Please note: the articles have been translated automatically.
When we talk about thread sizes in Europe, we usually talk about the metric standard thread. Of course, threads have an external thread (e.g. screw) and an internal thread (e.g. nut). The designation for threads is made up of the letter M (for metric threads) and a number, which designates the outside diameter of the solid tube on which the corresponding external thread is cut (i.e. the outside diameter of the screw before the thread is cut on it).
When it comes to describing a thread, there are many unfamiliar terms. The most important are:
- Core hole diameter (i.e. with which drill do I have to pre-drill for the thread)
- Pitch (indicates by how many mm a screw turns in when tightened one turn)
- Flank angle (difference Wirthworth thread / metric thread)
Now that the standard thread has been discussed above, there is also a fine thread. With a fine thread, the pitch is less, i.e. a screw with a fine thread needs more turns until it is screwed in just as far as a screw with a standard thread. If a fine thread is used at all, then it is primarily used for adjusting screws (i.e. to be able to adjust something precisely).
Metric threads have a flank angle of 60°, Wirthworth threads have a flank angle of 55°. In practice, this detail is only interesting insofar as a Wirthworth thread is usually used for tight screw connections on pipes, as this guarantees better sealing. Therefore, there are different thread cutting sets for this purpose.
Drilling from the core hole
The diameter of the core hole for pre-drilling for turning in the internal thread is usually the most important parameter in practice. Therefore, there is a drill hole table with different core hole diameters for different metric standard thread sizes.
Somewhat unusual drill sizes are used here (for M4, M5, M8 and M12). In practice, it is usually sufficient to use the next smaller drill (listed in brackets), since the drill diameter is then expanded accordingly when tapping.
Drilling the internal thread
When drilling internal threads by hand, three different taps are usually used for each thread size.
- Spurs: marked with a ring on the shank. The tapping teeth on the spur are tapered at the front so that the spur is largely self-centering.
- Center cutter: marked with two rings on the shank. in the execution between pre-cutter and final cutter
- Finish cutter: no ring on the shank. Used for recutting the thread so that the screw runs smoothly in the thread afterwards.
Author’s note: even if one or the other will probably stone me, for tapping in soft materials such as aluminum, plastic, etc. I would only use the finishing tap. On the one hand it is less work and on the other hand there is less risk of cutting a second thread if the finishing tap does not fit properly into the pre-cut thread.
When tapping, the respective taps are inserted into the tap wrench. It is advisable to use some drilling and cutting oil (or WD 40 if necessary). When applying, care should be taken to ensure that the tap is placed as straight as possible. Once a thread has been misaligned, it is difficult to correct. If required, there are various tapping guides for you. When drilling (especially in hard material such as steel), you should not drill through in one go, but make a counter-rotation after approx. 2-3 turns so that the chip on the cutting tooth can break.
Hand threads are typically used for through holes. With blind holes (i.e. holes that do not go through the entire material) it should be considered that the screw that is inserted into the thread cannot be screwed in to the end of the internal thread.
In addition to tapping with hand taps, there are also machine taps. These have little or no taper at the front end and only one cutter is required for each thread size. However, machine taps are operated with drill presses with the appropriate tapping device (power is transmitted via a clutch). A purchase in this direction only pays off if a thread should be drilled really often.
External thread cutting
When cutting the external thread, the solid pipe should be firmly clamped. It helps if you use a file to bevel the edges of the top of the pipe to make it easier to put the tap wrench on. Here, too, care should be taken to ensure that the tap wrench is placed as straight as possible. (There are also guided tours for this if required). Furthermore, with the external thread, not everything should be cut in one go, but again after 2-3 turns a counter-turn should be made so that the chip breaks. The use of lubricant is also recommended.
Tapping in stainless steel
Taps are usually made of HSS-G. These are well suited for normal use in steel, but if tough stainless steel is to be machined, then HSS-E is recommended as the tool material (or titanium-coated taps).
Author’s note: it is really advisable to use the right material. It’s really annoying to get a broken tap out of a nearly finished workpiece.
Strength class according to ISO 898
Of course you want to know how much a fastening point can withstand. This depends on the one hand on the material in which the screw is screwed in and on the other hand on the screw/nut itself.
ISO 898 specifies the mechanical properties of bolts and nuts. In the case of hexagon head screws, the strength class should be indicated on the head of the screw. Numbers from 3.6 (low strength) to 12.9 (very high strength, e.g. for engines) are used here. Grade 8.8 bolts are readily available and have good strength.
A thread table is available for download here as an aid
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