When milling complex workpieces on a milling machining center, the following issues should be noted when using CNC end mills.
End Mill Clamping
Most end mills used in machining centers are clamped by spring clip sets and are in a cantilever state when used. During the milling process, the end mill may gradually extend from the chuck or even fall off completely, causing the workpiece to be scrapped. The reason is generally because there is an oil film between the inner hole of the chuck and the outer diameter of the end mill shank, resulting in insufficient clamping force. End mills are usually coated with anti-rust oil when they leave the factory. If non-water-soluble cutting oil is used during cutting, a layer of mist-like oil film will also be attached to the inner hole of the chuck. When there is an oil film on both the shank and the chuck, it is difficult for the chuck to firmly clamp the shank, and the end mill is easy to loosen and fall during processing. Therefore, before clamping the end mill, the end mill shank and the inner hole of the chuck should be cleaned with cleaning fluid, and then clamped after drying.
When the diameter of the end mill is large, even if the shank and chuck are very clean, the tool may still fall off. At this time, a shank with a flattened notch and a corresponding side locking method should be selected.
Another problem that may occur after the end mill is clamped is that the end mill breaks at the end of the tool holder during processing. The reason is generally because the tool holder has been used for too long and the end of the tool holder has been worn into a cone. At this time, a new tool holder should be replaced.
Vibration of End Mill
Since there is a small gap between the end mill and the tool holder, the tool may vibrate during the processing. Vibration will make the cutting amount of the end mill circumferential edge uneven, and the cutting expansion amount will increase compared with the original value, affecting the processing accuracy and tool life. However, when the width of the processed groove is too small, the tool can also be vibrated on purpose. The required groove width is obtained by increasing the cutting expansion amount, but in this case, the maximum amplitude of the end mill should be limited to less than 0.02mm, otherwise stable cutting cannot be performed. In normal processing, the smaller the vibration of the end mill, the better.
When tool vibration occurs, consider reducing the cutting speed and feed speed. If there is still a large vibration after both have been reduced by 40%, consider reducing the cutting amount.
If resonance occurs in the processing system, the reason may be due to factors such as excessive cutting speed, low feed speed, insufficient rigidity of the tool system, insufficient clamping force of the workpiece, and workpiece shape or workpiece clamping method. At this time, measures such as adjusting the cutting amount, increasing the rigidity of the tool system, and increasing the feed speed should be taken.
End Cutting of End Mills
In CNC milling of workpiece cavities such as molds, when the cutting point is a concave part or a deep cavity, the extension of the end mill needs to be lengthened. If a long-edge end mill is used, due to the large deflection of the tool, it is easy to vibrate and cause tool breakage. Therefore, during the processing, if only the blade near the end of the tool is required to participate in the cutting, it is best to use a short-edge long-shank end mill with a longer total tool length. When using a large-diameter end mill to process workpieces on a horizontal CNC machine tool, due to the large deformation caused by the weight of the tool, special attention should be paid to the problems that are prone to end cutting. In the case where a long-edge end mill must be used, the cutting speed and feed speed must be greatly reduced.
Selection of Cutting Parameters
The selection of cutting speed mainly depends on the material of the workpiece being processed. The selection of feed speed mainly depends on the material of the workpiece being processed and the diameter of the end mill. The tool samples of some foreign tool manufacturers are accompanied by a tool cutting parameter selection table for reference. However, the selection of cutting parameters is also affected by many factors such as machine tools, tool systems, the shape of the workpiece being processed, and the clamping method. The cutting speed and feed speed should be adjusted appropriately according to the actual situation.
When tool life is the priority factor, the cutting speed and feed speed can be appropriately reduced. When the chip separation condition is not good, the cutting speed can be appropriately increased.
Selection of Cutting Method
Down milling is helpful to prevent blade damage and increase tool life. However, there are two points to note:
- If ordinary machine tools are used for processing, efforts should be made to eliminate the gap in the feed mechanism.
- When there is an oxide film or other hardened layer formed by casting or forging processes on the surface of the workpiece, reverse milling should be used.
Use of Carbide End Mills
The application scope and application requirements of high-speed steel end mills are relatively broad, and even if the cutting conditions are slightly inappropriate, there will not be too many problems. Although carbide end mills have good wear resistance during high-speed cutting, their application scope is not as wide as that of high-speed steel end mills, and the cutting conditions must strictly meet the use requirements of the tool.
