It is simply inconceivable to imagine modern technology without threaded connections. The thread has a number of advantages in comparison with other types of detachable and one-piece fasteners: the clamping force can be adjusted using a dynamometer, which eliminates the possible destruction of body parts. Such a connection tolerates dynamic loads, it is easy to disassemble and reassemble.
However, the use of threaded connections is not limited only to industry. At home, in everyday life, it is often necessary to cut threads. How and in what order is this done correctly? What tool is needed for this? The article contains useful information that will help people who do not have this experience in this.
Ways
A thread is a kind of helical groove on the outer surface of the cylinder (outer) or on the surface of the hole (inner).
A die is used to cut the external thread, and a tap is used for the internal thread.
But this method and tool are suitable only at home or in repair production, because they require a large investment of time for cutting. In the mass production of hardware (bolts), threads are not cut, but rolled on special equipment for metal forming. It is important that the workpiece is heated to a temperature of over 750 degrees (then dynamic recrystallization and texture removal will occur). This design is inaccurate. And therefore, for critical connections, the threads are cut.
The threaded surface can be obtained by machining on a screw-cutting machine. For this, both a die with a tap and special turning tools can be used.
In addition, the rapid development of electronics has made it possible to process threaded surfaces on turning and even CNC milling machines.
general information
In the CIS countries, the standard metric thread. Its angle is sixty degrees in cross section. In Western countries, inch thread is used (angle of 55 degrees). This circumstance should be remembered when buying any spare parts for a car or other equipment.
Depending on the geometric shape of the thread teeth, rectangular, triangular, trapezoidal and other options are distinguished in the section.
A special type of thread is a ball screw pair. It found application only in machine tools. In intensive use, the helical surface wears out. However, the ball screw design allows you to adjust and eliminate play.
There are threads with a right approach (the most common) and left (have limited and highly specialized use). You can hardly find a die or tap for cutting left-handed threads. Cutting on the machine is perhaps the only possible way. They are recommended to use it in the absence of other options.
External thread: how to cut a die
The easiest and most affordable way to thread on an outer cylindrical surface is to use a die. Depending on the design, the device can be round, prismatic, sliding.
A round die is somewhat reminiscent of a nut. Only the outer contour is not a hexagon, but a regular circle with small recesses for attaching to the winch. It differs from the nut by the presence of three ditches for chip output when threading.
Round dies form threads in just one tool pass. Therefore, during their operation, it is necessary to apply lubricant to the cutting zone. The maximum thread diameter obtained in this way is 52 millimeters.
The sliding die consists of two identical parts. They are installed in the screwdriver with a certain gap. In the process, the parts come together.
Selection of the diameter of the workpiece when cutting the external thread
The correct choice of the outer diametric size of the initial workpiece is the key to the quality of the resulting thread. Threading (both external and internal) on the surface (in the hole) is possible only if certain requirements are met. So that the die does not break and does not jam, the cylinder diameter should be several tenths of a millimeter less than the thread rating. During the cutting process, the metal will be extruded to some extent and fill the die in shape, so that the gap will be minimal.
Surface preparation for external cutting
How to thread a bar? It should be noted that the diameter of the bar must be at least a millimeter larger than the diameter of the thread so that there is an allowance for removing black on the lathe. Cutting on a black unprocessed surface is extremely undesirable: there is a high probability that the die will find a non-metallic inclusion and break.
After turning the outer diameter on the lathe, it is necessary to grind the chamfer. This element is necessary, firstly, in order to ensure die cutting, and secondly, to remove the sharp burr obtained by cutting the end on a lathe.
Sequencing
How to thread a thread? The execution of this operation manually is carried out in the following sequence:
- The original workpiece is mounted in a bench vise. In this case, there should be no distortions and deviations from the vertical position.
- The die is attached to the knob. The end face of the die should coincide with the end surface of the die holder.
- The first revolution is carried out with little effort: the main thing is to correctly set the direction and avoid skewing the die.
- After the thread is cut along its entire length, the knob should begin to rotate counterclockwise.
Thread cutting technology
The forming tool in this case is a tap. How to thread with this tool? In principle, it is also quite simple: a hole with a diameter slightly larger than the diameter of the thread depression is drilled in the metal, the tap itself is inserted into the knob by the shank, after which it starts to be screwed into the hole, cutting the thread. Externally, the tool looks like a bolt. Only it is made of high-strength alloy tool steel and has grooves for chip evacuation.
Manually cutting a thread - either with a die or with a tap - is a very difficult task, and without certain experience it will be difficult to do this operation (primarily physically). To facilitate the process, special tool kits are available. This set allows you to thread not in one pass, but in three, using three different taps (rough, semi-finished and fair).
Pipe surface thread
When carrying out plumbing work, it is often necessary to cut a thread on a pipe. How it's done? There are no fundamental differences. The only difference is that the pipe inside is hollow. That's all. For these purposes, a conventional die and screwdriver or a screw-cutting lathe can be used.
Also for the implementation of the process on the pipe surface using the so-called swabs. In this case, both sliding and solid dies can be used.
The cutting procedure is preceded by a reliable fastening of the pipe section (in a vice or in special prisms), after which the surface is cleaned of dirt and oxides, the burr of the band saw is eliminated. It is recommended to grease the surface with oil. Only after these preparatory operations can you start directly to the cutting.
Threading on the machine
Manual threading is hard physical labor. Therefore, if the master has the opportunity to resort to the help of metalworking equipment, it should be used.
How to thread a lathe? A universal screw-cutting lathe allows this operation to be carried out in several ways: with a die (external), a tap (internal), and also with a specially sharpened in the form of thread turning tool with a solder plate made of tool steel or a tool with a replaceable plate.
When cutting with a die and a tap, the workpiece is installed in a three-jaw self-centering chuck, after which the machine drive is turned on, and the spindle starts to rotate. The rotation speed should be minimal (no more than one or two revolutions per second) for safety reasons. A die (tap) with a collar is attached to the end face of the workpiece and slightly pressed against it. Then it is necessary to hold the knob until it cuts the thread of the required length. Then the reverse is turned on, and the die is twisted in the opposite direction.
Cutting with a cutter is carried out in several passes. In this case, the cutter for one revolution of the spindle moves in the axial direction by an amount that is equal to the step.