How to cut a thread

January 20, 2012

How to cut a threadThe thread is one of the most convenient detachable joints and, therefore, information on how to cut the thread can be useful in the household. There is an inch and metric thread, so it happens that the suitable screw at first glance turns out to be unsuitable - the diameter is wrong and the thread pitch is different. Even the profiles of metric and inch threads are different: the angle at the top in a metric thread is 60 degrees, and in inch threads - 55 degrees.

Any threaded connection consists of two parts, one of which has an internal thread in the hole, and the second external, on a cylindrical surface. Generally, right-handed threads are used, in which rotation occurs clockwise, but sometimes left-handed threads are found with counterclockwise rotation.

how to cut the threadFor a thread, the main dimensions are the pitch - the distance between its turns) and the outer diameter. They are interrelated and standard. Both internal and external threads are determined by the size of the diameter of the rod, that is, if M10 thread is indicated, then this thread is metric, right with a screw diameter of 10 mm and a thread pitch of 1.5 mm.Sometimes non-standardized threads are also used, for example, camera lenses are mounted on threads with a diameter of 42 mm. According to the standard, the pitch for this thread is 4.5 mm, but for an optical device this is too large a parameter, so a pitch of 1 mm is used. There are special standards for fine threads. Basic thread sizes can be found in reference books and standards.

Any threads are cut using special tools and machines, and at home you can cut internal and external threads on fasteners with a diameter of up to ten millimeters. To do this, use a simple tool like a set of dies (for external thread) or taps (1) (for internal thread). Dies (2) are a hard metal tool, similar to a nut with cutting grooves.

how to cut the thread into a dieThe taps (1) are also made of hard metal, this tool looks like a screw with cutting grooves. Usually they use not one tap, but a set of two or three tools, which are used for roughing and finishing passes, and have a different depth of cutting of the threaded grooves. And taps and dies when cutting threads are clamped into special tools - rollers (3), the size of which should fit the tool used.

Tools for threading should be of high quality and sharp. It will be impossible to cut a good thread with a dull rusty tap, and there is also a high probability that it will break in the first hole, in which case it will be more difficult to remove it from the workpiece. Low quality metal taps are suitable only for working with plastics. As for the dies, the blunt tool will cut uneven threads, and then the nut can be screwed only with great effort.

External threading

how to cut a thread on a coneA round metal rod with a diameter equal to the diameter of the required thread is firmly clamped in a vice in a vertical position. At the end of the workpiece, a file or chamfer is filed with a file or velvet file and the machine is abundantly lubricated with machine oil to facilitate thread cutting. A suitable die is fixed in a special winding and begin to wear on the end of the workpiece. The tool seems to be wrapped around the workpiece, making sure that there is no distortion. When the right-hand thread through one or one and a half turn in a clockwise direction, the plate must be rotated half a turn in the opposite direction.So pass all the necessary length of thread to the end of the work.

Internal threading

how to cut internal threadFirst, a hole is drilled in the workpiece of the desired diameter and the chamfer is removed to set the tool. The diameter of the drill is selected from reference data. Holes in the workpiece can be through (through the entire thickness of the part) or blind. If a hole with a female thread of a small length is required, in this case a hole is drilled with a greater length than the thread, since the taper of the tap at the bottom must be taken into account. You should also consider the allowances for the depth of drilling.

Internal threads for greater purity of the profile are formed by three taps. The first of them forms a preliminary pass, it is indicated by a risk in the upper smooth part of the tool. The second tap from the kit completes the formation of the thread (it is indicated by two risks), and the third tool finally forms the thread profile, it is indicated by three risks. Thus the thread is cut gradually, and as a result, a clear profile is formed. For holes up to three millimeters in diameter, use a set of two taps.The process of threading itself is the same as when working with dies - the repetition of reverse movements back and forth to chip chips from the tool with the use of grease. If the thread is cut in blind holes, then it is advisable to periodically turn out the tap in order to completely remove metal sawdust from its surface. They are removed with the help of an old toothbrush or cloth, and then it is necessary to lubricate the tool again and continue cutting to the stop.


  • If a thread accidentally hesitated during work, you can restore it by holding the part in a vice and driving the thread again with a tap or die.
  • If it is necessary to additionally lengthen the surface of the threaded screw, then it is necessary to ensure that the plate easily passes through the existing threads of the thread and does not damage its profile.
  • To prevent the thread from corrosion, it can be greased with technical petroleum jelly.
  • If at cutting it turned out to be skewed and the external thread “went away” to the side, then you can cut off a piece of the rod and start the process again.
  • If the tap is broken in the hole, and there is no possibility to drill another hole next to it, you can remove the broken tool in several ways.If part of the tap remains stuck out, then its end is ground in the form of a scapula and turned out with pliers. And if part of the tap is left inside, then nitric acid and a wooden stick are used: a drop of acid is applied on the broken tap and they wait until the acid destroys its cutting edges. Then the piece is pulled out with tweezers or knocked out on the other side of the hole (if it is through).

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