High-speed machining (HSM) is an important technique widely used in modern milling technology. By applying HSM milling techniques, not only can various soft and hard materials be milled, but excellent workpiece accuracy can be achieved. This article introduces the requirements for tools and tool holders in HSM.
Here are some requirements for cutting tools in HSM (High Speed Machining)
Tool vibration directly affects the surface quality achieved in machining. Therefore, maintaining uniform cutting forces on the tool is extremely important in HSM finishing to avoid exciting tool vibration.
Adjacent tool geometries affecting cutting forces: Good concentricity allows even load distribution along the cutting edge; Larger cutting edge overlap provides more uniform cutting force characteristics (larger helix and number of flutes); Shorter cutting length improves rigidity (axis diameter reduced somewhat relative to machine tapers); Best core condition with minimal stress concentration at the gullet.
HSM can machine high-strength materials, meaning deformation resistance increases with work material hardness. The increased loads on the cutting edge require a stable edge geometry design. However, the free surface areas on the workpiece also generate more frictional heat at high-speed cutting conditions, meaning the tool clearance angles must be reduced. Thus, increasing cutting-edge stability can only be achieved by decreasing the angle of inclination. In very hard materials and brittle tool materials, this can even lead to negative angles of inclination.
Precisely matched radii are ground on the cutting edge peaks and valleys to avoid reaching a glowing red-hot state or local edge breakage when sudden heat occurs.
If high shape accuracy requirements exist for the workpiece, the ball nose radius on the finishing tool directly impacts the form precision achieved on the machined part. Therefore, as a basic requirement, using tools with very tight radius tolerances (in the micron range) is very important in finishing highly precise components.
material and coating
The tool material must be harder than the workpiece material. The greater the hardness differential between work and tool materials, the lower the tool wear and longer the tool life. Oxidation resistance is also required due to the locally high temperatures.
The high thermal load fluctuations and demands on the oxidation resistance of the tool material ultimately require coatings on fine-grained tungsten carbide substrates.
Coating systems proven and tested in HSM like TiN, TiCN and TiAlCN quickly reach their limits. Thus, multi-component coating systems were developed based on high aluminum nitrides combined with other elements like yttrium, vanadium or tantalum. Even higher performance can be achieved using nano-layered structures, CBN and PKD.
Requirements for tool holders in HSM
HSK-A and HSK-E tool holder systems are preferable for the very high spindle speeds required in HSM. Since the tool holder flange is mounted on the spindle head, there is definite mechanical support for the holder in the Z direction. This prevents excessive pull-in from centrifugal forces at higher RPMs.
Fundamental errors in process preparation may already occur that make small vibrations and safe process control impossible. To achieve stable HSM, balancing tools and holders to spec and checking concentricity is critical. Limits related to unbalanced masses must also be considered for rotational speeds.
Poorly balanced or eccentric rotating tool systems lead to:
（1）Very poor surface quality
（2）Very short tool life
（3）Bad process stability and safety
（4）Potential damage to milling spindles
Imbalance and deviation from ideal concentricity arising in the machining process are seen very clearly in the principle sketches below:
Compared to perfect concentricity without deviation: smaller theoretical roughness
Compared to perfect concentricity with deviation: larger theoretical roughness
The balanced mass has a significant impact on the overall dynamic performance of the rotating system.
Imbalance is equivalent to having an eccentric object rotating. This eccentricity can excite centrifugal forces that increase with the square of rotational speed. This means the same imbalance on a 42,000 rpm spindle generates 441 times (212 = 441) the centrifugal force compared to a 2,000 rpm spindle. Thus, imbalance in tool holders has particularly adverse consequences in high-speed machining.
Tool clamping techniques for HSM should be used with holders such as collets and shrink-fit chucks. Alternative systems like Weldon flat contacts are not recommended due to significant disadvantages in HSM.
While collets offer good damping for roughing due to their clamping effect, maximum rigidity and repeatability are achieved with shrink-fit chucks. This is critical for perfect workpiece surfaces. Shrink-fit chucks enable very precise concentricity (deviations below 0.003 mm) and high transmission torque.
Design configurations of various shrink-fit holders: The transmitted torque depends on the design configuration of the clamping device. It can vary greatly depending on the configuration.
What are the special requirements for cutting tool geometry in high-speed milling?
High-speed milling requires stable tool geometries such as large helix angles, uniform cutting-edge overlap, and small rake angles to resist high temperatures, etc.
What are the requirements for oxidation resistance of tools in high-speed machining?
High-speed milling generates high temperatures, requiring the tools to have sufficient oxidation resistance, usually necessitating oxidation-resistant coatings on cemented carbide substrates.
What kinds of coatings are commonly used in high-speed milling?
TiAlN, TiSiN, AlCrN and other multi-component coatings have excellent high-temperature oxidation resistance.
Why does high-speed machining require special tool interfacing systems?
HSK interfaces provide higher rigidity to resist centrifugal forces at high RPMs and ensure precise concentricity.
Why does high-speed milling require special attention to tool balancing?
Imbalance at high speeds generates large centrifugal forces, damaging surface finish and impacting tool life.
How are suitable tool materials selected for high-speed machining?
High thermal strength tungsten carbides are preferred, combined with cobalt-based and iron-free materials to increase softening resistance.
What are the key performance indicators for tool holders in high-speed milling?
Rigidity, heat resistance, corrosion resistance, thermal conductivity, vibration damping, concentricity tolerances.