Cutting internal and external threads. External thread cutting. Diameters of threaded rods when cutting with dies. For cutting external threads use
A machining operation involving chip removal, as a result of which external or internal helical grooves with a given profile and dimensions are formed on cylindrical or conical surfaces, called thread cutting.
Thread cutting on screws, bolts, nuts and other parts is carried out mainly on machines. During installation and repair work In some cases, a mechanic has to cut threads manually or using pneumatic or electric machines - thread cutters.
The main elements of any carving, as noted in Chapter. 3, are profile, pitch, depth, outer, middle and inner diameter s.
According to the shape of the thread profile, they are divided into triangular, rectangular, trapezoidal, thrust and round (Fig. 4.14).
The thread type or profile is selected according to GOST depending on the purpose.
Rice. 4.14. Profiles and thread elements:
a - triangular;
b - rectangular;
c - trapezoidal;
g - persistent;
d - round;
d- outside diameter threads;
dcp- average diameter threads;
d1 - internal thread diameter.
In mechanical engineering, three thread systems are accepted: metric, in which the pitch and diameter are measured in millimeters; inch, having a different profile shape and characterized by the number of threads per inch of its length and diameter in inches;
pipe thread, which has a profile like an inch thread, but with a smaller pitch.
In plumbing practice, it often becomes necessary to determine the dimensions of thread elements on a finished part. The outer diameter is measured using a caliper or micrometer, and the thread pitch is measured using a millimeter or inch thread gauge (a set of templates with threads of various sizes).
Taps are used for cutting threads in holes, and for cutting external thread- dies.
It's called a tap cutting tool, which is a hardened screw on which several longitudinal straight or helical grooves are cut, forming cutting edges (Fig. 4.15). The tap has a working part and a shank ending in a square.
Rice. 4.15. Tap and its elements:
A - general form:
1 - cutting pen;
2 - cutting edge;
3 - square;
4 - shank;
5 - groove;
b - cross section:
1 - front surface;
2 - cutting edge;
3 - rear (back) surface;
4 - groove;
5 - cutting pen.
The tap shank is used to secure the tool in the chuck or driver during operation. Hand taps have a square end.
The working part is the threaded part of the tap that produces threads; it is divided into intake and calibrating parts.
The intake (cutting) part of the tap is the front conical part, which first enters the hole being cut and performs the main cutting work.
The calibration part protects and calibrates the hole being cut.
For education cutting edges The tap and chip exit are provided by longitudinal grooves. The threaded parts of the tap, delimited by grooves, are called cutting feathers.
According to the method of application, taps are divided into manual and machine. Hand taps are used for cutting threads by hand. They are usually produced in sets of two or three. A set of three taps includes rough, medium and finishing (or 1, 2, 3), and a set of two taps includes rough and finishing. They are used in the same order when cutting threads.
Taps are conventionally designated by marks (grooves): a rough tap has one circular mark on the shank, a medium tap has two, and a fine tap tap has three. The type of thread and its size are also indicated there.
Of great importance right choice diameters of holes for threading. Choice drill diameters for a given type and size of thread is made according to special tables. However, with sufficient accuracy for practice, the drill diameter can be determined using the formula
Dsv=dр - 2h
where Dsv - drill diameter, mm; dр - outer diameter of thread, mm; h - thread profile height, mm.
Threading with hand taps is done using cranks that fit onto the square ends of the shanks. There are collars various designs with fixed and adjustable tap holes.
The tool used for cutting external threads is called a die. The die is a hardened steel nut with chip grooves forming cutting edges (Fig. 4.16).
Rice. 4.16. Die and its elements:
a - general view;
b - geometric parameters of the die.
1 - calibrating part;
2 - fence part;
3 - chip groove.
The dies are round (sometimes called dies), sliding dies (clamp dies) and special for cutting pipes.
To work with round dies, cranks (lever holders) are used, which are a frame with two handles, into the hole of which a die is placed and is kept from turning by means of three locking screws, the conical ends of which fit into recesses on the side surface of the dies.
The clamps for sliding dies are an oblique frame with two handles. Half dies are inserted into the frame hole. The half-dies are installed to the required size using a special pressure screw.
To cut threads with a tap, perform the following techniques. The part is secured in a vice, the roughing tap is lubricated and, in a vertical position (without distortion), is inserted into the hole to be cut. Putting a knob on the tap and lightly pressing it against the part with your left hand, carefully turn the knob clockwise with your right hand (when cutting a Left thread - counterclockwise) until the tap cuts into the metal and its position in the hole becomes stable. Then the knob is taken with both hands and smoothly rotated (Fig. 4.17, a). After one or two full revolutions, the return movement of the tap by about a quarter of a turn breaks the chips, this greatly facilitates the cutting process. Having finished cutting, unscrew the tap from the hole (by rotating the knob in reverse side) or pass it through.
The second and third taps are lubricated and inserted into the hole without a driver. After the tap is correctly installed on the thread, put on the driver and continue threading.
When cutting deep holes, it is necessary to completely unscrew the tap 2-3 times during the cutting process and clear it of chips, since excess chips in the grooves can cause breakage of the tap or thread failure.
Before cutting external threads with dies, ground to required diameter the rod is secured in a vice. At the very end of the rod, a small chamfer is removed at an angle of 45° (Fig. 4.17.6). The rod must have a clean surface, since cutting threads through scale or rust will greatly wear out the dies.
Rice. 4.17. Working techniques for cutting threads with hand taps (a) and dies (b, c).
To obtain the correct thread, the diameter of the rod is usually made 0.2-0.4 mm less than the required thread diameter.
At the end of the rod, fixed in a vice so that its end protrudes from the jaws 15-20 mm more than the length of the section being cut, place a die fixed in the driver and, with slight pressure, begin to cut the thread, turning the driver with short movements clockwise (Fig. 4.17, c). The first 1.0-1.5 threads are usually cut without lubrication, since the die grips dry metal more easily; then the rod is lubricated with natural drying oil and the crank or clamp continues to be rotated one or two turns to the right and half a turn to the left to break the chips.
At the beginning of cutting threads with dies, it is necessary to apply some pressure on the die (during the working stroke), avoiding its distortion. During the cutting process, the pressure on both hands should be even.
During the cutting process, the sliding dies in the die should be pressed only at the beginning of the passage; after passing along the entire length of the thread, the dies are screwed together (or, as they say, “driven”), then the dies are tightened with a screw again and the threads are threaded a second time.
If it is necessary to obtain an accurate and clean thread, cutting is done with two dies - roughing and finishing.
Mechanized thread cutting is carried out hand drill or an electric thread-cutting machine, as well as on a drilling or threading machine. This work requires special attention and caution, especially when using a drill and an electric or pneumatic machine.
Hand drills cut threads with a diameter of up to 6 mm, and productivity is tripled compared to working with a wrench. The use of electric or pneumatic machines increases labor productivity by almost 5 times.
When cutting threads with a drill or machines, the tap is clamped in the chuck and special attention is paid to ensure that there is no misalignment of the tap relative to the axis of the hole.
In plumbing and assembly work, perhaps the most common connection is a threaded one, so every mechanic must not only be able to cut a thread, but also know what type of connection this or that type is intended for.
Tapping is the formation of threads by removing chips (as well as by plastic deformation - rolling) on external or internal surfaces. Screw thread cutting is one of the most common metalworking operations. A rod with an external thread is called a bolt, and a part with an internal thread is called a nut.
Threads can be single-start, formed by one helical line (thread), and multi-start, formed by two or more threads. According to the direction of the helical line, the threads are divided into right and left.
Thread pitch is the distance between two points of the same name on adjacent thread profiles, measured parallel to the thread axis.
Outside diameter - the greatest distance between two outermost points, measured in a direction perpendicular to the axis of the thread.
Inner diameter - the smallest distance between the outermost points of a thread, measured in a direction perpendicular to the axis.
According to the shape of the thread profile, they are divided into triangular (universal); trapezoidal and rectangular, designed for parts that transmit motion (lead screws, machine support screws, etc.); persistent, necessary in mechanisms that operate under high one-sided pressure (for example, in presses); round – very wear-resistant regardless of operating conditions, most often used during installation water fittings(Fig. 48).
Rice. 48. Types of thread: a – triangular; b – trapezoidal; c – rectangular; g – persistent; d – round; e – right; f – left.
Thread cutting, like almost any metalworking operation, can be done manually or mechanically. Our further conversation will mainly concern manual method perform this operation.
Cutting internal thread is preceded by drilling a hole and countersinking it, and it is very important to choose the right drill of the required diameter. It can be approximately determined by the formula:
dst = D – P,
where d sv – required drill diameter, mm;
D – outer diameter of thread, mm;
P – thread pitch, mm.
If the drill diameter is chosen incorrectly, then defects cannot be avoided: if the hole diameter is larger than the required one, the thread will not have full profile; with a smaller hole size, it will be difficult for the tap to enter it, which will lead either to thread failure or to jamming and breakage of the tap.
The internal thread cutting algorithm is as follows:
– mark the workpiece and either place it on a workbench or secure it in a vice;
– drill a hole (through or to the required depth) and countersink it approximately 1 mm with a 90 or 120° countersink;
– clean the hole from chips;
– select a rough tap of the required diameter (see Fig. 8, a, b), with the required pitch and type of thread, lubricate its working part with oil and install it with the intake part in the hole, check its position relative to the axis of the hole using a square, put it on square the shank of the knob and slowly, without jerking, rotate the tap clockwise until it cuts into the metal of the workpiece by several threads;
– further rotation of the tap should be as follows: one or two turns clockwise, then 1/2 turn counterclockwise (to crush chips). In this case, the tap is rotated clockwise with downward pressure, and counterclockwise - freely;
– thread cutting until the working part of the tap fully enters the hole;
– unscrew the rough tap from the hole and continue cutting the thread with a middle tap and then with a finishing tap (the finishing tap must be screwed into the hole without a driver. The driver is put on its shank only when the tap passes correctly along the thread).
The procedure for cutting threads in blind holes has some features: firstly, the depth of the hole for the blind thread must be drilled 5-6 threads more than what is provided for in the drawing; secondly, after a series of two or three working and reverse revolutions, the tap should be turned out of the hole and the hole cavity should be cleared of chips.
The quality of the cut thread is checked visually: to ensure that there are no burrs or torn threads, and the accuracy of the thread can be checked using threaded plug gauges for through holes and a control bolt for blind holes.
The main reason for defective parts when cutting internal threads is the breakage of the tap as a result of its incorrect selection or non-compliance with the cutting technique. In this case, fragments of the tap remain in the hole. There are several ways to extract them.
Firstly, if there is a protruding part of the tap left, you can grab it with pliers or a hand vise and turn it out of the hole.
Secondly, if there is no protruding part, then you can insert a three-pin plug into the grooves and, by rotating it counterclockwise, unscrew the tap.
In both the first and second cases, before you begin to remove the tap fragments, kerosene should be poured into the hole along the grooves.
Thirdly, if the tap is made of carbon steel, then the part (together with the fragments) must be heated red-hot, cooled slowly, a hole must be drilled in the fragment into which a special cone-shaped tap with left-hand thread must be screwed, and the fragments of the broken tap must be carefully unscrewed.
Fourthly, if it is not possible to heat the part (for example, the part is too large), then you can weld an electrode or a broken shank to the broken tap and unscrew the fragments.
Fifthly, there is chemical method removal of fragments. If the part in which the thread was cut is made of an aluminum alloy, then the fragments can be etched out with a solution of nitric acid: acid is poured into the hole through the grooves of the tap and a piece of iron wire is lowered there (iron in this case plays the role of a catalyst). After 8–10 minutes, the spent acid is removed with a pipette, a new portion is poured in, and so on until complete destruction metal of the tap, after which the hole is washed. This process is quite lengthy, takes several hours, but in this case the part does not receive defects, and after removing the fragments it is suitable for further use.
When cutting external threads, it is important to choose the diameter of the rod on which the cutting will be performed. At incorrect selection here, as in the case of internal threads, defects are possible: the diameter of the rod is less than the required one, resulting in an incomplete thread profile; When cutting a thread on a rod with a diameter larger than required, due to the high pressure on the die teeth, either thread failure or breakage of the die teeth is possible. In order not to make a mistake in selecting the diameter of the rod, you need to know a simple rule: its diameter should be 0.1 mm less than the outer diameter of the thread.
The procedure for cutting external threads is as follows:
– select a workpiece of the required diameter, secure it in a vice and at the end of the workpiece intended for threading, remove a chamfer 2–3 mm wide;
– fix the die (round or sliding) in the die-holder collar with stop screws so that the marking on the die is on outside(see Fig. 8, c, d, e, f, g);
– lubricate the end of the rod (workpiece) with machine oil and place a die on it strictly at an angle of 90° (the marking on the die should be at the bottom);
– pressing the die firmly against the workpiece, rotate the die holder handle clockwise until the thread is cut to the desired length. Rotational movements should be carried out in this order: one or two turns - clockwise, 1/2 turn - counter-clockwise;
– after cutting the thread to the required distance, remove the die from the workpiece using reverse rotational movements.
When cutting threads on pipes intended for laying pipelines, the order of rotational movements of the die holder has one feature. At the beginning of the thread, as usual, one or two turns forward (clockwise) and 1/2 turn back (counterclockwise), and when cutting the last few threads, you should not turn back. The thread cut in this way has a so-called run-out, that is, the last threads of the thread are cut to a shallower depth, which contributes to better locking of the pipeline.
To cut a thread to a specific, fixed length, you can proceed in two ways. Or periodically take measurements of the cut threads measuring instruments, or use a die holder with a guide flange and bushing: put the die holder on the workpiece until the die stops, unscrew the bushing to the required thread length and secure; during rotational movements of the die holder, the flange will be screwed onto the bushing, dragging the die along with it.
If it is necessary to cut particularly precise external threads on cylindrical workpiece with a diameter of 4 to 42 mm and in increments of 0.7 to 2 mm, then thread-rolling dies can be used instead of conventional ones (Fig. 49).
Rice. 49. Thread rolling die: 1 – body; 2 – knurling rollers with thread.
In addition to the fact that such dies produce cleaner threads, they are also more durable (metal fibers are not cut off during such an operation, but undergo plastic deformation and are, as it were, compressed).
The quality of the cut external thread is checked by external inspection to detect broken threads or burrs. To check the accuracy of the thread, use a control nut: it should screw on without effort, but have no play (swing).
From the book: Korshever N. G. Metal work
If it is necessary to manually cut external threads for pipe connection When making a stud or bolt for a nut with a metric thread, a simple but quite effective tool is most often used - a die. The thread preparation process is quite simple if you follow the instructions below exactly.
Preparing to cut threads with a die
Before cutting a thread with a die on a rod or pipe, you need to select a tool of the required diameter and pitch. The die itself is a nut with cutting teeth conical shape with holes for fastening in the collar. The dies can be solid, split, sliding, round, square or hexagonal.
First of all, you need to select the material and tool of the required diameter. Below is a table of correspondence between the most popular diameters for metric, imperial and pipe threads with an example of selecting a workpiece for an M6 thread
As we can see from the table, to cut an M6 thread we need a rod with a diameter of 5.8 mm. For larger thread diameters, there is an acceptable minimum and maximum value for the rod thickness. You cannot violate it - if the diameter is exceeded, you risk breaking the die; if the value is less than the minimum, you will get a weak thread.
Before you start cutting external threads with a die, in addition to the tool itself and the workpiece, you need a driver suitable in shape and size, as well as machine oil. After this, you can proceed directly to the cutting itself.
Thread cutting order
Before cutting, it is necessary to chamfer the outer part of the pipe or workpiece at an angle of 45°. This is necessary to facilitate the first turns and fix the die.
Further actions:
- Secure the pipe or workpiece in a strictly vertical position. The best option To avoid distortions, use a bench vice, but you can also use a gas wrench.
- Lubricate the tool with oil.
- Place the die on the head of the rod in a strictly horizontal position and start with the first few circles.
- If there is obvious misalignment in the first laps, remove the die, tap the workpiece and start again.
- When rotating on the first turns, simultaneously press evenly on the handles of the driver to begin the cutting process.
- After the first few turns, check that the thread is cut correctly. This can be done by checking the horizontality of the dies and the knob, which can be checked with a level. Next, when correct position tool, you can continue cutting pipe threads with a die to the entire required length.
- When approximately the middle of the length is reached, the pressing force can be weakened, then the self-tightening process begins.
- After one or two turns, it is necessary to turn the die half a turn back to remove chips.
- After cutting to the desired length, simply return the tool back along the finished thread.
It is necessary to take into account that the die can have several numbers, most often 2. In this case, after cutting the rough thread, it is necessary to go through each of the numbers in turn to finally form the thread profile.
Often, when performing home repairs, it becomes necessary to make threads - external or internal. In order not to involve specialists in this, but to do it yourself, you need to purchase special tool. Cutting with dies and taps does not require any special skills or abilities. It is enough to know their types and technical parameters.
A little about the features of the tool
The first step is to decide on the type of thread. It is divided into 2 main types: metric and inch. The first one with the left direction is most often used. In order not to measure the distance between the grooves, you can find out the type by the shape. Metric thread in cross-section it is an equilateral triangle, and an inch one is an isosceles triangle.
Which products use a certain type of thread? Fasteners use metric, while water pipes use inch. In addition, you need to take into account the following factors that influence the process of cutting connecting elements.
- If the thread is intended to be connected to a finished part, its geometric dimensions must be appropriate.
- When manufacturing complete fastening units, it is recommended to use the metric type.
- The diameter of the workpiece must be different from the thread size. For the outside - to a lesser extent, for the inside - to a greater extent.
Self-cutting threaded connections performed using dies and taps. They are made of high-strength steel with a standardized working fluid size.
Outdoor
The die is a nut with internal slots and external clamps for turning the tool. She may be various shapes- round, square or hexagonal. If the work is carried out at home, you will need a vice to fix the part.
Before performing work, the main thing is to choose the correct diameter of the workpiece. It should be 0.2-0.3 mm smaller than the size of the future thread. For metric, you can use the data from the table.
The workpiece should be prepared in advance. If its cross-section is not a circle, you need to turn it. Then a conical chamfer is removed from the end part to mark the first turn of the thread.
- Having secured the workpiece in a vice, the correctness of its location is checked.
- Installing the die into the die holder. Its surface must be in the same plane as the surface of the workpiece end.
- The first turn is performed with little effort. It is important to make turns in the right direction.
- Having reached the lower limit, the die must be turned in the opposite direction.
One such pass will not be enough to form a thread with good geometry. It is recommended to repeat the procedure 3-4 times until the die is freely screwed onto the workpiece. To check the quality, tighten a nut of the appropriate diameter. If effort is observed, it can be processed outer part workpieces with fine-grain sandpaper.
Internal
Taps must be used to form internal threads. They are a cylinder with an external notch. If you need to process small parts with a diameter of up to 20 mm, you can use manual view tap. For large sizes necessary mechanical restoration using .
Best to use full set taps. It includes three parts that are designed for roughing, intermediate and final shaping threaded holes. If you have a drill in your home kit, it is recommended to purchase taps with a tail mount.
The workpiece should be fixed. If this is a large-sized part, this can be done using clamps. For small products, a vice is used. Having secured the part, its location relative to the tap is checked. The axis of the latter must be strictly perpendicular to the plane of the workpiece. The first pass will be made with great effort. Do not forget that for this you need to use a tap for rough processing. Having formed the primary thread, a tap is used for the intermediate thread. And only after this can we begin final processing.
To reduce friction, specialists use solid oil or a similar product. In this way, mechanical damage to the part can be avoided. The formed thread must have the same geometry along its entire length. Otherwise, the connection strength may be lost.
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External thread cutting. Diameters of threaded rods when cutting with dies.
Before cutting a thread, it is necessary to select the diameter of the workpiece for this thread.
Slicing die thread, it must be borne in mind that when forming a thread profile metal products, especially steel, copper, etc., stretches and the product increases.
When choosing the diameter of a rod for external threads, you should be guided by the same considerations as when choosing holes for internal threads. The practice of cutting external threads shows that best quality
threads can be obtained if the diameter of the rod is slightly smaller than the outer diameter of the thread being cut. If the diameter of the rod is less than required, the thread will be incomplete; if it is more, then the die either cannot be screwed onto the rod and the end of the rod will be damaged, or during operation the teeth of the die may break due to overload, and the thread will be torn off.
In table Figure 27 shows the diameters of the rods used when cutting threads with dies. Table 27
Diameters of threaded rods when cutting with dies
The diameter of the workpiece should be 0.3-0.4 mm less than the outer diameter of the thread.
When cutting a thread with a die, the rod is secured in a vice so that the end of the vice protruding above the level of the jaws is 20-25 mm longer than the length of the part being cut. To ensure penetration, a chamfer is filed at the upper end of the rod. Then a die attached to the die is placed on the rod and with slight pressure the die is rotated so that the die cuts in approximately 0.2-0.5 mm. After this, the cut part of the rod is lubricated with oil and the die is rotated in exactly the same way as when working with a tap, i.e. one or two turns to the right and half a turn to the left (Fig. 152, b). Rice. 152.
Technique for cutting threads with a die (b)
To prevent defects and breakage of teeth, it is necessary that the die fits onto the rod without distortion.
