Application of the hottest spring collet

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Application of spring collet

in the modern complex manufacturing environment, if you want to maintain continuous cutting processing in the best state, most factories must receive strict training from machine tool manufacturers and carefully learn the processing motion principle, structural characteristics and use skills of new equipment before operation. This is especially true for top-level technical systems. For example, advanced machine tool control system, assembly of 5-axis machining program for complex shape parts, etc. Fortunately, there is an opposite treatment scheme, which is some process equipment that has been developed so far with powerful functions, high precision, but easy to operate (without special training) and long service life. The spring collet (or spring sleeve) with high positioning accuracy relative to the chuck used to quickly position and clamp the workpiece (or tool) belongs to this category. It has a long history of more than 100 years and a wide range of applications

the spring collet must remain the same as the positioning datum of the workpiece (or tool), such as the spindle. The relative movement between the spring collet and the workpiece (or

tool) will lead to incorrect part processing. The rotation or relative axial movement of the spring collet and the workpiece (or tool) will affect the dimensional consistency and geometric accuracy of the machined workpiece

the use of the first spring collet is not very ideal. But it did prove a fact at that time that the use of a good workpiece chuck can improve the production efficiency and the accuracy of cleaning and processing parts. Later, Hardinge company, which is famous in the field of lathe development and manufacturing, proudly used the spring collet developed by our company (in 1890) on the lathe they developed in 1901 to provide workpiece positioning and clamping, and published their product drawings and developed series of products. At that time, it was mainly developed to meet the market needs of mass production in the clock and lens manufacturing industry. It is really incredible that spring collet products can be so well provided for early (before 1920) ordinary lathes and cam multi shaft automatic lathes. This is like configuring a set of modern technology control system on the current advanced CNC lathe

let's review the past. With the continuous progress of processing and equipment technology, in the design reform trend that requires all systems to greatly improve production efficiency, spring collets, the most basic but important process equipment of machine tools, have never been given space and time for design. It seems very strange to invest in Maldives' tourism service industry

under the situation that the structure of the machine tool is also changing at a rapid speed, the miracle that the basic structure of the original spring collet can continue to remain unchanged is mainly due to its unique characteristics of dexterity, exquisite structure, powerful function, convenient use and good economy. Although the spring collet is small, it does play a very important role in the machine tool industry because it has the following strong functions:

1 It can accurately position and clamp the workpiece (or tool), and has the function of resisting torque and bearing cutting forces from multiple directions

2. It has the function of increasing the driving force (tension) and converting the driving force into the clamping force of the workpiece (or tool)

3. It has the function of quickly loosening the workpiece (or tool)

4. It has high repetition accuracy without reducing the machining accuracy and without damaging the workpiece

5. It has the ability to work in a wide range of spindle speeds with only minimal clamping force loss

6. The moment of inertia is the smallest in high-speed cutting

clamping force

in general, the design and use of spring collets cover a wide range of fields. It needs to correspond to a variety of machine tool series, and includes products designed to reflect their different styles and characteristics from machine tools, so the total number has reached tens of thousands. But now there is a common misconception that spring collets are only used to clamp cylindrical workpiece blanks. This is not true. In fact, it can locate and clamp workpieces (or tools) of almost any shape, including square or hexagonal workpieces

the following mainly introduces the relevant factors and working principles that affect the correct clamping positioning and clamping force of various spring collets

factors affecting the clamping force

the clamping force is the force exerted by the machine tool on the workpiece through the spring collet. The illustration in this paper is a spring collet used on the lathe to locate and clamp the workpiece under the current economic downward pressure. It can also be used to locate, clamp or many other occasions for cutting tools and ground workpieces. The external thread of the pull rod (not shown in the figure) is connected with the internal thread at the rear end of the spring collet to tension, producing axial tension. Then, the axial tension is converted into a clamping force perpendicular to the center of the spring collet by the conical surface called the locking angle at the front end of the machine tool spindle. Not only that, the clamping force can also be expanded by the locking angle. After calculation, it is known that the clamping force of the spring collet can be expanded by times according to different locking angles

spring collet is a process device with simple structure, but there are many main factors affecting the clamping force. Understanding the basic principles can help the workpiece (or tool) to be clamped correctly and find faults quickly. The following introduces several main factors affecting the clamping force and the summarized experience:

1 Axial force. The figure shows the axial tension applied by the pull rod to the spring collet. In the use of spring collets, the axial force and tension can be applied in different ways, but their working principles are basically the same. Obviously, a large axial tension will produce a large clamping force, and vice versa. Generally, the axial tension of the pull rod can be adjusted by the operator

2. In the use of spring collets, the designed locking angle (or head inclination) will determine the index that can be achieved after the clamping force is expanded, which is usually decided by machine tool manufacturers and spring collet manufacturers. When the new design of spring collet is still being explored, the designer suggests users to use the existing spring collet structure from the perspective of economy and reliability. The standard taper (or head inclination) has been determined at the time of design according to the type of machine tool (such as lathe), service conditions (dynamic and static) and purpose (workpiece and tool)

3. The total friction between the workpiece (or tool) and the spring collet will directly affect the clamping force. A small friction value will result in a small clamping force, and vice versa. The spring collet supplier can take various measures to overcome the relative rotation or axial movement between the spring collet and the workpiece (or cutter), such as deliberately making a sawtooth shape in the inner hole of the spring collet or impregnating cemented carbide microcrystalline particles on the clamping surface

4. Friction occurs between the conical surface of the spindle and the spring collet at the locking angle. Its friction force also directly affects the clamping force of the spring collet on the workpiece. If the friction force is too small, the clamping force is not enough. If the clamping force is too large, the wear of the spring collet will be accelerated. In use, the spring collet should be loosened frequently to exchange workpieces. For example, when it is used on a turning center, a thin layer of lubricant should be sprayed on the inner hole surface of the spring collet when loosening. If possible, it is better to use coolant for lubrication, because the coolant provides washable spring collet, and the lubrication effect is good. In particular, regular application of coolant at the locking angle can reduce long-term wear and increase the clamping force. Some more materials with better lubricating effect can also be used, including high-quality grease or wax based materials with EP (extreme pressure) characteristics. It seems strange that some smart operators choose lipstick used by women when it is difficult to process and the general cooling and lubrication effect is poor. It is said that the effect is very good

5. Choosing the appropriate nominal diameter of the spring collet to ensure that the spring collet fully supports the workpiece is a necessary condition for increasing the clamping force and reliable clamping to ensure high-quality processing. If the nominal hole diameter of the spring collet is too large, the workpiece is clamped only by the orifice part of the spring collet, which will cause the geometric mismatch between the outer circle of the workpiece and the inner hole of the spring collet, thus reducing the clamping force. If the nominal hole diameter of the collet is too small, only the interior of the head inclination angle contacts the workpiece. Relatively speaking, the clamping force increases, but it causes the non concentricity between the collet and the workpiece. Under a nominal size, it can clamp and locate the workpiece with the same nominal diameter. When selecting the nominal aperture size of the spring collet, Hardinge recommends that it can change within the range of 0.0254mm

it is proved by cutting tests and years of application that the spring collet can meet the processing needs of increasing cutting speed and has a long service life. We know that more than 100 years ago, the spindle speed was generally measured by hundreds of revolutions per minute. Today, the spindle speed is as high as tens of thousands of revolutions per minute, and the material removal rate is also increasing at an alarming rate. In more than 100 years of successful use, although many advanced high-end manufacturing technologies have been used and can be processed at high spindle speed, spring collets with existing structures are still used. This net profit of 28.3 billion yen in the manufacturing industry is indeed rare, and it is indeed something that Hardinge people should be proud of

working collets and factors that affect grip force (end)

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