INTERNAL THREADS – HOW AND WITH WHAT SHOULD THEY BE CUT CORRECTLY?

Cutting internal threads is a task where both the correct procedure and the right tool are essential. It is not enough to simply drill a hole and use a tap. The pilot hole diameter, chamfering the hole, the type of tap, the tapping method, and the material being threaded all play an important role.

In this article, we will show you step by step how to cut an internal thread correctly. We will explain when to use a hand tap and when a machine tap is the better choice, the difference between through holes and blind holes, and why it is worth using cutting oil or tapping paste. This will help you choose the right equipment more easily and reduce the risk of damaging either the thread or the tool itself.

Thread cutting is generally a simple process. It always consists of 3 basic steps:

1. Pre-drilling the hole
2. Countersinking the hole
3. Cutting the thread

WHAT TOOLS WILL YOU NEED?

To choose the right tools, you first need to determine whether you will be cutting the thread manually or by machine and what type of thread you need.

By thread type, we mean its size and pitch.

The material to be machined also plays an important role.

For most standard steels, tools made of high-speed steel (HSS) are sufficient.

For machining stainless steel and very hard steels, tools made of cobalt high-speed steel (HSS-Co.) should be used.

On our website, we offer drills, taps and countersinks in both versions.

Step 1: Pre-drilling the Hole

1. Mark the position where you want to cut the internal thread.
2. Using a drill and the appropriate drill bit, drill the hole. The drill bit diameter must always be smaller than the required thread. The exact drill diameter depends on the thread size and pitch. You can find more information in our article: How to determine the correct tapping drill size for a metric internal thread.
3. Depending on the workpiece material, choose the required size from our ground HSS drill bits (Item No. 3620)
   or from our HSS-Co cobalt drill bits (Item No. 3720).
4. Before drilling, apply cutting oil or cutting spray to the drill bit to reduce friction and improve cutting performance.

Step 2: Countersinking the Hole

1. After drilling the hole, use a 90° countersink to create a chamfer on the edge of the hole. (Item No. 3507 or Item No. 3717).
   This step makes it easier to start the tap and ensures a clean beginning of the thread-cutting process.
2. The thread size for which a particular countersink is intended can be identified directly from its product name.

Step 3: Thread Cutting

Whether cutting threads manually or by machine, you should always use cutting oil or cutting paste. This extends the service life of the tap and improves the quality of the finished thread surface.

Manual Thread Cutting

Selecting the Required Tools

If we tried to cut the thread in a single operation, it would require a considerable amount of force.

Therefore, during manual thread cutting, the required force is reduced by creating and deepening the thread gradually.

Dividing the process into several steps also reduces the risk of breaking the tap inside the hole.

In addition, if you make a mistake while cutting the thread, it can still be corrected during the following step.

For this reason, hand taps are supplied in sets of two or three pieces.

The individual taps in the set differ in their thread profile.

The process starts with taps having an incomplete thread profile, followed by taps with a higher and complete thread profile.

• Two-piece hand tap sets for metric fine threads. They include a taper tap and a bottoming tap.
• Three-piece hand tap sets for standard metric threads. They include a taper tap, an intermediate tap and a bottoming tap.

To make it easy to identify the correct order in which the taps should be used, they are marked with rings on the shank.

1. Taper tap – marked with one ring or one groove. It is used for the initial cutting operation and has the smallest thread profile diameter.
2. Intermediate tap – marked with two rings. It is used after the taper tap and has a partially formed thread profile that gradually completes the thread.
3. Bottoming tap – has no ring marking. It is used last and features a full thread profile that finishes and smooths the thread.

To rotate the hand tap in the pre-drilled hole, you will need a tap wrench.

As with the tap itself, the wrench size must match the size of the tap being used.

You should also consider how accessible the location is where the thread will be cut.

You can choose between ratchet tap wrenches, Ball Tap Wrenches or adjustable tap wrenches.

Manual Thread Cutting Procedure

1. Secure the taper tap in the tap wrench.
2. Apply cutting oil or cutting paste to the tap to reduce friction and improve cutting performance.
3. Carefully begin turning the tap in the pre-drilled hole. After every few turns, rotate the tap back by approximately half a turn to break the chips and prevent damage to the thread.
4. Continue turning and cutting until the required thread depth has been reached.
5. Carefully remove the tap and inspect the finished thread.
6. For fine threads, repeat the entire process using the bottoming tap. For standard threads, first use the Plug Tap and then the bottoming tap.

Machine Thread Cutting

Selecting the Required Tools

When choosing a tap for machine thread cutting, it is also important to determine whether you are cutting a through thread or a blind thread.

A through hole passes completely through the workpiece. A blind hole, on the other hand, ends at a certain depth and does not pass all the way through.

When cutting blind threads, it is particularly important to choose the correct tap from the range of machine taps to ensure the finished thread meets your requirements.

So, what are the differences between the various types of machine taps, what should you pay attention to, and why?

Spiral flute or straight flute?

When cutting blind holes, the chips produced must be carried upward out of the hole.

This is exactly what taps with a spiral flute are designed to do. The spiral flute removes the chips in the opposite direction to the cutting movement.

Therefore, spiral flute taps should be used for blind holes, while straight flute taps are suitable for through holes.

Pointed or flat chamfer?

Since a pointed tap reaches the bottom of a blind hole only with difficulty (see illustration), taps with a flat chamfer are used for cutting blind threads.

Therefore, use taps with a flat chamfer for blind holes. For through holes, both pointed and flat chamfer taps can be used.

Note:

For small-diameter taps intended for blind holes, you will still find a pointed tip instead of a flat chamfer.

This is due to the manufacturing process. During production, the tap must be clamped both at the shank and at the tip. The tip is normally clamped using a centre hole.

However, this is not possible with smaller diameters, so the tap has to be clamped by its pointed tip.

Chamfer Form and Length

The chamfer is located at the tip of the tap and extends over several thread turns.

These thread turns determine the amount of torque acting on the tap and therefore also influence its service life.

They also affect how the tap starts cutting and how well it centres itself.

The more thread turns the chamfer has, the lower the forces acting at the beginning of the cutting process.

You can easily identify how many thread turns the chamfer extends over from the tap designation. This is referred to as the chamfer form.

OREN taps are available as

• Form B: approximately 4–5 thread turns with chip breaker – suitable for through holes, tough materials and materials producing medium to long chips.
• Form C: approximately 2–3 thread turns – suitable for both through and blind holes with a short thread run-out.

Chip Breaker

The chamfer on both hand taps and machine taps may be equipped with a chip breaker. Its purpose is to break the chips so that they can leave the hole without difficulty.

Summary

As a general recommendation:

Use spiral flute machine taps, Form C, for blind threads, and straight flute machine taps with a chip breaker, Form B, for through threads.

To make it easier to choose the right product on our website, we have prepared the following table to help you select the correct tool depending on

whether you need a fine metric thread or a standard metric thread and whether you require an HSS or HSS-Co tap.

Machine Thread Cutting Procedure

1. Install the machine tap into a drill or another suitable machine.
2. Apply cutting oil or cutting paste to the tap to reduce friction and improve cutting performance.
3. Start the drill and slowly feed the tap into the pre-drilled hole. Maintain constant pressure and ensure that the tap enters the hole straight.
4. Once the required depth has been reached, switch off the drill and carefully remove the tap.
5. Inspect the finished thread.

Recommendations

• Safety precautions: Always wear safety glasses and protective gloves to protect yourself from chips and sharp edges.
• Tool maintenance: After use, clean and oil your tools so they are ready for the next job.

We hope this guide helps you successfully cut internal threads. If you have any questions, please do not hesitate to contact a professional.