3. Collets for feeding and clamping the bar

In automatic lathes, the extension (feed) and clamping of the bar are carried out using a mechanism, the essential links of which are the feed and clamping collets.

Fig. 91. Collet for pushing the bar.

The feed collet is a split sleeve, the jaws of which were in a compressed state during heat treatment (Fig. 91). The elasticity of its jaws must ensure that the bar is clamped with a force sufficient to move it in the axial direction with the clamping collet open. At the same time, the elastic force of the jaws of the feed collet should not be too great, since the feed collet must slide along the bar, moving to its original position, while the bar is held by the clamping collet.

The feed collet and feed pipe are threaded. When the bar is clamped by the clamping collet, the feed collet, moving backwards, slips to its original position. Before feeding, the clamping collet releases the bar and the feeding collet moves forward with it, gripping it with the force of the collet preload.

Recently, feed collets with interchangeable inserts have been used, which ensure their more rational use, since they make it possible to use the same collet for feeding bars of different diameters and profiles.

According to the working conditions, the feed collets must have a high wear resistance, as a result of which they are usually made of case-hardened steel or steel with a high content of alloying elements.

Automatic clamping of the bar on automatic lathes is carried out using a clamping collet placed at the front end of the spindle.

Fig. 92. Bar clamping collet: a — up to 5 mm in diameter; b - with a diameter of 6 to 36 mm.

There are various collet designs and, accordingly, clamping methods. On revolving machines, collets with a reverse cone were used (Fig. 92). This fastening of the bar provides greater rigidity, since the axial cutting force increases the clamping force of the bar by the collet.

The collet can rest against the machine spindle with its front part (Fig. 22, c). In this case, to clamp the bar on the collet, a sleeve is found, to which a compressive force must be applied. This fastening ensures accurate bar feeding, since during the clamping process the collet remains stationary in the axial direction, and the sleeve moves.

The main disadvantage of this scheme for fastening the rod is a comparative increase in the dimensions of the clamping mechanism.

In clamping collets for round and hexagonal material, three slots are made, for square material - four.

Clamping collets are usually made from steels U8, 9XC; after hardening, they should have a hardness of the working part RC ~ 58-60, and tail - RC ~ 38-50.

Recently, collets with transitional replaceable jaws have been used, which are attached to the main body of the collet with screws. In practice, such collets turn out to be very convenient, since bars of different diameters are clamped with sponges. Thus, a set of collets for different bar diameters is replaced by a set of jaws. In addition, the conditions for heat treatment of collets are improved, since the replaceable jaws are heat treated independently.

The disadvantage of collets with interchangeable jaws is the complexity of the design and somewhat reduced rigidity.

Collets used for processing parts that cause low cutting forces usually have a ground hole, so that the surface of the bar at the clamping points does not deteriorate. At high cutting forces, to increase friction between the bar and the collet, the clamping surface is made corrugated.

The main dimensions of the feed and clamping collets shown in Figs. 91 and 92 are given in table. 35.

Table 35 Main dimensions(in mm) feed and clamping collets used on revolving machines(figs. 91 and 92)

- This is a special fixing device for installing the tool. It is used as the main element of a collet chuck for clamping a tool or a cylindrical workpiece. Special collets for square or hex shank are also used.

Construction and application

The collet is a springy split sleeve with a truncated cone and a hole of the required diameter. The collet body has special cuts that allow the locking tabs to move when inserting or removing the tool. The compressive force causes the nut to rotate.

The illustration below shows a collet (1), a cutter with a cylindrical shank (2) and a collet chuck (3).

This fixing element is intended for installation in a collet chuck, which, most often, is a separate element of the equipment of a metal-cutting machine. The cartridge itself is installed by means of a Morse taper on the seat of the cartridge. It can also be an integral part of the machine.
Collets are widely used in modern metal-cutting machines - turning, drilling and milling. The main advantage of this type of fixation is the high speed of insertion or removal of the tool. The disadvantages include the need for strict compliance with the sizes of the chuck and collet.

Types of collets

In modern metalworking, various types of collets are used, differing in size and design. General purpose clamps are divided into:

• Through and deaf. Through can fix a part of unlimited size and are used, for example, to fix a turning rod.
• One or two clamping zones. The most rigid fixation is provided by collets with two clamping zones, completely eliminating deviation from the spindle axis.
• Collets for taps and other tools with square or hex shank. The design of this type of retainer provides axial compensation.

The use of collets in lathes

The collet and collet chuck are used as the main fixing equipment when it is necessary to longitudinally process small diameter workpieces. These tools can be fitted with ceramic or carbide inserts to improve wear resistance and prevent metal buildup.

To work with workpieces on automatic lathes, feed and clamping collets are used. The feeder is used in conjunction with the clamping one and provides sufficient clamping of the workpiece for moving in the axial direction, with the second fixing equipment in the open position. This type of collet has a threaded connection to the supply pipe. When the workpiece is held by the clamping bar, the feed moves back to its original position. Before feeding, the clamp releases the workpiece and the feeder moves forward with it.

The clamping collet provides automatic fixation of the bar and is located in the front of the spindle. On turret lathes, reverse taper collets are most commonly used, which provide higher rigidity due to increased clamping force during axial cutting.

GOSTs

GOST 17201-71 applies to clamp-type collets designed for cylindrical shanks. Other commonly used standards define the basic and connecting dimensions of collets. For clamping collets, GOST 2876-80 applies, for feeders - GOST 2877-80. Of the international standards, DIN 6499, ISO 15488 are generally applicable, regulating the parameters of ER collets - with two clamping zones and a through hole.

A device such as a collet chuck is a type of clamping mechanism used to hold a cutting tool rotating at high speed. Unlike keyless type models, collet chucks do not require the use of additional devices such as screws, studs or locking elements, which is why such devices are often called self-clamping.

Collet-type clamping elements are equipped with lathe chucks, in which parts are fixed during processing. Such chucks, depending on the geometric parameters of the workpiece being processed, may differ both in their dimensions and in the shape of the clamping elements. The most common of all currently in use are collet clamps of the ER type.

Main varieties

To complete modern metalworking machines (turning, milling and drilling), self-centering chucks are used, equipped with two, three or four jaws. The clamping elements in such chucks can be operated manually or by means of a mechanical drive. If we talk about the areas of application of such clamping mechanisms, then two-jaw chucks are used mainly for fixing shaped castings during their processing, and with the help of three-jaw clamping mechanisms, reliable fastening of workpieces and shanks of round, conical and hexagonal tools is ensured. Clamping mechanisms equipped with four jaws are used in cases where it is necessary to fix an asymmetrical workpiece or a rectangular workpiece in order to perform processing.

In general, all mechanisms for fixing a tool or workpiece during processing are divided into the following categories:

• collet type devices;
• lever devices;
• quick-clamping devices (BZP);
• wedge clamping devices;
• hydraulic cartridges;
• membrane clamps;
• thermochucks;
• two-, three- and four-jaw clamping devices.

Self-tightening chucks that use a collet mechanism are needed to equip both specialized and universal machines. The advantage of using chucks of this type is that their design provides sufficient clamping force even with a small amount of torque, which cannot be said about similar devices of other types. When choosing a collet chuck, you should pay attention to the fact that the main elements of its design are made of hardened steel. In this case, it can last much longer.

How the clamping collet mechanism works

A milling or turning chuck, which is based on a collet clamp, works according to the following principle:

• The self-tightening nut, which is tightened, creates pressure on the end of the collet mechanism.
• Under the influence of pressure, the collet moves into a cone-shaped hole, which leads to compression of the petals that make up its working part.
• Compressing, the petals of the collet securely fix the tool shank or workpiece.

Accordingly, in order to remove a tool or workpiece from such a chuck, it is necessary to loosen the self-locking nut, reducing the pressure it exerts on the collet.

The great advantage of milling machine (or lathe) collet chucks, when compared to similar devices of other types, is that the tool or workpiece that is held in them is perfectly centered, which ensures their minimum radial runout. In addition, to use a collet-type milling or turning chuck, there is no need to use special keys, which tend to get lost often.

Collet chucks can be used to hold parts whose cross-sectional configuration does not match the shape of their inlet. In order to fix such a part in a collet chuck, special cartridges are used, supplied with the device.

Design features

The main structural elements of any collet chuck are the clamp nut and the collet clamp mechanism, which can be retractable, retractable or fixed. Depending on their purpose, collet mechanisms can be clamping and feeding. Collet-type mechanisms are also used to fix workpieces on the surface of their inner hole. This fixation is provided by a special device - a collet mandrel. In addition to fixing parts during their processing, a collet mandrel can be used to fix shell-type milling cutters on it.

Feed-type collet mechanisms are used in cases where the part fixed in them during its processing must be periodically pulled out of the clamping device. The design of such a collet consists of a steel sleeve, on the side surface of which three cuts are made, forming springy clamping petals, as well as a tube, through the inner hole of which the workpiece is fed into the processing zone. Before using such a mechanism, the collet sleeve is screwed into a threaded hole in the tube, and the end of the workpiece is passed between the petals of the clamping mechanism. A special rod is responsible for feeding the workpiece into the processing zone, driven by a cam or hydromechanical mechanism.

The clamping collet, which is also made in the form of a sleeve with springy petals, provides only fixation of the workpiece, without feeding it into the processing area. Depending on the diameter of the tool shank or the workpiece being processed, which are fixed using a collet mechanism, the number of petals may be different. So, for fixing tools and workpieces with a diameter of not more than 3 mm, three-lobed collets are used, with a diameter of 3 to 80 mm - four-petal, with a diameter of more than 80 mm - six-petal.

To fix tools and workpieces of the smallest diameter, detachable collets are needed, in the design of which there are special springs responsible for opening the cams. Collets of this type can be additionally equipped with inserts, selected depending on the diameter of the tool or workpiece.

Collet chucks, which are designed with an additional nut, can be used to hold small diameter tools with a Morse taper shank. The disadvantage of this type of collets is that a separate clamping mechanism must be used for tools of different diameters.

Turning workpieces of small diameter, performed on longitudinal type machines, can also be performed using collet clamping mechanisms. The collet chucks used in such cases differ in their design from conventional models.

The first thing to consider when choosing a collet type chuck is how it locks onto the spindle of the machine being used. There can be two options for such fastening: using a special adapter or winding the cartridge onto the threaded end of the spindle. If a collet-type milling or turning chuck will be attached to the machine spindle using a flange or a Morse taper, you must first find out their exact parameters (belt diameter and taper parameters).

As mentioned above, depending on the diameter of the tool shank or the workpiece being processed, the number of clamping elements that the collet is equipped with is selected. In addition, you should pay attention to the material from which such elements are made: the harder it is, the longer the cartridge will last you.