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Drilling and Cutting Threads

Sometimes a DIY enthusiast has to cut their own threads. You'll learn how to do that, and why it isn't at all difficult

Femal threads

You need female threads to tighten specific screws when you can't use nuts, or where the joint should be inconspicuous, such as when joining two pieces of steel.

Material list

  • Set of taps
  • Tap wrench
  • Suitable drill for pre-drilling
  • Countersink
  • Cutting oil

For thread tapping, you need a set of taps which consists of three taps. Each tap is marked with rings. You start with the tap with one ring, then you use the one with two, and then finally the one with three rings. There are also so-called cutting taps or machine taps, which cut a complete thread in only one pass. Before tapping, you have to drill a suitable hole, which is called the core hole. As a rule of thumb, the core hole size is 0.8 times the screw size.

Nominal dimension Female thread core hole diameter mm
M3 2,50
M4 3,30
M5 4,20
M6 5,00
M8 6,80
M10 8,50
M12 10,20

Once you have drilled the right size hole, you should countersink it. This acts as a guide for the tap, and later also the screws, and they centre themselves better.

Fix the tap into the tap wrench and screw it into the drilled hole in a clockwise direction with some force. I recommend that you use cutting oil to make everything a bit easier, but normal lubricating oil also works. After every turn, you should back off by a quarter turn to break up the chippings. 

This will allow the thread to be cut cleanly. Screw the tap in all the way, until it comes out again on the other side. And your female thread is finished!

Cutting Male Threads

Cutting male threads always makes sense if you are not using a continuously threaded rod, but only want to attach a nut on the outside. 

Material list

  • Die
  • Die holder
  • File or bench grinder
  • Cutting oil

Male threads are cut into round steel rods. Ideally the rod's outer diameter is a little bit smaller than the nominal size of the thread to be cut. In order to determine the appropriate rod diameter for the respective thread size, we have prepared an summary for you here.

Nominal dimension Male thread core hole diameter mm
M3 2,92
M4 3,91
M5 4,90
M6 5,88
M8 7,87
M10 9,85
M12 11,83

Before you start with the die, you should file down the end of the rod or grind it with the bench grinder. Using the bench grinder is recommended, because you can obtain a more even shape by evenly turning the rod on the grinding disc. This way the die will catch better. 

Now fix your die into the appropriate holder and secure it with the screw provided. Set the die directly on the clamped rod, apply enough pressure and slowly turn it clockwise.

The use of cutting oil is also essential here. In the first two turns in particular, the die must be kept straight. To ensure this, you should make the turns using your body. Keep your arms bent and tight against your body. Now turn your whole body, as far as it will go, in a clockwise direction around the rod. Lower your arms, reach over and hold the die from the other side. Now turn further. Repeat this movement until you can feel that the die has caught and is sitting firmly on the rod. From here, you can carry on turning it normally. After every turn, back off by half a turn to break up the chippings. 

With this ability to make a thread exactly where you need it, maybe you can come up with the solution to make progress with your projects.
Frank Rath
Enthusiastic DIY-ler
Reading time5 minutes