The drilling and countersinking operations described above precede cutting internal thread. A thread is a helical groove of constant cross-section on the internal or external cylindrical surface; in the first case, the thread is called internal, in the second - external. Before describing the thread cutting process, we will briefly describe its main types.
According to the direction of the helix, the thread is divided into right and left. The thread profile is the cross-section of its turn in a plane passing through the axis of the cylinder on which the thread is cut. The main parameters of the thread are shown on 52. The profile shape is as follows: triangular (shown on 52), rectangular, trapezoidal, persistent (with a profile in the form of an unequal trapezoid) and round. In metric threads, the angle of the triangular profile is 60°, and the thread parameters are expressed in millimeters. For example, the designation M20x 1.5 is “translated” as follows: M - metric thread, 20 - outer diameter in mm, 1.5 - pitch in mm. There are other thread systems - inch and pipe.
But let's get back to threading. Let's start with the internal one. It is cut with a tap, the tail part of which is secured in the collar. For through holes, use a tap with a intake (bottom) part on the first 4-5 threads, which guide the movement of the tap along the walls of the hole. For blind holes, taps with a shorter chamfer (2-3 threads) are needed so that the effective (cutting) thread zone reaches almost to the bottom of the hole.
For manual thread cutting, taps are usually produced in sets that include 2-3 tools: roughing, semi-finishing and finishing. The first and second ones pre-cut the thread, the third ones give it the final size and shape. This step-by-step thread cutting significantly reduces the cutting force. Taps are distinguished by the number of marks on the tail: a rough tap has one mark, a semi-finish tap has two, a finishing tap has three or none. The double set includes roughing and finishing taps.
Of no small importance is right choice the diameter of the drill used to drill a hole for an internal thread, and the diameter of the rod for an external thread. The diameter of the drill (and rod) should be slightly smaller than the outer diameter of the thread.
Internal thread cutting is done as follows. The workpiece (part) with a drilled hole is secured in a vice so that the axis of the hole is strictly vertical. The intake part of the rough tap is inserted into the hole and its installation is checked using a square. The surface of the hole and the cutting part of the tap should be lubricated with a cutting fluid (machine oil for steel, kerosene for cast iron). A knob is placed on the tail part of the tap.
With your left hand you press the driver against the tap, and with your right hand you turn it until it cuts several turns into the metal. After this, take the knob with two hands and begin to rotate it slowly in this mode: 1 - 1.5 turns clockwise, 0.5 turns counterclockwise (53). A reverse turn is needed to break the chips.
After cutting the thread with a rough tap, semi-finish and then finishing taps are placed, and the same manipulations are performed with each of them as with the rough tap. All the time, using a square, you need to control the position of the tap axis relative to the surface of the workpiece.
For cutting external thread use dies with a die holder. The same tool is used to update damaged threads on bolts, screws and studs. The cutting thread of the die has a intake (initial) part on one or both sides. In the first case, the die must be adjacent to the stop of the die-holder with the opposite side (without the intake part). To avoid distortion of the thread, chamfer the end of the rod (after securing it vertically in a vice). Then the die is placed on the end of the rod perpendicular to its axis and, lightly pressing right hand onto the die holder, turn it with the left hand (54) until the die is securely embedded in the metal. This is achieved after cutting in the first threads. After this, pressure is no longer needed, you just need to slowly rotate the die. The cutting process can be facilitated, while simultaneously increasing the cleanliness of the thread, by dropping a few drops of machine oil or cutting fluid onto the rod and die.
External thread cutting continues until the die passes the entire required length of the rod. After this, the die is rolled off the rod, cleaned of chips and grease, and the cut thread is checked with a standard nut. Cleaning of chips should be done with a brush, not with your hands, to avoid cuts from the sharp cutting edges of the tap or die.
When cutting threads with taps and dies (both manually and on metal cutting machines) or using special power tools not only a layer of material is removed from the surface of the workpiece, but also plastic deformation of the outer part of the treated surface occurs. This deformation is accompanied by extrusion of the workpiece material from the thread cavity into its protrusions. This phenomenon must be taken into account when determining the diameter of the rod or hole for threading. Therefore, it is most advisable to determine the dimensions of rods and holes for threading using reference tables, in which these dimensions are given taking into account all the factors that arise during cutting.
In practice, when cutting threads, the diameter of the hole is taken to be equal to the nominal diameter of the thread, reduced by its pitch. For example, when cutting a metric M10 thread, the hole diameter should be respectively 1.0... 1.5 mm, i.e. should be 8.5 mm.
When cutting external threads, the diameter of the rod should be 0.1... 0.2 mm less than the nominal diameter of the thread, depending on its size.
When rolling threads, the diameter of the rod is selected based on the average diameter of the thread, which must be specified in the thread processing task, or is determined using special tables. To facilitate insertion of the die, it is necessary to chamfer the top of the rod with an angle of approximately 60°.
Editing the processing of external and internal threaded surfaces
1. Thread cutting must be done with the die or tap generously lubricated with machine oil.
2. When cutting threads, you should periodically cut off the resulting chips by moving the tap or die back 1/2 turn.
3. After cutting a thread on a rod or hole, you need to check its quality:
external inspection - avoiding scuffing and torn threads;
threaded gauge (or standard bolt, nut) - the passage part of the gauge (bolt, nut) is screwed on by hand, rolling in the bolt-nut pair is not allowed.
Typical defects when cutting threads, reasons for their occurrence and methods of prevention
|
Method of warning |
||
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Torn thread |
The diameter of the rod is larger than the nominal diameter, and the diameter of the hole is smaller. Thread cutting without lubrication. The chips are not crushed by the reverse stroke of the tool. Dull cutting tool |
Carefully check rod and hole diameters before cutting threads. Lubricate the cutting area generously. Strictly follow the thread cutting rules. Monitor status cutting edges tools and when they become dull, replace the tool |
|
Incomplete |
The diameter of the rod is less than required. Hole diameter is larger than required |
Carefully check the diameters of the rod and holes for threading |
|
Thread misalignment |
Misalignment of the die or tap during cutting |
Carefully control the position of the tool when plunging |
|
Seizures on the thread surface |
Small rake angle of the tap. Insufficient length of the fence cone. Severe dullness and improper sharpening of the tap. Low quality coolant. High viscosity of the workpiece material. Using excessively high cutting speeds |
Use taps of the required design and geometry. Use appropriate coolant. Select a rational cutting speed using reference tables |
|
Failure due to caliber plugs. Backlash in the screw-nut pair |
Breaking the thread with a tap if it is installed incorrectly. Large tap runout. Removing chips with a tap when turning out. Application increased speeds cutting Use of random cutting fluids. Incorrect adjustment of the floating chuck or its unsuitability |
Correctly (without wobbling) install the tool. Choose normal speeds cutting Use the most effective coolant for the given processing conditions. Choose a working cartridge |
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End of table. 3.3
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Rules for cutting external threads
1. Before cutting threads, you should check the diameter of the rod (bolt, stud, screw); it should be 0.1... 0.2 mm less than the nominal thread diameter.
2. It is necessary to file the chamfer at the top of the rod (if it is not on the workpiece). When filing a chamfer, you need to ensure its concentricity relative to the axis of the rod, as well as its diameter, which should not exceed the internal diameter of the thread along the end surface. In addition, the angle of inclination of the chamfer relative to the axis of the rod should not exceed 60°. The rod should be secured in a vice firmly and perpendicular to the jaws. The perpendicularity of the rod must be checked using a square.
3. It is necessary to strictly monitor the perpendicularity of the end of the die to the axis of the rod when cutting into the die.
4. Before rolling a thread on a rod, it is necessary to check its diameter; it should be equal to the average diameter of the thread being cut.
5. When cutting threads on gas and water pipes Special attention Care should be taken to maintain the length of the cut part for couplings and bends.
When cutting internal threads, the following must be observed: right-handed.
1. Before cutting threads, you should check:
correspondence of the hole diameter to the size of the thread being cut.
It must match the thread table data;
hole depth for cutting blind threads. It must correspond to the size indicated on the drawing.
2. When inserting a tap, it is necessary to ensure that its axis is perpendicular to the upper plane of the workpiece in which the thread is being cut.
3. When cutting threads, you should use the entire set of taps: the first - rough; the second is semi-finish; the third is finishing.
4. When cutting threads in a blind hole, it is necessary to periodically clear it of chips.
5. Particular care should be taken when cutting small diameter threads (5 mm or less) to avoid breakage of the tap.
6. When cutting threads with a machine tap on a machine, it is necessary to secure it in a safety chuck.
Typical defects when cutting threads, the reasons for their occurrence and methods of prevention are given in table. 3.3.
If it is necessary to manually cut external threads for a pipe connection, make a stud or bolt for a nut with metric thread, most often a simple but quite effective tool is 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 required diameter and step. The die itself is a nut with conical cutting teeth 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, with the correct position of the tool, you can continue cutting the pipe thread 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 into desired length simply return the tool back to 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.
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 is the outer diameter of the thread;
dcp - average thread diameter;
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, having a profile like the inch one, but smaller in 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 to cut threads in holes, and dies are used to cut external threads.
A tap is a cutting tool that 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.
Longitudinal grooves are used to form the cutting edges of the tap and release chips. 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.
The correct choice of hole diameters for threading is of great importance. Choice drill diameters under this type and the thread size 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 dies.
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 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 a thread with a tap, the following techniques are performed. 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 the external thread with dies, the rod turned to the required diameter 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).
For getting 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.
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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. As a result, the pressure on the surface of the die increases, which leads to heating and adhesion of metal particles, so the thread may become torn.
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.
Checking the cut internal threads is done with thread plug gauges, and external threads are checked with thread micrometers or thread ring gauges.
