As with internal taps, the external taps (round dies) are standardized according to DIN. Depending on your internal thread, you will need the suitable external thread (round die).
An external thread, also known as a screw thread, is cut by hand with a round die which is fixed in a die stock.
Round dies have three or more cutting edges, depending on their size. Between these cutting edges there are cavities which remove the chips. The cutting edges are serrated and geometrically represent the nut thread.
A round rod is usually used as the counterpart to the internal thread.The diameter of the round bar must be slightly smaller than the outside diameter of the thread as the die presses the material slightly outwards during the cutting process.
Note: If you have a turning lathe, then turn the diameter about 0.1 times the pitch smaller than the thread outer diameter. This means that if your thread outside diameter has a pitch of 1.5 mm, this results in a reduction of 0.15 mm (1.5 mm x 0.1). After turning, the round rod has a diameter of 9.85 mm.
There are hexagonal square dies for use in interlocks. They are mainly used for repair purposes and to move the die to inaccessible places.
By the way, we explain how you cut an internal thread in our article "How to cut an internal thread in 3 steps".
To cut an external thread you need the following tools:
- Round rod
- Vise for clamping
- Round die (external thread)
- Die stock
- Flat-tip screwdriver
- Cutting spray, for optimum surface quality
The working procedure
As a first step file the edges of the round rod and chamfer the round rod (45°) which should be slightly larger than the thread depth.
This makes it easier for the die to start and the cutting edges to grip the round rod better.
As a second step, you clamp the round die into a die stock (fix it by tightening the screws with the flat-tip screwdriver) and cut the thread. When fixing, make sure that the holes for the screw connection are in the correct position.
When threading, carefully turn the round die clockwise onto the round rod with sufficient pressure. Make sure you cut a straight thread.
Use the cutting spray here. Spray the head of the round die with the cutting spray. This increases the service life of the round die and improves the surface quality.
Note: Turned back a third turn after approx. two turns to break the chip. This reduces the load on the round die and fresh lubricant reaches the cutting edges.
When you have cut the thread, unscrew the round die in the opposite direction.
Done: The thread is made.
You can test the result with a suitable nut.
Summary and conclusion
An external thread is cut in only two steps.
First you need to know which internal thread matches the external thread.
When selecting the round die, it is important to consider which material you want to cut into. The harder the material, the harder the tool steel of the round die has to be.
Make sure that you turn back the round die a third turn after two turns (to break the chip).
Don't forget to use cutting spray to increase the service life of the round die and to improve the surface quality.
If you have trouble finding the right round die, please do not hesitate to contact us.