Cut internal thread - this is how it works

Sometimes it is necessary to make individual screw connections yourself. Cutting a suitable internal thread for a certain screw size is basically unproblematic. However, some requirements must be observed. The following detailed instructions provide information on how to do it correctly and what to look out for.

This is important

  • The material into which the thread will be cut
  • The right drill size
  • Loosen chips
  • Also read - pre-drilling a thread is precision work
  • Also Read - Tapping And Cutting
  • Also Read - Correctly Cutting a Thread

The material into which the thread will be cut

Internal threads can be cut in metal, plastic and wood. Depending on the material, different tools and procedures are necessary.

metal

Special taps are used for cutting internal threads in metal. All metals can be machined that show traces of filing even when filing with very light pressure.

The taps are always combined in three versions to form a set of the same size: a coarse spur, a fine spur and a finishing cutter. They are used one after the other in this order.

The taps are either inserted in a so-called tap wrench or in a special tool holder. The tool holder has the same function as a ratchet.

It is easier to use, but unlike the tap wrench, it cannot be turned back to break the chips. In most cases, this makes it the worse alternative.

The drill hole must be countersunk in order to give the tap sufficient support. Cutting oil must always be used to protect the taps. When pre-drilling in metal, the drill should also be cooled with drilling oil if possible.

plastic

The same tools as for metalworking can be used to cut threads in plastic. The use of a wrench is always more advantageous here.

Wood

Special tools are used when threads are to be cut in wood. In most cases, the diameters 10, 16 and 22 mm are used.

Instead of the cutting oil common with metals, linseed oil is used when cutting threads in wood. Cooling of the drill when pre-drilling is of course not necessary for wood.

The right drill size

The drill size for pre-drilling results from the thread diameter of the screw that is to fit into the thread. There are tables where you can look up the drilling diameter based on the screw size.

As a rough rule of thumb, the thread diameter of the screw x 0.8 applies. However, it does not always apply exactly - there are significant deviations in the tables.

Loosen chips

The chips must be removed again and again during cutting, otherwise the newly cut thread will be damaged by the chips. Regularly turning back the wrench by half or a quarter of a turn breaks the chips and makes them easier to remove.

Cutting internal threads by hand in metal - this is how it works

  • workpiece
  • Cutting oil
  • Drilling oil
  • Tap replacement
  • Countersink
  • Drill (€ 48.00 at Amazon *)
  • Appropriate size drill bit
  • small brass brush
  • suitable fixation option for the workpiece

1. Pre-drill thread

Use the drill to pre-drill at the desired location with a drill of the appropriate size. Drill exactly vertically, work at slow speeds and cool the drill with drilling oil.

2. Countersink the borehole

Carefully countersink the drill hole with the countersink. In this way, the thread cutter can grip precisely.

3. threading

Place the coarse precutter in the tap wrench and fix it. Moisten with cutting oil. Roughly pre-cut the thread exactly vertically. Turn the tap back at regular intervals to break the chips. Remove chips regularly and wet with cutting oil.

Then cleanly remove the chips with the small brass brush and use the fine pre-cutter and then the finishing cutter in the same way.

Tips & Tricks The drill hole must be exactly vertical, otherwise the tap could break. A column drill or drill stand are therefore best suited for pre-drilling.