Modern technology is simply unthinkable without threaded connections. The thread has a number of advantages compared to 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 well, it is easy to disassemble and reassemble it.
However, the use of threaded connections is not limited to industry. At home in everyday life, it is very often necessary to cut threads. How and in what sequence is it correct to do this? What tool is needed for this? The article contains useful information that will help people who do not have this experience.
Ways
Thread - a kind of helical groove on the outer surface of the cylinder (outer) or on the surface of the hole (inner).
For cutting external threads, a die is used, forinternal - tap.
But this method and tool are suitable only at home or in the repair industry, because they require a lot of time for cutting. In the mass production of hardware (bolts), the threads are not cut, but rolled on special metal forming equipment. At the same time, it is important that the workpiece be heated to a temperature above 750 degrees (then dynamic recrystallization and texture elimination will occur). Such a performance is inaccurate. And therefore, threads are cut for critical connections.
The threaded surface can be obtained by processing on a screw-cutting lathe. 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
On the territory of the CIS countries there is a metric thread standard. Its angle is sixty degrees in cross section. In Western countries, an inch thread is used (angle 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 in the section, rectangular, triangular, trapezoidal and other options are distinguished.
A special type of thread is a ball screw. It has found application only in the machine tool industry. With intensive use, the helical surface wears out. However, the design of the ball screw allowsadjust and eliminate backlash.
There are right-hand threads (the most common) and left-hand threads (they have limited and highly specialized applications). You can hardly find a die or tap for cutting left-hand threads. Cutting on a machine is perhaps the only possible way. It is recommended to use it in the absence of other options.
External thread: how to cut with a die
The easiest and most affordable way to thread on the outer cylindrical surface is to use a die. Depending on the design, the device can be round, prismatic, sliding.
The round die is somewhat reminiscent of a nut. Only the outer contour is not a hexagon, but an ordinary circle with small indentations for attaching to a collar. Differs from a nut in having three grooves for chips to escape when threading.
Round dies form a thread in just one pass of the tool. Therefore, during their work, lubricant must be supplied to the cutting zone. The maximum thread diameter obtained in this way is 52 mm.
The sliding die consists of two identical parts. They are installed in klupp with a certain gap. In the process, the parts come together.
Selection of workpiece diameter when cutting external threads
The correct choice of the outer diametrical size of the original workpiece is the key to the quality of the resulting thread. Cut threads (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 diameter of the cylinder should be several tenths of a millimeter less than the thread rating. During the cutting process, the metal will extrude to some extent and fill the die in shape, so that the gap will be minimal.
Surface preparation for external cutting
How to cut a thread on a bar? It should be noted that the diameter of the rolled bar must be at least a millimeter larger than the diameter of the thread, so that there is an allowance for removing blackness on the lathe. Cutting on a black raw surface is highly undesirable: there is a high probability that the die will find a non-metallic inclusion and break.
After turning the outer diameter on a lathe, it is necessary to machine a chamfer. This element is necessary, firstly, in order to ensure the insertion of the die, and secondly, to remove the sharp burr obtained when cutting the end on a lathe.
Sequence of actions
How to cut a thread correctly? This operation is performed manually in the following sequence:
- The original workpiece is fixed in a bench vise. In this case, there should be no distortions and deviations from the vertical position.
- The die is attached to the collar. The end of the die must match the end surface of the die holder.
- The first turn is carried out with little effort: the main thing is to set the direction correctly and avoid skewdice.
- After the thread is cut along the entire length, the knob should be rotated counterclockwise.
Internal threading technology
The shaping tool in this case is a tap. How to cut a thread with this tool? In principle, it is also quite simple: a hole is drilled in the metal with a diameter slightly larger than the diameter of the thread cavity, the tap itself is inserted into the knob with a shank, after which it begins to be screwed into the hole, while cutting the thread. Externally, the tool looks like a bolt. Only it is made of high strength alloy tool steel and has chip evacuation grooves.
Manually cutting threads - both with a die and a tap - is a very difficult task, and without some experience it will be difficult to do this operation (primarily physically). To facilitate the process, special tool kits are available. Such a set allows you to cut threads not in one pass, but in three, using three different taps (roughing, semi-finishing and finishing).
Thread on pipe surface
When carrying out plumbing work, it is often necessary to cut threads on a pipe. How it's done? There are no fundamental differences. The only difference is that the tube is hollow inside. That's all. For these purposes, an ordinary die and a wrench or a screw-cutting lathe can be used.
Also, for the implementation of the process on the pipe surface, so-called clods are used. At the same time, it is possibleuse both sliding and solid dies.
The cutting procedure is preceded by a reliable fixing of the pipe section (in a vice or in special prisms), after which the surface is cleaned of dirt and oxides, and the burr from the band saw is removed. The surface is recommended to be lubricated with oil. Only after these preparatory operations can you proceed directly to cutting.
Threading on a machine
Cutting threads by hand 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 cut threads on a lathe? A universal screw-cutting lathe allows this operation to be carried out in several ways: with a die (outer), a tap (inner), as well as a turning cutter specially sharpened according to the shape of the thread with a brazed tool steel plate or a cutter with an interchangeable 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 speed of rotation should be minimal (no more than one or two revolutions per second) for safety reasons. A die (tap) with a knob is attached to the end of the workpiece and is slightly pressed against it. After that, 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 chiselcarried out in several passes. In this case, the cutter for one revolution of the spindle moves in the axial direction by an amount equal to the pitch.
