Safety technology for high speed milling tools

1 Introduction

High-speed milling processes are used in a wide range of automotive, aircraft and mold manufacturing industries. Because the centrifugal force of each part of the cutter when the milling cutter rotates at a high speed has far exceeded the effect of the cutting force itself, it becomes the main load of the tool. When the centrifugal force reaches a certain level, the tool will be deformed or even broken. Therefore, the safety technology of the high-speed milling cutter is studied. It is extremely important for the development of high-speed milling technology.

2 Status of research on safety technology of high speed milling cutter

In the early 1990s, Germany began research on the safety technology of high-speed milling cutters, and drafted the draft standard of DIN6589-1 "Safety requirements for high-speed milling cutters", which stipulated the test methods and standards for the failure of high-speed milling cutters. The guiding opinions on the design, manufacture and use of high-speed milling cutters are proposed, and a unified safety inspection method is specified. This draft standard has become the guiding document for the safety of high-speed milling cutters in various countries.

2.1 Safety failure modes and test methods of high speed milling cutters

The draft standard specifies the speed limit for high-speed cutting. After this speed, the centrifugal force will become the main load of the milling cutter, and safety technology must be adopted. In the relationship between the tool diameter and the high-speed cutting range, the area above the curve is the high-speed cutting range that the milling cutter specified by the standard must pass the safety test: for a single-piece tool (integral or welding tool) with a diameter d1 ≤ 32 mm, the cutting speed exceeds 10000m/mm is the high-speed cutting range; for the assembled machine tool with diameter d1>32mm, the high-speed cutting range is above the line segment BC.

There are two types of high-speed milling cutters that fail safely: deformation and cracking. The safety test methods for different types of milling cutters are also different. For the machine clamp indexable milling cutter, there are two safety test methods: one method is to test at 1.6 times the maximum use speed, the permanent deformation of the tool or the displacement of the part does not exceed 0.05mm; the other method is Tested at 2 times the maximum speed of use, the tool does not rupture (including the screw that clamps the blade is cut, the blade or other clamping element is smashed, the body bursts, etc.). For a monolithic milling cutter, it must be tested at twice the maximum operating speed without bending or breaking.

2.2 High-speed milling cutter strength calculation model

The key to whether the high-speed tool fails under the action of centrifugal force is whether the strength of the cutter body is sufficient and whether the clamp of the clamp is reliable. When the centrifugal force is used as the main load to calculate the strength of the tool body, the error calculated by the classical mechanics theory is very large due to the complexity of the tool shape, and often cannot meet the requirements of safety design.

In order to qualitatively and quantitatively analyze the force and deformation of the structural strength under the centrifugal force during the tool design stage, the stress at different speeds can be calculated by the finite element method, and the failure process and the improved design scheme can be simulated. The finite element calculation model of the high-speed milling cutter includes a cutter body, a cutter body seat, a blade and a clamping screw. Firstly, the elastic deformation of the blade body (including the quality of the parts such as screws and blades) is calculated, and then the separated tool holder is analyzed in detail, and the obtained elastic deformation of the blade body is added as a boundary condition to the blade seat separation body; The tool holder, the blade, the screw and the massless friction pair form a model of the blade clamping system for reliability analysis of the clamping. The finite element model simulates the tilting, sliding, turning of the blade in the holder and the deformation of the screw during clamping. The displacement of the blade and the force of the screw at different speeds can be calculated.

3 Measures to improve the safety of high-speed milling cutter

In combination with the high-speed milling cutter safety standard, through the analysis of the finite element calculation model, in order to meet the safety requirements, the following measures can be taken:

(1) Reduce tool quality, reduce the number of tool components, and simplify tool structure

The relationship between the fracture limit of different tools of the same diameter and the number of tool bodies, the number of tool components and the number of contact faces of the components obtained by the test. It is found that the lighter the tool quality, the smaller the number of components and the contact faces of the components, the less the tool The higher the limit speed of the rupture. It has been found that the use of titanium alloy as the tool body material reduces the quality of the component and increases the cracking limit and the limit speed of the tool. However, due to the sensitivity of titanium alloy to the slit, it is not suitable to manufacture the cutter body. Therefore, some high-speed milling cutters have used high-strength aluminum alloy to manufacture the cutter body.

In the structure of the cutter body, care should be taken to avoid and reduce the stress concentration. The grooves on the cutter body (including the cutter seat groove, the chip pocket and the keyway) may cause stress concentration and reduce the strength of the cutter body. Therefore, the groove and the groove should be avoided as much as possible. The bottom of the groove has a sharp corner. At the same time, the structure of the cutter body should be symmetrical to the rotary shaft so that the center of gravity passes through the axis of the milling cutter. The clamping and adjustment structure of the blade and the tool holder should eliminate the play as much as possible, and the repeatability is required. At present, high-speed milling cutters have widely used HSK tool holders to connect with the machine tool spindles, which greatly improves the rigidity and repeat positioning accuracy of the tool system, which is beneficial to the improvement of the tool breaking limit speed. In addition, the diameter of the machine-clamped high-speed milling cutter reveals a trend that the diameter becomes smaller and the number of teeth is reduced, which also contributes to the improvement of the strength and rigidity of the tool.

(2) Improve the clamping method of the tool

Simulation calculations and fracture test studies have shown that the clamping method of high-speed milling inserts does not allow for the usual frictional clamping, using a blade with a central hole, screw clamping, or a specially designed tool structure to prevent blade 甩fly. The clamping force of the blade seat and the blade is preferably in the same direction as the centrifugal force. At the same time, the pre-tightening force of the screw should be controlled to prevent the screw from being damaged in advance due to overload. For small diameter shank milling cutters, hydraulic chucks or thermal expansion shrink chucks are used for high precision and high stiffness.

(3) Improve the dynamic balance of the tool

Improving the dynamic balance of the tool can greatly improve the safety of the high-speed milling cutter. Because the unbalanced amount of the tool creates an additional radial load on the spindle system, its magnitude is proportional to the square of the speed.

If the mass of the rotating body is m, and the eccentricity of the center of mass and the center of the rotating body is e, the inertial centrifugal force F caused by the unbalanced amount is:

F=emω2=U(n/9549)2

Where: U is the tool system unbalance amount (g·mm), e is the tool system centroid eccentricity (mm), m is the tool system mass (kg), n is the tool system speed (r/min), ω is the tool system Angular velocity (rad/s).

It can be seen from the above formula that improving the dynamic balance of the tool can significantly reduce the centrifugal force and improve the safety of the high speed tool. Therefore, in accordance with the requirements of the draft standard, the milling cutter for high-speed cutting must pass the dynamic balance test and should meet the G4.0 balance quality level specified in ISO1940-1.

4 Conclusion

High-speed milling cutter safety technology is an important part of researching high-speed tools. It should strengthen the quantitative analysis of tool safety, accurately determine the micro-factors that affect the safety of high-speed milling cutters, and solve them from the aspects of tool materials, structure and manufacturing process. Safety of high speed milling cutters.