When milling complex workpieces on a machining center, it's essential to pay attention to several key factors when using CNC end mills:
1. End Mill Clamping
CNC end mills for machining centers typically use a spring collet clamping system, which puts the tool in a cantilevered position during operation. Sometimes, the end mill can gradually protrude from the holder or even fall out, leading to workpiece damage. This usually happens due to an oil film between the tool holder and the end mill shank, reducing the clamping force. End mills are often coated with anti-rust oil at the factory, and if non-water-soluble cutting oil is used, the tool holder may also get oily. This makes it difficult to securely clamp the shank, increasing the risk of the end mill loosening or falling during cutting. Before clamping, both the end of the tool and the inside of the holder should be cleaned with a cleaning solution and dried thoroughly. For larger-diameter end mills, even clean surfaces may not prevent slippage, so using a shank with a flattened section and a side-locking method is recommended. Another issue that can occur is the breakage of the end mill at the holder port, which is often caused by wear on the holder’s port. In such cases, replacing the tool holder is necessary.
2. Vibration of the End Mill
Vibration is common due to the tight fit between the end mill and the holder. This vibration can cause uneven cutting edges, leading to increased cut depth and enlargement, affecting accuracy and tool life. However, in some cases, controlled vibration can be used to achieve a specific groove width by increasing the cutting amount, provided the amplitude remains under 0.02 mm. The less vibration, the better. If vibration occurs, reduce the cutting speed and feed rate. If the problem persists after reducing both by 40%, consider reducing the cutting depth. Resonance issues may stem from excessive cutting speed, low feed rate, insufficient tool rigidity, weak workpiece clamping, or improper workpiece shape. Adjustments like increasing tool rigidity, feed rate, or cutting depth may help resolve this.
3. End Cutting of the End Mill
When machining cavities like molds, especially deep or concave areas, the end mill may need to extend beyond its normal length. Using a long-edge end mill increases the risk of vibration and tool breakage due to deflection. In such cases, a short-blade, long-shank end mill with extended total length is preferable. When using large-diameter end mills on horizontal CNC machines, the weight of the tool can cause deformation, making end edge cutting more likely. If a long-edge end mill is unavoidable, significantly reduce the cutting speed and feed rate to minimize risks.
4. Selection of Cutting Parameters
The choice of cutting speed mainly depends on the material being machined, while the feed rate depends on the material and the end mill diameter. Some foreign tool manufacturers provide cutting parameter tables for reference. However, actual conditions such as machine tool type, tool system, workpiece shape, and clamping method also influence these parameters. Adjust cutting speed and feed rate according to the situation. If tool life is critical, reduce speed and feed slightly. If chip formation is poor, increase speed appropriately.
5. Choice of Cutting Method
Using down-cutting helps protect the blade and improve tool life. Two things to note: first, if using a conventional machine, try to eliminate any play in the feed mechanism; second, if the workpiece has an oxide layer or hardened surface from casting or forging, up-cut milling is recommended.
6. Use of Carbide End Mills
High-speed steel end mills have a broader range of applications and are more forgiving if cutting conditions are slightly off. However, carbide end mills, while highly wear-resistant at high speeds, require strict adherence to cutting conditions. They are not as versatile as HSS tools and must be used within specified limits.
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