Form B or Form C: Which tap is right for which hole?

The basic principle is easy to explain: Form B with a spiral point is intended for through holes. Form C has a shorter chamfer and is used primarily for blind holes. With Form C, however, a more precise distinction is required. A straight-fluted design does not actively convey the chips out of the tap drill hole, whereas a right-hand helix transports them back towards the opening of the tap drill hole. The correct version therefore depends not only on the type of tap drill hole, but also on the material and the resulting chip form.

Form B and Form C initially refer to the chamfer

The chamfer is the front section of the machine tap in which the cutting teeth gradually produce the thread profile. The first teeth do not immediately cut the complete thread profile. Initially, they remove only part of the material. With each subsequent tooth in the chamfer, the profile is cut more deeply until the complete thread form is finally achieved. This allows the tap to enter the tap drill hole more easily. At the same time, the load generated during tapping is distributed across several cutting edges instead of acting fully on individual teeth right from the start.

With Form B, this cutting work is usually distributed across four to five threads. Form C, by contrast, has a shorter chamfer of around two to three threads. This allows the complete thread profile in a blind hole to extend closer to the bottom of the tap drill hole. However, the letters B and C alone do not provide a complete description of the tool. The flutes, spiral point, helix direction and helix angle must also be considered.

“When selecting a tool, it is not enough simply to read Form B or Form C on the tool. Form B is the clear standard solution for through holes, while with Form C it is also necessary to check whether a straight flute or a right-hand helix is suitable for the material. Short-breaking chips can remain in the flutes (Form C), whereas long chips must be conveyed out of the blind hole in a controlled manner (Form C with right-hand helix). The decisive factor is therefore always the interaction between the type of tap drill hole, the material, the thread depth and chip evacuation.”

Form B with spiral point for through holes

Form B has a spiral point that pushes the chips generated during tapping forward in the cutting direction. In a through hole, this path is clear. The chips can exit the component on the opposite side instead of remaining in the thread that has already been cut.

This chip flow makes Form B the preferred solution for through holes, particularly in materials producing medium and long chips. At the same time, the tool can be designed with comparatively shallow flutes, giving it greater stability. This is especially beneficial for recurring machining operations and series production because the chip path is clearly defined and the threaded section is not unnecessarily burdened with chips.

A Form C tap can also be used in a through hole in principle. However, it would be too broad a statement to say that Form C can always be used there without restriction. Straight-fluted Form C designs, for example, are also suitable for through holes in materials producing short chips. With longer chips, Form B is normally the more appropriate and economical choice because it directs the chips forwards in a controlled manner.

Practical example: M6 through thread in 42CrMo4

42CrMo4, an M6 through thread and a clear forward chip path: For this combination, a VÖLKEL practical test used a GRÜNRING HSSE machine tap, Form B with spiral point. The threads were produced in a component for front axles at an automotive supplier.

The tool geometry was directly suited to the application. The through tap drill hole allowed the chips generated during tapping to be directed forwards through the component in the cutting direction. At the same time, the GRÜNRING machine tap used was designed for machining this material.

The user was satisfied with the quality of the threads produced and achieved a longer tool life with the tools used.

Further information on the tool, machine, lubrication and test result can be found in the article “M6 thread in 42CrMo4: GRÜNRING machine tap in practical testing”.

Form C without a right-hand helix for blind holes

Form C has a short chamfer of around two to three threads. This means that the complete thread profile is produced earlier than with Form B. This is important for blind holes because only a limited amount of space is available beyond the required thread.

With a straight-fluted Form C design, however, the chips are not actively conveyed out of the tap drill hole. During machining, they collect in the flutes and partly in the remaining space beyond the thread. This works reliably only if the material produces short, easily controlled chips or if only a relatively short thread is being cut.

The volume of the flutes must be sufficient to accommodate the chips produced. If the chips are longer than expected or the thread is too deep, the flutes can become increasingly full. The chips are then no longer accommodated cleanly and may become trapped between the tool and the thread flanks. A Form C design without a right-hand helix is therefore not automatically suitable for every blind hole.

Form C with right-hand helix for blind holes

With materials that produce long chips, it is not sufficient merely to accommodate the chips in the flutes. They must be actively conveyed out of the blind hole. For this purpose, Form C is combined with a right-hand helix.

The helical flutes convey the chips back towards the tool shank and the opening of the tap drill hole. This prevents the chips from being compacted ahead of the tool at the closed bottom of the tap drill hole. This is particularly important with tough materials that produce long or continuous chips.

The required helix angle depends on the specific application. In general, the demands placed on chip evacuation increase as the material becomes tougher, the chips become longer or the thread depth increases. Depending on the tool range and field of application, VÖLKEL therefore offers different Form C designs with a variety of helix angles. These include versions with a 15°, 35°, 39° or 40° right-hand helix, for example.

The precise wording is important: Form C alone does not convey the chips out of the blind hole. The right-hand helix is responsible for this. Form C initially refers to the short chamfer. Only the combination of a short chamfer and a right-hand helix produces the typical tool for blind holes in materials that generate long chips.

The material determines the appropriate Form C design

The tool should not be selected solely on the basis of the general material designation. The decisive factor is how the specific material can be machined under the existing cutting conditions.

If the material produces short, broken chips, these can often be accommodated in the flutes of a straight-fluted Form C design. If, on the other hand, the chips are long, continuous or difficult to control, they must be actively conveyed out of the blind hole. In this case, a Form C design with a right-hand helix is the more suitable choice.

Grey cast iron and tougher steels are typical examples of different chip formation. Due to its material structure, grey cast iron generally produces short, broken chips that can be accommodated comparatively well in the flutes. Tougher steels, by contrast, can produce longer or continuous chips that require active chip evacuation from the blind hole. However, this is not a rigid rule based on material alone. The actual chip behaviour under the specific cutting conditions is always decisive.

Even two workpieces that are both generally described as steel can behave very differently during tapping. Strength, toughness, alloy, heat treatment and the actual condition of the material all influence chip formation. Thread depth, cutting speed, lubrication, machine and tool geometry are further factors. The material designation is therefore only the starting point for the selection, not the complete answer.

“In practice, the problem is often not that Form C was selected in principle, but that the chip behaviour of the material was not sufficiently considered when choosing the specific design. Especially with blind holes, it makes a decisive difference whether the chips can be safely accommodated in the flutes or must be actively conveyed out of the tap drill hole.”

Thread depth is not the same as hole depth

For blind holes, three different depths are often confused with one another:

1. The usable thread depth describes the section containing a fully formed thread.
2. The tool depth describes how far the tap actually has to travel into the tap drill hole.
3. The tap drill hole depth additionally includes the necessary safety clearance and the area of the drill point.

These three values are not identical. The chamfer does not immediately produce the thread to its full profile depth. The first cutting teeth initially remove only part of the material. Only as the chamfer progresses is the thread profile gradually completed. The tap must therefore enter the tap drill hole more deeply than the required usable thread depth.

With Form C, this normally requires an additional two to three threads for the chamfer. A safety clearance from the bottom of the tap drill hole is also required. The tap must not make contact with the bottom of the tap drill hole during machining.

The following simplified basic formula applies:

The conical section produced by the tip of the twist drill must also be taken into account. This section is not fully available to the tap as usable working space. The specific allowance therefore depends on the thread size, pitch, tool used and the geometry at the bottom of the tap drill hole.

With a Form C design without a right-hand helix, there is another factor to consider: The chips are not actively conveyed out of the tap drill hole. They remain mainly in the flutes and partly in the free space beyond the thread. This area serves as a chip reservoir and must be large enough to accommodate the chips produced during machining.

A deeper tap drill hole can provide additional chip space. However, it does not automatically replace the correct tool geometry. If the material produces long or continuous chips, even a larger chip reservoir can quickly reach its limits. A Form C design with a right-hand helix is then generally the more reliable solution because it actively conveys the chips back towards the opening of the tap drill hole.

What are the consequences of selecting the wrong tool?

Form B in a conventional blind hole

Form B pushes the chips generated during tapping forwards in the cutting direction. In a through hole, this chip path is clear. In a blind hole, however, it ends at the closed bottom of the tap drill hole. The chips collect ahead of the machine tap and become increasingly compacted as machining continues.

This can cause the torque to rise significantly. At the latest when the direction of rotation is reversed, the chip that is still connected to the material must be sheared off cleanly. Because Form B has a longer chamfer and a geometry designed for through holes, there is a risk that a thin chip will not be completely sheared off but will instead become trapped between the chamfer and the thread flank. Possible consequences include damaged thread flanks, chipping of the cutting edges and, in extreme cases, a broken machine tap.

Form C without a right-hand helix with long chips

A straight-fluted Form C design does not actively convey the chips out of the tap drill hole. With short-breaking chips, this can work as long as the volume of the flutes and the available chip space are sufficient. However, if the material produces long or continuous chips, the flutes become increasingly full.

The chips can become entangled, move between the tool and the thread flank, or be carried along and crushed when the machine tap retracts. This makes the process unstable, increases the torque and can damage the surface of the thread. In this case, a Form C design with a right-hand helix is required so that the chips are conveyed to the opening of the tap drill hole in a controlled manner.

Form C in a through hole

A Form C machine tap can also be used for a through hole in principle. With materials that produce short chips, a straight-fluted Form C design can even be suitable for both blind and through holes. Using Form C in a through hole is therefore not automatically an incorrect tool choice.

With materials that produce medium or long chips, however, Form B is normally the more appropriate solution. The spiral point directs the chips forwards in the cutting direction and therefore makes use of the clear exit from the through hole. A Form C design with a right-hand helix, by contrast, would convey the chips back towards the tool shank even though they could exit the tap drill hole forwards. The thread can still be cut in this way, but the chip flow is unnecessarily unfavourable for the application and can encourage chip wrapping around the shank.

Absolute exception: Form B in a very deep blind hole

In certain special cases, a tool with a spiral point can also be used in a blind hole. The prerequisite is that the tap drill hole is made significantly deeper than the required thread. Below the usable thread depth, there must be sufficient free space to accommodate the chips pushed forwards in the cutting direction.

However, this is not a general recommendation for a conventional Form B machine tap. Depending on the application, an adapted tool geometry may be required, for example a shorter chamfer and a chamfer relief angle suitable for the blind hole. The chips still remain in the tap drill hole and may have to be removed in an additional operation.

This solution is normally not suitable for planned series production. The tap drill hole must be drilled deeper even though this additional depth is not required for the function of the thread. This increases the machining time, places greater strain on the twist drill and occupies the machine for longer. Depending on the component, the required tap drill hole depth may also be impossible to achieve because of the available material thickness.

Together with the potential cleaning effort, this results in additional process costs. If the machining operation can be planned from the outset, a suitable Form C design should therefore be selected directly. Form B in a very deep blind hole remains an absolute exception for special conditions and is not a standard alternative for series production.

Typical warning signs of an unsuitable tool design include a sudden increase in torque, unstable tool operation, noticeable chip wrapping, rough or damaged thread flanks, chipping at the chamfer or failure to achieve the full thread depth. In this case, the process should be stopped rather than simply continued with greater force. The tool geometry, chip flow and tap drill hole depth must be checked first.

Selecting Form B or Form C in five steps

1. Determine the type of tap drill hole: A through hole generally indicates Form B, while a blind hole indicates Form C.
2. Check the chip behaviour: Short, broken chips can be machined with Form C without a right-hand helix. Long or continuous chips require Form C with a right-hand helix.
3. Match the tool geometry: The chamfer, flutes and, where applicable, helix angle must be suitable for the application.
4. Check the thread depth and tap drill hole depth: The chamfer, safety clearance, drill point and required chip space must be taken into account.
5. Consider the overall process: The machine, lubrication, cutting parameters and cost-effectiveness are also part of tool selection.