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How to grasp the matching between tools and machine tools?

October 15, 2022

When it comes to the matching of tools and machine tools, you may first think of the matching of shape and size. Indeed, the matching of the shape and size is the basis for the correct installation of the tool on the machine tool. Without this foundation, the tool cannot be correctly installed on the machine tool, so it is impossible to complete any processing task.

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However, this alone is not enough.
After the tool is installed on the machine tool, it is necessary to complete certain processing tasks. In the process of completing this machining task, it is necessary to ensure the machining accuracy, bear and transfer the cutting force and cutting torque, bear, transfer and export the cutting heat, consider the possible transmission of cutting waste (chips and heads) and even the workpiece, as well as the digital transmission of modern tool parameters.
Although some of these tasks are not common, they are also possible tasks for the tool. If we can consider the matching between tools and machine tools when selecting tools, it will increase our thinking to solve processing problems.
To ensure the machining accuracy, transfer cutting force and torque, and provide the channel for cutting fluid are the problems we often encounter after ensuring the matching of shape and size. For example, on machining centers, we often use cylindrical (usually called straight shank) as the clamping method. As for the cylindrical tool handle, in addition to the typical complete cylindrical shape, there are also some changes that add some other elements to the cylindrical shape, such as the flat straight handle (the milling cutter is divided into single cutting plane and double cutting plane according to the diameter, and the common full cutting plane is drilled, which is called the side pressing type), the inclined flat handle with 2 ° inclination, and the straight handle with a flat tail (commonly used for drills), Straight shank with square body (commonly used for taps and reamers), etc.

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As far as the connection mode of this kind of tool handle and machine tool is concerned, it is not rare that only the cylindrical part is used for positioning and clamping. Spring sleeve systems of various pressure angles, powerful collet systems, hydraulic locking systems, thermal expansion clamping systems, and force deformation locking systems are all used to lock cylindrical tool handles. However, each clamping method has its advantages and disadvantages. Take the most common spring sleeve system as an example. A large pressure angle (defined here as the angle between the positive pressure of the cone surface locking and the cylinder axis), that is, a large cone angle represents a short locking stroke, which is conducive to rapid locking and loosening. However, the positive pressure decomposed to the cylinder surface under the same locking torque is small, resulting in a small friction distance and a correspondingly small cutting force distance that can be resisted, The tool is easy to slip in the tool handle, which affects the stability of the machining process and the quality of the machined surface; At the same time, the tool handle diameter that can be clamped by this kind of chuck has a wide range of changes, which is conducive to reducing the inventory of spring sleeves and optimizing management. A small pressure angle is the opposite. The spring sleeve with small pressure angle can clamp a small range of tool handle diameter, and the locking stroke is long during clamping, which is not conducive to rapid clamping and loosening. However, its clamping accuracy is slightly higher, the clamping force is large, and it can withstand greater cutting load.

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The hydraulic locking system is a new clamping system, which uses the incompressibility of high viscosity hydraulic oil to make the inner wall of the tool clamping chamber produce elastic deformation, thus locking the tool. The hydraulic locking system has high accuracy, and it is convenient to lock and release without special equipment. The locking torque is usually better than that of the spring sleeve system, but its inner wall can only work within the range of elastic deformation. Once the range is exceeded, irreversible plastic deformation will occur on the inner wall, which will cause permanent failure of the clamping cavity of the tool handle. Therefore, the flat tool handle, especially the full cut flat tool handle commonly used for drilling tools, cannot be used in the hydraulic locking system. The common reasons for damage and failure of the system are the pressure applied to the cavity and the tool handle not inserted into the bottom of the chamber.
The thermal expansion clamping system usually requires special equipment, which can control heating and cooling according to multiple predetermined modes. Non professional heating equipment (even flame heating) may be used, but the temperature and heating curve cannot be well controlled, which will affect other parts of the tool handle, or even change its metallographic structure, so that the system will soon become invalid. In addition, the tool length of the thermal expansion clamping system is difficult to adjust, and special auxiliary tools are required, which adds some trouble to the situation where multiple tools need to work synchronously.

On the other hand, the tool clamping mode may also determine the possible value of production efficiency.
Cylindrical tool shank, hydraulic pressure and thermal expansion are all balanced designs that can adapt to high speed, while the flat clamping is a typical unbalanced design, which is not recommended for high-speed cutting by tool manufacturers.
As far as the tool shank itself is concerned, when a part of the material is milled (or ground) to form a pressure surface, the center of gravity of the tool shank is not coincident with the rotation center of the tool. In the process of tool clamping, the flattening handle is pushed to the side that has deviated from the center by the locking screw, and the center of gravity of the tool will further deviate from the rotation center of the tool on the machine tool, which increases the imbalance of the tool. In addition, some users often do not care about the length of a screw after the original locking screw is damaged or lost, which also adds uncertainty to the balance performance of the tool. Therefore, the flat type (including the bevel type) is not recommended to be used at high speed.

However, the flattening type is a tool handle with forced driving property, which is more reliable than the pure cylinder driven by friction force at high torque. Therefore, it is suitable for rough machining (rough machining generally has large torque, but low speed).