How to cut a thread with a cutter on a lathe

Cutting threads on a lathe with a cutter

» Machine » Thread cutting on a lathe with a cutter

Thread cutting on a lathe is one of the most common operations performed on thousands of lathes every day. Cutters, thread-cutting heads, dies and taps are used as working tools. Carving using a cutter requires special skill and knowledge.

Types of incisors

types of incisors: a - straight; b - round; c - curved

The design of the lathe allows you to cut internal and external threads. For this purpose, many varieties of incisors have been created, belonging to three large groups:

• prismatic;
• round;
• rod-shaped

Rods are the simplest type of cutting tool. This is a rod (of any cross-sectional shape) with a working head. The profile is determined by the shape of the head. One of the varieties of rod cutters is with soldered carbide working edges. They are more resistant to abrasion and do not require frequent sharpening. Round and prismatic belong to the shaped category.

Prismatic cut only the outer side. Compared to rod ones, they are able to work with large surfaces. The cutter is held in a dovetail holder. Capable of more regrinding than rod ones.

Round ones make external and internal threads. They are more convenient to cut and have a wider range of applications than prismatic ones. They can be sharpened more times. They are installed in the holder relative to the end and the hole.

Threading technique

The cutter moves evenly along the workpiece and with its tip cuts a line in the shape of a screw. The inclination of the line to the axis perpendicular to the movement of the cutter is called the helix angle. This indicator depends on:

• speed of rotation of the part in the machine;
• features of the cutter feed.

As the cutter is inserted into the blank, it becomes covered with a helical groove. The thread provides reliable fastening, sealing or movement of elements in the planned direction.

Depending on the configuration of the blank, the name of the fastening is given: cylindrical or conical. The profile, which is also the diameter of the projection onto the thread plane, is selected taking into account the purpose. The following profiles are most applicable:

• rectangular;
• trapezoidal;
• acute-angled.

They are cut single-pass or multi-pass. The latter are created by several grooves located at equal intervals from each other, the former by a single groove.

The properties of a thread are determined by such concepts as pitch and stroke - the distance between identical elements of the thread. It is equal to the product of the number of entries per step.

Slicing rules

a thread template is used to check that threads are cut correctly

1. The cleanliness of the thread is determined by the position and direction of movement of the tool. There is a template to use the tool correctly. It is placed parallel to the center of the machine on the workpiece. The projections of the instrument and the template are superimposed and the lumen is examined. The cutter is placed exclusively along the center line of the machine.
2. Internal threads are cut with curved tools (with a small hole diameter). You can use a flat tool by inserting it into the mandrel. External - often bent, in rare cases even. The choice of cutter depends on the type of metal and thread parameters.
3. The rules for placing the working edge of the tool depend on the planned profile. Triangular is performed at an apex of 60 or 55 degrees. (for metric or inch).

   Since errors are sometimes possible during the movement of the caliper, use a tool with a tip of 59 degrees 30 minutes.

4. The rake angle of the tool is selected in accordance with the material of the part within the range of 0 - 25 degrees. The harder and more brittle the material, the smaller the angle. So, for steel blanks, use a tool with an angle of 0 degrees; high-alloy steels can be cut with an angle of up to 10 degrees.

5. The depression is made smooth, rounded or with a notch; the top of the cutter is selected to match the shape.
6. The lateral angles of the tool at the rear must be such that during cutting the lateral edges do not come into contact with the thread surface. Usually they are equal on the right and left: when the thread lift angle is above 4 degrees - from 6 to 8 degrees and from 3 to 5 degrees, if the angle is up to 4 degrees.
7. Internal threads are cut directly after boring or drilling. Due to the fact that the metal is deformed during operation, the diameter of the hole is taken to be slightly larger than the internal diameter. Therefore, up to 0.02 is added to the calculated indicator for brittle metals and up to 0.4 for viscous ones.
8. Sometimes the thread should end with a groove.

   The internal diameter of the groove should be made 0.3 mm smaller than the same characteristic of the thread.

9. In order to make a high-quality thread at the end, it is necessary to make a shoulder of up to 3 mm without changing the diameter. This protrusion marks the final pass of the cutter. At the end of cutting, the ledge is removed.
10. Roughing passes are performed at speeds of up to 30 meters per minute, finishing at speeds of up to 55 meters per minute.

    When processing cast iron, the speed is no more than 25 meters per minute. Carbide tools cut steel at speeds of up to 150 m/minute. When the thread pitch is less than 2 mm, increased speeds are used, when the pitch is from 6 mm, reduced speeds are used (exact indicators can be found in special tables).

11. The thread is cut in several passes; at the end of each pass, the tool is set to the initial position.

12. When it is necessary to apply an external thread to a long workpiece, the caliper is manually set to the initial position by disconnecting the connector nut. To make the next cutting pass, you must get exactly into the groove.

Parameters and corresponding GOSTs

Instructions for cutting threads in the video:

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Thread cutting on a lathe: classification, diagrams

Thread cutting on a universal lathe is a common operation, especially in repair departments. The presence of a lead screw and a large feed range make it possible to cover basic needs without readjustments and special tools - a few threaded cutters are enough. The possibility of mechanized cutting with manual, machine taps, and lechers simplifies setup and speeds up the process while maintaining acceptable quality of fasteners.

Classification

In practice, cutters for external and internal threads with a rectangular section holder are used. Less common are disc, prismatic, sharpened along the front surface. The working profile of all corresponds to the dimensions of the screw groove. In the direction of the cut spiral, left and right ones are released.

There are solid and prefabricated instruments. The first ones are mainly made of high-speed steel, small section or disk. The bulk is equipped with cutting plates secured by soldering with refractory solder or mechanically, allowing replacement when worn.

Threaded cutters: external (Fig. 1), internal (Fig. 2)

Materials

For the manufacture of the cutting part are used:

• high-speed steels;
• hard alloys;
• mineral ceramics;
• superhard tool materials (STM).

The former are used for thread cutting of steels, non-ferrous metal alloys, and plastics. They are distinguished by high strength, thermal conductivity, but lower, compared to others, hardness, red-hardness, wear resistance, which limit the cutting speed.

The largest proportion of thread cutters used are those equipped with carbide inserts. This is due to high durability, hardness, sufficient strength and rigidity, and reasonable cost. Processing productivity is 2-3 times higher than with rapid.

A wide range allows you to select the optimal grade for processing in most cases.

Ceramics are relatively cheap, quite fragile, and are used for processing fine pitch threads of steel and cast iron parts, with a rigid AIDS system, with limited allowance removal per pass.

STMs based on polycrystalline diamond (PCD) or cubic boron nitride (CBN) are extremely hard, heat-resistant, but expensive. Indispensable for precision work on difficult-to-cut materials. PCD is used for cutting copper, aluminum, and tungsten carbide. CBN work on hardened steels and hardened cast irons. Successful application requires high rigidity and smooth running of the equipment.

Decoding the writing of threads

Regulatory documents: GOST, OST, MN for a specific type contain samples of conditional recording.

Graphic materials are designed in accordance with the instructions of GOST 2.311-68 “Image of threads”.

A typical designation structure contains:

• the literal part defining the type;
• numbers corresponding to the nominal size in millimeters or inches;
• pitch (mm) is indicated only as fine, after the “×” sign;
• for multi-start ones, instead of the previous paragraph, the stroke (mm) is given, then the step in parentheses;
• direction: right is the default, left is LH;
• tolerance range or accuracy class;
• make-up length other than normal.

Example 1: М16×1.5LH–6H. Explanation:

• M – metric cylindrical;
• 16 – nominal diameter, mm;
• 1.5 – fine pitch, mm;
• LH – left;
• 6Н – tolerance range, where 6 – degree of accuracy; H – main deviation. Capital letters are used for internal (nuts), hence the threads in the hole.

The screw-in length is not indicated, which means it is normal.

Example 2: G1/2–A

• G – cylindrical pipe;
• 1/2 – thread size, inches; corresponds to the internal diameter of the pipe;
• A – accuracy class.

The designation options are illustrated below.

Thread cutting using lathe equipment

Shaping on the machine is carried out by copying the working profile of the tool onto the part along a helical line. Translational movement is transmitted to the cutter, tap, die, comb. In combination with the rotation of the workpiece, a screw movement is obtained, the tool surface coincides with the cut surface.

As a rule, cutting small batches of fasteners and fittings up to M36 is carried out using taps and lechers. It is more profitable to produce large orders using specialized machines. Large-diameter threads, running threads, power threads, and precision threads are processed with cutters on universal lathes when CNC models are not available or the production program is insufficient.

Cutting internal and external threads with a cutter

Threads with high coaxiality to other surfaces, transmitting movement and force are performed with a cutter. The rotation of the spindle is connected kinematically to the lead screw, which moves the support with the tool holder.

The general procedure includes:

• Grooving the surface along the length of the cut, forming a groove for the tool to exit.
• Selection, if necessary: ​​sharpening, finishing of the cutter with checking using corner templates.
• Setting modes on the machine, tuning the guitar to a pitch not provided by the box.

The movement of the cutter per revolution of the workpiece is equal to the step P or the move H for multi-start ones.

• Installation of the cutter according to the template.

•  Cutting for the number of passes selected from the directory.

Thread cutting of a batch of parts is divided into rough and finishing. For the latter, the tool is carefully sharpened. Threads with pitches greater than 2 mm are produced by lateral cutting. The left helical groove is obtained by switching the bit so that the lead screw rotates in the direction opposite to the spindle. The support with the cutter moves from left to right.

Average speeds when threading steel are 20–35 m/min with high-speed cutting tools, 100–150 m/min with carbide tools. Finishing strokes are performed at a speed increased by 50–100%. Internal threads are processed at 30% reduced conditions.

Using taps

The widespread grade R6M5 allows cutting workpieces with a hardness of up to 240 HB; taps made from tool alloy steels are used for “raw” parts. Carbide ones are rarely used, since the edges are chipped due to distortions and misalignment, which increase bending loads.

The range of typical sizes is limited to M36 – 42, G2. Large diameters with large pitches are processed with a set of 2, preferably 3, taps. The hole is bored slightly larger than the inner diameter of nut D1 (see Fig. 2), taking into account the bulging of the metal from the groove. When drilling, take into account the breakdown. Recommended values ​​are given in reference books.

There are several processing methods:

• Often, nuts smaller than M12 are cut by holding the knob with your hands. Strictly speaking, taking it is a violation of TB and can lead to injury. At the beginning of screwing, press the tap with the center of the tailstock for direction, then self-tightening occurs. Stop, unscrew in reverse.
• The tap is installed in the driver, rests against a bar fixed in the tool holder, and is supported by a center holder or rear center. Turn on low speed, cut on self-tightening. To eliminate runout of the turns, it is recommended to press the tap to half the working length until screwing, smoothly extending the quill.

• A swinging self-retracting tap holder is used.

• Large diameters are obtained by feeding the caliper along the lead screw, and the box is adjusted to the appropriate pitch. The tap is inserted into a mandrel clamped in the tool holder. A safety cartridge installed in the quill prevents the tap from breaking when reaching the bottom of a blind hole. Speeds up serial operations. Taps with staggered teeth are optimal for tough stainless and heat-resistant alloys. Recommended cutting speeds for steel are 3 – 15 m/min, for bronze, cast iron 4 – 22 m/min, work with cooling. For left-handed nuts, use tools with a left-hand thread, the rotation is opposite, the rest is the same.

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Thread cutting with cutters

The most common method of cutting threads on lathes is threading with thread cutters.

Design of cutters for thread cutting. The profile of the cutting part of the cutter must correspond to the thread profile. The cutting angle for metric threads should be 60°, for inch and pipe threads - 55°.

To avoid distortion of its profile when cutting a thread, thread cutters are sharpened along the front surface with a rake angle y = 0° and the top of the cutter is set at the height of the machine center line; on cutters for roughing passes, the rake angle is from 5 to 25°, depending on the mechanical properties of the material being processed; the clearance angle on the side surfaces of the cutter is from 5 to 10°.

Types of cutters

There are thread cutters for cutting external threads (Fig. 215, a) and cutters for cutting internal threads (Fig. 215, b). Both can be solid or inserted. The head of the thread cutter for internal threads must be

Rice. 215. Cutters for cutting triangular threads: a - external, b - internal

perpendicular to the axis of the cutter shaft. The length and cross-section of the rod depend on the diameter of the hole.

Innovative turner V.K. Seminsky uses special thread cutters to cut internal threads. For these incisors (Fig. 216), the head is rotated relative to the rod by

Rice. 216. Thread cutter for cutting internal threads designed by V.K. Seminsky

twisting the holder by 45°. This gives the cutter increased rigidity and ensures quieter operation compared to a conventional threaded cutter (Fig. 215, b).

Finishing pass

For finishing passes when cutting threads, spring holders and spring cutters are sometimes used to produce clean and smooth threads. Such a cutter, encountering a harder part of the metal on its way, is slightly pressed out and does not spoil the thread.

Rice. 217. Spring holder with threaded cutter

In Fig. 217 shows a spring holder 1. Bolt 2 is used to secure the insert threaded cutter 3 in the holder. The peculiarity of this holder is that it can work both as a spring and as a rigid one. This is achieved using screw 4: when the screw is tightened, the holder acts as a rigid one; when the screw is released, it works like a spring.

Rough cutting is done with a cutter fixed in a rigid holder, and finishing cutting is done with a cutter fixed in a spring holder.

Rice. 218. Installation of thread cutter: a - correct, b - incorrect

Installation of cutter

Install the thread cutter exactly at the height of the centers, otherwise the thread profile will turn out incorrect. In addition, the middle line of the cutter profile must be perpendicular to the axis of the part (Fig. 218, a).

These requirements remain in force when cutting not only external but also internal threads. If these requirements are neglected, the thread profile will be turned to the side (asymmetrical), as shown in Fig. 218, b.

The thread cutter is installed using a template, as shown in Fig. 219 (when cutting external threads) and in Fig. 220 (when cutting internal threads).

Correct installation

The correct installation of the thread cutter according to the template is checked as follows: apply the template to the cylindrical

surface of the part in the horizontal plane exactly at the height of the axis of the part, then insert the cutter into the cutout of the template and determine in the light whether there is a gap between its cutting edges and the cutout. If there is a gap, then it is eliminated by rearranging the cutter, after which the cutter is firmly fixed in the cutting head. When installing a cutter for cutting internal threads, the template can also be installed at the end of the part (see Fig. 220).

Fig. 219. Installing a thread cutter using a template when cutting external threads

Checking with templates

Rice. 220. Installing a thread cutter according to a template when cutting internal threads

Templates (see Fig. 219 and 220) are also used to check the correct sharpening of threaded cutters.

Similar materials

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Cutting threads on a lathe with a cutter and a die | mk-soyuz.rf

One of the well-known and universal metalworking methods is thread cutting on a lathe. This technological process is performed using special turning tools:

• incisors,
• dies,
• taps,
• threading heads.

The quality of the thread affects the reliability of the connection of various elements. Thread cutting is a job that requires precision, skill and experience.

Thread profiles

The shapes of the workpieces, depending on the type of surface, can be cylindrical or conical. Threads can be external (on the surface) and internal (in the hole of the blank). The type of threaded connection depends on the thread profile.

There are five types of profile:

• triangular,
• rectangular,
• trapezoidal,
• persistent,
• round.

The thread can be single-start or multi-start:

• single-thread - formed by a single threaded thread,
• multi-thread - created by two or more threads placed at the same distance from one another.

The main indicators of both types of thread are pitch and stroke. Thread pitch is the distance between the nearest threads and is measured along the center line of the workpiece being processed. The stroke of one thread is the gap between two peaks of one turn on the working surface of the product. The stroke is equal to the product of the thread pitch and the number of starts.

Cutting tool - die

A die is a cutting turning tool made in the shape of a nut. The holes drilled in it are made to remove chips. The tool consists of cutting elements located on each side and forming a fence cone. A die on a lathe forms threads on bolts, studs, screws and other metal products.

The most used types of dies:

• metric,
• left,
• pipe

Thread cutting with a die should be preceded by processing the desired area of ​​the part. In this case, the outer cross-section of the thread must be larger than the machined area of ​​the part. The difficulty of cutting a thread with a die is that it does not have a special lead, which makes it difficult to cut the initial turns smoothly.

You can make the task easier by making a small chamfer at the end so that its height coincides with the height of the thread profile. The size of the die must match the diameter of the part.

Then the die of the required size is fixed in a die holder fixed in the tailstock of the lathe. The thread cutting speed depends on the material for the workpiece: brass products are processed at about 15 m/min, cast iron - 3 m/min, steel blanks - 4 m/min.

At this processing speed, the die wears less. During the cutting process, each rotation of the die must be replaced by turning it back a third of the circle to clear the holes from chips.

How to cut threads on a lathe

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How to cut trapezoidal threads on a lathe

Thread cutting on a lathe is one of the most common operations performed on thousands of lathes every day. Cutters, thread-cutting heads, dies and taps are used as working tools. Carving using a cutter requires special skill and knowledge.