By Dan Doiron
High-speed cutting (HSC) is defined as a process where cutting parameters such as speed and feed rate are increased several times in order to reach a higher cutting removal rate. Typical applications are hard milling or milling of parts with thin walls. Notably, the trochoidal milling processes in titanium or other materials presents challenges due to using long tools and generating small chips. The machine tool, cutting tool and toolholder need to be adapted in an optimal way for this process.
Toolholders are stressed during high-speed operations, and so maintaining good control on parameters such as run-out accuracy, vibration dampening, and balancing quality are equally important. When trochoidal milling, due to the long tools (up to 5xd) and number of flutes (up to 7) on the end mill, the clamping force is extremely important and a mechanical safe guide is recommended for additional support.
Traditionally, there are several approaches for toolholding in high-speed applications, and each has its own strengths and weaknesses.
Conventional chucks such as standard ER-collet holders or side-Lock chucks (Weldon) have been readily used for many years. However, ER collets are weak in clamping force and run-out, while side-lock chucks are strong in clamping force and weak in runout and dampening.
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Shrinking (Shrink-Fit) is quite popular and has very good runout and balancing behavior but does not excel at vibration dampening because it is very often limited by the maximum feed rate. Tool pull-out is possible, but can be prevented with a special spiral ground groove applied to the tool shank. Note that this ground configuration is usually not a standard item feature.
Improved hydraulic chucks have been developed during the last few years, and due to the oil package, the advantage is very good vibration dampening. The limitations, though, are the maximum dynamic clamping force during radial engagement, and the lack of pull-out safety-features.
A new FPC toolholding technology offers advantages in high-speed applications. This new mechanical toolholding system has a powerful worm gear (1:16 ratio) pulling a special collet with extremely high ratio into a flat angled cone. Clamping forces are extremely high delivering three tons of traction force, and are independent from the tolerances of the tool shaft. Runout deviation is at 3µm which is critical for hard materials and trochoidal applications, and the toolholder has incomparable vibration dampening. The toolholder offers increased accuracy with a 3 X D tool length, and concentricity is ≤ 3 µm to guarantee long tool life and quality workpiece surface finishes.
Because the chuck has a simple mechanical drive actuated with a hex wrench design, quick tool changes can be done in seconds. For maximum safety, the strongest clamping force is provided to prevent the possibility of pull-out with the use of the pin lock collet.
The new toolholders are the fastest on the market today. In a speed comparison with four chuck technologies, using a 20 mm end mill in the same material, the new toolholder enabled the feed rate to be increased by 30% with no loss in performance.
Here is a chart that indicates the strengths (+) and weaknesses (-) of each toolholding solution for high-speed cutting applications:
Looking forward, it is an ongoing initiative to optimize toolholders for maximum speed and power of the spindles and cutting tools in materials.
Toolholder manufacturers will continue to develop systems to improve and reduce any deficiencies, resulting in higher cutting removal rates.
Dan Doiron, is the milling product manager, Emuge-Franken, USA.
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