The cross-sectional flute geometry is a decisive factor for determining the way chips flow.
The image to the right (a) shows a general-purpose cross-sectional of a general 4-flute end mill. Due to the geometry of the flute surface (convex heel part B, and a hollow A) chip flow is restricted. However (b) shows a cross-sectional view of a flute geometry with good chip discharge properties. As there is no convex heel part B the chip flow becomes smoother and the chip separation is improved. This is especially effective for high efficiency slotting.
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Cross-sectional flute geometry of rib type end mills
The image to the right shows a cross sectional flute geometry of a rib type end mill. (a) is regular general-purpose geometry and (b) is a special octagonal geometry. When rib slotting, thin, long end mills are usually used but they tend to bend and vibrate, making it difficult to obtain a good surface finish. To improve surface finishes, it is necessary to increase the rigidity of the end mill by increasing the web thickness.
Additionally, to reduce the tool contact length to avoid vibration, the helix angle has to be decreased. One type of geometry that satisfies these requirements is a special octagonal cross-sectional type (b). Using this type of end mill with a low helix angle will reduce vibration and produce better surface finishes as shown in the image below.