How to cut an inch thread on a lathe

Classification of threads

A thread is a helical line formed on the surface of a body of revolution by the apex of a protrusion of a certain shape. The distance between adjacent protrusions is equal to the thread pitch. The shape of the protrusion depends on its type. There is a depression between two adjacent protrusions.

  • Direction of the spiral cutter line:
  • right (it is lifted from left to right, and the bolt is screwed in clockwise);
  • left (screwing takes place counterclockwise).
  • The shape of the protrusion in the form:
  • triangle,
  • trapezium,
  • unequal trapezoid,
  • rectangle,
  • semicircle.
  • The outer surface of the part (cylindrical or conical).
  • Locations on the part (internal or external).
  • Number of visits (one, two, three);
  • Purpose (mounting and chassis).
  • with a profile angle:
  • 60⁰ (metric, tapered inch);
  • 55⁰ (tubular cylindrical, tubular conical);
  • stubborn;
  • round;
  • trapezoidal (angle 30⁰).
  • Trapezoidal thread is characterized by greater strength than rectangular thread, with less laboriousness of manufacture. The purpose of the trapezoidal thread is to convert rotary motion into translational.

    In addition, a thread called modular is used. It is used where rotation is transmitted from a worm to a worm wheel, the axes of which are at an angle of 90⁰.

    Threading technique

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

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

    When the cutter is buried in the blank, it becomes covered with a helical groove. The thread ensures secure fastening, sealing or movement of elements in the planned direction.

    Depending on the configuration of the blank, the name of the mount is given: cylindrical or conical. The profile, which is at the same time the cross section of the projection onto the plane of the thread, is selected taking into account the purpose. The following profiles are most applicable:

    • rectangular;
    • trapezoidal;
    • acute-angled.

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

    inch, thread, lathe

    Thread properties are defined by concepts such as pitch and stroke. the distance between the same elements of the thread. It is equal to the product of the number of calls per step.

    Decoding the spelling of the threads

    The thread is designated: М12х1.25–7Н. This means: metric internal (7H), diameter 12 mm, pitch. 1.25 mm, tolerance field 7H. For an external thread, the tolerance field is recorded: 6h. The coarse step value is not given in the designation (М16–8g). The letters LH (М16–8g – LH) are added to the left thread in the designation. GOST 8724-81 contains a table indicating the diameter and pitch of the required size.

    Methods for obtaining threads

    Threaded connections are used in mechanical engineering and construction. The use of threaded hardware allows you to connect parts of mechanisms and structural elements during construction by using bolts, nuts, screws, studs. Threaded products are also designed to transmit forces or movement in mechanisms such as a jack, gearbox, press, machine.

    Tapping scheme on a lathe

    Jacks and lead screws are made with trapezoidal thread.

    There are the following methods for making threads:

    • manually (tap or die);
    • on machines:
    • screw-cutting lathe (mod. 16K20);
    • thread rolling with rollers and flat dies;
    • milling (mod. RTS 161F4) to obtain threads with a large pitch;
    • grinding wheels with a given profile for the manufacture of fine and precise threads;
    • screw-cutting (mod. 1622);
    • for threading nuts (mod. 2064);
    • providing vortex threading using multi-cutter heads.

    Vortex cutting is provided by 4 cutters located in the rotating head from its own motor. This device is fixed to the lathe support. Due to the alternate plunge of the cutters, a high machining speed is ensured, since the cutters do not heat up significantly. This also provides a lower roughness of the resulting threaded surface and increased profile accuracy.

    How to cut a thread on a lathe with a cutter, the number of revolutions, dimension

    Threading 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 a working tool. Carving with a cutter requires special dexterity and knowledge.


    There are many types of threads. They differ in profile shape, location, direction of entry, step size and many other parameters.

    By the shape of the thread profile there are

    • Trapezoidal.
    • Rectangular.
    • Round.
    • Persistent.
    • Triangular: metric; inch; pipe.
    • Outside (like a bolt).
    • Internal (like a nut).
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    Along the surface on which the thread is made

    • Cylindrical.
    • Conical.
    • By operational purpose.
    • Fasteners.
    • Fastening and sealing.
    • Running gear.
    • Special.

    Equipment used

    The thread is cut by hand or on a screw-cutting lathe. When cutting by hand, a vice and a tool are used: dies and taps. You can cut a thread using a screw-cutting lathe 16K20 with a cutter, a tap, a die and other types of tools. They are used for cutting both external and internal threads.

    On the feed box of the 16K20 machine there is a table in which the positions of the handles are indicated in order to adjust the desired thread pitch. Setting the processing modes allows you to get a high-quality part. Cutter technology provides for:

    • its correct sharpening;
    • setting up the operating modes of the machine;
    • correct installation of the cutter in the center of the part using a fixture. a template;
    • measurement of the obtained dimensions with threaded gauges or gauges.

    When threading is not allowed: scuffing, crushing, torn threads, risks.

    Special attachment or separately driven swirl heads expand the functionality of the lathes.

    Turret lathes increase productivity by reducing tool change times.

    Using die heads

    Die heads are used for pipes and bolts. They are installed in the machine, then the required diameter is adjusted and moved along the pipe. They are able to carry out work with high precision.

    Universal heads allow you to install dies from different companies. It is also possible to lubricate the combs and adjust the dimensions, manual clamping is often found.


    Taps are the main tool for tapping internal threads in blind and through holes manually or mechanically. From the point of view of accessibility, the tap for threading is a relatively simple tool, the use of which is available to both amateurs and professionals, allowing to obtain, in an ordinary workshop, a standard thread that meets the requirements of GOST.

    Tap device

    This type of threading tool is a screw with straight and helical flutes required for threading and evacuation of chips generated in the process. The working area includes the gauging and cutting parts. There is a square tail section for fastening the tool in hand wrenches or chucks. The cutting process itself is the rotation of the tool in a pre-drilled hole (blind or through) by a reciprocating motion.

    Depending on the version, the tap device allows you to cut threads in one, two or three passes. Preparation of threads in more than one pass is carried out with sets of rough, medium and fine tools. Five-tool sets are available for cutting holes in particularly tough materials such as titanium alloy.

    How to tap tapered threads

    Threaded connections are simple, reliable, make it possible to adjust the tightening, as well as disassemble and assemble parts and mechanisms. They are widely used in various mechanisms, devices, devices.

    The thread is external (screw) and internal (nut). Distinguish between cylindrical triangular (sawtooth), conical triangular, rectangular, trapezoidal, thrust, and round threads. The most widely used is cylindrical triangular or, as it is also called, fastening thread drawing

    External threading:

    At home, it is performed manually with dies. The diameter of the rod for the external thread should be 0.3-0.4 mm less than the external diameter of the thread to be cut. Deviations from this rule are not allowed.

    Cutting the external thread with a round die is performed in the following order: at the upper end of the rod, a chamfer is removed, which ensures the plunging of the die into the metal;

    The rod is clamped in a vice vertically so that its protruding end is 20-25 mm longer than the length of the cut part (figure); a die fixed in the knob is placed on the rod and rotated with slight pressure so that the die cuts about 1-2 threads without skewing. After that, the rod is lubricated with oil and smoothly rotate the crank for 1-2 turns to the right and 1/2 turn to the left.

    Thread cutting with sliding prismatic dies (figure) is performed as follows: a chamfer is cut at the end of the rod; dies are installed in the die; clamp the rod in a vice; put on the stems on the rod and tightly move the nuts with a clamping screw; grease the dies and the rod with oil; the klupp is turned 1. 1.5 turns clockwise, then 1-4, 1-2 turns back and so on until the end of the thread; having cut the thread, screw the screw to the end of the rod, tighten the dies with a screw and pass the thread again; check the thread with a nut of the appropriate diameter; at the end of the work, the dies are removed from the die, cleaned of shavings, wiped and lubricated with oil; wipe the klupp.

    inch, thread, lathe

    Figure. Tapping with a die

    a. Tapping with a round die b. Tapping with prismatic dies

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    How to properly cut a thread with a tap: algorithm, recommendations, size tables

    The question of how to tap a thread arises when a pre-drilled hole must be prepared to accommodate a bolt, screw, stud, and any other type of threaded fastener. It is the tap in such situations that is the main tool that allows you to quickly and accurately cut an internal thread with the required geometric parameters.

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    Manual threading of internal threads is performed with metal taps complete with a knob


    Rules for working with taps:

    When threading in deep holes, in soft and viscous metals (copper, aluminum, bronze, etc.), the tap must be periodically unscrewed from the hole and the grooves must be cleaned of chips; you need to cut the thread with a full set of taps. rough, medium and fine.

    Medium and finishing taps are inserted into the hole without a wrench, and only after the tap has gone correctly along the thread, a wrench is put on the head and the threading continues; during the cutting process, you need to carefully monitor with a square so that there is no misalignment of the tap; the threading point should be lubricated with oil.

    Types of taps for threading

    Depending on the cutting method, there are the following types of taps. hand, machine-hand and machine. The manual ones are equipped with a square tail part for fixation in the crank, are available in 2 and 3-set.

    Machine-hand taps can be installed in both the driver and the chuck of the machine. For tapping in a through hole, they are produced with six threads on the working part, for blind holes. with three.

    Machine taps are designed exclusively for installation in the chuck of the machine, have screw and short flutes.

    How To Cut Threads On A Lathe

    Let’s briefly list some special types of taps:

    • Nut. have a long tail for stringing nuts, which allows you to cut threads in several products without turning the tool out. Designed for machine cutting.
    • Taps for left-hand threads. designated by the letters LH, have the same pitch and diameter as a tool with a metric thread.
    • Inch taps. a tool for threading in the inch measurement system. Available in 2 or 3 sets in three accuracy classes.
    • Pipe Taps. Allows you to cut pipe threads with an inch gauge system ranging from 1/8 to 2 inches. Available in manual and machine-hand execution, in two sets with a sharp and blunt entry.
    • Tapered taps are used to prepare the corresponding type of metric (K) and pipe (Rc) threads. They differ in the presence of an elongated initial intake part with an incomplete thread, gradually turning into a full.


    Tapping taps are the main threading tools for the domestic, semi-professional and professional use. They can be used to prepare virtually all types of threads. metric, inch, trapezoidal, etc.

    • Manufacturing material. The best option is alloy steels that provide high tool life.
    • Cutting method. for manual or machine-manual use.
    • Hole diameter. determined according to the corresponding table.
    • Threads to be cut. metric, inch, main or fine pitch, etc.

    Features of internal threading

    Different types of screw-cutting equipment can work on such a thread. Unlike surface external machining, such operations require the initial creation of the hole by drilling. In some cases, an appropriate boring is also required, which will allow you to use the thread in the future to mate with parts of a specific diameter.

    But if the diameter is incorrectly calculated, cutting an internal thread on a lathe can ruin the workpiece. This happens in cases where the original diameter itself matches the dimensions of the target part to be integrated into the target cavity. This can be avoided by allowing the technological thickening of the inner walls before drilling. This tolerance should correspond to the height of the thread flange.

    Application of taps

    Taps are used specifically for internal metric threads. In this case, the diameter should be no more than 50 mm. On lathes, machine taps are usually used, allowing you to perform an operation in one go.

    This opportunity largely appears due to the single use of several tools that are installed in multipurpose universal cartridges. If, in the case of a conventional cutter, the process is divided into several stages by passes, then tapping on a lathe can be segmented by the quality of processing by different types of tools. Again, they work at the same time, following one after the other, which compensates for the load that falls on one cutter. Allocate taps for rough and fine threads. over, the first tool removes about 75% of excess metal from the target ditch.

    Threading on a lathe: technology

    The formation of threads on metal workpieces in the form of bodies of revolution is one of the most popular and at the same time complex turning operations. The difficulty lies in making the basic settings of the equipment, and in the preparation of the auxiliary tool. In order for threading on a lathe to comply with the terms of reference, you should adhere to the technology for its implementation and do not forget about safety rules.

    Thread types

    Turners distinguish between even and odd threads. In the first case, we are talking about cuts, which ultimately form an even integer number of cutting lines in steps. Accordingly, an odd thread leaves an odd thread. In terms of performing the operation, even threading on a lathe will have its advantages. For example, the operator, after each approach, can launch the sliding nut in the support and quickly return it back with the cutter manually, without interrupting the operation of the equipment. Further, with each new pass of the sliding nut, the tool will automatically be guided to the original cavity, which ensures a certain accuracy of the operation.

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    In turn, odd cutting requires, after each pass, the return of the caliper to its original position together with the cutter, which does not do without starting the reverse stroke.

    Multiple and vortex cuts can be subdivided into separate categories. So, in the first case, cutting a multi-start thread on a lathe will require accurate angular separation of the workpiece during transitions from one groove to another. The initial calculation of the pitch and parameters of the cutting line of the cut is important here. As for the vortex thread, it requires additional installation on the carriage of the carriage of the rotating cutter head. Several separate incisors can be fixed on it, each of which will cover its own area of ​​work.

    Slicing quality control

    After completing the operation, the operator checks the thread for compliance with the required parameters. For this, a template is used, on the surface of which the imprints of the teeth-ridges of the thread are applied. There are different templates for metric and inch threads, while both groups also distinguish between strips for checking the pitch, depth and angle of grooves.

    Calibers give a comprehensive assessment. these are devices that allow you to assess the correctness of the profile. If, for example, threading on a lathe with a pick was carried out with a critical slope, this will be fixed with a suitable gauge. Micrometers and inserts are also used for such tasks. Accurate verification readings are given only in the case of preliminary cleaning of the thread surfaces.

    Application of dies

    Dies are purposefully used to work with the same hardware in the form of bolts and studs, but only when creating external threads. The zone that is planned for cutting is pre-processed and cleaned. Also, the surface cut diameter is calculated so that it is less than the outer diameter of the applied thread. In the case of metric technology, this difference can be 0.2 mm for small threads. To form a thread start, a chamfer is previously removed at the end of the workpiece, corresponding to the height of the threaded profile.

    Although the process itself can be carried out manually, most often machine threading with a die is performed on a lathe using a special die holder. The cartridge itself is fixed in the rear quill or in the socket of the head with several mounting niches. The average speed of such cutting will be 5-6 m / min.

    Cutting with die heads

    To use several cutters at the same time, special heads with chucks are also used. This is a revolving tooling into which the same screw nozzles can be integrated. tangential, radial and round. After the completion of cutting, their combs diverge on the return stroke and no longer come into contact with the made thread. The most common are round cutters of this type, since they are able to carry out several regrinds, have increased durability and are easier to maintain.

    However, internal threading on a turret-type lathe is carried out only with prismatic combs with a special lead-in taper. To work with “worms” and long screws, cutting heads are used, which are integrated directly into the machine support. They allow you to form both external and internal threads.

    Preparing the machine

    Regardless of the type of lathe and working tool, the thread will be carried out by machining. Through the settings of the machine, the operator determines the angle of the helical mowing line of impact on the surface of the workpiece, which will have a perpendicular position relative to the axis of rotation. Here it should be noted that the machines have different power and, accordingly, the spindle speed. so that the cutting tool can cope with its task efficiently, it is important from the beginning to correctly correlate the angle of impact and the speed of the engine.

    inch, thread, lathe

    An important parameter is the step between the cutting lines. it is also taken into account in the equipment settings and appropriate adjustments are made in terms of the position of the tool relative to the workpiece. Since threading on a screw-cutting lathe is usually performed in several approaches, it is advisable to keep the primary parameters until the operation is complete. Even if the step along the mowing line of threading is maintained, there will be a risk of violation of the positions of the beginning and end of deformation sections, which may not coincide with each other. It is important to keep these nuances in mind before starting work.