High Speed Machining (HSM) is an important technology widely used in modern milling technology. By applying HSM milling technology, it is possible not only to mill various soft and hard materials, but also to achieve excellent workpiece precision. This article describes the HSM requirements for tools and holders.
1. HSM requirements for cutting tools
1. Geometry
Tool vibration directly affects 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 tool vibration.
The influence of the adjacent geometrical characteristics of the tool on the cutting force:
• Good concentricity facilitates even load distribution on the cutting edge
• Greater cutting edge overlap for uniform cutting force characteristics (larger helix angle and number of flutes)
• Short cutting length for better rigidity (diameter of shaft is reduced a bit compared to steep machine walls)
• Best core cross-section condition with minimum stress concentration at the notch
High-strength materials can be processed using HSM, which means that the resistance to deformation increases with the hardness of the material to be processed. The increased load on the cutting edge requires a stable design of the cutting edge geometry. However, more frictional heat will be generated in the free area of the workpiece surface under high cutting speed, which means that the clearance angle of the tool 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 material is brittle, it may even result in a negative bevel angle.
Precisely fitting radii are ground at the tip of the blade to avoid red hot conditions or partial edge breakage when suddenly heated.
If the shape accuracy of the workpiece is required to be very high, the radius of the ball portion of the finishing tool used has a direct impact on the shape accuracy of the workpiece to be processed. Therefore, as a basic condition, it is very important to use tools with very tight radius tolerances (in the micron range) during the finishing of very delicate parts.
2. Materials and Coatings
The tool material must be harder than the material to be machined. The greater the hardness difference between the workpiece material and the tool material, the less tool wear and the longer the tool life. Because of the high local temperatures, it is also necessary to ensure that the tool material is resistant to oxidation.
Large fluctuations in thermal load and the need for oxidation resistance of the tool material lead to the eventual need for coatings on fine-grained tungsten carbide tool bodies.
Tried and tested coating systems such as TiN, TiCN and TiAlCN quickly reach their limits in HSM processing. Therefore, multi-component coating systems have been developed, based on nitrides with high aluminum content, in combination with other elements such as yttrium, vanadium or tantalum. Higher performance can also be achieved using nanolayer structures, CBN and PKD.
2. HSM's requirements for tool holders
Due to the high spindle speeds required in HSM machining, it is best to use the HSK-A and HSK-E tool holder systems. Since the tool holder flange is mounted on the spindle head, the tool holder has a defined mechanical support in the Z direction, so at higher speeds it is not dragged into the spindle due to increased centrifugal forces.
Fundamental errors may already have occurred in the process preparation phase, rendering less vibration and safe process control impossible. To achieve stable HSM machining, it is essential to balance and check the alignment of the tool and toolholder assembly as required. The rotational speed limit associated with the unbalanced mass must also be considered.
A poorly balanced or misaligned rotary tool system will result in:
• very poor surface quality
• very low tool life
• Poor process stability and safety
• Possible damage to the milling spindle
The unbalance and deviation from the ideal concentricity caused by abrupt changes in the process can be seen very clearly in the schematic diagram below:
No deviation compared to perfect concentricity: smaller theoretical roughness
Deviation from perfect concentricity: greater theoretical roughness
The balance mass has an important influence on the dynamic performance of the whole rotating system.
Unbalance is equivalent to having an eccentric object rotating. This eccentric body can induce a centrifugal force which increases quadratically with the rotational speed. This means that the same imbalance induces 441 times as much centrifugal force on a spindle at 42,000 rpm as on a spindle at 2,000 rpm (212 = 441). Therefore, an unbalance of the tool holder arrangement in high-speed machining has particularly pronounced adverse consequences.
Applying tool clamping technology in HSM, you can use tool holders with:
• Collets and
• Reducers
Alternative systems such as Weldon connectors are not recommended as they have significant drawbacks in HSM processing.
Due to the good damping properties of the colleted toolholders which give good results during the roughing process, together with the reducing joints a very high degree of rigidity and repeatability can be achieved. This is essential to obtain a perfect workpiece surface. Using reducers allows you to achieve very precise concentricity (less than 0.003 mm deviation) and high transfer torque.
Design structure of various reducing tool holders: the transmission torque depends on the design structure of the clamping equipment; different design structures, they may be very different.
