How to cut a left-hand thread on a lathe

The design of a lathe allows you to cut both inside and outside threads. There are many varieties of cutters for this purpose, belonging to three big groups:

Rods are the easiest type of tool for slicing. It is a rod (of any cross-sectional shape) with a working head. Profile is determined by the shape of the head. One variety of rod cutters with soldered carbide working faces. They are more resistant to abrasion and do not require frequent sharpening. Circular and prismatic tools belong to the shaped category.

Prismatic only cut on the outside. Capable of tackling larger surfaces than solid tools. The cutter is held in the toolholder by a dovetail. Capable of more resharpening than picks.

Round threads can be made into male or female threads. They are more convenient to cut and have a wider application range than prismatic ones. They can be resharpened a greater number of times. Mounted in the toolholder relative to the face and bore.

Types and properties of cutters


Internal and external thread picks with a rectangular head are used in practice. Less common: disc, prismatic, front-surface sharpening. All have a working profile that matches the screw groove dimensions. Left and right-hand picks are available in accordance with the direction of the spiral to be cut.

A distinction is made between solid and prefabricated tools. The first, mostly made of high-speed steel, small cross-section or circular. Main mass is equipped with cutting inserts that are soldered with refractory solder or mechanically replaceable in case of wear.

Thread cutters: external (fig. 1), internal (fig. 2)

Tools for threading

Thread cutting with turning equipment

Forming on the lathe is realized by copying the working profile of the tool on the workpiece along the helical scythe. Progressive motion is used for cutters, taps, dies, combs. In combination with the rotation of the workpiece, a helical movement is obtained, the tool surface coincides with the tapped surface.

As a rule, small lots of fasteners and fittings up to M36 are threaded with taps, lancers. It is more advantageous to make large orders on specialized automatic machines. Larger diameter, running, power, precision threads are machined with cutters on universal lathes, when CNC models are not available or the production program is insufficient.

MACHINE SHOP TIPS #16 Clausing Lathe LH threading tubalcain

Internal and external thread cutting with pick

Threads with high alignment to other surfaces, transferring movements and forces are cut with a pick. Spindle rotation is linked kinematically to the lead screw that moves the slide with the toolholder.

  • Surface turning along the length of the thread, creating a tool exit groove.
  • Selection, if necessary: Grinding, lapping, checking with contour templates.
  • Setting the modes on the machine, adjusting the guitars to a pitch not provided by the box.

Movement of the pick per revolution of the workpiece is equal to pitch P or travel H for multipass cuts.

Batch threading of the parts is divided into roughing and finishing cutting. For the latter, the tool is carefully sharpened. Tapping of threads with a pitch greater than 2 mm is carried out by a side plunge cutter. Left helical groove is produced by switching the tripod so that the lead screw rotates in the opposite direction to the spindle. The plunger and cutter move from left to right.

Average speeds when tapping steel are 20. 35 m/min with a high-speed tool, 100. 150 m/min with a carbide tool. Finishing strokes are performed at speed increased by 50. 100%. The internal threads are machined at 30% reduced speed.

Use of Taps

The popular P6M5 grade allows cutting of workpieces with hardness up to 240 NV, taps from tool alloy steels are used for “raw” parts. Carbide tools are seldom used since the edges are bent out due to misalignment, misalignment that increases bending stresses.

Typical sizes are limited to M36 through 42, G2. Large coarse pitch diameters are machined with a set of 2, preferably 3 taps. The bore is slightly larger than the inside diameter of the nut D1 (see counterbore and header description). Fig. 2), taking into account metal bulging from the groove. Drilling operations require considerations for parting-off. Recommended values are given in reference books.

There are several ways of machining:

  • Often, nuts less than M12 are threaded by holding the screwdriver with the hands. Strictly speaking, reception is a safety violation and can lead to injury. At the start of screwing, tap is pushed up by the tailstock center for guidance, then self-tightening takes place. Stop, unscrew on reverse.
  • The tap is set into the screwdriver, rested against the bar fixed in the toolholder, and supported with the center holder or back center. Switch on low revolution, thread at self-tightening. To eliminate run-out of coils, it is recommended to hold the tap until screwing in half of the working length, smoothly advancing the quill.

Thread cutting on turning lathe with pick, tap

Threaded connections are actively used in almost all branches of human activity, from agriculture to the space industry. For thread cutting in small amounts, a lathe is used in combination with special tools: cutters, dies, taps and threading heads. Stroke is selected according to the required strength, size, profile and location of the helical surface.

When the work is done on a lathe with a cutter mounted, the tip of the tool traces a helical depression, moving along the axis of rotation of the workpiece. This groove is called a thread. It is characterized by the following parameters:

thread, lathe
  • Pitch. the distance between the adjacent threads. Measured along the axis of the workpiece.
  • Angle of increase of the helical scythe is a measure of the inclination of the scythe line relative to the plane perpendicular to the axis of rotation of the workpiece. Determined by the rotation frequency of the part and the cutting tool travel speed along its axis.
  • Stroke (for multi-turn threads). distance between the nearest coils formed by one thread.

There are 5 thread profiles which affect the characteristics of a threaded connection:

The surface of the part itself can have a tapered or cylindrical shape. The helical line cut by the tool on the lathe is formed in one or more runs. For threads with two or more threads, the latter are positioned equidistant from each other. To count their number, just look at the beginning of the thread surface.

The technology of using taps and dies

Taps, which are a screw with several longitudinal grooves that form cutting edges and contribute to chip evacuation, are used on a lathe to cut mostly metric threads in small-diameter holes. If machine taps are used for threading, the operation is carried out in one pass.

Machine taps differ from conventional taps in that they consist of two parts. a fence and a gauge. If ordinary taps are used for threading with a lathe, the technology of this process involves the use of a set of tools. A set for tapping internal threads includes three types of taps: roughing taps, which do 60% of the work, semi-finishing taps (30%), and finishing taps (10%). Sometimes there can be two tools in a set: a roughing tool that does 75% of the work and a finishing tool that does 25% of the work. To distinguish a roughing tap from a finishing tap, just look at its intake part: it is much longer than that of a finishing tap.

The design of a tap for tapping

The speed of threading on a lathe using taps can be quite high:

With dies, which are a ring with an internal thread and several chip grooves, external threads are made on screws, bolts and studs. The surface of the part should be pre-ground to the size of the required diameter, which must necessarily take into account the tolerance:

  • 0.14-0.28 mm. for threads with a diameter of 20-30 mm;
  • 0.12-0.24 mm. for threads with a diameter of 11-18 mm;
  • 0.1-0.2 mm. for threads having a diameter of 6-10 mm.

Dies with which external threads are threaded are fastened in a special chuck (die holder) located in the quill of the tailstock of the lathe.

Threading dies

Using dies, threads are cut at the following speeds (their setting also takes into account the minimum wear of the tool during operation):

To make sure the die goes smoothly on the workpiece, a chamfer is removed at the end of the latter, the height of which coincides with the height of the thread profile.

Characteristics of threading methods;

thread, lathe

Threads are formed on parts using the following basic methods:

Threading tools are divided into three groups:

Bladed (thread cutters, chisels and cutters, taps, dies and heads);

Abrasives (single- and multi-strand grinding wheels);

Rolling (dies, rollers, heads, rollers).

When forming a thread, the workpiece and the tool make two relative movements:

Rotation around the longitudinal axis of the thread;

thread, lathe

A feed rate that is equal to the thread pitch P.

In the work of bladed and abrasive tools, threads are formed by cutting chips, and in the work of knurling tools. by plastic deformation of the surface layers of the workpiece. the thread on the part, obtained by extrusion, is stronger, because in this case, the fibers of the metal are not cut, but deformed and hardened by the tool.

If we consider the methods of creating threads in more detail, we can distinguish the following:

Tapping with thread cutters or thread combs;

tapping with dies, threading heads and taps;

Threading with flat or circular knurled dies;

Milling with special thread mills;

It is possible to use such a method as threading with chisels. Thread cutters and combs are used on screw-turning machines for cutting both external and internal threads (internal thread from 12 mm in diameter and up).

The method of thread cutting with picks is characterized by a relatively low productivity, therefore, at present it is applied mainly in small-batch and individual production, as well as during the creation of precision screws, gauges, lead screws, etc. д. The advantage of this method is the simplicity of the cutting tool and the relatively high accuracy of the threads produced. Schematically it is as follows: with simultaneous rotary motion of the part to be threaded and translational motion of the cutter (on a lathe. II) the latter removes (cuts out) a part surface in the form of a helical scythe (I).

I is the helical fishing line formed when threading with cutters;

II. Forward motion of a pick on a lathe during thread cutting.

The most common threads are cut with dies and taps. Fig. 15 shows taps, which by their design features are divided into round. I and II (lerki) and sliding. III (kluppovye).

Circular dies for use in assembly, production and other operations are designed for cutting external threads with a diameter of up to 52 mm in one pass. Specially designed dies are used for larger threads, which are actually used only for trimming the thread after it has been previously threaded with other tools. Sliding dies consist of two halves that are inserted in a die and gradually come together in the process of cutting.

When threading on metal-cutting machines (II) dies are mounted and fastened in a special chuck or fixture Parts are fed into a calibrating part of a rotating die. The vast majority of internal fastener threads are tapped.

A tap is a steel rod with a thread divided by longitudinal straight or helical grooves which form the cutting edges of the tap. These grooves also serve as a chip exit. According to the method of application taps are divided into manual and machine.

The sequence of threading in blind holes is as follows: first, a socket is drilled where the stud or screw will be screwed in later on. Diameter of drill bit should be chosen according to table below. Recommended in GOST 9150-81 standard. The thread is tapped with a set of two or three taps (small, medium and normal, finishing), depending on the size of the thread. It is not possible to thread with one tap (normal) in one trip. This leads to tapping failure.

The kit consists of three taps for coarse pitch metric threads and one inch thread, and two taps for fine pitch metric threads and pipe threads.

The main industrial method of thread production today is thread rolling on special thread-rolling machines with a three-roller head body 1, a roller-holder 2 and a rolling roller 3 The part 4 is clamped in the vise of the slide. In this case, high productivity ensures high quality of the product (shape, dimensions and surface roughness).

three-roller head, 2. roller holder, 3. knurling roller, 4. workpiece.

The process of thread rolling consists in creating a thread on the surface of a part without removing a chip by plastic deformation of the surface of the workpiece. Schematically it looks like this. The part is rolled between two flat dies (fig. 20. I) or with cylindrical rollers (Fig. 20. II, III) having a thread profile and a thread of the same profile is pressed out on the rod. The largest thread diameter is 25 mm, the smallest. 1 mm; the length of the thread is 6080 mm.

I. by flat dies; II and III. by cylindrical rollers.

Thread milling with comb milling cutters is used. Internal and external thread milling are carried out on special thread milling machines. A rotating milker with radial feed plunges into the workpiece and threads the surface of the workpiece. The axial displacement of a part or a cutter from a special copying device by the amount equal to the thread pitch during one revolution of the part is periodically

Grinding as a method of thread creation is mainly used to produce precise threads on relatively short threaded parts such as threaded plugs. gauges, thread rollers, etc. д.

The essence of the process is that the grinding wheel, placed against the workpiece at an angle of the thread lift, during rapid rotation and a simultaneous slow rotation of the part with a feed along the axis by the thread pitch, per one revolution, cuts (grinds out) a part surface. Depending on the machine design and a number of other factors, the threads are ground in two to four or more passes

Thread grinding is used for the production of small threads or to improve the accuracy of the thread. The grinding wheels used have a thread profile in the cross-section. Grinding with single thread grinding wheels compared with multi thread grinding wheels leads to a higher thread precision. But the production rate is higher with multi-strand grinding wheels. Ring threaded discs are also used.

Use of cutters

Thread-cutting tools are required for tapping with the lathe. They are made of high-speed steel, and the requirements for their characteristics are stipulated by the relevant GOST (18876-73). Such cutters are divided into the following types according to their construction:

The helical thread groove on the surface of the workpiece is cut with a bent or straight cutter, and the formation of internal threads requires straight and curved tools, which are fixed in a special mandrel. The apex of the lathe cutter, which is used for thread cutting, must have a configuration that fully corresponds to the profile of the thread to be formed.

Cutters for thread cutting: a. rod cutter; b. multi-threaded prismatic; c. single-threaded prismatic; d. multi-threaded disc; e. single-threaded disc; f. disc for internal thread; α. back angle; γ. front angle; φ. angle of fence cone; h. height of cutter axis setting

When shaping a thread with a pick, there are a number of peculiarities of this technology should be taken into account.

  • The front angle of the turning tool for tapping depends on the characteristics of the material to be processed. Such angle can be chosen within a rather wide range: 0-250. Thus, if a thread is threaded with the machine on workpieces made of conventional steels, the feed angle must be 0 degrees. With high-alloy steels that are well resistant to temperature stress, the feed angle may be 5-100 degrees. It can be the greater the higher the toughness of the material, and the lower the higher the hardness and brittleness of the metal of which the workpiece is made on the machine.
  • The apex of the lathe cutter, which forms the screw line on the workpiece, must have a shape identical to the profile of the thread.
  • The trailing lateral corners of the tool are selected so that the surfaces of the cutter with which they are formed do not rub against the newly formed helical groove. Usually these angles on both sides of the lathe cutter are made equal. If the angle of elevation, which characterizes the thread, is less than 4 degrees, then such angles are chosen within the range of 3-50, if more than 40, then 6-8 degrees.
  • Internal type threads are cut in already prepared holes, which are obtained by boring or drilling.

Thread-cutting cutters

Blanks that are made of steel are processed on a lathe with tools with plates made of hard alloys T15K6, T14K8, T15K6, T30K4. If the part is made of cast iron, then for thread cutting tools with plates of the following grades of hard alloys are used: BK4, B2K, BK6M, BK3M.

Cutting methods

You can use a tap to create an outside thread. With this technology, the surface of the part undergoes additional machining before starting. The diameter of the workpiece must be slightly larger compared to the inside diameter of the tap.

Tapping begins with chamfering, the size of which corresponds to the depth of the profile of the thread of the die. After that, the die is clamped in a special chuck and the threading proceeds directly. The speed of rotation of the workpiece must match the pitch of the thread. If the part is more than a few inches in size, special cutters are used to make the threads.

Taps are used to make pipe internal threads. They cut the pipe with the required parameters from the inside. Usually such threading is done on a lathe in one pass. If the metal is processed has a high hardness or toughness, a set of two or three taps is used.

In this case, the first one does most of the work, and the second and third do the thread, which fully meets the specified parameters. A set of two tools does 75% of the work when the first tool is fed, and a set of three tools does 60% of the work.

Threading heads are also used for cutting. There are four cutters mounted on them at the same time, and the workpiece is placed between them. Tapping is done similarly to how it is done with a tap.

Taps consisting of two sections can also be used when working with a tap on a lathe. The first of them is a cutting one, the second one is a calibration one. The task of the cutting part is to create a profile, and the calibration part is to perform precise machining so that the thread corresponds to the necessary standards.

Making a screw with a single cutter

Trapezoidal single thread is made as follows:

  • the workpiece is prepared and the channels for sharpening are set;
  • The cutter is ground on a special prepared template;
  • The sharpened element is set and clamped. It should be located so that the centers coincide and are parallel to the cutting axis;
  • equipment is turned on and the workpiece is fed for threading;
  • the finished part is checked according to the finished template.