6 features of internal taps at a glanceMay 17, 2018
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
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 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.
|Work processes||Hand taps||Machine taps|
|Single-cut hand taps |
|2-piece hand tap sets |
|3-piece hand tap sets |
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.
|Thread type||Hand taps||Machine taps||Application|
|M = metric, DIN ISO 13||Standard threads, mainly used in Europe, most frequently used threads|
|MF = metric fine, DIN ISO 13||Set screws, mainly used in Europe, deep and narrower thread profile than M, it can transfer significantly more tensile force|
|Ww (BSW) = British Standard Whitworth thread according to BS 84||Pipe connections, mainly used in England, thread is indicated in inches|
|UNC = American UNC coarse thread ANSI / ASME B 1.1||Computer parts, American counterpart to the European M, mainly used in North America, thread is indicated in inches|
|UNF = American UNF fine thread ANSI / ASME B 1.1||American counterpart to the European MF, mainly used in North America, thread is indicated in inches|
|PG = DIN 40 430 steel conduit thread||Formerly electrical installation, replaced by M|
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.
With drills for blind holes, the head end for very small diameters is pointed instead of flat. This is due to the manufacturing process. During manufacture, the tap must be clamped to the shank as well as to the head. This is done at the head end via a centering hole. With smaller dimensions, however, this is not possible and the tap must be accommodated via the pointed head end.
On the other hand, taps with a flat head end can also be used in through holes.
|Head shape||Through hole||Blind hole|
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.
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:
|Type||Threads and point cut||Application|
|B||approx. 4 - 5 threads with progressive tap||Suitable for through holes, tough, medium and long chipping materials|
|C||approx. 2-3 threads||Suitable for through hole and blind hole with short thread outlet|
|D||approx. 4-6 threads||Rarely used, suitable for through holes|
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.
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.
|Flank angles||Hand taps||Machine taps||Thread type|
|60°||M, MF, UNC, UNF|
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.
Since the reinforced shank of machine taps is thicker than the thread itself, they cannot be used for deep holes.
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.
Colour ring on machine tap
We use the color ring on the shank of the machine tap to identify the tool steel from which the tap is made. The ring therefore serves as an orientation aid for the tool steel used.
The blue ring indicates the high-speed steel HSS and the green ring indicates the high-speed steel HSSE-Co 5 with 5% cobalt content.
The red ring marks the high-speed steel HSSE-Co 5 (high-speed steel with 5% cobalt content) with the TiAlN coating.
Rings for hand tap sets
The taper tap is marked with a ring and the second tap with two rings on the shank. The final tap has no ring (in rare cases three rings).
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.
|Thread type||Threads are standardized according to an ISO norm. Find the desired standard. This makes your circle of possible internal taps extremely small.|
|Head shape||The head shape can be pointed or flat. With the pointed head shape you should only cut into a through hole. With the flat head shape you cut into blind holes as well as through holes.|
|Point cut||The point cut is made via threads. The more threads the internal tap has, the better the centering in the workpiece and less force is applied on the drill.|
|Thread profile||The teeth on the thread profile take over the actual cutting process. Which pitch and which flank angle you need is already determined by the type of thread.
|Flute||The flute profile can be straight or spiral. They are responsible for how the chip flows and in which direction it is removed (important for blind holes). Spiral fluted taps promote the removal of chips, especially at high speeds.|
|Shank||In contrast to the reinforced shaft, the overflow shaft has the same diameter throughout. This is suitable for long through holes or blind holes. For internal taps with reinforced shank you have to pay attention to the cutting depth, you could hit the workpiece with the reinforced shank.|
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.
The RUKO Blog
Tips, tricks, and useful information about metal cutting.