Which tool for internal threads? Hand tap, machine tap, thread former or Kombi-Bit?

Which tools are available for producing an internal thread?

For an initial overview, the four tool types can be reduced to their different strengths:

Hand taps are mainly used when an internal thread is to be produced manually or retapped. Typical applications include one-off parts, workshop work, assembly and maintenance. Classic three-piece hand tap sets distribute the cutting work across taper, second and bottoming taps.

We explain the differences between HSS-G, HSS-E and HSS-E VAP and which version is suitable for which application in detail in our article HSSG, HSSE or HSSE VAP? Which hand tap is the right one?.

Machine taps are designed for fast and repeatable machining. The thread is cut by machine in a single operation. The required tool geometry depends, among other things, on the hole type, material, thread depth and chip behaviour.

The distinction between through holes and blind holes is particularly important. We explain in detail which geometry is suitable and why Form B and Form C handle the resulting chips differently in our article Form B or Form C: Which tap suits which hole?.

Thread formers work completely without producing chips. Instead of removing material, the tool displaces the material and forms it into the thread profile. The prerequisite is a material with sufficient formability. The process is particularly interesting when chips are to be avoided or when repeatable series production is required.

We explain how the process works, what advantages it offers and where its limits lie in detail in Thread forming: chipless, clean, reliable.

Kombi-Bits, on the other hand, are a special solution with a much narrower application window. They combine tap-hole drilling, thread cutting and deburring in a single tool. This can be particularly practical for mobile assembly and service work.

You can find more about the application and the limits of the tool in our article VÖLKEL Kombi-Bit: tap-hole drilling, thread cutting and deburring in one operation.

One-off production, assembly or series production: How many threads are to be produced?

Quantity is an important selection criterion. Someone who occasionally produces a single thread or reworks an existing thread has different requirements for the tool than a company producing several hundred or thousands of identical threads every day.

For one-off production and repair work, flexibility is usually the priority. Here, the hand tap is often the obvious solution. It does not require an elaborate machine setup and allows controlled manual machining.

For damaged threads, however, the following applies: retapping only works as long as the existing thread still has sufficient material and a basically intact thread profile. If the thread is severely damaged or stripped, it must actually be repaired, for example with a thread insert.

For assembly and service work, the Kombi-Bit can become interesting. Tap-hole drilling, thread cutting and subsequent deburring are carried out with the same tool in one continuous workflow. Especially for short through threads on site, this can simplify the process considerably. Its limit, however, remains clear: a maximum thread depth of 1 × D and through threads only.

For recurring machine processing and series production, machining time, repeatability, tool life and process reliability become the main priorities. Machine taps or, with suitable materials and process conditions, thread formers are generally the more obvious solutions here.

Is the work carried out manually or by machine?

The available drive also clearly narrows down the tool selection.

The classic hand tap is designed for manual machining and is guided, for example, with a tap wrench. This allows the cutting process to be controlled well. At the same time, the manual time required increases with every additional thread.

Machine taps, by contrast, are intended for machine-based processes. They allow internal threads to be produced quickly and repeatably. The prerequisite is that tool geometry, material, tap hole, cutting data and lubrication are matched to the process.

Thread formers are also generally used by machine. During forming, the material is displaced, which means the required torque is higher than during thread cutting. The machine, tool holder, tap hole and lubrication must therefore be matched particularly carefully.

The Kombi-Bit occupies a special position. It can be used, for example, with a cordless drill or hand drill with forward and reverse rotation, making it particularly interesting for mobile work. Here too, however, its narrow application limits in terms of thread depth and hole type apply.

Thread cutting or thread forming: chips or chipless?

Hand taps, machine taps and Kombi-Bits work by cutting. Material is removed from the tap hole when the thread profile is produced, creating chips.

These chips must be removed in a controlled manner. Especially in blind holes, poorly evacuated chips can lead to increasing torque, damage to the thread or tool breakage.

Thread forming eliminates this problem. The material remains in the component and is pressed into the thread profile by cold forming. No chips are produced. This can be particularly interesting for automated processes, blind holes or applications where chips inside the component would be problematic.

However, this does not mean that thread forming is always the better solution. The material must be sufficiently formable, the required torque is higher and the process parameters must be precisely correct, particularly for the tap hole and lubrication.

The decisive question is therefore not: Cutting or forming, which is better? But rather: Which process suits the material, component and manufacturing process?

Through hole or blind hole: Which tools are even suitable?

The hole type can exclude some tools from the selection at an early stage.

With a through hole, the hole is open on the opposite side. With suitable tool geometry, chips can therefore be transported forward out of the tap hole. For machine taps, Form B with a spiral point is a typical solution for this.

The Kombi-Bit is also explicitly designed for through threads. However, the maximum thread depth of 1 × D also applies. A Kombi-Bit for M8, for example, is therefore only suitable for a maximum thread depth of 8 mm within its intended range of application.

With a blind hole, the hole ends inside the workpiece. The chips cannot simply leave the hole towards the front. With machine taps, a geometry must therefore be selected that suits the material and chip formation. Form C with a right-hand spiral can, for example, transport long chips back towards the tap-hole opening.

Hand taps and suitable thread formers can also be used for blind holes. The Kombi-Bit, on the other hand, is ruled out.

This makes the question through hole or blind hole? one of the fastest filters when selecting a tool.

Why does the tap hole also influence tool selection?

A clean internal thread starts before the actual thread cutting or forming. The tap-hole diameter must match the thread and the selected machining process.

If the tap hole is too small, a cutting tap has to remove more material. Torque increases, chip formation becomes more difficult and the tool is subjected to greater loads. If the tap hole is too large, on the other hand, the thread flanks become shallower and the finished thread loses load-bearing capacity.

Thread forming requires different tap-hole diameters than thread cutting. Because no material is removed but displaced, the tap hole must be larger. For a metric M8 thread, for example, the tap-hole diameter is 6.8 mm for thread cutting and 7.45 mm for thread forming.

With the Kombi-Bit, there is no need to select a separate tap drill within the workflow. The correct drilling diameter is already part of the tool.

We explain in detail which tap-hole diameter you need for a particular thread size and what to consider when drilling in Clean threads start with the tap hole. There you can use our interactive tap-hole calculator to determine the correct diameter directly.

How important are machining time and process reliability?

The tool price alone is not enough to assess which process is economical. The entire machining process is what matters.

A simple example shows the difference: if a single internal thread has to be produced during a repair, a machine process optimized for high quantities would usually be disproportionate. A hand tap can be the simpler and more economical solution here.

If, by contrast, several hundred identical threads have to be produced every day, the calculation looks completely different. Machining time, tool life, repeatability and process reliability become decisive. Machine taps or thread formers are then much more interesting.

With thread formers, another factor is added: because no chips are produced, conventional chip evacuation is eliminated. Particularly in automated processes, this can increase process stability. The prerequisite remains, however, that the material, tap hole, lubrication and machine are suitable for the process.

With the Kombi-Bit, the time advantage arises elsewhere. Tap-hole drilling, thread cutting and deburring are performed with one tool. For short through threads in assembly and service, this can simplify work steps. For deep threads, blind holes or highly optimized series processes, however, the tool is not an alternative because of its design-related application limits.

Economy therefore does not automatically mean choosing the cheapest or fastest tool. Quantity, machining time, required work steps, tool life, scrap risk and process reliability must be considered together.

Hand tap, machine tap, thread former or Kombi-Bit: the technical comparison

What suits my application?

The technical comparison matrix shows the fundamental differences. For practical selection, the question can be asked even more directly:

Above all, the two overviews show one thing: There is no single best tool for internal threads.

At first glance, the Kombi-Bit may appear particularly universal because three operations are combined in one tool. In reality, however, its application window is significantly narrower than that of a classic machine tap because it is limited exclusively to through threads and a maximum thread depth of 1 × D. Conversely, a machine tap designed for series production would not automatically be the most sensible solution for a single repair.