Flute length and overhang
Usually, the flute length of an end mill has to be a little longer than the length to be machined. However, too much flute length is not desirable. This is because with a longer length of cut, the overhang also has to be increased and will decrease rigidity.
Overhang affects the rigidity of end mills.
End mill deflection is proportional to the length of overhang, L (cubed) and is found by using formula shown in the previous chapter.
This means that even with a slightly longer overhang, the amount of deflection will become much larger. For example, if the overhang doubles with the same diameter, the deflection will be 8 fold as shown in figure to the right. For example, the deflection of a machined surface when using high-speed steel end mill is approx. 0.2mm, if the tool overhang is doubled, the deflection will be increased to 0.2mm × 8 = 1.6mm under the same cutting loads. This shows that a very large deflection will be produced on the machined surface.
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Y2 = 8Y1
Y1, Y2: Deflection
W : Load
Effect of length of cut on tool life
The image to the right shows a tool life comparison when 2 end mills (medium and short length) are used to perform a slotting operation with the same axial depth of cut.
It shows that even with the same depth of cut, as the overhang increases, the deflection increases and machining becomes unstable thereby shortening tool life.
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