and hot tapping with the twist drill.

If you have cutting spray at hand, you should use it here. Spray the twist drill with the cutting spray. This increases the service life of the drill and improves the surface quality.

The correct diameter for the twist drill can be found in the following table (PDF).

1. Drill core hole (twist drill)

2. Countersink core hole (cpuntersink)

2. Countersink core hole (cpuntersink)

Then use the 90° countersink to countersink into the core hole and make a chamfer. The size of the countersink should be 10 % larger than the diameter of the tap.

This will give you a better entry into the hole when cutting the thread. It also prevents the first and last thread from being pressed out of the hole.


3. Cut internal thread (hand tap)

As a final step, put the tap into a tap wrench (ball tap or tool holder with ratchet) and cut the thread.

Carefully turn the tap into the core hole.

With the tap sets you must observe the sequence (taper, second, and final tap).

To cut a straight internal thread, you can use a try square and place it at right angles to the tap.

Here you can use cutting spray again.

Friction between the chip and the tap cutting edges and thus also the necessary torque is reduced. This ensures better chip removal.

When you have cut the thread, unscrew the tap in the opposite direction.

Done: The thread is made.

3. Cut internal thread (machine tap)

As a final step, tighten the tap in the machine and cut the thread.

Insert the tap slowly and straight into the core hole at the appropriate speed.

Here you can use cutting spray again.

Friction between the chip and the tap cutting edges and thus also the necessary torque is reduced. This ensures better chip removal.

After a few turns, the tap pulls itself into the workpiece and cuts the thread.

Once the tap is through, stop the machine, switch to the opposite direction and restart the machine. The tap then guides itself through its already cut thread.

Done: The thread is made.


Instruction

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Download instruction as PDF (0,6 MB) downloaden


Summary and conclusion

Basically, a thread can be cut easily.

All you need to know is what kind of internal tap you need and whether you want to cut the thread by hand or machine.

When selecting the internal tap, it is important to consider whether you are drilling in a through hole or a blind hole. Our detailed article 6 features of internal taps at a glance can help you choose the right tap.

This results in the required internal tap and the corresponding size for the twist drill and countersink.

If you cut the thread by hand, make sure that the drill is turned back a third turn after two turns (to break the chip).

When cutting by hand as well as by machine, you should use cutting spray if possible. This increases the service life of the tap and improves the surface quality.

In one of the next articles we will explain how to cut an external thread. You can then screw an element into your internal thread.

If you have trouble finding the right tap, please do not hesitate to contact us.

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