To cut an internal thread, the appropriate tap must be selected. In this article you will learn the characteristics of internal taps and what influence they have on the selection.

We will look at the following six features:

• Thread type
• Head shape
• Point cut
• Thread profile
• Flute
• Shank

Male taps are basically differentiated between hand taps and machine taps.

Hand taps

Hand taps are divided into:

• Single-cut hand taps
• 2-piece hand tap sets with taper and final tap
• 3-piece hand tap sets with taper, second, and final tap

As the name suggests, threads are cut by hand. For this purpose, for example, a tap wrench is used in which the hand tap is clamped.

With single cut taps it is possible to cut internal threads in one work process.

An advantage of a multi-part hand tap set is the lower force on each individual tap (reduction of the risk of breakage). Should you ever cut crooked with the taper tap, a correction with the next tap is possible.

Machine taps

Machine taps are basically single cut taps and cut the thread in one work process.

Since the drill is aligned vertically to the workpiece in the machine, it simplifies the cutting of a straight thread.

Which internal tap you use depends primarily on the factors of application, time and quantity. For example, it makes sense to use a machine tap if you want to cut many threads quickly (series production).

Right and left cutting threads

You should also note in which direction a screw or the opposite element is turned. There are right and left cutting threads. A screw or the counterpart are always screwed in the same direction as the thread is cut. Screws are often tightened clockwise, which is why most taps are right-handed.

Left-hand cutting threads are used if the screw could loosen due to stress. This happens e.g. with the attachment of grinding wheels and circular saw blades, with the hose attachment to the gas cylinder (here as a safety feature) or with the left pedal of a bicycle. In these cases, a left-hand thread (counterclockwise) is cut.

1. Feature: Thread type

Threads are standardized and normed in an ISO standard. These describe, among other things, flank angle, thread diameter, pitch, shank, tolerance zone.

You will find a short overview of the most common standards we carry in the following table.

2. Feature: Head shape

The head can be pointed or flat. Which one is used depends on whether you are threading a through hole or blind hole.

One through hole runs completely through the workpiece.

The blind hole however ends in the workpiece at a certain drilling depth and is therefore not continuous. In this case it is advantageous that the chip is transported upwards out of the hole. This is the case with machine taps with a left-hand twisted groove, which push the chips against the cutting direction.

In the picture below you can see that with a pointed head it is not possible to drill all the way into a blind hole. Therefore taps with flat head end are used, which reach the bottom of the blind hole.

On the other hand, taps with a flat head end can also be used in through holes.

Now you surely ask yourself the question why you need a machine tap especially for through holes. Taps for through holes may have more threads at the point cut as the top of the tap can exit at the end of the hole. Thus the thread is cut completely despite the missing cutting edges at the top.

In a blind hole, the tap head would hit the end of the hole and not cut the thread completely.

The advantages more threads have, will be discussed in the next section.

3. Feature: Point cut

The point cut is located at the head of the tap and takes place over multiple threads.

The threads influence the load of the torque on the tap, and thus also the service life. They also influence the tapping and centering behavior. The more threads a tap has, the less forces act during tapping.

The point cut for manual and machine taps can have a progressive tap. It is used to break the chip so that it can exit the drilling hole easily.

Type

The type specifies via how many threads the point cut takes place, e.g. 2-3 threads.

There are three forms B, C, and D at RUKO:

4. Feature: Thread profile

The thread profile is defined by teeth, which cut the thread. This is clearly visible in the hand tap sets.

The 3-piece hand tap set consists of taper, second, and final tap. The thread profiles are different here; the taper and second tap do not have a complete profile, only the final tap cuts the entire thread.

The figure shows the incomplete thread profiles for taper and second tap as well as the complete teeth of the final tap.

The thread is cut in three work steps. This means that materials that are difficult to cut and large thread diameters can be cut easily by hand. In contrast to the single-cut tap, the effort required for tapping and the force applied to the tap is greatly reduced.

Since the force factor does not come into play with machine taps due to the machine use, the additional two work steps are saved and single-cut taps are used.

In addition to the effort required, the three working steps of the hand tap protect the tap from excessive wear (drill breakage). The graph shows the percentage of workpiece removal during cutting with the various cutters.

Flank angles

The flank angle measures the angle from thread flank to thread flank.

The angle as well as the pitch are decisive for the application of the thread type. The most common angle in Europe is the 60° angle for ISO metric threads. We also offer the Whitworth thread with 55° degrees and the steel conduit thread with 80° degrees.

The pitch is the axial distance covered by one rotation, i.e. the distance between the thread tips in mm.

5. Feature: Flute

The grooved profile is used as a channel system for the collection and removal of chips. There are two types of flutes: straight and spiral.

The straight flute is used for through holes as the chip crumbles and falls through the hole. For blind holes it is advantageous to use taps with spiral flutes as the chip is removed from the hole against the cutting direction.

Hand taps are generally straight fluted. Machine taps can have both flute profiles.

The spirally grooved machine taps promote the removal of chips, especially at high speeds.

6. Feature: Shank

There are two shank variants: The overflow shank and the reinforced shank.

The overflow shank has the same diameter throughout and is always smaller than the core diameter of the thread (through holes).

In contrast, the reinforced shank increases towards the end. The reinforced shank is used for hard and difficult to machine workpieces. Since this is usually the case with machine-guided drills, there are machine taps in both versions.

The difference between the two shanks is clearly visible in the picture:

At the end of the shank there is a square spindle on all taps for clamping in tap wrenches or machine holders.

Summary

When choosing the right tap, you must first answer the following questions:

1. How much time do you have?
If you do not have much time, you should use a single-cut tap.

2. How many threads do you want to cut?
If you want to drill a lot of threads, you should reach for the machine tap.

3. Is it a left or right cutting thread??
Look at the screw or the counterpart to be screwed in. Your thread must also be cut in the same direction.

4. Do you cut a through hole or blind hole?
Here you decide how many threads you need and in which direction the chip should be removed from the hole. Which hole the tap is suitable for is usually written on the packaging.

5. Which material will you cut into (application)?
In case of hard materials, a longer point cut is advantageous because less force is applied to the tap. The required tool steel also depends on the application. With internal taps made of high speed steel HSSE-Co 5 you can cut e.g. in stainless steel and bronze.

In the table we compare the features and their influences on the selection of the suitable internal tap.

On both sides of our hand taps and machine taps you will find a product filter in the left column. Here you can filter the taps according to applications (steel, aluminium, plastics, etc.) or technical data (type of thread, cutting direction, shank, etc.) and thus select the suitable internal tap.

If you have trouble finding the right tap, you can of course contact us.

We explain how to cut an internal thread in our article How to cut an internal thread in three steps.