When machining complex workpieces on a milling center, it's essential to pay attention to several key factors when using CNC end mills. Here are some important considerations:
1. End Mill Clamping
CNC end mills typically use a spring collet system for clamping, and they operate in a cantilevered position. During the cutting process, the end mill may gradually extend from the tool holder or even fall out, which can damage the workpiece. This often happens because of oil residue between the tool holder’s inner hole and the end mill’s shank, reducing the clamping force. End mills are usually coated with anti-rust oil before shipping, and if non-water-soluble cutting oil is used, the tool holder may also get oily. To avoid this, always clean both the end mill shank and the tool holder with a suitable cleaning solution, then dry them thoroughly before clamping. For larger diameter end mills, even clean surfaces might not prevent slippage, so consider using a shank with a flattened section and a side-locking method. Another issue could be the end mill breaking at the tool holder port, which is often due to wear on the tool holder’s interface. In such cases, replacing the tool holder is recommended.
2. Vibration of the End Mill
Vibration is common due to the tight fit between the end mill and the tool holder. This can lead to uneven cutting edges, increasing the cut amount and causing inaccuracies. When working on narrow grooves, controlled vibration can help achieve the desired width, but the amplitude should not exceed 0.02 mm. The less vibration, the better. If vibration occurs, try reducing the cutting speed and feed rate. If the problem persists after reducing both by 40%, consider decreasing the depth of cut. Resonance may also occur due to high cutting speeds, low feed rates, poor rigidity, or improper workpiece clamping. Adjustments like increasing the tool rigidity or feed rate can help resolve these issues.
3. End Cutting of the End Mill
When machining deep cavities or concave areas, longer end mills may be needed. However, long-edge tools are more prone to vibration and breakage. In such cases, using a short-flute, long-shank end mill is preferable. On horizontal CNC machines, large-diameter end mills can experience deformation due to their own weight, making end edge cutting more likely. If a long-edge end mill must be used, significantly reduce the cutting speed and feed rate to minimize the risk of damage.
4. Selection of Cutting Parameters
The choice of cutting speed depends mainly on the material being machined. Feed rate is influenced by both the material and the end mill’s diameter. Some foreign tool manufacturers provide reference tables for optimal parameters, but actual conditions—such as machine tool capabilities, tool system rigidity, and workpiece shape—must also be considered. Adjust the speed and feed rate based on real-time performance. If tool life is a priority, reduce speed and feed slightly. If chip evacuation is poor, increase the cutting speed appropriately.
5. Choice of Cutting Method
Down-cutting helps protect the cutting edges and extends tool life. However, there are two things to keep in mind: first, ensure that the feed mechanism has no play if using conventional machines; second, if the workpiece surface has an oxide layer or hardened layer from casting or forging, up-cut milling is recommended.
6. Use of Carbide End Mills
Carbide end mills offer excellent wear resistance at high speeds, but they require strict adherence to cutting conditions. Unlike high-speed steel end mills, which are more forgiving, carbide tools need precise control over speed, feed, and depth of cut to perform optimally and avoid premature failure.
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