High speed machining (HSM) is a high-tech processing technology integrating material science, engineering mechanics, mechanical dynamics and manufacturing science.
It has been widely used in automobile manufacturing, aerospace and mechanical processing industries.
As we all know, high-speed milling plays a very important role in high-speed machining.
It is understood that 40% of high-speed machining comes from high-speed milling, and high-speed milling tool system is an important part of high-speed milling machine tools.
Its performance will affect the processing quality and efficiency.
Therefore, the research and development of high speed milling tool system has attracted the attention of mechanical engineering experts and scholars at home and abroad.
（1） Brief introduction of high speed milling tool system
For more than half a century, the traditional BT (7 ∶ 24 taper) tool system has played an important role in machining.
Fig. 8-54 shows the working diagram of BT tool system during high-speed machining.
During high-speed machining, the working speed of the spindle reaches tens of thousands of revolutions per minute.
Under the centrifugal force, the expansion of the spindle hole is greater than that of the solid tool handle, which reduces the contact area between the taper handle and the spindle, leading to the decline of the radial stiffness and positioning accuracy of the BT tool system;
Under the action of the tension of the clamping mechanism, the axial position of the BT tool holder changes and the axial accuracy decreases, thus affecting the machining accuracy;
When the machine tool is stopped, it is difficult to disassemble the tool handle if it is trapped in the spindle hole.
In addition, because the BT tool system only uses the conical surface for positioning and clamping, there are also shortcomings such as low tool change repetition accuracy, low connection stiffness, poor torque transmission ability, large size, heavy weight, and long tool change time.
In order to solve the above problems, Germany, the United States, Japan and other industrial developed countries have successively developed several new tool systems to meet the requirements of modern machining production, such as HSK tool system, KM tool system and BIG plus tool system.
Fig. 8-54 Schematic diagram of traditional BT tool system
（2） Dynamic Balance of High Speed Milling Tool System
In the process of tool system development, dynamic balance has gradually become one of the important indicators to evaluate the quality of a tool system.
In the process of high-speed cutting, the spindle speed is very high.
Therefore, in the high-speed rotating tool system, there is a residual imbalance (asymmetric mass), which will generate a centrifugal force that is square with the speed.
This dynamic load will stimulate the vibration of the tool and the machine tool, resulting in the decline of the surface quality, tool life and spindle bearing life, and even affect the normal processing.
In order to reduce or limit the influence of the dynamic load caused by the residual unbalance, the tool system should be dynamically balanced.
In the process of high speed cutting, the dynamic balance of tool shank is a process to improve the mass distribution of integral parts, so as to reduce the unbalanced mass and its force to an acceptable level.
Dynamic balancing can be achieved by adding weight, removing material (e.g. EPB tool shank), or adjusting (e.g. dynamic balancing ring on EPB boring head).
Material removal is often used for dynamic balancing of the tool system.
The dynamic balancing of the EPB tool handle is shown in Fig. 8-55.
Fig. 8-55 Dynamic balance of EPB tool handle
The quality of dynamic balance can be measured by the quality of dynamic balance, which is determined by the dynamic balance mass G, unit unbalance e and speed n.
The relationship between the above three is shown in Figure 8-56 (excerpted from ISO1940 standard).
The unit unbalance e is also called unbalance eccentricity, that is, the deviation distance of the center of gravity from the rotation axis of the tool holder.
Dynamic balancing reduces e, in other words, making the center of gravity as close to the axis of rotation as possible.
This also reduces the lateral vibration of the rotary tool.
Fig. 8-56 Relationship between dynamic balance mass G, unit unbalance e and speed n
In the process of high speed cutting, the dynamic balance mass G of the tool system depends on the rotating speed n, the tool handle mass M and the residual unbalance U, and the dynamic balance mass G cannot be determined without reference to the rotating speed n.
For the tool handle, it is better to use the unit unbalance e and residual unbalance U to determine its dynamic balance mass, e or U can be generally used for all tool handles, and G must be expressed relative to specific n, where e can be easily calculated by dividing U (obtained by the dynamic balancing machine) by the tool handle mass M.
In general, when the speed of ISO40 tool handle is greater than 10000r/min, it is necessary to seriously consider the dynamic balance problem.
When the speed is above 15000r/min, it is strongly recommended to use the dynamic balance tool system.
When the speed of ISO50 tool handle is greater than 8000r/min, it is necessary to consider the dynamic balance problem.
After the adjustable balancing tool system has been accurately balanced on the dynamic balancing machine, the 40 tool handle is pre balanced to G2.5 at 20000 r/min;
At 15000r/min, 50 toolholders are pre balanced to G2.5.
With the development of high speed machining technology, the importance of tool holder in machining is becoming more and more prominent.
As an important part of tool holder research, dynamic balance will be paid more and more attention by mechanical engineering experts and tool manufacturers.