On our machine training pages, in the Manufacturing Processes category, we’re covering the topic of tapping. We’ll cover how tapping is done, and the steps of the tapping process.
Tapping
Internal threads can be cut with a tap either by hand or on a machine. Depending on the quantity of parts to be produced and the desired precision and surface quality, various manufacturing methods can be used.
To cut internal threads, the tap-drill hole needs to be drilled first. For a metric ISO thread, the diameter of the tap-drill hole (drill diameter) corresponds to the value obtained by subtracting the thread pitch from the thread’s major diameter.
Hand Tapping
1- To tap a hole, the hole to be tapped first needs to be drilled to the correct diameter.
2- The mouth of the hole to be tapped needs to be chamfered with a countersink drill.
3- The appropriate tap set and tap wrench are selected for the job.
4- From the tap set, the tap to be inserted into the hole first is chosen (the taper tap).
A 3-piece hand tap set consists of a taper tap, a plug tap, and a bottoming (finishing) tap. To obtain a clean thread cut to the correct size, and to avoid straining any single tap with too much material removal, the cutting work is split across the set’s 3 taps.
Taps have marking lines to show the order in which they should be used. The taps are used in sequence according to the number of lines. On some sets, the 3rd tap has no line at all. If none of them have a line, look at the teeth and the lead-in chamfer of the taps in the set. The one with the most tapered tip and the least-defined teeth is the first. The others follow as the 2nd and 3rd accordingly.
5- The tap is mounted onto the tap wrench.
How to Tap a Hole
You need to make sure the tap is aligned with the hole’s axis. The tap is placed into the hole perpendicular to the part’s surface. A few drops of oil are applied at the mouth of the hole to make cutting easier. Then, while pressing down lightly on the tap wrench, it’s turned clockwise at the same time.
Lubricating the inside of the hole while tapping makes cutting easier.
Starting the tap correctly is important during tapping. To start it, make sure the tap is perpendicular to the part’s surface, and turn it clockwise using the tap wrench while pressing down lightly.
If, when you let go of the tap wrench, the tap stays gripped in the part and doesn’t fall out, this means the starting step was successful.
6- Once the tap is started, it’s turned half a turn clockwise (cutting) and then a quarter turn counterclockwise (breaking off the chip being cut from the part).
This is repeated until the end of the hole is reached.
The tap needs to start and advance exactly along the hole’s axis. Thanks to the reversing motion, repeated roughly every quarter turn around the tap’s own axis, the chips are broken off and fresh cutting oil reaches the cutting edges again.
7- The same process is repeated with taps number 2 and 3. This continues until the tap can turn and advance easily inside the part with just a very light finger force.
How to Tap a Hole – Video
You can watch how tapping is done in the video prepared on how to tap a hole.
Machine Tapping
When tapping on a lathe, drill press, or thread-cutting machine, the threads are cut exactly along the axis of the threaded hole. Thanks to appropriately high cutting speeds, machine tapping significantly shortens the time compared to hand tapping.
On a tapping attachment mounted to a drill press, the allowable torque can be adjusted depending on the size of the thread and the material. Once the set thread depth is reached, the attachment automatically switches to a reverse motion at an increased speed.
Drill Diameters for Tapping
The table below gives the tap-drill hole diameters for tapping. The tap-drill holes need to be drilled to the permitted size. This makes tapping easier and prevents the tap from breaking.
Hole diameter for M3 is 2.5 / drill diameter for M3 is 2.5
Hole diameter for M4 is 3.5 / drill diameter for M4 is 3.5
Hole diameter for M5 is 4.2 / drill diameter for M5 is 4 or 4.5
Hole diameter for M6 is 5.0 / drill diameter for M6 is 5
Hole diameter for M8 is 6.8 / drill diameter for M8 is 7
Hole diameter for M10 is 8.5 / drill diameter for M10 is 8.5
Hole diameter for M12 is 10.2 / drill diameter for M12 is 10 or 10.5
Hole diameter for M16 is 14.0 / drill diameter for M16 is 14
Hole diameter for M20 is 17.5 / drill diameter for M20 is 17.5
Hole diameter for M24 is 21.0 / drill diameter for M24 is 21
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