CNC Knowledge: What are the requirements for cutting tools and tool holders for high-speed milling?

High-speed machining (HSM) is an important technology widely used in modern milling technology. By applying HSM milling technology, it is not only possible to mill various soft and hard materials, but also achieve excellent workpiece accuracy. This article presents HSM requirements for tools and tool holders.

1

HSM requirements for cutting tools

01. Geometry

Tool vibrations directly affect the surface quality obtained by machining. Therefore, it is extremely important to maintain a uniform cutting force on the tool during HSM finishing to avoid causing tool vibration.

The influence of adjacent geometric properties of the tool on cutting forces:

1) Good concentricity helps the load to be evenly distributed across the cutting edge;

2) Larger cutting edge overlap achieves uniform cutting force characteristics (larger helix angle and number of grooves);

3) A short cutting length promotes obtaining better rigidity (compared to the steep wall of the machine tool, the diameter of the shaft is slightly reduced);

4) The cross section of the core is in the best condition and the stress concentration at the notch is the lowest.

High strength materials can be machined using HSM, which means that the resistance to deformation increases with the hardness of the material being machined. Increased loads on the cutting edge require a stable design of the cutting edge geometry. However, high-speed cutting also generates more frictional heat in the free area of ​​the workpiece surface, which means that the tool clearance angle must be reduced. Therefore, increasing the stability of the cutting edge can only be achieved by reducing the bevel angle. In cases where the material is very hard and the tool brittle, this can even result in a negative bevel angle.

A precisely adjusted radius is ground at the tip of the cutting edge to avoid reaching a red-hot state or local breakage of the cutting edge during sudden heating.

If the shape accuracy of the workpiece is very high, the spherical radius of the finishing tool used has a direct impact on the shape accuracy of the workpiece. Therefore, as a basic requirement, it is very important to use tools with very tight radius tolerances (in the micron range) during the finishing process of very high precision parts.

02. Materials and coatings

The tool material must be harder than the material to be machined. The greater the difference in hardness between the workpiece material and the tool material, the lower the tool wear and the longer the tool life. Since the local temperature is very high, it is also necessary to ensure that the tool material is resistant to oxidation.

Large thermal load fluctuations and the requirement for tool materials to resist oxidation ultimately led to the need for coatings on tool bodies made from fine-grained tungsten carbide.

Proven coating systems such as TiN, TiCN and TiAlCN quickly reach their limits in HSM processing. This is why multi-component coating systems have been developed, based on nitrides with a high aluminum content, combined with other elements such as yttrium, vanadium or tantalum. Higher performance can also be achieved using nanolayer, CBN and PKD structures.

2

HSM requirements for tool holders

Due to the high spindle speeds required for HSM machining, it is best to use the HSK-A and HSK-E toolholder systems. Since the tool holder flange is installed on the spindle head, the tool holder has clear mechanical support in the Z direction. Therefore, at higher speeds it will not be driven into the spindle due to the increase in centrifugal force.

Fundamental errors may have occurred during the machining preparation phase, making vibration reduction and safe process control impossible. To achieve stable HSM machining, it is crucial to balance the tool and tool holder assembly as required and check their coaxiality. The rotation speed limits associated with unbalance must also be taken into account.

An improperly balanced or non-concentric rotating tool system will result in:

1) Very poor surface quality

2) Very short lifespan

3) Poor treatment stability and safety

4) Possible damage to the milling spindle

Imbalance and deviation from ideal concentricity are caused by sudden changes in the machining process. The results can be seen very clearly in the schematic diagram below:

No deviation from perfect concentricity: lower theoretical roughness

Deviation from perfect concentricity: greater theoretical roughness

The quality of balance has an important influence on the dynamic performance of the entire rotating system.

Imbalance is equivalent to the rotation of an eccentric object. This eccentric object can induce centrifugal force, which actually increases as the rotation speed increases. This means that the same unbalance induces 441 times more centrifugal force on a spindle rotating at 42,000 rpm than on a spindle rotating at 2,000 rpm (212 = 441). Therefore, the imbalance of the tool holder device during high-speed machining has particularly obvious adverse consequences.

By applying tool clamping technology in HSM, you can use tool holders with:

• Clamp

• Reducing joint

Alternative systems such as Weldon connectors are not recommended as they have significant disadvantages in HSM processing.

Since the good damping properties of the chuck tool holder bring good results to the roughing process, extremely high rigidity and repeatability can be achieved with the reduction joint. This is essential to achieve a perfect room surface. The use of reducing couplings makes it possible to obtain very precise concentricity (deviation less than 0.003 mm) and high transmissible torques.

The design structure of various reduction tool holders: the transmission torque depends on the design structure of the clamping device, they may differ significantly depending on the design structure;